The Solar System

Planets, moons, asteroids, comets and meteoroids—the solar system we call home is located in an outer spiral arm of the vast Milky Way galaxy. It consists of the sun (our star) and everything that orbits around it. This includes the eight planets and their natural satellites (such as our moon), dwarf planets and their satellites, as well as asteroids, comets and countless particles of smaller debris.

### Size and Distance

The solar system extends much farther than the eight planets that orbit the sun. The solar system also includes the Kuiper Belt that lies past Neptune's orbit. This is a sparsely occupied ring of icy bodies, almost all smaller than the most popular Kuiper Belt Object, dwarf planet Pluto.

And beyond the fringes of the Kuiper belt is the Oort Cloud and the outer reach. This giant spherical shell surrounds our solar system. It has never been directly observed as of yet, but its existence is predicted based on mathematical models and observations of comets that likely originate there.

The Oort Cloud is made of icy pieces of space debris the sizes of mountains and sometimes larger, orbiting our sun as far as 1.6 light years away. This shell of material is thick, extending from 5,000 astronomical units to 100,000 astronomical units. One astronomical unit (or AU) is the
distance from the sun to Earth, or about 93 million miles (150 million kilometers). The Oort Cloud is the boundary of the sun's gravitational influence, where orbiting objects can turn around and return closer to our sun.

## Planets & Moons

### Mercury and Venus

Up first are Mercury and Venus. Neither of them has a moon.

Because Mercury is so close to the sun and its gravity, it wouldn’t be able to hold on to its own moon. Any moon would most likely crash into Mercury or maybe go into orbit around the sun and eventually get pulled into it.

Why Venus doesn’t have a moon is a mystery for scientists to solve.

### Earth (That's us!)

Up next is Earth, and of course we have one moon.

### Mars

Mars has two moons. They’re names are Phobos and Deimos. Named for the mythological sons of Ares, the Greek counterpart of the Roman god, Mars. Phobos means fear or panic (think "phobia"), and Deimos means flight (as in running away after an overwhelming defeat). Fitting names for the sons of a war god.

Mars' moons are among the smallest in the solar system. Phobos is a bit larger than Deimos, and orbits only 6,000 km (3,700 miles) above the Martian surface. No known moon orbits closer to its planet. It whips around Mars three times a day, while the more distant Deimos takes 30 hours for each orbit. Phobos is gradually spiraling inward, drawing about 1.8 m closer to the planet each century. Within 50 million years, it will either crash into
Mars or break up and form a ring around the planet.

To someone standing on the Mars-facing side of Phobos, Mars would take up a large part of the sky. And people may one day do just that. Scientists have discussed the possibility of using one of the Martian moons as a base from which astronauts could observe the Red Planet and launch robots to its surface, while shielded by miles of rock from cosmic rays and solar radiation for nearly two-thirds of every orbit.

### Jupiter

Next are the giant outer planets. They have lots of moons. Jupiter, for instance, has 53 moons!  The most well-known are Io (pronounced eye-oh), Europa, and Callisto. Jupiter also has the biggest moon in our solar system, Ganymede.

Jupiter is the biggest planet and has the biggest moon, Ganymede.

The planet Jupiter's four largest moons are called the Galilean satellites after Italian astronomer Galileo Galilei, who first observed them in 1610. The German astronomer Simon Marius claimed to have seen the moons around the same time, but he did not publish his observations and so Galileo is given the credit for their discovery. These large moons, named Io, Europa, Ganymede, and Callisto, are each distinctive worlds.

* Io -- is the most volcanically active body in the solar system. Io's surface is covered by sulfur in different colorful forms. As Io travels in its slightly elliptical orbit, Jupiter's immense gravity causes "tides" in the solid surface that rise 100 m (300 feet) high on Io, generating enough heat for volcanic activity and to drive off any water. Io's volcanoes are driven by hot silicate magma.
* Europa -- surface is mostly water ice, and there is evidence that it may be covering an ocean of water or slushy ice beneath. Europa is thought to have twice as much water as does Earth. This moon intrigues astrobiologists because of its potential for having a "habitable zone." Life forms have been found thriving near subterranean volcanoes on Earth and in other extreme locations that may be analogues to what may exist on Europa.
* Ganymede -- is the largest moon in the solar system (larger than the planet Mercury), and is the only moon known to have its own internally generated magnetic field.

* Callisto -- surface is extremely heavily cratered and ancient -- a visible record of events from the early history of the solar system. However, the very few small craters on Callisto indicate a small degree of current surface activity.

The interiors of Io, Europa and Ganymede have a layered structure (as does Earth). Io has a core, and a mantle of at least partially molten rock, topped by a crust of solid rock coated with sulfur compounds. Europa and Ganymede both have a core; a rock envelope around the core; a thick, soft ice layer; and a thin crust of impure water ice. In the case of Europa, a global subsurface water layer probably lies just below the icy crust. Layering at Callisto is less well defined and appears to be mainly a mixture of ice and rock.

### Saturn

Saturn has 53 moons, and that’s not counting Saturn’s beautiful rings. Saturn’s moons have great names like Mimas, Enceladus, and Tethys. One of these moons, named Titan, even has its own atmosphere, which is very unusual for a moon.

Each of Saturn's moons bears a unique story. Two of the moons orbit within gaps in the main rings. Some, such as Prometheus and Pandora, interact with ring material, shepherding the ring in its orbit. Some small moons are trapped in the same orbits as Tethys or Dione. Janus and Epimetheus occasionally pass close to each other, causing them to periodically exchange orbits.

Here's a sampling of some of the unique aspects of the
moons:

* Titan -- is so large that it affects the orbits of other near-by moons. At 5,150 km (3,200 miles) across, it is the second largest moon in the solar system. Titan hides its surface with a thick nitrogen-rich atmosphere. Titan's atmosphere is similar to the Earth's atmosphere of long ago, before biology took hold on our home planet. Titan's atmosphere is approximately 95% nitrogen with traces of methane. While the Earth's atmosphere extends about 60 km (37 miles) into space, Titan's extends nearly 600 km (ten times that of the Earth's atmosphere) into space.

* Iapetus -- has one side as bright as snow and one side as dark as black velvet, with a huge ridge running around most of its dark-side equator.
* Phoebe -- orbits the planet in a direction opposite that of Saturn's larger moons, as do several of the more recently discovered moons.
* Mimas -- has an enormous crater on one side, the result of an impact that nearly split the moon apart.
* Enceladus -- displays evidence of active ice volcanism: Cassini observed warm fractures where evaporating ice evidently escapes and forms a huge cloud of water vapor over the south pole.

* Hyperion -- has an odd flattened shape and rotates chaotically, probably due to a recent collision.
* Pan -- orbits within the main rings and helps sweep materials out of a narrow space known as the Encke Gap.
* Tethys -- has a huge rift zone called Ithaca Chasma that runs nearly three-quarters of the way around the moon.
* Four moons orbit in stable places around Saturn called Lagrangian points. These places lie 60 degrees ahead of or behind a larger moon and in the same orbit. Telesto and Calypso occupy the two Lagrangian points of Tethys in its orbit; Helene and Polydeuces occupy the corresponding Lagrangian points of Dione.
* Sixteen of Saturn's moons keep the same face toward the planet as they orbit. Called "tidal locking," this is the same phenomenon that keeps our Moon always facing toward Earth.

### Uranus

Uranus, the seventh planet of the Solar System, has 27 known
moons, all of which are named after characters from the works of William Shakespeare and Alexander Pope.  Uranus's moons are divided into three groups: thirteen inner moons, five major moons, and nine irregular moons.

The inner moons are small dark bodies that share common properties and origins with Uranus's rings. The five major moons are massive enough to have reached hydrostatic equilibrium, and four of them show signs of
internally driven processes such as canyon formation and volcanism on their surfaces. The largest of these five, Titania, is 1,578 km in diameter and the eighth-largest moon in the Solar System, and about one-twentieth the mass of the Moon. Lastly, Neptune has 13 moons.  One of them, Triton, is as big as dwarf planet Pluto.

All of Uranus's inner moons appear to be roughly half water ice and half rock. The composition of the moons outside the orbit of Oberon remains unknown, but they are likely captured asteroids.

Here's a sampling of some of the unique aspects of the
moons:

* Miranda -- the innermost and smallest of the five major satellites, has a surface unlike any other moon that's been seen. It has giant fault canyons as much as 12 times as deep as the Grand Canyon, terraced layers and surfaces that appear very old, and others that look much younger.
* Ariel -- has the brightest and possibly the youngest surface among all the moons of Uranus. It has few large craters and many small ones, indicating that fairly recent low-impact collisions wiped out the large craters that would have been left by much earlier, bigger strikes. Intersecting valleys pitted with craters scars its surface.
* Umbriel -- is ancient, and the darkest of the five large moons. It has many old, large craters and sports a mysterious bright ring on one side.
* Oberon -- the outermost of the five major moons, is old, heavily cratered and shows little signs of internal activity. Unidentified dark material appears on the floors of many of its craters.
* Cordelia and Ophelia -- are *shepherd moons* that keep Uranus' thin, outermost "epsilon" ring well defined.

Between them and Miranda is a swarm of eight small satellites unlike any other system of planetary moons. This region is so crowded that astronomers don't yet understand how the little moons have managed to avoid crashing into each other. They may be shepherds for the planet's 10 narrow rings, and scientists think there must be still more moons, interior to any known, to confine the edges of the inner rings.

"Well shone, Moon," wrote Shakespeare, "truly, the moon shines with a good grace."

### Neptune

Lastly, Neptune has 13 moons. One of them, Triton (not to be confused with Saturn's moon, Titan), is as big as dwarf planet Pluto.

All of Neptune's inner moons are dark objects. Their spectra indicate that they are made from water ice contaminated by some very dark material, probably complex organic compounds. In this respect, the inner Neptunian moons are similar to the inner moons of Uranus.

In order of their distance from the planet, the irregular moons are Triton, Nereid, Halimede, Sao, Laomedeia, Psamathe, and Neso, a group that includes both prograde and retrograde objects. The five outermost moons are similar to the irregular moons of other giant planets, and are thought to have been gravitationally captured by Neptune, unlike the regular satellites, which probably formed in situ.

Fascinating details about Triton. Part of its surface resembles the rind of a cantaloupe. Ice volcanoes spout what is probably a mixture of liquid nitrogen, methane and dust, which instantly freezes and then snows back down to the surface. One Voyager 2 image shows a frosty plume shooting 8 km (5 miles) into the sky and drifting 140 km (87 miles) downwind.