Jupiter is the fifth planet from our sun and the largest planet in the solar system.
Jupiter's stripes and swirls are cold, windy clouds of ammonia and water.
The atmosphere is mostly hydrogen and helium, and its iconic Great Red Spot is a giant storm bigger than Earth that has raged for hundreds of years.
Jupiter is surrounded by 53 confirmed moons, as well as 14 provisional ones — for a possible total of 67 moons.
Scientists are most interested in the "Galilean satellites" — the four largest moons discovered by Galileo Galilei in 1610: Europa, Callisto, Ganymede and Io.
Jupiter also has three rings, but they are very hard to see and not nearly as intricate as Saturn's.
Jupiter is named for the king of ancient Roman gods.
With a radius of 43,440.7 miles (69,911 kilometers), Jupiter is 11 times wider than Earth.
If Earth were the size of a nickel, Jupiter would be about as big as a basketball.
From an average distance of 484 million miles (778 million kilometers), Jupiter is 5.2 astronomical units away from the sun.
One astronomical unit (abbreviated as AU), is the distance from the sun to Earth.
From this distance, it takes sunlight 43 minutes to travel from the sun to Jupiter.
Jupiter has the shortest day in the solar system.
One day on Jupiter takes only about 10 hours (the time it takes for Jupiter to rotate or spin around once), and Jupiter makes a complete orbit around the sun (a year in Jovian time) in about 12 Earth years (4,333 Earth days).
Jupiter took shape when the rest of the solar system formed about 4.5 billion years ago, when gravity pulled swirling gas and dust in to become this gas giant.
Jupiter took most of the mass left over after the formation of the sun, ending up with more than twice the combined material of the other bodies in the solar system.
In fact, Jupiter has the same ingredients as a star, but it did not grow massive enough to ignite.
About 4 billion years ago, Jupiter settled into its current position in the outer solar system, where it is the fifth planet from the sun.
Its equator is tilted with respect to its orbital path around the sun by just 3 degrees.
This means Jupiter spins nearly upright and does not have seasons as extreme as other planets do.
The composition of Jupiter is similar to that of the sun — mostly hydrogen and helium.
Deep in the atmosphere, pressure and temperature increase, compressing the hydrogen gas into a liquid.
This gives Jupiter the largest ocean in the solar system — an ocean made of hydrogen instead of water.
Scientists think that, at depths perhaps halfway to the planet's center, the pressure becomes so great that electrons are squeezed off the hydrogen atoms, making the liquid electrically conducting like metal.
Jupiter's fast rotation is thought to drive electrical currents in this region, generating the planet's powerful magnetic field.
It is still unclear if, deeper down, Jupiter has a central core of solid material or if it may be a thick, super-hot and dense soup.
It could be up to 90,032 degrees Fahrenheit (50,000 degrees Celsius) down there, made mostly of iron and silicate minerals (similar to quartz).
As a gas giant, Jupiter doesn't have a true surface.
The planet is mostly swirling gases and liquids.
While a spacecraft would have nowhere to land on Jupiter, it wouldn't be able to fly through unscathed either.
The extreme pressures and temperatures deep inside the planet crush, melt and vaporize spacecraft trying to fly into the planet.
Jupiter's appearance is a tapestry of colorful cloud bands and spots.
The gas planet likely has three distinct cloud layers in its "skies" that, taken together, span about 44 miles (71 kilometers).
The top cloud is probably made of ammonia ice, while the middle layer is likely made of ammonium hydrosulfide crystals.
The innermost layer may be made of water ice and vapor.
The vivid colors you see in thick bands across Jupiter may be plumes of sulfur and phosphorus-containing gases rising from the planet's warmer interior.
Jupiter's fast rotation — spinning once every 10 hours — creates strong jet streams, separating its clouds into dark belts and bright zones across long stretches.
With no solid surface to slow them down, Jupiter's spots can persist for many years.
Stormy Jupiter is swept by over a dozen prevailing winds, some reaching up to 335 miles per hour (539 kilometers per hour) at the equator.
The Great Red Spot, a swirling oval of clouds twice as wide as Earth, has been observed on the giant planet for more than 300 years.
More recently, three smaller ovals merged to form the Little Red Spot, about half the size of its larger cousin.
Scientists do not yet know if these ovals and planet-circling bands are shallow or deeply rooted to the interior.
Jupiter's environment is probably not conducive to life as we know it.
The temperatures, pressures and materials that characterize this planet are most likely too extreme and volatile for organisms to adapt to.
While planet Jupiter is an unlikely place for living things to take hold, the same is not true of some of its many moons.
Europa is one of the likeliest places to find life elsewhere in our solar system.
There is evidence of a vast ocean just beneath its icy crust, where life could possibly be supported.
With four large moons and many smaller moons, Jupiter forms a kind of miniature solar system.
Jupiter has 53 confirmed moons, as well as 14 provisional ones — for a possible total of 67 moons.
These newly discovered, provisional moons are reported by astronomers and acknowledged with a temporary designation by the International Astronomical Union.
Once their orbits are confirmed, they are included in Jupiter's confirmed moon count.
Jupiter's four largest moons — Io, Europa, Ganymede and Callisto — were first observed by the astronomer Galileo Galilei in 1610 using an early version of the telescope.
These four moons are known today as the Galilean satellites, and they're some of the most fascinating destinations in our solar system.
Io is the most volcanically active body in the solar system.
Ganymede is the largest moon in the solar system (even bigger than the planet Mercury).
Callisto's very few small craters indicate a small degree of current surface activity.
A liquid-water ocean with the ingredients for life may lie beneath the frozen crust of Europa, making it a tempting place to explore.
Discovered in 1979 by NASA's Voyager 1 spacecraft, Jupiter's rings were a surprise, as they are composed of small, dark particles and are difficult to see except when backlit by the sun.
Data from the Galileo spacecraft indicate that Jupiter's ring system may be formed by dust kicked up as interplanetary meteoroids smash into the giant planet's small innermost moons.
The Jovian magnetosphere is the region of space influenced by Jupiter's powerful magnetic field.
It balloons 600,000 to 2 million miles (1 to 3 million kilometers) toward the sun (seven to 21 times the diameter or Jupiter itself) and tapers into a tadpole-shaped tail extending more than 600 million miles (1 billion kilometers) behind Jupiter, as far as Saturn's orbit.
Jupiter's enormous magnetic field is 16 to 54 times as powerful as that of the Earth.
It rotates with the planet and sweeps up particles that have an electric charge.
Near the planet, the magnetic field traps swarms of charged particles and accelerates them to very high energies, creating intense radiation that bombards the innermost moons and can damage spacecraft.
Jupiter's magnetic field also causes some of the solar system's most spectacular aurorae at the planet's poles.
While Jupiter has been known since ancient times, the first detailed observations of this planet were made by Galileo Galilei in 1610 with a small telescope.
More recently, this planet has been visited by passing spacecraft, orbiters and probes.
Pioneer 10 and 11 and Voyager 1 and 2 were the first to fly by Jupiter in the 1970s, and since then we've sent Galileo to orbit the gas giant and drop a probe into its atmosphere.
Cassini took detailed photos of Jupiter on its way to neighboring Saturn, as did New Horizons on its quest for Pluto and the Kuiper Belt.
The next mission is Juno, which arrived in the Jovian system in July 2016.
The biggest planet in our solar system, Jupiter also has a large presence in pop culture, including many movies, TV shows, video games and comics.
Jupiter was a notable destination in the Wachowski siblings' science fiction spectacle Jupiter Ascending, while various Jovian moons provide settings for Cloud Atlas, Futurama, Power Rangers, and Halo, among many others.
In Men in Black when Agent J — played by Will Smith — mentions he thought one of his childhood teachers was from Venus, Agent K — played by Tommy Lee Jones — replies that she is actually from one of Jupiter's moons.
The sun at the heart of our solar system is a yellow dwarf star, a hot ball of glowing gases.
Its gravity holds the solar system together, keeping everything from the biggest planets to the smallest particles of debris in its orbit.
Electric currents in the sun generate a magnetic field that is carried out through the solar system by the solar wind — a stream of electrically charged gas blowing outward from the sun in all directions.
The connection and interactions between the sun and Earth drive the seasons, ocean currents, weather, climate, radiation belts and aurorae.
Though it is special to us, there are billions of stars like our sun scattered across the Milky Way galaxy.
With a radius of 432,168.6 miles (695,508 kilometers), our sun is not an especially large star — many are several times bigger — but it is still far more massive than our home planet: 332,946 Earths match the mass of the sun.
The sun's volume would need 1.3 million Earths to fill it.
The sun is 93 million miles (about 150,000,000 kilometers) from Earth.
Its nearest stellar neighbor is the Alpha Centauri triple star system: Proxima Centauri is 2.24 light years away, and Alpha Centauri A and B — two stars orbiting each other — are 4.37 light years away.
A light year is the distance light travels in one year, which is equal to 5,878,499,810,000 miles or 9,460,528,400,000 kilometers.
The sun, and everything that orbits it, is located in the Milky Way galaxy.
More specifically, our sun is in a spiral arm called the Orion Spur that extends outward from the Sagittarius arm.
From there, the sun orbits the center of the Milky Way Galaxy, bringing the planets, asteroids, comets and other objects along with it.
Our solar system is moving with an average velocity of 450,000 miles per hour (720,000 kilometers per hour).
But even at this speed, it takes us about 230 million years to make one complete orbit around the Milky Way.
The sun rotates as it orbits the center of the Milky Way.
Its spin has an axial tilt of 7.25 degrees with respect to the plane of the planets' orbits.
Since the sun is not a solid body, different parts of the sun rotate at different rates.
At the equator, the sun spins around once about every 25 days, but at its poles the sun rotates once on its axis every 36 Earth days.
The sun and the rest of the solar system formed from a giant, rotating cloud of gas and dust called a solar nebula about 4.5 billion years ago.
As the nebula collapsed because of its overwhelming gravity, it spun faster and flattened into a disk.
Most of the material was pulled toward the center to form our sun, which accounts for 99.8% of the mass of the entire solar system.
Like all stars, the sun will someday run out of energy.
When the sun starts to die, it will swell so big that it will engulf Mercury and Venus and maybe even Earth.
Scientists predict the sun is a little less than halfway through its lifetime and will last another 6.5 billion years before it shrinks down to be a white dwarf.
The sun's enormous mass is held together by gravitational attraction, producing immense pressure and temperature at its core.
The sun has six regions: the core, the radiative zone, and the convective zone in the interior; the visible surface, called the photosphere; the chromosphere; and the outermost region, the corona.
At the core, the temperature is about 27 million degrees Fahrenheit (15 million degrees Celsius), which is sufficient to sustain thermonuclear fusion.
This is a process in which atoms combine to form larger atoms and in the process release staggering amounts of energy.
Specifically, in the sun's core, hydrogen atoms fuse to make helium.
The energy produced in the core powers the sun and produces all the heat and light the sun emits.
Energy from the core is carried outward by radiation, which bounces around the radiative zone, taking about 170,000 years to get from the core to the top of the convective zone.
The temperature drops below 3.5 million degrees Fahrenheit (2 million degrees Celsius) in the convective zone, where large bubbles of hot plasma (a soup of ionized atoms) move upwards.
The surface of the sun — the part we can see — is about 10,000 degrees Fahrenheit (5,500 degrees Celsius).
That's much cooler than the blazing core, but it's still hot enough to make carbon, like diamonds and graphite, not just melt, but boil.
The sun, like others stars, is a ball of gas.
In terms of the number of atoms, it is made of 91.0% hydrogen and 8.9% helium.
By mass, the sun is about 70.6% hydrogen and 27.4% helium.
The surface of the sun, the photosphere, is a 300-mile-thick (500-kilometer-thick) region, from which most of the sun's radiation escapes outward.
This is not a solid surface like the surfaces of planets.
Instead, this is the outer layer of the gassy star.
We see radiation from the photosphere as sunlight when it reaches Earth about eight minutes after it leaves the sun.
The temperature of the photosphere is about 10,000 degrees Fahrenheit (5,500 degrees Celsius).
Above the photosphere lie the tenuous chromosphere and the corona (crown), which make up the thin solar atmosphere.
This is where we see features such as sunspots and solar flares.
Visible light from these top regions is usually too weak to be seen against the brighter photosphere, but during total solar eclipses, when the moon covers the photosphere, the chromosphere looks like a red rim around the sun, while the corona forms a beautiful white crown with plasma streamers narrowing outward, forming shapes that look like flower petals.
Strangely, the temperature in the sun's atmosphere increases with altitude, reaching as high as 3.5 million degrees Fahrenheit (2 million degrees Celsius).
The source of coronal heating has been a scientific mystery for more than 50 years.
The sun itself is not a place conducive to living things, with its hot, energetic mix of gases and plasma.
But the sun has made life on Earth possible, providing warmth as well as energy that organisms like plants use to form the basis of many food chains.
The sun doesn't have any moons; instead, it has planets and their moons, along with asteroids, comets, and other objects.
The sun does not have rings.
The electric currents in the sun generate a complex magnetic field that extends out into space to form the interplanetary magnetic field.
The volume of space controlled by the sun's magnetic field is called the heliosphere.
The sun's magnetic field is carried out through the solar system by the solar wind — a stream of electrically charged gas blowing outward from the sun in all directions.
Since the sun rotates, the magnetic field spins out into a large rotating spiral, known as the Parker spiral.
The sun doesn't behave the same way all the time.
It goes through phases of its own solar cycle.
Approximately every 11 years, the sun's geographic poles change their magnetic polarity.
When this happens, the sun's photosphere, chromosphere and corona undergo changes from quiet and calm to violently active.
The height of the sun's activity, known as solar maximum, is a time of solar storms: sunspots, solar flares and coronal mass ejections.
These are caused by irregularities in the sun's magnetic field and can release huge amounts of energy and particles, some of which reach us here on Earth.
This space weather can damage satellites, corrode pipelines and affect power grids.
A number of ancient cultures built stone structures or modified natural rock formations to mark the motions of the sun.
They charted the seasons, created calendars and monitored solar and lunar eclipses.
Modern heliophysics research (the study of the sun) and exploration aims to explore the complex sun-Earth system.
This includes the sun and its effects on Earth and the solar system, as well as the conditions in space that future explorers will experience.
For more information, visit Heliophysics at the NASA Science Mission Directorate.
The sun has inspired mythological stories in cultures around the world, including those of the ancient Egyptians, the Aztecs of Mexico, Native American tribes of North and South America, the Chinese, and many thers.
In more recent times, the sun adorns everything from album covers, such as Sublime's iconic 1992 debut, to packages of raisins, while it influences stories in comics, theatrical films and everything in between.
If you're Superman (or a fellow Kryptonian), your powers are heightened by the yellow glow of our sun, and you can even dispose of dangerous materials like Superboy once did, by hurling them into the sun.
And in the 2007 film Sunshine, the sun is dying, leaving Earth in a state of deep freeze.
To save humanity, a crewed spacecraft is on its way to reignite the sun with a nuclear bomb, though things don't go quite as planned.
Mercury's eccentric orbit takes the small planet as close as 47 million km (29 million miles) and as far as 70 million km (43 million miles) from the sun.
If one could stand on the scorching surface of Mercury when it is at its closest point to the sun, the sun would appear more than three times as large as it does when viewed from Earth.
Temperatures on Mercury's surface can reach 800 degrees Fahrenheit (430 degrees Celsius).
Because the planet has no atmosphere to retain that heat, nighttime temperatures on the surface can drop to -290 degrees Fahrenheit (-180 degrees Celsius).
Because Mercury is so close to the sun, it is hard to directly observe from Earth except during dawn or twilight.
Mercury makes an appearance indirectly -- 13 times each century, observers on Earth can watch Mercury pass across the face of the sun, an event called a transit.
These rare transits fall within several days of 8 May and 10 November.
The first two transits of Mercury in the 21st century occurred 7 May 2003, and 8 November 2006.
The next are 9 May 2016, and 11 November 2019.
Mercury speeds around the sun every 88 days, traveling through space at nearly 50 km (31 miles) per second, faster than any other planet.
One Mercury solar day (one day-night cycle) equals 175.97 Earth days.
Instead of an atmosphere, Mercury possesses a thin exosphere made up of atoms blasted off the surface by the solar wind and striking micrometeoroids.
Because of solar radiation pressure, the atoms quickly escape into space and form a tail of neutral particles.
Though Mercury's magnetic field at the surface has just one percent the strength of Earth's, it interacts with the magnetic field of the solar wind to episodically create intense magnetic tornadoes that funnel the fast, hot solar wind plasma down to the surface.
When the ions strike the surface, they knock off neutrally charged atoms and send them on a loop high into the sky.
Mercury's surface resembles that of Earth's Moon, scarred by many impact craters resulting from collisions with meteoroids and comets.
Very large impact basins, including Caloris (1,550 km, or 960 miles, in diameter) and Rachmaninoff (306 km, or 190 miles), were created by asteroid impacts on the planet's surface early in the solar system's history.
While there are large areas of smooth terrain, there are also lobe-shaped scarps or cliffs, some hundreds of miles long and soaring up to a mile high, formed as the planet's interior cooled and contracted over the billions of years since Mercury formed.
Mercury is the second densest planet after Earth, with a large metallic core having a radius of about 2,000 km (1,240 miles), about 80 percent of the planet's radius.
In 2007, researchers used ground-based radars to study the core, and found evidence that it is partly molten (liquid).
Mercury's outer shell, comparable to Earth's outer shell (called the mantle and crust), is only about 400 km (250 miles) thick.
The first spacecraft to visit Mercury was Mariner 10, which imaged about 45 percent of the surface.
NASA's MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission flew by Mercury three times in 2008-2009 and has been in orbit around the planet since 18 March 2011.
Almost the entire planet has now been imaged, revealing a surface that has been shaped both by extensive volcanism and impacts.
Data from MESSENGER's scientific instruments have provided a trove of scientific discoveries.
These include the identification of a new landform known as hollows, measurements indicating that Mercury has a remarkably high abundance of the volatile elements sulfur and potassium, and the discoveries that Mercury's magnetic field is offset relative to the planet's equator and that the planet has a highly unusual internal structure.
In 1991, astronomers on Earth using radar observations showed that Mercury may have water ice at its north and south poles inside deep craters.
MESSENGER observations have shown that the materials identified by radar are present only in regions of permanent shadow, consistent with the idea that they are cold enough to preserve water ice, despite the extreme high temperatures experienced by sunlit parts of the planet.
Mercury is appropriately named for the swiftest of the ancient Roman gods.
Mercury, the god of commerce, is the Roman counterpart to the ancient Greek god Hermes, the messenger of the gods.
Venus is the second planet from the sun and our closest planetary neighbor.
Similar in structure and size to Earth, Venus spins slowly in the opposite direction most planets do.
Its thick atmosphere traps heat in a runaway greenhouse effect, making it the hottest planet in our solar system with surface temperatures hot enough to melt lead.
Glimpses below the clouds reveal volcanoes and deformed mountains.
Venus is named for the ancient Roman goddess of love and beauty, the counterpart to the Greek goddess Aphrodite.
With a radius of 3,760 miles (6,052 kilometers), Venus is roughly the same size as Earth, just slightly smaller.
From an average distance of 67 million miles (108 million kilometers), Venus is 0.7 astronomical units away from the sun.
One astronomical unit (abbreviated as AU), is the distance from the sun to Earth.
From this distance, it takes sunlight 6 minutes to travel from the sun to Venus.
Venus' rotation and orbit are unusual in several ways.
Venus is one of just two planets that rotate from east to west.
Only Venus and Uranus have this "backwards" rotation.
It completes one rotation in 243 Earth days — the longest day of any planet in our solar system, even longer than a whole year on Venus.
But the sun doesn't rise and set each "day" on Venus like it does on most other planets.
On Venus, one day-night cycle takes 117 Earth days because Venus rotates in the direction opposite of its orbital revolution around the sun.
Venus makes a complete orbit around the sun (a year in Venusian time) in 225 Earth days or slightly less than two Venusian day-night cycles.
Its orbit around the sun is the most circular of any planet — nearly a perfect circle.
Other planet's orbits are more elliptical, or oval-shaped.
With an axial tilt of just 3 degrees, Venus spins nearly upright, and so does not experience noticeable seasons.
When the solar system settled into its current layout about 4.5 billion years ago, Venus formed when gravity pulled swirling gas and dust in to become the second planet from the sun.
Like its fellow terrestrial planets, Venus has a central core, a rocky mantle and a solid crust.
Venus is in many ways similar to Earth in its structure.
It has an iron core that is approximately 2,000 miles (3,200 kilometers) in radius.
Above that is a mantle made of hot rock slowly churning due to the planet's interior heat.
The surface is a thin crust of rock that bulges and moves as Venus' mantle shifts and creates volcanoes.
From space, Venus is bright white because it is covered with clouds that reflect and scatter sunlight.
At the surface, the rocks are different shades of grey, like rocks on Earth, but the thick atmosphere filters the sunlight so that everything would look orange if you were standing on Venus.
Venus has mountains, valleys, and tens of thousands of volcanoes.
The highest mountain on Venus, Maxwell Montes, is 20,000 feet high (8.8 kilometers), similar to the highest mountain on Earth, Mount Everest.
The landscape is dusty, and surface temperatures reach a scalding 880 degrees Fahrenheit (471 degrees Celsius).
It is thought that Venus was completely resurfaced by volcanic activity 300 to 500 million years ago.
Venus has two large highland areas: Ishtar Terra, about the size of Australia, in the north polar region; and Aphrodite Terra, about the size of South America, straddling the equator and extending for almost 6,000 miles (10,000 kilometers).
Venus is covered in craters, but none are smaller than 0.9 to 1.2 miles (1.5 to 2 kilometers) across.
Small meteoroids burn up in the dense atmosphere, so only large meteoroids reach the surface and create impact craters.
Almost all the surface features of Venus are named for amazing Earth women.
A volcanic crater is named for Sacajawea, the Native American woman who guided Lewis and Clark's exploration.
A deep canyon is named for Diana, Roman goddess of the hunt.
Venus' atmosphere consists mainly of carbon dioxide, with clouds of sulfuric acid droplets.
The thick atmosphere traps the sun's heat, resulting in surface temperatures higher than 880 degrees Fahrenheit (470 degrees Celsius).
The atmosphere has many layers with different temperatures.
At the level where the clouds are, about 30 miles up from the surface, it's about the same temperature as on the surface of the Earth.
As Venus moves forward in its solar orbit while slowly rotating backwards on its axis, the top level of clouds zips around the planet every four Earth days, driven by hurricane-force winds traveling at about 224 miles (360 kilometers) per hour.
Atmospheric lightning bursts light up these quick-moving clouds.
Speeds within the clouds decrease with cloud height, and at the surface are estimated to be just a few miles per hour.
On the ground, it would look like a very hazy, overcast day on Earth.
And the atmosphere is so heavy it would feel like you were 1 mile (1.6 kilometers) deep underwater.
No human has visited Venus, but the spacecraft that have been sent to the surface of Venus do not last very long there.
Venus' high surface temperature overheat electronics in spacecraft in a short time, so it seems unlikely that a person could survive for long on the Venusian surface.
There is speculation about life existing in Venus' distant past, as well as questions about the possibility of life in the top cloud layers of Venus' atmosphere, where the temperatures are less extreme.
Venus has no moons.
Venus has no rings.
Even though Venus is similar in size to the Earth and has a similarly-sized iron core, Venus' magnetic field is much weaker than the Earth's due to Venus' slow rotation.
The brightest object in the night sky on Earth (besides our moon), Venus has been observed for millennia.
And as one of just two bodies between Earth and the sun, Venus periodically passes across the face of the sun — a phenomenon called a transit.
Observing transits of Venus has helped astronomers study the nearby planet and better understand the solar system and our place in it.
Transits of Venus occur in pairs with more than a century separating each pair, occurring in 1631 and 1639; 1761, 1769; 1874, 1882; and 2004, 2012.
The next transit isn't until December 2117.
Such long gaps occur between transits because Earth's and Venus' orbits around the sun are inclined differently, so Venus much more often passes between Earth and the sun without crossing the face of the sun from our perspective.
Named after the goddess of love and beauty, Venus has become nearly synonymous with "woman" in popular culture, as referenced by the famous relationship guide Men are from Mars, Women are from Venus.
As a solar system locale, Venus was a popular destination for early 20th century science fiction writers; before we knew about what lay beneath Venus' mysterious cloud cover, writers could speculate about a more hospitable planet and its possible inhabitants.
More recently, Venus has been a backdrop for video games such as Transhuman Space, Battlezone and Destiny.
And in the Disney animated film The Princess and the Frog, Ray the firefly falls in love with Venus, "the evening star," as he has mistaken it for another firefly.
Earth is the third planet from the sun and the fifth largest in the solar system.
Just slightly larger than nearby Venus, Earth is the biggest of the terrestrial planets.
Our home planet is the only planet in our solar system known to harbor living things.
The name Earth is at least 1,000 years old.
All of the planets, except for Earth, were named after Greek and Roman gods and goddesses.
However, the name Earth is an English/German word, which simply means the ground.
This page provides a brief overview of our home planet.
For a comprehensive look at Earth, visit NASA's Earth Science Division.
With a radius of 3,959 miles (6,371 kilometers), Earth is the biggest of the terrestrial planets, and the fifth largest planet overall.
From an average distance of 93 million miles (150 million kilometers), Earth is exactly one astronomical unit away from the sun because one astronomical unit (abbreviated as AU), is the distance from the sun to Earth.
This unit provides an easy way to quickly compare planets' distances from the sun.
It takes about eight minutes for light from the sun to reach our planet.
As Earth orbits the sun, it completes one rotation every 23.9 hours.
It takes 365.25 days to complete one trip around the sun.
That extra quarter of a day presents a challenge to our calendar system, which counts one year as 365 days.
To keep our yearly calendars consistent with our orbit around the sun, every four years we add one day.
That day is called a leap day, and the year it's added to is called a leap year.
Earth's axis of rotation is tilted 23.4 degrees with respect to the plane of Earth's orbit around the sun.
This tilt causes our yearly cycle of seasons.
During part of the year, the northern hemisphere is tilted toward the sun and the southern hemisphere is tilted away.
With the sun higher in the sky, solar heating is greater in the north producing summer there.
Less direct solar heating produces winter in the south.
Six months later, the situation is reversed.
When spring and fall begin, both hemispheres receive roughly equal amounts of heat from the sun.
When the solar system settled into its current layout about 4.5 billion years ago, Earth formed when gravity pulled swirling gas and dust in to become the third planet from the sun.
Like its fellow terrestrial planets, Earth has a central core, a rocky mantle and a solid crust.
Earth is composed of four main layers, starting with an inner core at the planet's center, enveloped by the outer core, mantle and crust.
The inner core is a solid sphere made of iron and nickel metals about 759 miles (1,221 kilometers) in radius.
There the temperature is as high as 9,800 degrees Fahrenheit (5,400 degrees Celsius).
Surrounding the inner core is the outer core.
This layer is about 1,400 miles (2,300 kilometers) thick, made of iron and nickel fluids.
In between the outer core and crust is the mantle, the thickest layer.
This hot, viscous mixture of molten rock is about 1,800 miles (2,900 kilometers) thick and has the consistency of caramel.
The outermost layer, Earth's crust, goes about 19 miles (30 kilometers) deep on average on land.
At the bottom of the ocean, the crust is thinner and extends about 3 miles (5 kilometers) from the sea floor to the top of the mantle.
Like Mars and Venus, Earth has volcanoes, mountains and valleys.
Earth's lithosphere, which includes the crust (both continental and oceanic) and the upper mantle, is divided into huge plates that are constantly moving.
For example, the North American plate moves west over the Pacific Ocean basin, roughly at a rate equal to the growth of our fingernails.
Earthquakes result when plates grind past one another, ride up over one another, collide to make mountains, or split and separate.
Earth's global ocean, which covers nearly 70 percent of the planet's surface, has an average depth of about 2.5 miles (4 kilometers) and contains 97 percent of Earth's water.
Almost all of Earth's volcanoes are hidden under these oceans.
Hawaii's Mauna Kea volcano is taller from base to summit than Mount Everest, but most of it is underwater.
Earth's longest mountain range is also underwater, at the bottom of the Arctic and Atlantic oceans.
It is four times longer than the Andes, Rockies and Himalayas combined.
Near the surface, Earth has an atmosphere that consists of 78 percent nitrogen, 21 percent oxygen, and 1 percent other gases such as argon, carbon dioxide and neon.
The atmosphere affects Earth's long-term climate and short-term local weather and shields us from much of the harmful radiation coming from the sun.
It also protects us from meteoroids, most of which burn up in the atmosphere, seen as meteors in the night sky, before they can strike the surface as meteorites.
Earth has a very hospitable temperature and mix of chemicals that have made life possible here.
Most notably, Earth is unique in that most of our planet is covered in water, since the temperature allows liquid water to exist for extended periods of time.
Earth's vast oceans provided a convenient place for life to begin about 3.8 billion years ago.
Some of the features of our planet that make it great for sustaining life are changing due to the ongoing effects of climate change.
To find out more visit our sister website, climate.nasa.gov.
Earth is the only planet that has a single moon.
Our moon is the brightest and most familiar object in the night sky.
In many ways, the moon is responsible for making Earth such a great home.
It stabilizes our planet's wobble, which has made the climate less variable over thousands of years.
Earth sometimes temporarily hosts orbiting asteroids or large rocks.
They are typically trapped by Earth's gravity for a a few months or years before returning to an orbit around the sun.
Some asteroids will be in a long "dance" with Earth as both orbit the sun.
Some moons are bits of rock that were captured by a planet's gravity, but our moon is likely the result of a collision billions of years ago.
When Earth was a young planet, a large chunk of rock smashed into it, displacing a portion of Earth's interior.
The resulting chunks clumped together and formed our moon.
With a radius of 1,080 miles (1,738 kilometers), the moon is the fifth largest moon in our solar system (after Ganymede, Titan, Callisto and Io).
The moon is farther away from Earth than most people realize.
The moon is an average of 238,855 miles (384,400 kilometers) away.
That means 30 Earth-sized planets could fit in between Earth and the moon.
Earth has no rings.
Our planet's rapid rotation and molten nickel-iron core give rise to a magnetic field, which the solar wind distorts into a teardrop shape in space.
The solar wind is a stream of charged particles continuously ejected from the sun.
When charged particles from the solar wind become trapped in Earth's magnetic field, they collide with air molecules above our planet's magnetic poles.
These air molecules then begin to glow and cause aurorae, or the northern and southern lights.
The magnetic field is what causes compass needles to point to the North Pole regardless of which way you turn.
But the magnetic polarity of Earth can change, flipping the direction of the magnetic field.
The geologic record tells scientists that a magnetic reversal takes place about every 400,000 years on average, but the timing is very irregular.
As far as we know, such a magnetic reversal doesn't cause any harm to life on Earth, and a reversal is very unlikely to happen for at least another thousand years.
But when it does happen, compass needles are likely to point in many different directions for a few centuries while the switch is being made.
And after the switch is completed, they will all point south instead of north.
Earth is made up of complex, interactive systems that create a constantly changing world that we are striving to understand.
From the vantage point of space, we are able to observe our planet globally, using sensitive instruments to understand the delicate balance among its oceans, air, land and life.
NASA satellite observations help study and predict weather, drought, pollution, climate change, and many other phenomena that affect the environment, economy and society.
This is a small sampling of the history of exploration of Earth from space.
For more, visit NASA's Earth Science Division
Storytellers explore the nature of our planet and possible alternate realities in many books, movies and television shows.
The iconic Planet of the Apes film (and many sequels) takes place in a future in which astronauts "discover" a planet inhabited by highly intelligent apes and primitive humans, only to realize later, much to their dismay, that — spoiler alert! — it was Earth all along.
In the long-running and re-booted television series Battlestar Galactica, tired survivors of a war with highly evolved robots called Cylons are on a quest to find Earth, a long-lost colony.
In other stories, Earth has been abandoned or destroyed, such as in the Joss Whedon series Firefly or the book and its film adaptation The Hitchhiker's Guide to the Galaxy.
In the animated feature Titan A.E., Earth has been destroyed by an alien species, but a well-placed planet builder recreates it and all the species that live on it.
The fifth largest moon in the solar system, Earth's moon is the only place beyond Earth where humans have set foot.
The brightest and largest object in our night sky, the moon makes Earth a more livable planet by moderating our home planet's wobble on its axis, leading to a relatively stable climate.
It also causes tides, creating a rhythm that has guided humans for thousands of years.
The moon was likely formed after a Mars-sized body collided with Earth.
Earth's only natural satellite is simply called "the moon" because people didn't know other moons existed until Galileo Galilei discovered four moons orbiting Jupiter in 1610.
With a radius of 1,079.6 miles (1,737.5 kilometers), the moon is less than a third the width of Earth.
If Earth were the size of a nickel, the moon would be about as big as a coffee bean.
The moon is farther away from Earth than most people realize.
The moon is an average of 238,855 miles (384,400 kilometers) away.
That means 30 Earth-sized planets could fit in between Earth and the moon.
The moon is slowly moving away from Earth, getting about an inch farther away each year.
The moon is rotating at the same rate that it revolves around Earth (called synchronous rotation), so the same hemisphere faces Earth all the time.
Some people call the far side — the hemisphere we never see from Earth — the "dark side," but that's misleading.
As the moon orbits Earth, different parts are in sunlight or darkness at different times.
The changing illumination is why, from our perspective, the moon goes through phases.
During a "full moon," the hemisphere of the moon we can see from Earth is fully illuminated by the sun.
And a "new moon" occurs when the far side of the moon has full sunlight, and the side facing us is having its night.
The moon makes a complete orbit around Earth in 27 Earth days and rotates or spins at that same rate, or in that same amount of time.
Because Earth is moving as well — rotating on its axis as it orbits the sun — from our perspective, the moon appears to orbit us every 29 days.
The leading theory of the moon's origin is that a Mars-sized body collided with Earth about 4.5 billion years ago.
The resulting debris from both Earth and the impactor accumulated to form our natural satellite 239,000 miles (384,000 kilometers) away.
The newly formed moon was in a molten state, but within about 100 million years, most of the global "magma ocean" had crystallized, with less-dense rocks floating upward and eventually forming the lunar crust.
Earth's moon has a core, mantle and crust.
The moon's core is proportionally smaller than other terrestrial bodies' cores.
The solid, iron-rich inner core is 149 miles (240 kilometers) in radius.
It is surrounded by a liquid iron shell 56 miles (90 kilometers) thick.
A partially molten layer with a thickness of 93 miles (150 kilometers) surrounds the iron core.
The mantle extends from the top of the partially molten layer to the bottom of the moon's crust.
It is most likely made of minerals like olivine and pyroxene, which are made up of magnesium, iron, silicon and oxygen atoms.
The crust has a thickness of about 43 miles (70 kilometers) on the moon's near-side hemisphere and 93 miles (150 kilometers) on the far-side.
It is made of oxygen, silicon, magnesium, iron, calcium and aluminum, with small amounts of titanium, uranium, thorium, potassium and hydrogen.
Long ago the moon had active volcanoes, but today they are all dormant and have not erupted for millions of years.
With too sparse an atmosphere to impede impacts, a steady rain of asteroids, meteoroids and comets strikes the surface of the moon, leaving numerous craters behind.
Tycho Crater is more than 52 miles (85 kilometers) wide.
Over billions of years, these impacts have ground up the surface of the moon into fragments ranging from huge boulders to powder.
Nearly the entire moon is covered by a rubble pile of charcoal-gray, powdery dust and rocky debris called the lunar regolith.
Beneath is a region of fractured bedrock referred to as the megaregolith.
The light areas of the moon are known as the highlands.
The dark features, called maria (Latin for seas), are impact basins that were filled with lava between 4.2 and 1.2 billion years ago.
These light and dark areas represent rocks of different composition and ages, which provide evidence for how the early crust may have crystallized from a lunar magma ocean.
The craters themselves, which have been preserved for billions of years, provide an impact history for the moon and other bodies in the inner solar system.
If you looked in the right places on the moon, you would find pieces of equipment, American flags, and even a camera left behind by astronauts.
While you were there, you'd notice that the gravity on the surface of the moon is one-sixth of Earth's, which is why in footage of moonwalks, astronauts appear to almost bounce across the surface.
The temperature reaches about 260 degrees Fahrenheit (127 degrees Celsius) when in full sun, but in darkness, the temperatures plummets to about -280 degrees Fahrenheit (-173 degrees Celsius).
The moon has a very thin and weak atmosphere, called an exosphere.
It does not provide any protection from the sun's radiation or impacts from meteoroids.
The many missions that have explored the moon have found no evidence to suggest it has its own living things.
However, the moon could be the site of future colonization by humans, though there are no immediate plans to do so.
Earth's moon has no moons of its own.
The early moon may have developed an internal dynamo, the mechanism for generating global magnetic fields for terrestrial planets, but today, the moon has a very weak magnetic field.
The magnetic field here on Earth is many thousands of times stronger than the moon's magnetic field.
Human beings have studied the moon for millennia, watching its phases change and observing eclipses — both solar and lunar.
During a solar eclipse, our moon moves between Earth and the sun and blocks the sunlight.
In a lunar eclipse, Earth blocks the sun's light that normally lights up the moon, so we see Earth's shadow over the face of the moon.
From Earth, we see the moon get dark and often turn red.
This happens because Earth's atmosphere scatters blue and green light while it bends yellow, orange and red wavelengths toward the moon.
The moon is the most explored body in our solar system besides Earth, having been visited by numerous spacecraft from multiple space agencies around the world.
It's also the only place besides Earth where human beings have set foot.
Our lunar neighbor has inspired stories since the first humans looked up at the sky and saw its grey, cratered face.
Some observers saw among the craters the shape of a person's face, so stories refer to a mysterious "man in the moon." Hungrier observers compared its craters to cheese and dreamed of an entire sphere made of delicious dairy products.
The moon made its film debut in a 1902 black and white silent French film called Le Voyage Dans la Lune (a trip to the moon).
And a year before astronauts walked on the moon, 2001: A Space Odyssey (1968) told the story of astronauts on an outpost on the moon.
Decades later, it is still widely regarded as the best science fiction movie ever made.
In reality, while we do not yet have a moon colony, spacecraft have left lots of debris on the lunar surface, and astronauts have planted six American flags on the moon.
But that doesn't mean the United States has claimed it; in fact, an international law written in 1967 prevents any single nation from owning planets, stars, or any other natural objects in space.
Though details of Mars' surface are difficult to see from Earth, telescope observations show seasonally changing features and white patches at the poles.
For decades, people speculated that bright and dark areas on Mars were patches of vegetation, Mars was a likely place for advanced life forms, and water might exist in the polar caps.
When the Mariner 4 spacecraft flew by Mars in 1965, photographs of a bleak, cratered surface shocked many - Mars seemed to be a dead planet.
Later missions, however, showed that Mars is a complex planet and holds many mysteries yet to be solved.
Chief among them is whether Mars ever had the right conditions to support small life forms called microbes.
Mars is a rocky body about half the size of Earth.
As with the other terrestrial planets - Mercury, Venus, and Earth - volcanoes, impact craters, crustal movement, and atmospheric conditions such as dust storms have altered the surface of Mars.
Mars has two small moons, Phobos and Deimos, that may be captured asteroids.
Potato-shaped, they have too little mass for gravity to make them spherical.
Phobos, the innermost moon, is heavily cratered, with deep grooves on its surface.
Like Earth, Mars experiences seasons due to the tilt of its rotational axis.
Mars' orbit is about 1.5 times farther from the sun than Earth's and is slightly elliptical, so its distance from the sun changes.
That affects the length of Martian seasons, which vary in length.
The polar ice caps on Mars grow and recede with the seasons.
Layered areas near the poles suggest that the planet's climate has changed more than once.
Volcanism in the highlands and plains was active more than 3 billion years ago.
Some of the giant shield volcanoes are younger, having formed between 1 and 2 billion years ago.
Mars has the largest volcano in the solar system, Olympus Mons, as well as a spectacular equatorial canyon system, Valles Marineris.
Mars has no global magnetic field today.
However, NASA's Mars Global Surveyor orbiter found that areas of the Martian crust in the southern hemisphere are highly magnetized, indicating traces of a magnetic field from 4 billion years ago that remain.
Scientists believe that Mars experienced huge floods about 3.5 billion years ago.
Though we do not know where the ancient flood water came from, how long it lasted, or where it went, recent missions to Mars have uncovered intriguing hints.
In 2002, NASA's Mars Odyssey orbiter detected hydrogen-rich polar deposits, indicating large quantities of water ice close to the surface.
Further observations found hydrogen in other areas as well.
If water ice permeated the entire planet, Mars could have substantial subsurface layers of frozen water.
In 2004, Mars Exploration Rover Opportunity found structures and minerals indicating that liquid water once existed at its landing site.
The rover's twin, Spirit, also found the signature of ancient water near its landing site, halfway around Mars from Opportunity's location.
The cold temperatures and thin atmosphere on Mars do not allow liquid water to exist at the surface for long.
The quantity of water required to carve Mars' great channels and flood plains is not evident today.
Unraveling the story of water on Mars is important to unlocking its climate history, which will help us understand the evolution of all the planets.
Water is an essential ingredient for life as we know it.
Evidence of long-term past or present water on Mars holds clues about whether Mars could ever have been a habitat for life.
In 2008, NASA's Phoenix Mars lander was the first mission to touch water ice in the Martian arctic.
Phoenix also observed precipitation (snow falling from clouds), as confirmed by Mars Reconnaissance Orbiter.
Soil chemistry experiments led scientists to believe that the Phoenix landing site had a wetter and warmer climate in the recent past (the last few million years).
NASA's Mars Science Laboratory mission, with its large rover Curiosity, is examining Martian rocks and soil at Gale Crater, looking for minerals that formed in water, signs of subsurface water, and carbon-based molecules called organics, the chemical building blocks of life.
That information will reveal more about the present and past habitability of Mars, as well as whether humans could survive on Mars some day.
Mars was named by the Romans for their god of war because of its red, bloodlike color.
Other civilizations also named this planet from this attribute; for example, the Egyptians named it "Her Desher," meaning "the red one."
Asteroids, sometimes called minor planets, are rocky remnants left over from the early formation of our solar system about 4.6 billion years ago.
Most of this ancient space rubble can be found orbiting the sun between Mars and Jupiter within the main asteroid belt.
Asteroids range in size from Vesta - the largest at about 329 miles (530 kilometers) in diameter - to bodies that are less than 33 feet (10 meters) across.
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The total mass of all the asteroids combined is less than that of Earth's Moon.
Editor's note: Even with more than one-half million asteroids known (and there are probably many more), they are still much more widely separated than sometimes seen in Hollywood movies: on average, their separation is in excess of 1-3 million km (depending on how one calculates it).
Most asteroids are irregularly shaped, though a few are nearly spherical, and they are often pitted or cratered.
As they revolve around the sun in elliptical orbits, the asteroids also rotate, sometimes quite erratically, tumbling as they go.
More than 150 asteroids are known to have a small companion moon (some have two moons).
There are also binary (double) asteroids, in which two rocky bodies of roughly equal size orbit each other, as well as triple asteroid systems.
The three broad composition classes of asteroids are C-, S-, and M-types.
The C-type (chondrite) asteroids are most common, probably consist of clay and silicate rocks, and are dark in appearance.
They are among the most ancient objects in the solar system.
The S-types ("stony") are made up of silicate materials and nickel-iron.
The M-types are metallic (nickel-iron).
The asteroids' compositional differences are related to how far from the sun they formed.
Some experienced high temperatures after they formed and partly melted, with iron sinking to the center and forcing basaltic (volcanic) lava to the surface.
Only one such asteroid, Vesta, survives to this day.
Jupiter's massive gravity and occasional close encounters with Mars or another object change the asteroids' orbits, knocking them out of the main belt and hurling them into space in all directions across the orbits of the other planets.
Stray asteroids and asteroid fragments slammed into Earth and the other planets in the past, playing a major role in altering the geological history of the planets and in the evolution of life on Earth.
Scientists continuously monitor Earth-crossing asteroids, whose paths intersect Earth's orbit, and near-Earth asteroids that approach Earth's orbital distance to within about 45 million kilometers (28 million miles) and may pose an impact danger.
Radar is a valuable tool in detecting and monitoring potential impact hazards.
By reflecting transmitted signals off objects, images and other information can be derived from the echoes.
Scientists can learn a great deal about an asteroid's orbit, rotation, size, shape, and metal concentration.
Several missions have flown by and observed asteroids.
The Galileo spacecraft flew by asteroids Gaspra in 1991 and Ida in 1993; the Near-Earth Asteroid Rendezvous (NEAR-Shoemaker) mission studied asteroids Mathilde and Eros; and the Rosetta mission encountered Steins in 2008 and Lutetia in 2010.
Deep Space 1 and Stardust both had close encounters with asteroids.
In 2005, the Japanese spacecraft Hayabusa landed on the near-Earth asteroid Itokawa and attempted to collect samples.
On June 3, 2010, Hayabusa successfully returned to Earth a small amount of asteroid dust now being studied by scientists.
NASA's Dawn spacecraft, launched in 2007, orbited and explored asteroid Vesta for over a year.
Once it left in September 2012, it headed towards dwarf planet Ceres, with a planned arrival of 2015.
Vesta and Ceres are two of the largest surviving protoplanet bodies that almost became planets.
By studying them with the same complement of instruments on board the same spacecraft, scientists will be able to compare and contrast the different evolutionary path each object took to help understand the early solar system overall.
Main asteroid belt: The majority of known asteroids orbit within the asteroid belt between Mars and Jupiter, generally with not very elongated orbits.
The belt is estimated to contain between 1.1 and 1.9 million asteroids larger than 1 kilometer (0.6 mile) in diameter, and millions of smaller ones.
Early in the history of the solar system, the gravity of newly formed Jupiter brought an end to the formation of planetary bodies in this region and caused the small bodies to collide with one another, fragmenting them into the asteroids we observe today.
Trojans: These asteroids share an orbit with a larger planet, but do not collide with it because they gather around two special places in the orbit (called the L4 and L5 Lagrangian points).
There, the gravitational pull from the sun and the planet are balanced by a trojan's tendency to otherwise fly out of the orbit.
The Jupiter trojans form the most significant population of trojan asteroids.
It is thought that they are as numerous as the asteroids in the asteroid belt.
There are Mars and Neptune trojans, and NASA announced the discovery of an Earth trojan in 2011.
Near-Earth asteroids: These objects have orbits that pass close by that of Earth.
Asteroids that actually cross Earth's orbital path are known as Earth-crossers.
As of June 19, 2013, 10,003 near-Earth asteroids are known and the number over 1 kilometer in diameter is thought to be 861, with 1,409 classified as potentially hazardous asteroids - those that could pose a threat to Earth.
The International Astronomical Union's Committee on Small Body Nomenclature.is a little less strict when it comes to naming asteroids than other IAU naming committees.
So out there orbiting the sun we have giant space rocks named for Mr. Spock (a cat named for the character of "Star Trek" fame), rock musician Frank Zappa, regular guys like Phil Davis, and more somber tributes such as the seven asteroids named for the crew of the Space Shuttle Columbia killed in 2003.
Asteroids are also named for places and a variety of other things.
The IAU discourages naming asteroids for pets, so Mr. Spock stands alone.
Asteroids are also given a number, for example (99942) Apophis.
The Harvard Smithsonian Center for Astrophysics keeps a fairly current list of asteroid names.
Shooting stars, or meteors, are bits of interplanetary material falling through Earth's atmosphere and heated to incandescence by friction.
These objects are called meteoroids as they are hurtling through space, becoming meteors for the few seconds they streak across the sky and create glowing trails.
Scientists estimate that 44 tonnes (44,000 kilograms, about 48.5 tons) of meteoritic material falls on the Earth each day.
Several meteors per hour can usually be seen on any given night.
Sometimes the number increases dramatically - these events are termed meteor showers.
Some occur annually or at regular intervals as the Earth passes through the trail of dusty debris left by a comet.
Meteor showers are usually named after a star or constellation that is close to where the meteors appear in the sky.
Perhaps the most famous are the Perseids, which peak around 12 August every year.
Every Perseid meteor is a tiny piece of the comet Swift-Tuttle, which swings by the Sun every 135 years.
Other meteor showers and their associated comets are the Leonids (Tempel-Tuttle), the Aquarids and Orionids (Halley), and the Taurids (Encke).
Most comet dust in meteor showers burns up in the atmosphere before reaching the ground; some dust is captured by high-altitude aircraft and analyzed in NASA laboratories.
Chunks of rock and metal from asteroids and other planetary bodies that survive their journey through the atmosphere and fall to the ground are called meteorites.
Most meteorites found on Earth are pebble to fist size, but some are larger than a building.
Early Earth experienced many large meteorite impacts that caused extensive destruction.
One of the most intact impact craters is the Barringer Meteorite Crater in Arizona, about 1 kilometer (0.6 mile) across, formed by the impact of a piece of iron-nickel metal approximately 50 meters (164 feet) in diameter.
It is only 50,000 years old and so well preserved that it has been used to study impact processes.
Since this feature was recognized as an impact crater in the 1920s, about 170 impact craters have been identified on Earth.
A very large asteroid impact 65 million years ago, which created the 300-kilometer-wide (180-mile-wide) Chicxulub crater on the Yucatan Peninsula, is thought to have contributed to the extinction of about 75 percent of marine and land animals on Earth at the time, including the dinosaurs.
Well-documented stories of meteorite-caused injury or death are rare.
In the first known case of an extraterrestrial object to have injured a human being in the U.S., Ann Hodges of Sylacauga, Alabama, was severely bruised by a 3.6-kilogram (8-pound) stony meteorite that crashed through her roof in November 1954.
Meteorites may resemble Earth rocks, but they usually have a burned exterior.
This fusion crust is formed as the meteorite is melted by friction as it passes through the atmosphere.
There are three major types of meteorites: the "irons," the "stones," and the stony-irons.
Although the majority of meteorites that fall to Earth are stony, more of the meteorites that are discovered long after they fall are irons - these heavy objects are easier to distinguish from Earth rocks than stony meteorites.
Meteorites also fall on other solar system bodies.
Mars Exploration Rover Opportunity found the first meteorite of any type on another planet when it discovered an iron-nickel meteorite about the size of a basketball on Mars in 2005, and then found a much larger and heavier iron-nickel meteorite in 2009 in the same region.
In all, Opportunity has discovered six meteorites during its travels on Mars.
More than 50,000 meteorites have been found on Earth.
Of these, 99.8 percent come from asteroids.
Evidence for an asteroid origin includes orbits calculated from photographic observations of meteorite falls projected back to the asteroid belt; spectra of several classes of meteorites match those of some asteroid classes; and they are very old, 4.5 to 4.6 billion years.
However, we can only match one group of meteorites to a specific asteroid - the eucrite, diogenite, and howardite igneous meteorites come from the third-largest asteroid, Vesta.
Asteroids and the meteorites that fall to Earth are not pieces of a planet that broke apart, but instead are the original diverse materials from which the planets formed.
The study of meteorites tells us much about the earliest conditions and processes during the formation and earliest history of the solar system, such as the age and composition of solids, the nature of the organic matter, the temperatures achieved at the surface and interiors of asteroids, and the degree to which materials were shocked by impacts.
The remaining 0.2 percent of meteorites is split roughly equally between meteorites from Mars and the moon.
The over 60 known martian meteorites were blasted off Mars by meteoroid impacts.
All are igneous rocks crystallized from magma.
The rocks are very much like Earth rocks with some distinctive compositions that indicate martian origin.
The nearly 80 lunar meteorites are similar in mineralogy and composition to Apollo mission moon rocks, but distinct enough to show that they have come from other parts of the moon.
Studies of lunar and martian meteorites complement studies of Apollo Moon rocks and the robotic exploration of Mars.
Saturn was the most distant of the five planets known to the ancients.
In 1610, Italian astronomer Galileo Galilei was the first to gaze at Saturn through a telescope.
To his surprise, he saw a pair of objects on either side of the planet.
He sketched them as separate spheres, thinking that Saturn was triple-bodied.
Continuing his observations over the next few years, Galileo drew the lateral bodies as arms or handles attached to Saturn.
In 1659, Dutch astronomer Christiaan Huygens, using a more powerful telescope than Galileo's, proposed that Saturn was surrounded by a thin, flat ring.
In 1675, Italian-born astronomer Jean-Dominique Cassini discovered a "division" between what are now called the A and B rings.
It is now known that the gravitational influence of Saturn's moon Mimas is responsible for the Cassini Division, which is 4,800 kilometers (3,000 miles) wide.
Like Jupiter, Saturn is made mostly of hydrogen and helium.
Its volume is 755 times greater than that of Earth.
Winds in the upper atmosphere reach 500 meters (1,600?feet) per second in the equatorial region.
In contrast, the strongest hurricane-force winds on Earth top out at about 110 meters (360 feet) per second.
These super-fast winds, combined with heat rising from within the planet's interior, cause the yellow and gold bands visible in the atmosphere.
In the early 1980s, NASA's two Voyager spacecraft revealed that Saturn's rings are made mostly of water ice, and they imaged "braided" rings, ringlets, and "spokes" - dark features in the rings that form and initially circle the planet at different rates from that of the surrounding ring material.
Saturn's ring system extends hundreds of thousands of kilometers from the planet, yet the vertical height is typically about 10 meters (30 feet) in the main rings.
During Saturn's equinox in autumn 2009, when sunlight illuminated the rings edge-on, Cassini spacecraft images showed vertical formations in some of the rings; the particles seem to pile up in bumps or ridges more than 3 kilometers (2 miles) tall.
We've discovered 53 confirmed moons and another 9 provisional moons (for a possible total of 62 moons).
Saturn's largest satellite, Titan, is a bit bigger than the planet Mercury.
Titan is the second-largest moon in the solar system; only Jupiter's moon Ganymede is bigger.
Titan is shrouded in a thick, nitrogen-rich atmosphere that might be similar to what Earth's was like long ago.
Further study of this moon promises to reveal much about planetary formation and, perhaps, about the early days of Earth.
Saturn also has many smaller "icy" satellites.
From Enceladus, which shows evidence of recent (and ongoing) surface changes, to Iapetus, with one hemisphere darker than asphalt and the other as bright as snow, each of Saturn's satellites is unique.
At Saturn's center is a dense core of rock, ice, water, and other compounds made solid by the intense pressure and heat.
It is enveloped by liquid metallic hydrogen, inside a layer of liquid hydrogen - similar to Jupiter but considerably smaller.
Saturn's magnetic field is smaller than Jupiter's but still 578 times as powerful as Earth's.
Saturn, the rings, and many of the satellites lie totally within Saturn's enormous magnetosphere, the region of space in which the behavior of electrically charged particles is influenced more by Saturn's magnetic field than by the solar wind.
Aurorae occur when charged particles spiral into a planet's atmosphere along magnetic field lines.
On Earth, these charged particles come from the solar wind.
Cassini showed that at least some of Saturn's aurorae are like Jupiter's and are largely unaffected by the solar wind.
The next chapter in our knowledge of Saturn is being written right now by the Cassini-Huygens mission.
The Huygens probe descended through Titan's atmosphere in January 2005, collecting data on the atmosphere and surface.
The Cassini spacecraft, orbiting Saturn since 2004, continues to explore the planet and its moons, rings, and magnetosphere.
The Cassini Equinox Mission studied the rings during Saturn's autumnal equinox, when the Sun was shining directly on the equator, through 2010.
Now the spacecraft is seeking to make exciting new discoveries in a second extended mission called the Cassini Solstice Mission, which continues until September 2017.
Saturn is named for the Roman god of wealth and agriculture, among other things.
The Greek equivalent was Cronos, father of Zeus/Jupiter.
Other civilizations have given different names to Saturn, which is the farthest planet from Earth that can be observed by the unaided human eye.
The seventh planet from the sun with the third largest diameter in our solar system, Uranus is very cold and windy.
The ice giant is surrounded by 13 faint rings and 27 small moons as it rotates at a nearly 90-degree angle from the plane of its orbit.
This unique tilt makes Uranus appear to spin on its side, orbiting the sun like a rolling ball.
The first planet found with the aid of a telescope, Uranus was discovered in 1781 by astronomer William Herschel, although he originally thought it was either a comet or a star.
It was two years later that the object was universally accepted as a new planet, in part because of observations by astronomer Johann Elert Bode.
William Herschel tried unsuccessfully to name his discovery Georgium Sidus after King George III.
Instead the planet was named for Uranus, the Greek god of the sky, as suggested by Johann Bode.
With a radius of 15,759.2 miles (25,362 kilometers), Uranus is 4 times wider than Earth.
If Earth was the size of a nickel, Uranus would be about as big as a softball.
From an average distance of 1.8 billion miles (2.9 billion kilometers), Uranus is 19.8 astronomical units away from the sun.
One astronomical unit (abbreviated as AU), is the distance from the sun to Earth.
From this distance, it takes sunlight 2 hours and 40 minutes to travel from the sun to Uranus.
One day on Uranus takes about 17 hours (the time it takes for Uranus to rotate or spin once).
And Uranus makes a complete orbit around the sun (a year in Uranian time) in about 84 Earth years (30,687 Earth days).
Uranus is the only planet whose equator is nearly at a right angle to its orbit, with a tilt of 97.77 degrees — possibly the result of a collision with an Earth-sized object long ago.
This unique tilt causes the most extreme seasons in the solar system.
For nearly a quarter of each Uranian year, the sun shines directly over each pole, plunging the other half of the planet into a 21-year-long, dark winter.
Uranus is also one of just two planets that rotate in the opposite direction than most of the planets (Venus is the other one), from east to west.
Uranus took shape when the rest of the solar system formed about 4.5 billion years ago, when gravity pulled swirling gas and dust in to become this ice giant.
Like its neighbor Neptune, Uranus likely formed closer to the sun and moved to the outer solar system about 4 billion years ago, where it is the seventh planet from the sun.
Uranus is one of two ice giants in the outer solar system (the other is Neptune).
Most (80 percent or more) of the planet's mass is made up of a hot dense fluid of "icy" materials - water, methane and ammonia - above a small rocky core.
Near the core, it heats up to 9,000 degrees Fahrenheit (4,982 degrees Celsius).
Uranus is slightly larger in diameter than its neighbor Neptune, yet smaller in mass.
It is the second least dense planet; Saturn is the least dense of all.
Uranus gets its blue-green color from methane gas in the atmosphere.
Sunlight passes through the atmosphere and is reflected back out by Uranus' cloud tops.
Methane gas absorbs the red portion of the light, resulting in a blue-green color.
As an ice giant, Uranus doesn't have a true surface.
The planet is mostly swirling fluids.
While a spacecraft would have nowhere to land on Uranus, it wouldn't be able to fly through its atmosphere unscathed either.
The extreme pressures and temperatures would destroy a metal spacecraft.
Uranus' atmosphere is mostly hydrogen and helium, with a small amount of methane and traces of water and ammonia.
The methane gives Uranus its signature blue color.
While Voyager 2 saw only a few discrete clouds, a Great Dark Spot and a small dark spot during its flyby in 1986, more recent observations reveal that Uranus exhibits dynamic clouds as it approaches equinox, including rapidly changing bright features.
Uranus' planetary atmosphere, with a minimum temperature of 49K (-224.2 degrees Celsius) makes it even colder than Neptune in some places.
Wind speeds can reach up to 560 miles per hour (900 kilometers per hour) on Uranus.
Winds are retrograde at the equator, blowing in the reverse direction of the planet's rotation.
But closer to the poles, winds shift to a prograde direction, flowing with Uranus' rotation.
Uranus' environment is not conducive to life as we know it.
The temperatures, pressures and materials that characterize this planet are most likely too extreme and volatile for organisms to adapt to.
Uranus has 27 known moons.
While most of the satellites orbiting other planets take their names from Greek or Roman mythology, Uranus' moons are unique in being named for characters from the works of William Shakespeare and Alexander Pope.
All of Uranus' inner moons appear to be roughly half water ice and half rock.
The composition of the outer moons remains unknown, but they are likely captured asteroids.
Uranus has two sets of rings.
The inner system of nine rings consists mostly of narrow, dark grey rings.
There are two outer rings: the innermost one is reddish like dusty rings elsewhere in the solar system, and the outer ring is blue like Saturn's E ring.
In order of increasing distance from the planet, the rings are called Zeta, 6, 5, 4, Alpha, Beta, Eta, Gamma, Delta, Lambda, Epsilon, Nu and Mu.
Some of the larger rings are surrounded by belts of fine dust.
Uranus has an unusual, irregularly shaped magnetosphere.
Magnetic fields are typically in alignment with a planet's rotation, but Uranus' magnetic field is tipped over: the magnetic axis is tilted nearly 60 degrees from the planet's axis of rotation, and is also offset from the center of the planet by one-third of the planet's radius.
Auroras on Uranus are not in line with the poles (like they are on Earth, Jupiter and Saturn) due to the lopsided magnetic field.
The magnetosphere tail behind Uranus opposite the sun extends into space for millions of miles.
Its magnetic field lines are twisted by Uranus' sideways rotation into a long corkscrew shape.
Only one spacecraft has visited distant Uranus.
After traveling more than 1.8 billion miles (3 billion kilometers) in nine years, NASA's Voyager 2 gathered much of its critical information about the mysterious planet, including its rings and moons, in just six hours.
Uranus is the "butt" of more than a few jokes and witty (and not so witty) puns, but it's also a frequent destination in various fictional stories, such as the video game Mass Effect and TV shows like Doctor Who.
The radioactive element uranium was named after Uranus when it was discovered in 1789, just eight years after the planet was discovered.
The ice giant Neptune was the first planet located through mathematical predictions rather than through regular observations of the sky.
Galileo had recorded it as a fixed star during observations with his small telescope in 1612 and 1613.
When Uranus didn't travel exactly as astronomers expected it to, a French mathematician, Urbain Joseph Le Verrier, proposed the position and mass of another as yet unknown planet that could cause the observed changes to Uranus' orbit.
After being ignored by French astronomers, Le Verrier sent his predictions to Johann Gottfried Galle at the Berlin Observatory, who found Neptune on his first night of searching in 1846.
Seventeen days later, its largest moon, Triton, was also discovered.
Nearly 4.5 billion kilometers (2.8 billion miles) from the Sun, Neptune orbits the Sun once every 165 years.
It is invisible to the naked eye because of its extreme distance from Earth.
Interestingly, the highly eccentric orbit of the dwarf planet Pluto brings Pluto inside Neptune's orbit for a 20-year period out of every 248 Earth years.
Pluto can never crash into Neptune, though, because for every three laps Neptune takes around the Sun, Pluto makes two.
This repeating pattern prevents close approaches of the two bodies.
The main axis of Neptune's magnetic field is tipped over by about 47 degrees compared with the planet's rotation axis.
Like Uranus, whose magnetic axis is tilted about 60 degrees from the axis of rotation, Neptune's magnetosphere undergoes wild variations during each rotation because of this misalignment.
The magnetic field of Neptune is about 27 times more powerful than that of Earth.
Neptune's atmosphere extends to great depths, gradually merging into water and other melted ices over a heavier, approximately Earth-size solid core.
Neptune's blue color is the result of methane in the atmosphere.
Uranus' blue-green color is also the result of atmospheric methane, but Neptune is a more vivid, brighter blue, so there must be an unknown component that causes the more intense color.
Despite its great distance and low energy input from the Sun, Neptune's winds can be three times stronger than Jupiter's and nine times stronger than Earth's.
In 1989, Voyager 2 tracked a large, oval-shaped, dark storm in Neptune's southern hemisphere.
This "Great Dark Spot" was large enough to contain the entire Earth, spun counterclockwise, and moved westward at almost 1,200 kilometers (750 miles) per hour.
Subsequent images taken by the Hubble Space Telescope showed no sign of this Great Dark Spot, but did reveal the appearance and then fading of two other Great Dark Spots over the last decade.
Voyager 2 also imaged clouds casting shadows on a lower cloud deck, enabling scientists to visually measure the altitude differences between the upper and lower cloud decks.
Neptune has six known rings.
Voyager 2's observations confirmed that these unusual rings are not uniform but have four thick regions (clumps of dust) called arcs.
The rings are thought to be relatively young and short-lived.
Neptune has 13 known moons, six of which were discovered by Voyager 2.
A 14th tiny, very dim, moon was discovered in 2013 and awaits official recognition.
Triton, Neptune's largest moon, orbits the planet in the opposite direction compared with the rest of the moons, suggesting that it may have been captured by Neptune in the distant past.
Triton is extremely cold - temperatures on its surface are about -235 degrees Celsius (-391 degrees Fahrenheit).
Despite this deep freeze at Triton, Voyager 2 discovered geysers spewing icy material upward more than 8 kilometers (5 miles).
Triton's thin atmosphere, also discovered by Voyager, has been detected from Earth several times since, and is growing warmer - although scientists do not yet know why.
Neptune was predicted by John Couch Adams and Urbain Le Verrier.
The men independently accounted for the irregularities in the motion of Uranus by correctly predicting the orbital elements of a trans-Uranian planet.
Using the predicted parameters of Le Verrier (Adams never published his predictions), Johann Galle discovered the planet in 1846.
Galle wanted to name the planet for Le Verrier, but that was not acceptable to the international astronomical community.
Instead, this planet is named for the Roman god of the sea.
There may be dozens of dwarf planets in our solar system.
So far, we've classified just a handful:
Closest to home, Ceres is the largest and most unique resident of the main asteroid belt between Jupiter and Mars.
It also is the first dwarf planet to be visited by a spacecraft - NASA's Dawn mission.
Pluto is the most famous dwarf planet.
Discovered in 1930, it was long classified as our solar system's ninth planet.
Pluto and its busy system of moons orbits the sun in the Kuiper belt, a region of icy debris beyond Neptune.
Astronomers discovered Eris, a Pluto-sized world, in 2003.
It takes icy Eris 557 Earth years to complete a single orbit around our sun.
Two more confirmed orbit the sun in the icy zone beyond Neptune - Haumea and Makemake.
In the distant past, people were both awed and alarmed by comets, perceiving them as long-haired stars that appeared in the sky unannounced and unpredictably.
Chinese astronomers kept extensive records for centuries, including illustrations of characteristic types of comet tails, times of cometary appearances and disappearances, and celestial positions.
These historic comet annals have proven to be a valuable resource for later astronomers.
We now know that comets are leftovers from the dawn of our solar system around 4.6 billion years ago, and consist mostly of ice coated with dark organic material.
They have been referred to as "dirty snowballs." They may yield important clues about the formation of our solar system.
Comets may have brought water and organic compounds, the building blocks of life, to the early Earth and other parts of the solar system.
As theorized by astronomer Gerard Kuiper in 1951, a disc-like belt of icy bodies exists beyond Neptune, where a population of dark comets orbits the sun in the realm of Pluto.
These icy objects, occasionally pushed by gravity into orbits bringing them closer to the sun, become the so-called short-period comets.
Taking less than 200 years to orbit the sun, in many cases their appearance is predictable because they have passed by before.
Less predictable are long-period comets, many of which arrive from a region called the Oort Cloud about 100,000 astronomical units (that is,?100,000 times the distance between Earth and the Sun) from the Sun.
These Oort Cloud comets can take as long as 30 million years to complete one trip around the Sun.
Each comet has a tiny frozen part, called a nucleus, often no larger than a few kilometers across.
The nucleus contains icy chunks, frozen gases with bits of embedded dust.
A comet warms up as it nears the Sun and develops an atmosphere, or coma.
The sun's heat causes the comet's ices to change to gases so the coma gets larger.
The coma may extend hundreds of thousands of kilometers.
The pressure of sunlight and high-speed solar particles (solar wind) can blow the coma dust and gas away from the Ssun, sometimes forming a long, bright tail.
Comets actually have two tails - a dust tail and an ion (gas) tail.
Most comets travel a safe distance from the sun - comet Halley comes no closer than 89 million kilometers (55 million miles).
However, some comets, called sungrazers, crash straight into the Sun or get so close that they break up and evaporate.
Scientists have long wanted to study comets in some detail, tantalized by the few 1986 images of comet Halley's nucleus.
NASA's Deep Space 1 spacecraft flew by comet Borrelly in 2001 and photographed its nucleus, which is about 8 kilometers (5 miles) long.
NASA's Stardust mission successfully flew within 236 kilometers (147 miles) of the nucleus of Comet Wild 2 in January 2004, collecting cometary particles and interstellar dust for a sample return to Earth in 2006.
The photographs taken during this close flyby of a comet nucleus show jets of dust and a rugged, textured surface.
Analysis of the Stardust samples suggests that comets may be more complex than originally thought.
Minerals formed near the Sun or other stars were found in the samples, suggesting that materials from the inner regions of the solar system traveled to the outer regions where comets formed.
Another NASA mission, Deep Impact, consisted of a flyby spacecraft and an impactor.
In July 2005, the impactor was released into the path of the nucleus of comet Tempel 1 in a planned collision, which vaporized the impactor and ejected massive amounts of fine, powdery material from beneath the comet's surface.
En route to impact, the impactor camera imaged the comet in increasing detail.
Two cameras and a spectrometer on the flyby spacecraft recorded the dramatic excavation that helped determine the interior composition and structure of the nucleus.
After their successful primary missions, the Deep Impact spacecraft and the Stardust spacecraft were still healthy and were retargeted for additional cometary flybys.
Deep Impact's mission, EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation), comprised two projects: the Deep Impact Extended Investigation (DIXI), which encountered comet Hartley 2 in November 2010, and the Extrasolar Planet Observation and Characterization (EPOCh) investigation, which searched for Earth-size planets around other stars on route to Hartley 2.
NASA returned to comet Tempel 1 in 2011, when the Stardust New Exploration of Tempel 1 (NExT) mission observed changes in the nucleus since Deep Impact's 2005 encounter.
Comet naming can be complicated.
Comets are generally named for their discoverer -- either a person or a spacecraft.
This International Astronomical Union guideline was developed only in the last century.
For example, comet Shoemaker-Levy 9 was so named because it was the ninth short-periodic comet discovered by Eugene and Carolyn Shoemaker and David Levy.
Since spacecraft are very effective at spotting comets many comets have LINEAR, SOHO or WISE in their names.
The Kuiper Belt is a disc-shaped region beyond Neptune that extends from about 30 to 55 astronomical units (compared to Earth which is one astronomical unit, or AU, from the sun).
This distant region is probably populated with hundreds of thousands of icy bodies larger than 100 km (62 miles) across and an estimated trillion or more comets.
Dwarf planet Pluto may be the best known of the larger objects in the Kuiper Belt.
Comets from the Kuiper Belt take less than 200 years to orbit the sun and travel approximately in the plane in which most of the planets orbit the sun.
.Objects in the Kuiper Belt are presumed to be remnants from the formation of the solar system about 4.6 billion years ago.
The first of these strange bodies, which astronomers call Kuiper Belt Objects (KBOs), came to light in 1992, discovered by Dave Jewitt and Jane Luu -- a pair of scientists who didn't believe the outer solar system was empty.
Beginning in 1987 they had doggedly scanned the heavens in search of dim objects beyond Neptune.
It took five years, looking off-and-on through the University of Hawaii's 2.2 m telescope, but they finally found what they were after: a reddish-colored speck 44 AU from the Sun -- even more distant than Pluto! Jewitt (University of Hawaii) and Luu (UC Berkeley) wanted to name their find "Smiley," but it has since been cataloged as "1992 QB1."
That discovery marked our first glimpse of the long-sought Kuiper Belt, named after Gerard Kuiper who, in 1951, proposed that a belt of icy bodies might lay beyond Neptune.
It was the only way, he figured, to solve a baffling mystery about comets: Some comets loop through the solar system on periodic orbits of a half-dozen years or so.
They encounter the Sun so often that they quickly evaporate -- vanishing in only a few hundred thousand years.
Astronomers call them "short-period comets," although "short-lived" is more to the point.
Short-period comets evaporate so quickly compared to the age of the solar system that we shouldn't see any, yet astronomers routinely track dozens of them.
It was a real puzzle.
Kuiper's solution was a population of dark comets circling the Sun in the realm of Pluto -- leftovers from the dawn of our solar system when planetesimals were coalescing to make planets.
The ones beyond Neptune, Kuiper speculated, never stuck together, remaining instead primitive and individual.
Nowadays they occasionally fall toward the Sun and become short-period comets.
Because KBOs are so distant, their sizes are difficult to measure.
The calculated diameter of a KBO depends on assumptions about how reflective the object's surface is.
With infrared observations by the Spitzer Space Telescope, most of the largest KBOs have known sizes.
One of the most unusual KBOs is Haumea, which is a part of a collisional family orbiting the sun.
The parent body, Haumea, apparently collided with another object that was roughly half its size.
The impact blasted large icy chunks away and sent Haumea reeling, causing it to spin end-over-end every four hours.
It spins so fast that it has pulled itself into the shape of a squashed American football.
Haumea and two small moons -- Hi'iaka and Namaka -- make up the family.
In March 2004, a team of astronomers announced the discovery of a planet-like transneptunian object orbiting the sun at an extreme distance, in one of the coldest known regions of our solar system.
The object (2003VB12), since named Sedna for an Inuit goddess who lives at the bottom of the frigid Arctic ocean, approaches the sun only briefly during its 10,500-year solar orbit.
It never enters the Kuiper Belt, whose outer boundary region lies at about 55 AU -- instead, Sedna travels in a long, elliptical orbit between 76 and nearly 1,000 AU from the sun.
Since Sedna's orbit takes it to such an extreme distance, its discoverers have suggested that it is the first observed body belonging to the inner Oort Cloud.
In July 2005, a team of scientists announced the discovery of a KBO that was initially thought to be about 10 percent larger than Pluto.
The object, temporarily designated 2003UB313 and later named Eris, orbits the sun about once every 560 years, its distance varying from about 38 to 98 AU.
For comparison, Pluto travels from 29 to 49 AU in its solar orbit.
Eris has a small moon named Dysnomia.
More recent measurements show it to be slightly smaller than Pluto.
The discovery of Eris -- orbiting the sun and similar in size to Pluto (which was then designated the ninth planet) -- forced astronomers to consider whether Eris should be classified as the tenth planet.
Instead, in 2006, the International Astronomical Union created a new class of objects called dwarf planets, and placed Pluto, Eris and the asteroid Ceres in this category.
In 2015, NASA's New Horizons spacecraft flew past Pluto, making the first up-close exploration of a Kuiper Belt Object.
The spacecraft is continuing deeper into this region of icy debris and may be able to explore at least one more object.
The region is named for the astronomer who predicted its existence -- Gerard Kuiper.
It is sometimes called the Edgeworth-Kuiper Belt, recognizing the independent and earlier discussion by Kenneth Edgeworth.
Objects discovered in the Kuiper Belt get their names from diverse mythologies.
Eris is named for the Greek goddess of discord and strife.
Haumea is named for a Hawaiian goddess of fertility and childbirth.
Comets from both regions are generally named for the person who discovered them.
The Oort Cloud is believed to be a thick bubble of icy debris that surrounds our solar system.
This distant cloud may extend a third of the way from our sun to the next star -- between 5,000 and 100,000 astronomical units.
Earth is about one astronomical unit from the sun (roughly 93 million miles or 150 million kilometers).
Consider this: At its current speed of about a million miles a day, NASA's Voyager 1 spacecraft won't reach the Oort Cloud for about 300 years.
And it will take about 30,000 years to reach the other side.
Dutch astronomer Jan Oort first proposed the idea of this region of space to explain the origins of comets with that take thousands of years to orbit the sun.
These are called long-period comets and most have been seen only once in recorded history.
More frequent visitors to the inner solar system are called short-period comets.
There may be hundreds of billions, even trillions, of icy bodies in the Oort Cloud.
Every now and then, something disturbs one of these icy worlds and it begins a long fall toward our sun.
Two recent examples are comets C/2012 S1 (ISON) and C/2013 A1 Siding Spring.
ISON was destroyed when it passed too close the the sun.
Siding Spring, which made a very close pass by Mars, will not return to the inner solar system for about 740,000 years.
This region of space is named for Dutch astronomer Jan Oort who predicted its existence in 1950.
The Oort Cloud still hasn't been discovered.
It is just a theory, although we have studied several comets believed to have come from this distant region of our solar system and on into our galaxy.
Our sun is one of at least 100 billion stars in the Milky Way, a spiral galaxy about 100,000 light years across.
The stars are arranged in a pinwheel pattern with four major arms, and we live about two-thirds of the way up one of them.
Many if not most of the stars host their own families of planets.
More than a thousand of these extrasolar (or exoplanets) have been discovered and thousands more are awaiting confirmation.
All of the stars in the Milky Way orbit a supermassive black hole at the galaxy's center, which is estimated to be some 4 million times as massive as our sun.
Fortunately, it is a safe distance of around 28,000 light years away Earth.
The Milky Way zips alonga galactic orbit at an average speed of about 514,000 miles per hour (828,000 km/hr).
It takes about 230 million years for our solar system to make one revolution around the galactic center.
The Milky Way is part of the Local Group, a neighborhood about 10 million light years across, consisting of more than 30 galaxies that are gravitationally bound to each other.
Aside from our galaxy, the most massive one in this group is Andromeda, which appears to be on course to collide with the Milky Way in about 4 billion years.
Scientists studying galaxies observed that the stars in the outer parts are orbiting the galactic centers just as quickly as the stars further in, a violation of Newton's well-established laws of gravitation.
They deduced that something other than the stars and clouds of gas and dust known to comprise galaxies was providing extra gravity – lots of it.
They calculated that there must be five times as much of this mysterious dark matter, detectable only by its gravitational pull, as there is of the matter we already knew about.
The Local Group is only one of many, many clusters of galaxies, and they are all moving away from each other as more and more space comes into being between them.
This means the universe, itself, is expanding.
That discovery is what led to the theory of the Big Bang origin of the universe.
Scientists expected that the gravitational attraction of everything in the universe would put the brakes on the rate of expansion, and eventually the expansion would stop or even reverse.
But in the 1990s, scientists discovered that the expansion is actually getting faster.
The force responsible for this surprising acceleration was dubbed dark energy.
No one is sure what it is, but one possibility is that it is energy contained within the very vacuum of space.
Since matter and energy are equivalent (as expressed in Einstein's famous equation, E=MC2) scientists have been able to calculate that whatever dark energy is, it comprises about 68 percent of everything in the universe.
Dark matter accounts for another 27 percent, leaving only five percent for protons, neutrons, electrons and photons – in other words, everything we see and understand.
Scientists calculate that there are at least 100 billion galaxies in the observable universe, each one brimming with stars.
On a very large scale, they form a bubbly structure, in which vast sheets and filaments of galaxies surround gargantuan voids.
Moons -- also called satellites -- come in many shapes, sizes and types.
They are generally solid bodies, and few have atmospheres.
Most of the planetary moons probably formed from the discs of gas and dust circulating around planets in the early solar system.
Astronomers have found at least 149 moons orbiting planets in our solar system.
Another 24 moons are awaiting official confirmation of their discovery.
This number does not include the six moons of the dwarf planets, nor does this tally include the tiny satellites that orbit some asteroids and other celestial objects.
Of the terrestrial (rocky) planets of the inner solar system, neither Mercury nor Venus have any moons at all, Earth has one and Mars has its two small moons.
In the outer solar system, the gas giants Jupiter and Saturn and the ice giants Uranus and Neptune have numerous moons.
As these planets grew in the early solar system, they were able to capture objects with their large gravitational fields.
Earth's Moon probably formed when a large body about the size of Mars collided with Earth, ejecting a lot of material from our planet into orbit.
Debris from the early Earth and the impacting body accumulated to form the Moon approximately 4.5 billion years ago (the age of the oldest collected lunar rocks).
Twelve American astronauts landed on the Moon during NASA's Apollo program from 1969 to 1972, studying the Moon and bringing back rock samples.
Usually the term moon brings to mind a spherical object, like Earth's Moon.
The two moons of Mars, Phobos and Deimos, are different.
While both have nearly circular orbits and travel close to the plane of the planet's equator, they are lumpy and dark.
Phobos is slowly drawing closer to Mars and could crash into the planet in 40 or 50 million years.
Or the planet's gravity might break Phobos apart, creating a thin ring around Mars.
Jupiter has 53 known moons (plus 14 awaiting official confirmation), including the largest moon in the solar system, Ganymede.
Many of Jupiter's outer moons have highly elliptical orbits and orbit backwards (opposite to the spin of the planet).
Saturn, Uranus and Neptune also have some irregular moons, which orbit far from their respective planets.
Saturn has 53 known moons (plus 9 awaiting official confirmation).
The chunks of ice and rock in Saturn's rings (and the particles in the rings of the other outer planets) are not considered moons, yet embedded in Saturn's rings are distinct moons or moonlets.
These shepherd moons help keep the rings in line.
Saturn's moon Titan, the second largest in the solar system, is the only moon with a thick atmosphere.
In the realm of the ice giants, Uranus has 27 known moons.
The inner moons appear to be about half water ice and half rock.
Miranda is the most unusual; its chopped-up appearance shows the scars of impacts of large rocky bodies.
Neptune has 13 known moons.
And Neptune's moon Triton is as big as the dwarf planet Pluto and orbits backwards compared with Neptune's direction of rotation.
Pluto's large moon Charon is about half the size of Pluto.
Like Earth's Moon, Charon may have formed from debris resulting from an early collision of an impactor with Pluto.
In 2005, scientists using the Hubble Space Telescope to study Pluto found two additional, but very small, moons.
The little moons Nix and Hydra are about two to three times as far from Pluto as Charon and roughly 5,000 times fainter than Pluto.
Eris, another dwarf planet even more distant than Pluto, has a small moon of its own, named Dysnomia.
Haumea, another dwarf planet, has two satellites, Hi'iaka and Namaka.
Most moons in our solar system are named for mythological characters from a wide variety of cultures.
Uranus is the exception.
Uranus' moons are named for characters in William Shakespeare's plays and from Alexander Pope's poem "Rape of the Lock." Moons are given provisional designations such as S/2009 S1, the first satellite discovered at Saturn in 2009.
The International Astronomical Union approves an official name when the discovery is confirmed.
Pluto is classified as a dwarf planet and is also a member of a group of objects that orbit in a disc-like zone beyond the orbit of Neptune called the Kuiper Belt.
This distant realm is populated with thousands of miniature icy worlds, which formed early in the history of our solar system.
These icy, rocky bodies are called Kuiper Belt objects or transneptunian objects.
Pluto is about two-thirds the diameter of Earth's moon and probably has a rocky core surrounded by a mantle of water ice.
More exotic ices like methane and nitrogen frost coat its surface.
Owing to its size and lower density, Pluto's mass is about one-sixth that of Earth's moon.
Pluto is more massive than Ceres -- the dwarf planet that resides in the asteroid belt between Mars and Jupiter -- by a factor of 14.
Pluto's 248-year-long elliptical orbit can take it as far as 49.3 astronomical units (AU) from the sun.
One AU is the mean distance between Earth and the sun: about 93 million miles or 150 million kilometers.
From 1979 to 1999, Pluto was actually closer to the sun than Neptune, and in 1989, Pluto came to within 29.8 AU of the sun, providing rare opportunities to study this small, cold, distant world.
Since its orbit is so elliptical, when Pluto is close to the sun, its surface ices sublimate, changing directly from solid to a gas, and rise and temporarily form a thin atmosphere.
Pluto's low gravity (about six percent of Earth's) causes the atmosphere to be much more extended in altitude than our planet's atmosphere.
Pluto becomes much colder during the part of each orbit when it is traveling far away from the sun.
During this time, the bulk of the planet's atmosphere is thought to freeze and fall as snow to the surface.
Pluto has a very large moon that is almost half its size named Charon, which was discovered in 1978.
This moon is so big that Pluto and Charon are sometimes referred to as a double dwarf planet system.
The distance between them is 12,200 miles (19,640 kilometers).
The Hubble Space Telescope photographed Pluto and Charon in 1994 when Pluto was about 30 AU from Earth.
These photos showed that Charon is grayer than Pluto (which is redder), indicating that they have different surface compositions and structure.
Charon's orbit around Pluto takes 6.4 Earth days, and one Pluto rotation (a Pluto day) takes 6.4 Earth days.
Charon neither rises nor sets, but hovers over the same spot on Pluto's surface, and the same side of Charon always faces Pluto -- this is called tidal locking.
Compared with most of the planets and moons, the Pluto-Charon system is tipped on its side, like Uranus.
Pluto's rotation is retrograde: it rotates backwards, from east to west (Uranus and Venus also have retrograde rotations).
Because Pluto and Charon are so small and far away, they are extremely difficult to observe from Earth.
In the late 1980s, Pluto and Charon passed in front of each other repeatedly for several years.
Observations of these rare events allowed astronomers to make rudimentary maps of each body showing areas of relative brightness and darkness.
In 2005, scientists photographing Pluto with the Hubble Space Telescope in preparation for the New Horizons mission found two tiny moons orbiting in the same plane as Charon.
These two moons, named Nix and Hydra, are two to three times farther away from Pluto than Charon.
In 2011 and 2012, scientists used Hubble to spot two more moons (originally designated P4 and P5).
In 2013, the two moons were named Kerberos (P4) and Styx (P5).
Late in 2014 and early in 2015, image animations displayed the mutual orbital waltz of Pluto and Charon around their center of mass.
Beginning in the spring of 2015, New Horizons started its detailed studies of Pluto including searches for additional moons and for rings.
Various studies continued through its close approach on July 14 at a distance of 8507 miles (13,691 kilometers) and after.
The return of New Horizons' best data began shortly after its close approach and will continue for more than a year due to the large transmission distance.
Because of the speed of the flyby and Pluto's slow rotation rate only one hemisphere of this dwarf planet has been photographed and measured at high resolution, limiting generalizations about all of Pluto's surface.
Still, Pluto's diameter could be measured and had to be revised upward to 1475 miles (2,374 kilometers) based on New Horizons' imagery.
Results from New Horizons, besides forcing the revision of textbooks, left planetary scientists struggling to explain this cold, distant world and its system of moons.
Pluto's surface exhibits craters as large as 162 miles (260 kilometers) in diameter on the dayside, near encounter hemisphere.
Craters are widely distributed there and show degradation or infill.
This was a surprise to find on a dwarf planet that was anticipated to be heavily cratered and lacking activity that might affect craters.
Dense cratering is seen in some areas, but so are tectonic features including scarps and troughs as long as about 370 miles (600 kilometers).
Mountains are also seen rising 6500 to 9800 feet (2 to 3 kilometers) above their surroundings.
The likely materials that can hold up the mountains and maintain their shapes over millions of years in Pluto's cold is limited.
The science team concludes that the mountains are made of "water ice-based 'bedrock'." Frozen gases on Pluto's surface (the temperature there is about -391 F = -235 C = 38 K) include nitrogen (N2), carbon monoxide (CO),and methane (CH4).
These were detected by ground-based telescopes and are now thought to be thin layers on top of the 'bedrock' water ice.
Dark surface coloring appears to be due carbon residues called tholins.
These are created by solar ultraviolet rays or charged particles falling on mixtures of nitrogen and methane.
Detailed views of a plain on this dwarf planet show no confirmed craters but it has large (tens of kilometers) polygonal or egg-shaped features defined and separated by troughs between neighbors.
Features similar in look to glaciers on Earth are seen in this region.
This is consistent with the strength and flow properties of the frozen gases mentioned above.
Radio transmission measurements from New Horizons measured Pluto's atmosphere having a pressure of 10 microbars (millionths of a bar).
For comparison, Earth's atmospheric pressure at sea level is about 1 bar, 100,000 times greater than Pluto's surface pressure.
Those measurements also showed that Pluto has a shallow tropospheric boundary layer (Earth has a troposphere too).
Imagery from the cameras shows a haze layer and even some structure in it.
It isn't known whether Pluto has a magnetic field, but its small size and slow rotation suggest little or none.
Data from two of New Horizons instruments may give an indirect answer to this question.
Charon was also studied in detail.
Its diameter came out slightly larger than expected, at 753 miles (1212 kilometers).
Surface variations of 9800 feet (3 kilometers) seen on this moon imply that it, like Pluto, has water ice that runs deep in its structure.
Also similar to Pluto, Charon exhibits cratered and smooth plains, fault scarps, and an extensive system of faults and graben.
A graben [Grah-ben] is a block of surface material that has dropped lower than its surroundings, creating a wide valley whose walls are the fault planes along which the block dropped.
Rolling plains are moderately cratered.
They even show several rille-like structures (first observed on Earth's moon).
Craters with rays (both light, as on Earth's moon, and dark, as sometimes seen on Mars) indicating freshness are found on Charon.
Other craters show evidence of aging.
The two largest fractures (visible during the flyby) on Charon extend at least 650 miles (1050 kilometers) across the surface and at least one other, with a depth of 3 miles (5 kilometers) is seen going over the horizon to Charon's night side.
One of the dayside fractures is seen as a double-walled graben-like structure.
There is no evidence that Charon has an atmosphere.
Nix is not spherical.
It has three different diameters (making it a tri-axial ellipsoid): 67 x 51 x 45 miles (108 x 82 x 72 kilometers).
There are variations in composition over its surface but it reflects more light than Charon.
This suggests that its ice is cleaner than Charon's.
Hydra is also not spherical with rough diameters of 53 x 41 miles (86 x 66 kilometers) with the third dimension not well determined.
It also is more reflective than Charon, leaving scientists to puzzle over how the ice on these small moons could stay so bright over billions of years given the processes known to darken material over time.
More information on Kerberos and Styx awaits the return of more data from New Horizons.
Data used for searches for additional moons and rings has not detected any.
Pluto is the only world named by an 11-year-old girl.
In 1930, Venetia Burney of Oxford, England, suggested to her grandfather that the new discovery be named for the Roman god of the underworld.
He forwarded the name to the Lowell Observatory and it was selected.
Pluto's moons are named for other mythological figures associated with the underworld.
Charon is named for the river Styx boatman who ferries souls in the underworld (as well as honoring Sharon, the wife of discoverer James Christy); Nix is named for the mother of Charon, who is also the goddess of darkness and night; Hydra is named for the nine-headed serpent that guards the underworld; Kerberos is named after the three-headed dog of Greek mythology (and called Fluffy in the Harry Potter novels); and Styx is named for the mythological river that separates the world of the living from the realm of the dead.
Pluto's place in mythology can get a little muddled, so we asked Dr.
Elizabeth Vandiver, chair of the Department of Classics in Whitman College in Walla Walla, Washington, to clarify the origins of the name: "Pluto is the name of the Roman god of the Underworld, equivalent to the Greek Hades.
However, the Greek name "Plouton" (from which the Romans derived their name "Pluto") was also occasionally used as an alternative name for Hades.
But Pluto is definitely the Roman spelling."
Dwarf planet Ceres is the largest object in the asteroid belt between Mars and Jupiter and the only dwarf planet located in the inner solar system.
It was the first member of the asteroid belt to be discovered when Giuseppe Piazzi spotted it in 1801.
And when Dawn arrived in 2015, Ceres became the first dwarf planet to receive a visit from a spacecraft.
Called an asteroid for many years, Ceres is so much bigger and so different from its rocky neighbors that scientists classified it as a dwarf planet in 2006.
Even though Ceres comprises 25 percent of the asteroid belt's total mass, tiny Pluto is still 14 times more massive.
Ceres is named for the Roman goddess of corn and harvests.
The word cereal comes from the same name.
With a radius of 296 miles (476 kilometers), Ceres is 1/13 the radius of Earth.
If Earth were the size of a nickel, Ceres would be about as big as a poppy seed.
From an average distance of 257 million miles (413 million kilometers), Ceres is 2.8 astronomical units away from the sun.
One astronomical unit (abbreviated as AU), is the distance from the sun to Earth.
From this distance, it takes sunlight 22 minutes to travel from the sun to Ceres.
Ceres takes 1,682 Earth days, or 4.6 Earth years, to make one trip around the sun.
As Ceres orbits the sun, it completes one rotation every 9 hours, making its day length one of the shortest in the solar system.
Ceres' axis of rotation is tilted just 4 degrees with respect to the plane of its orbit around the sun.
That means it spins nearly perfectly upright and doesn't experience seasons like other more tilted planets do.
Ceres formed along with the rest of the solar system about 4.5 billion years ago when gravity pulled swirling gas and dust in to become a small dwarf planet.
Scientists describe Ceres as an "embryonic planet," which means it started to form but didn't quite finish.
Nearby Jupiter's strong gravity prevented it from becoming a fully formed planet.
About 4 billion years ago, Ceres settled into its current location among the leftover pieces of planetary formation in the asteroid belt between Mars and Jupiter.
Ceres is more similar to the terrestrial planets (Mercury, Venus, Earth and Mars) than its asteroid neighbors, but it is much less dense.
One of the similarities is a layered interior, but Ceres' layers aren't as clearly defined.
Ceres probably has a solid core and a mantle made of water ice.
In fact, Ceres could be composed of as much as 25 percent water.
If that is correct, Ceres has more water than Earth does.
Ceres' crust is rocky and dusty with large salt deposits.
The salts on Ceres aren't like table salt (sodium chloride), but instead are made of different minerals like magnesium sulfate.
Ceres is covered in countless small, young craters, but none are larger than 175 miles (280 kilometers) in diameter.
This is surprising, given that the dwarf planet must have been hit by numerous large asteroids during its 4.5 billion-year lifetime.
The lack of craters might be due to layers of ice just below the surface.
The surface features could smooth out over time if ice or another lower-density material, such as salt, is just below the surface.
It's also possible that past hydrothermal activity, such as ice volcanoes, erased some large craters.
Within some of Ceres' craters, there are regions that are always in shadow.
It's possible that without direct sunlight, these "cold traps" could have water ice in them for long periods of time.
Ceres has a very thin atmosphere, and there is evidence it contains water vapor.
The vapor may be produced by ice volcanoes or by ice near the surface sublimating (transforming from solid to gas).
Ceres is one of the few places in our solar system where scientists would like to search for possible signs of life.
Ceres has something a lot of other planets don't: water.
Here on Earth, water is essential for life, so it's possible that with this ingredient and a few other conditions met, life could maybe exist there.
Living things on Ceres, if they are there at all, would likely be very small microbes similar to bacteria.
And while Ceres might not have living things today, there could be signs it harbored life in the past.
Ceres does not have any moons.
Ceres does not have any rings.
Scientists don't think Ceres has a magnetosphere.
Ceres is a good example of how challenging it can be to categorize bodies in our solar system.
When Giuseppe Piazzi first spotted it in 1801, he assumed Ceres was the "missing" planet between Mars and Jupiter.
Within a few years, Pallas, Juno and Vesta were also discovered in the region, and they too were called planets.
Starting in the 1840s, astronomers discussed reclassifying the increasing number of bodies in this area, since they didn't quite fit the definition of a planet.
By 1860 a total of 62 bodies had been discovered in the space between Mars and Jupiter.
And by 1863 astronomers accepted the classification of these objects as asteroids and called the area the asteroid belt.
That's the way it stayed for over 140 years.
Then in 2006, astronomers designated Ceres a dwarf planet, since it matched the criteria used to classify Pluto and similar-sized objects in the outer solar system.
Ceres holds the honor of being the first dwarf planet to be orbited by a spacecraft.
Dawn reached it in 2015 to study its surface, composition and history.
The largest body in the asteroid belt, Ceres has amassed a number of references in science fiction stories of the 20th and 21st centuries.
In the TV series The Expanse, Ceres is inhabited by humans, and in the PC Game Descent, one of the secret levels takes place on Ceres.
In the video game Destiny, Ceres was colonized by an alien race called the Fallen at the end of humanity's Golden Age.
Ceres was later destroyed by a civilization of post-humans who inhabit the Asteroid Belt.
Jupiter's moon Europa is slightly smaller than Earth's moon.
Its surface is smooth and bright, consisting of water ice crisscrossed by long, linear fractures.
Like our planet, Europa is thought to have an iron core, a rocky mantle and an ocean of salty water beneath its ice crust.
Unlike Earth, however, this ocean would be deep enough to extend from the moon's surface to the top of its rocky mantle.
Being far from the sun, the ocean's surface would be globally frozen over.
While evidence for this internal ocean is quite strong, its presence awaits confirmation by a future mission.
Europa orbits Jupiter every 3.5 days and is locked by gravity to Jupiter such that the same hemisphere of the moon always faces the planet.
Because Europa's orbit is slightly stretched out from circular, or elliptical, its distance from Jupiter varies, creating tides that stretch and relax its surface.
The tides occur because Jupiter's gravity is just slightly stronger on the near side of the moon than on the far side, and the magnitude of this difference changes as Europa orbits.
Flexing from the tides supplies energy to the moon's icy shell, creating the linear fractures across its surface.
If Europa's ocean exists, the tides might also create volcanic or hydrothermal activity on the seafloor, supplying nutrients that could make the ocean suitable for living things.
Based on the small number of craters observed, the surface of this moon appears to be no more than 40 to 90 million years old, which is quite youthful in geologic terms.
Along Europa's many fractures, and in splotchy patterns across its surface, is a reddish-brown material whose composition is not known, but may hold clues to the moon's potential as a habitable world.
NASA's Galileo mission, which explored the Jupiter system from 1995 to 2003, made numerous flybys of Europa.
It obtained the closest images to date of the moon's fractured surface, revealing strange pits and domes that suggested the ice could be slowly turning over, or convecting, due to heat from below.
Also of particular interest were regions of "chaos terrain," which contained broken, blocky landscapes covered in the mysterious reddish material.
Scientists think that chaos regions represent places where geologic activity has disrupted the otherwise smooth surface.
In 2011, scientists studying Galileo mission data announced the intriguing idea that chaos terrains are places where the surface has collapsed above lens-shaped lakes perched within the ice.
In 2013, NASA announced startling evidence from researchers using the Hubble Space Telescope that Europa might be actively venting plumes of water into space.
The finding generated considerable excitement among scientists, as it provided evidence that the moon is geologically active in the present day.
If confirmed by follow-up observations, the plumes could be investigated by future missions in similar fashion to the Cassini spacecraft's flights through plumes of Enceladus.
Europa is special among the bodies of our solar system in having a potentially enormous volume of liquid water, along with geologic activity that could promote the exchange of chemicals from the surface with the watery environment beneath the ice.
Indeed, it could be the most promising place in the solar system to search for signs of present-day life.
One of the most important measurements made by the Galileo mission showed how Jupiter's magnetic field was disrupted in the space around Europa.
This measurement strongly implied that a special type of magnetic field is being created (induced) within Europa by a deep layer of some electrically conductive fluid beneath the surface.
Based on Europa's icy composition, scientists think the most likely material to create this magnetic signature is a global ocean of salty water.
Future missions to Europa will likely seek to confirm the presence of its ocean.
For example, measurements of the amount of flexing due to the tides are one important indicator -- if the ocean exists, the tides should deform the surface by about 30 m (100 feet); if the moon is frozen through, the tides should stretch the surface by only one meter (3 feet).
Also of great interest will be the composition of the reddish material on the surface.
Scientists would like to know if this material holds clues to the composition of the ocean and whether material is cycling between the surface and the interior.
Europa was discovered on 8 January 1610 by Galileo Galilei.
The discovery, along with three other Jovian moons, was the first time a moon was discovered orbiting a planet other than Earth.
The discovery of the four Galilean satellites strengthened the view that planets in our solar system orbit the sun, instead of Earth.
Galileo apparently had observed Europa on 7 January 1610, but had been unable to differentiate it from Io until the next night.
Galileo originally called Jupiter's moons the Medicean planets, after the Medici family and referred to the individual moons numerically as I, II, III, and IV.
Galileo's naming system would be used for a couple of centuries.
The names Io, Europa, Ganymede, and Callisto were officially adopted after it became apparent that naming moons by number would be very confusing as new additional moons were being discovered.
Europa was originally designated Jupiter II by Galileo because it was the second satellite of Jupiter.
Europa is named for the daughter of Agenor, in ancient Greek mythology.
Europa was abducted by Zeus (the Greek equivalent of the Roman god Jupiter), who had taken the shape of a spotless white bull.
Europa was so delighted by the gentle beast that she decked it with flowers and rode upon its back.
Seizing his opportunity, Zeus rode away with her into the ocean to the island of Crete, where he transformed back into his true shape.
Europa bore Zeus many children, including Minos.
Saturn's largest moon Titan is the second largest moon in our solar system, second only to Jupiter's Ganymede, which is only 2 percent larger.
With a mean radius of 1,600 miles (2,575 km), Titan is bigger than Earth's moon, and even larger than the planet Mercury.
Titan is the only moon in our solar system that has clouds and a dense atmosphere, mostly nitrogen and methane.
It is also the only other place in the solar system known to have an earthlike cycle of liquids flowing across its surface.
Titan orbits Saturn at a distance of about 759,000 miles (1.2 million km), taking 15 days and 22 hours to complete a full orbit.
Titan is tidally locked in synchronous rotation with Saturn, and permanently presents one face to the planet as it completes its orbit.
Cassini has revealed that Titan's surface is shaped by rivers and lakes of liquid ethane and methane (the main component of natural gas).
These liquids form clouds from which the liquid gases sometimes rain from the sky as water does on Earth.
On Titan it is so cold (-290 degrees Fahrenheit or -179 degrees Celsius) that water plays the role of rock and lava, and flowing methane carves river channels and fills great lakes with liquid natural gas.
Vast regions of tall dunes stretch across the landscape -- dunes whose "sand" is composed of dark hydrocarbon grains.
The dunes are not unlike those seen in the desert of Namibia in Africa.
Volcanism may occur as well, but with liquid water as the lava.
Titan has few impact craters, meaning that its surface must be relatively young and some combination of processes erases evidence of impacts.
This is the case for Earth as well; craters on our planet are eroded by the relentless forces of flowing liquid (water, in Earth's case) and wind.
These forces are present on Titan as well.
Tectonic forces -- the movement of the ground due to pressures from beneath -- also appear to be at work on Saturn's largest moon.
Titan is the only moon in the solar system with a thick atmosphere.
Titan's atmospheric pressure is about 60 percent greater than Earth's -- roughly the same pressure found at the bottom of a swimming pool.
Because Titan is less massive than Earth, its gravity doesn't hold onto its gaseous envelope as tightly, so the atmosphere extends to an altitude 10 times higher than Earth's - nearly 370 miles (600 km) into space.
Titan's atmosphere is mostly nitrogen (about 95 percent) and methane (about 5 percent), with small amounts of other carbon-rich compounds.
High in the atmosphere, methane and nitrogen molecules are split apart by the sun's ultraviolet light and high-energy particles accelerated by Saturn's magnetic field; the products of this splitting recombine to form a variety of organic molecules.
Organic molecules contain carbon and hydrogen, and often include nitrogen, oxygen and other elements important to life on Earth.
Some of the compounds produced by the splitting and recycling of methane and nitrogen in the upper atmosphere create a kind of smog - a thick, orange-colored haze that obscures the moon's surface from view.
And some of the heavy, carbon-rich compounds also fall to the surface.
Some of these hydrocarbons go on to form grains that make up the "sand" of vast dune fields on Titan's surface.
One of Titan's great mysteries is the source of its methane, which makes all of this complex chemistry possible.
Since sunlight breaks down methane in the atmosphere, there must be a source that replenishes what is lost.
Researchers suspect methane could be belched into Titan's atmosphere by cryovolcanism, or volcanoes with water as lava.
In 1994, NASA's Hubble Space Telescope recorded pictures of Titan, which suggested that a huge bright continent exists on the hemisphere that faces forward in orbit.
These Hubble results didn't prove that liquid seas existed, however; only that Titan has large bright and dark regions on its surface.
Titan's surface remained shrouded in secrecy below the clouds until July 2004.
That's when NASA's Cassini spacecraft arrived.
Cassini was specially designed to peer through Titan's haze with radar and in certain colors of light, called spectral windows, that allow a glimpse of what lies below.
During dozens of flybys, the Cassini orbiter has mapped a large fraction of Titan's surface and made detailed studies of its atmosphere.
Cassini also carried the European-built Huygens probe, which parachuted through Titan's atmosphere in 2005 to make the first landing on a body in the outer solar system.
Titan was discovered on 25 March 1655 by the Dutch astronomer Christiaan Huygens.
The name Titan comes from a generic term for the children of Ouranos (Uranus) and Gaia in ancient Greek mythology.
In the stories, the Titans were the ancestors of the human race.
The Titans were known to have devoured the limbs of Dionysus, the son of Zeus.
Enraged, Zeus struck the Titans with lightning.
Zeus had intended this child to have dominion over the world.
The lightning burned the Titans to ashes, and from the ashes, mankind was formed.
Enceladus is Saturn's sixth largest moon, only 157 miles (252 km) in mean radius, but it's one of the most scientifically compelling bodies in our solar system.
Hydrothermal vents spew water vapor and ice particles from an underground ocean beneath the icy crust of Enceladus.
This plume of material includes organic compounds, volatile gases, carbon dioxide, carbon monoxide, salts and silica.
With its global ocean, unique chemistry and internal heat, Enceladus has become a promising lead in our search for worlds where life could exist.
In 2005, Cassini's multiple instruments discovered that this icy outpost is gushing water vapor geysers out to a distance of three times the radius of Enceladus.
The icy water particles are roughly one ten-thousandth of an inch, or about the width of a human hair.
The particles and gas escape the surface at jet speed at approximately 800 miles per hour (400 meters per second).
The eruptions appear to be continuous, refreshing the surface and generating an enormous halo of fine ice dust around Enceladus, which supplies material to one of Saturn's rings, the E-ring.
Several gases, including water vapor, carbon dioxide, methane, perhaps a little ammonia and either carbon monoxide or nitrogen gas make up the gaseous envelope of the plume.
In 2015, researchers analyzing years of data from Cassini found the magnitude of the moon's very slight wobble as it orbits Saturn can only be accounted for if its outer ice shell is not frozen solid to its interior, meaning a global ocean must be present.
The finding implies the plumes are being fed by this vast liquid water reservoir.
The measurements suggested a large sea about 6 miles (10 kilometers) deep beneath the southern polar region, under an ice shell about 19 to 25 miles (30 to 40 kilometers) thick.
Enceladus is one of the brightest objects in our solar system.
Covered in water ice that reflects sunlight like freshly fallen snow, Enceladus reflects almost 100 percent of the sunlight that strikes it.
Because Enceladus reflects so much sunlight, the surface temperature is extremely cold, about -201 degrees C (-330 degrees F).
About as wide as Arizona, Enceladus displays at least five different types of terrain.
Parts of Enceladus shows craters up to 22 miles (35 km) in diameter.
Other areas show regions with no craters, indicating major resurfacing events in the geologically recent past.
There are fissures, plains, corrugated terrain, and geysers that are likely all related to the liquid interior of the moon, even though it should have frozen eons ago.
It is possible Enceladus is heated by a tidal mechanism similar to Jupiter's moon Io.
The south polar region of Enceladus is almost entirely free of impact craters.
The area is littered with house-sized ice boulders and surfaces carved by tectonic patterns unique to this region of the moon.
A huge cloud of water vapor over the area and relatively warm fractures in the crust supply the cloud of water vapor and ice particles that extend into space.
Scientists came to informally call the deep crevasses "tiger stripes." The jets originate near the hottest spots on the tiger stripe fractures (also known as "sulci").
Enceladus is one of the major inner moons of Saturn along with Dione, Tethys, and Mimas.
It orbits Saturn at a distance of 148,000 miles (238,000 km), falling between the orbits of Mimas and Tethys.
It is tidally locked with Saturn, keeping the same face toward the planet.
It completes one orbit every 32.9 hours within the densest part of Saturn's E Ring, the outermost of its major rings, and is its main source.
Enceladus is, like many moons in the extensive systems of the giant planets, trapped in an orbital resonance.
Its resonance with Dione excites its orbital eccentricity, which is damped by tidal forces, tidally heating its interior, and possibly driving the geological activity.
With its global ocean, unique chemistry and internal heat, Enceladus has become a promising lead in our search for worlds where life could exist.
Discovered in 1789 by William Herschel little was known about Enceladus until the two Voyager spacecraft passed nearby in the early 1980s.
Then in 2005, the Cassini spacecraft started multiple close flybys of Enceladus, revealing its surface and environment in greater detail.
In particular, Cassini discovered water-rich plumes venting from the south polar region.
British astronomer William Herschel spotted Enceladus orbiting Saturn on 28 August 1789.
Enceladus is named after the giant Enceladus of Greek mythology.
The name Enceladus -- like the names of each of the first seven moons of Saturn to be discovered -- was suggested by William Herschel's son John Herschel in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope.
He chose these names because Saturn, known in Greek mythology as Cronus, was the leader of the Titans.
With a diameter of over 2,985 miles (4,800 km), Callisto is the third largest moon in our solar system and is almost the size of Mercury.
Callisto is the outermost of the Galilean satellites, and orbits beyond Jupiter's main radiation belts.
It has the lowest density of the Galilean satellites (1.86 grams/cubic cm).
Its interior is probably similar to Ganymede except the inner rocky core is smaller, and this core is surrounded by a large icy mantle.
Callisto's surface is the darkest of the Galileans, but it is twice as bright as our own Moon.
Callisto is the most heavily cratered object in our solar system.
It is thought to be a long dead world, with hardly any geologic activity on its surface.
In fact, Callisto is the only body greater than 1000 km in diameter in the solar system that has shown no signs of undergoing any extensive resurfacing since impacts have molded its surface.
With a surface age of about 4 billion years, Callisto has the oldest landscape in the solar system.
Callisto was discovered on 7 January 1610 by Galileo Galilei.
The discovery, along with three other Jovian moons, was the first time a moon was discovered orbiting a planet other than Earth.
The discovery of the four Galilean satellites eventually led to the understanding that planets in our solar system orbit the sun, instead of our solar system revolving around Earth.
Galileo originally called Jupiter's moons the Medicean planets, after the Medici family and referred to the individual moons numerically as I, II, III, and IV.
Galileo's naming system would be used for a couple of centuries.
It wouldn't be until the mid-1800s that the names of the Galilean moons, Io, Europa, Ganymede, and Callisto, would be officially adopted, and only after it became apparent that naming moons by number would be very confusing as new additional moons were being discovered.
Callisto was originally designated Jupiter IV by Galileo because it is the fourth satellite of Jupiter.
Callisto is named for the beautiful daughter of Lycaon, who followed the chaste goddess of the hunt, Artemis.
Unfortunately, since Callisto was seduced by Zeus (the Greek equivalent of the Roman god Jupiter) and became pregnant she was banished by Artemis.
Zeus changed Callisto into a bear to protect her from his wife Hera's jealousy.
Later, Zeus placed Callisto and their son in the sky, and mother and son became Ursa Major and Ursa Minor (Great Bear and Little Bear).
Looking like a giant pizza covered with melted cheese and splotches of tomato and ripe olives, Io is the most volcanically active body in the solar system.
Volcanic plumes rise 300 km (190 miles) above the surface, with material spewing out at nearly half the required escape velocity.
A bit larger than Earth's Moon, Io is the third largest of Jupiter's moons, and the fifth one in distance from the planet.
Although Io always points the same side toward Jupiter in its orbit around the giant planet, the large moons Europa and Ganymede perturb Io's orbit into an irregularly elliptical one.
Thus, in its widely varying distances from Jupiter, Io is subjected to tremendous tidal forces.
These forces cause Io's surface to bulge up and down (or in and out) by as much as 100 m (330 feet)! Compare these tides on Io's solid surface to the tides on Earth's oceans.
On Earth, in the place where tides are highest, the difference between low and high tides is only 18 m (60 feet), and this is for water, not solid ground!
This tidal pumping generates a tremendous amount of heat within Io, keeping much of its subsurface crust in liquid form seeking any available escape route to the surface to relieve the pressure.
Thus, the surface of Io is constantly renewing itself, filling in any impact craters with molten lava lakes and spreading smooth new floodplains of liquid rock.
The composition of this material is not yet entirely clear, but theories suggest that it is largely molten sulfur and its compounds (which would account for the varigated coloring) or silicate rock (which would better account for the apparent temperatures, which may be too hot to be sulfur).
Sulfur dioxide is the primary constituent of a thin atmosphere on Io.
It has no water to speak of, unlike the other, colder Galilean moons.
Data from the Galileo spacecraft indicates that an iron core may form Io's center, thus giving Io its own magnetic field.
Io's orbit, keeping it at more or less a cozy 422,000 km (262,000 miles) from Jupiter, cuts across the planet's powerful magnetic lines of force, thus turning Io into a electric generator.
Io can develop 400,000 volts across itself and create an electric current of 3 million amperes.
This current takes the path of least resistance along Jupiter's magnetic field lines to the planet's surface, creating lightning in Jupiter's upper atmosphere.
As Jupiter rotates, it takes its magnetic field around with it, sweeping past Io and stripping off about 1,000 kg (1 ton) of Io's material every second! This material becomes ionized in the magnetic field and forms a doughnut-shaped cloud of intense radiation referred to as a plasma torus.
Some of the ions are pulled into Jupiter's atmosphere along the magnetic lines of force and create auroras in the planet's upper atmosphere.
It is the ions escaping from this torus that inflate Jupiter's magnetosphere to over twice the size we would expect.
Io was discovered on 8 January 1610 by Galileo Galilei.
The discovery, along with three other Jovian moons, was the first time a moon was discovered orbiting a planet other than Earth.
The discovery of the four Galilean satellites eventually led to the understanding that planets in our solar system orbit the sun, instead of our solar system revolving around Earth.
Galileo apparently had observed Io on 7 January 1610, but had been unable to differentiate between Io and Europa until the next night.
Galileo originally called Jupiter's moons the Medicean planets, after the Medici family and referred to the individual moons numerically as I, II, III, and IV.
Galileo's naming system would be used for a couple of centuries.
It wouldn't be until the mid-1800s that the names of the Galilean moons, Io, Europa, Ganymede, and Callisto, would be officially adopted, and only after it became apparent that naming moons by number would be very confusing as new additional moons were being discovered.
Io was originally designated Jupiter I by Galileo because it is the first satellite of Jupiter.
Io is named for the daughter of Inachus, who was raped by Jupiter.
Jupiter, in an effort to hide his crime from his wife, Juno, transformed Io into a heifer.