Comet 9P/Tempel 1
Comet 9P/Tempel 1 is one of many smaller celestial bodies that hang around our Solar System. No one has ever visited a comet, but that doesn't mean we don't know something about such bodies. They're believed to have a core of ice and imbedded debris, and they typically orbit the Sun in one of three areas.
The most-distant comets inhabit the Oort Cloud, a spherical shell of comets and space debris that's well out beyond the orbit of Pluto and perhaps 10,000 times more distant from the Sun than the Earth. Scientists believe passing stars exert gravitational tugs on these objects and either catapult them off into interstellar space or send them spiraling in toward the Sun. Some of the displaced objects become "long-period" comets whose orbits take them within a few Astronomical Units (about 93 million miles, which is the distance from Earth to the Sun) of the Sun and then back out again, to perhaps a quarter of the distance to the nearest star. These comets travel so far that their orbit periods are measured in centuries or millennia.
Closer in, you'll find comets in the Kuiper Belt, a region beyond the orbit of Neptune and extending out perhaps 20 times the distance from Earth to the Sun. It's believed that gravitational tugs from the outer planets alter the orbits of some Kuiper Belt objects, causing them to journey sunward and form medium-period comets like the famous Halley's Comet, as well as short-period comets that orbit the Sun in 20 years or less.
The Deep Impact probe captured this image of Comet 9P/Tempel1 moments before impact.
Comet 9P/Tempel 1 is one of the latter. Originally a Kuiper Belt Object, Tempel 1's orbit has been altered significantly. It now revolves about the Sun between the orbits of Mars and Jupiter and is no longer gravitationally tied to the Kuiper Belt. Its trip round the Sun takes only 5.5 years, and it approaches as close as 1.5 AU, or about 139.5 million miles, from the Sun. It's one of the third class of comets, those that inhabit the middle of the solar system.
Regardless of origin, most comets spend the majority of their time in the cold and dark. It's only when they approach the Sun more closely that they "light up" themselves and become visible. Solar radiation transforms portions of the material in the comet's icy nucleus into gas. Some of this becomes ionized, forming a luminous, bluish tail that streams straight out, in the direction away from the Sun. Dust imbedded in the comet's nucleus, meanwhile, is released by solar radiation, as well. This forms a second tail that's white or pinkish with reflected sunlight and curved in the direction of the comet's orbit.
The tails are what most folks think of when someone mentions the word "comet". And no wonder. For the clouds of ionized gas and dust can be spectacular, reaching out into space hundreds of thousands of miles or more, exciting wonder, and even sometimes fear, from onlookers the world over. Once they were thought of as harbingers of bad times. Now they're recognized for what they are: solitary, but beautiful, wanderers through the Solar System.