Rings and Satellites
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The Planets
The Terrestrial Planets (Inner Planets) | |
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The terrestrial planets are the four innermost planets in the solar system, Mercury, Venus, Earth, and Mars. They are called terrestrial because they have a compact, rocky surface like the Earth's. The planets, Venus, Earth, and Mars have significant atmospheres while Mercury has almost none. |
The Jovian Planets (Outer Planets) | |
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Jupiter, Saturn, Uranus, and Neptune are known as the Jovian (Jupiter-like) planets, because they are all gigantic compared with Earth, and they have a gaseous nature like Jupiter's. The Jovian planets are also referred to as the gas giants, although some or all of them might have small solid cores. |
A World its Own | |
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Pluto with its moon Charon, used to be considered by some to be a double planet. Composed mostly of rock and ice it is perhaps closer in composition to the terrestrial planets, but its orbit is beyond most of the gas giants and little is known about its composition. Its eccentric path moves it inside the orbit of Neptune periodically, but because its orbit is highly inclined and it is in synchrony with Neptune, they will never collide. It is tipped so far over on its side that its North pole points below the ecliptic. Pluto might have an atmosphere from evaporated methane ice for a few decades when it is closest the sun, but it would be sure to freeze and vanish as Pluto moves away again later in its orbit. It is now, no longer considered a planet at all. In 2006 the IAU declared Pluto a "dwarf planet". |
Asteroids
Asteroids are rocky and metallic objects that orbit the Sun but are too small to be considered planets. They are known as minor planets. Asteroids range in size from Ceres, which has a diameter of about 1000 km, down to the size of pebbles. Sixteen asteroids have a diameter of 240 km or greater. They have been found inside Earth's orbit to beyond Saturn's orbit. Most, however, are contained within a main belt that exists between the orbits of Mars and Jupiter. Some have orbits that cross Earth's path and some have even hit the Earth in times past. One of the best preserved examples is the Barringer Meteor Crater near Winslow, Arizona. Asteroids are material left over from the formation of the solar system. One theory suggests that they are the remains of a planet that was destroyed in a massive collision long ago. More likely, asteroids are material that never coalesced into a planet. In fact, if the estimated total mass of all asteroids was gathered into a single object, the object would be less than 1,500 km (932 mi) across — less than half the diameter of our Moon.
Much of our understanding about asteroids comes from examining pieces of space debris that fall to the surface of Earth. Asteroids that are on a collision course with Earth are called meteoroids. When a meteoroid strikes our atmosphere at high velocity, friction causes this chunk of space matter to incinerate in a streak of light known as a meteor. If the meteoroid does not burn up completely, what's left strikes Earth's surface and is called a meteorite.
Of all the meteorites examined, 92.8% are composed
of silicate (stone),
and 5.7% are composed of iron and nickel; the rest are a mixture of the
three materials.
Stony meteorites are the hardest to identify since they look
very much like terrestrial rocks. Since asteroids are material from the very
early solar system, scientists are interested in their composition.
Spacecraft that have flown through the asteroid belt have found that the
belt is really quite empty and that asteroids are separated by very large
distances. Recently the Galileo spacecraft has made close encounters with
asteroids Gaspra and Ida. Ida, it has been found, has a tiny asteroid moon
of its own.
Comets
Unlike the other small bodies in the solar system, comets have been known since antiquity. There are Chinese records of Comet Halley going back to at least 240 BC. The famous Bayeux Tapestry, which commemorates the Norman Conquest of England in 1066, depicts an apparition of Comet Halley. As of 1995, 878 comets have been catalogued and their orbits at least roughly calculated. Of these 184 are periodic comets (orbital periods less than 200 years); some of the remainder are no doubt periodic as well, but their orbits have not been determined with sufficient accuracy to tell for sure. Comets are sometimes called dirty snowballs or icy mudballs. They are a mixture of ices (both water and frozen gases) and dust that for some reason didn't get incorporated into planets when the solar system was formed. This makes them very interesting as samples of the early history of the solar system.
When they are near the Sun and active, comets have several distinct parts:
- Nucleus: relatively solid and stable, mostly ice and gas with a small
amount of dust and other solids;
- Coma: dense cloud of water, carbon dioxide and other neutral gases sublimed
off of the nucleus;
- Hydrogen Cloud: huge (millions of km in diameter) but very sparse envelope
of neutral hydrogen;
- Dust Tail: up to 10 million km long composed of smoke-sized dust particles
driven off the nucleus by escaping gases; this is the most prominent part
of a comet to the naked eye;
- Ion Tail: up to 100 million km long composed of plasma and laced with rays and streamers caused by interactions with the solar wind.
Comets are believed to originate from the Oort Cloud, an area beyond the orbit of Neptune. Their highly eccentric orbits bring them racing in towards the sun where they become visible for a month or two and then shoot away again only to disappear for millenia. Only the short and intermediate period comets like Comet Halley, stay within the orbit of Neptune for a significant fraction of their orbits. Repeated passes near the Sun boils off most of a comet's ice and gas leaving a rocky object very much like an asteroid in appearance. A comet whose orbit takes it near the Sun is also likely to either impact one of the planets or to be ejected out of the solar system by a close encounter with a planet; especially with Jupiter.
By far the most famous comet is Comet Halley but Shoemaker Levy 9 was a big hit for a week in the summer of 1994 when it collided with Jupiter. Caught in the gigantic planet's gravity, the comet was ripped apart into a string of pearls months before colliding with the planet. The small pieces of the comet unleashed destructive forces on Jupiter that each easily engulfed Earth sized areas.
Meteor showers sometimes occur when the Earth passes through the orbit of a comet. Some occur with great regularity: the Perseid meteor shower occurs every year between August 9 and 13 when the Earth passes through the orbit of Comet Swift-Tuttle. Comet Halley is the source of the Orionid shower in October. Many comets are first discovered by amateur astronomers.
