Note: You can click on each image below to view a larger version.
As well as the eight (formerly nine) planets the solar system also contains a large number of smaller bodies known as asteroids. Several hundred thousand have been discovered and thousands more are found each year, although most are so small that the total mass of every asteroid combined would still be less than that of the moon. The majority of those currently known are found between the orbits of Mars and Jupiter, in a region known as the asteroid belt.
Asteroids are small, irregularly-shaped lumps of rock, none of which are large enough to retain any sort of atmosphere. They are identified by a name, which is normally chosen by the person who discovers it, and a number that indicates the order in which it was discovered (e.g. 951 Gaspra).
The most massive objects in the asteroid belt are Ceres, approximately 950km in diameter, and Vesta, with a diameter of about 530km. Ceres was recently reclassified as a Dwarf Planet, so Vesta is now the largest asteroid. NASA's Dawn probe will visit Vesta in 2011 and Ceres in 2015.
 951 Gaspra, as seen by NASA's Galileo spaceprobe in 1991, on its way to Jupiter |
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 243 Ida, visited by Galileo in 1993, was orbited by its own "moon", a tiny rock that was named Dactyl |
A comet is essentially a dirty snowball of rock, dust and ice, orbiting the Sun. Many of them have highly elliptical orbits that take them close to the Sun and then far away into the outer reaches of the solar system. The famous Halley's comet takes 76 years to complete an orbit, and comet Hale-Bopp, which was a spectacular sight in the sky in 1997, won't be back for another 40,000 years.
As a comet moves towards the Sun ice evaporates from its surface, surrounding it with a cloud of dust and gas called a "coma" or "halo". This is pushed away from the comet by the pressure of the solar wind, a stream of sub-atomic particles flowing out from the Sun, to form the tail. A comet's tail therefore always points away from the Sun.
 Halley's comet, photographed by W. Liller in 1986 (photo from NASA) |
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 The nucleus of Halley's comet, as seen by Giotto; it is about 16x8x8km in size |
During Halley's last visit in 1986 there was a major international scientific effort to learn as much about it as possible. Several space probes made fly-bys, the closest being by the European Space Agency's Giotto probe which passed the nucleus at a distance of only 596km. The probe met the comet head-on at a relative speed of over 68km per second and was seriously damaged by impacts from dust particles in the coma, despite having Kevlar shielding 12mm thick.
It was named after the great Italian artist Giotto di Bondone, whose painting of the Adoration of the Magi (i.e. the three wise men visiting the infant Jesus) from 1304-6 shows the star of Bethlehem as a comet blazing across the sky. However, Halley's comet would not actually have been visible during the lifetime of Christ, the nearest encounters being 11BC and 66AD.
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LEFT: The nucleus of comet Wild 2 as seen by NASA's
Stardust probe, which flew past it in January 2004
and took samples from the coma that it successfully returned to Earth in January 2006.
Even more ambitious missions are planned for the future, with the European Space Agency intending to put a lander onto comet Churyumov-Gerasimenko in 2014. |
Asteroids and comets are thought to be leftovers from the formation of the solar system which never formed into anything as large as a moon or planet because of disruption from the gravitational pull of the larger planets, in particular Jupiter. They are therefore particularly interesting as samples of material that has remained largely unchanged since the beginnings of the solar system.
It is believed that objects that formed in the inner solar system would have been flung out by gravitational encounters with the planets, creating a spherical cloud of debris around the solar system known as the Oort Cloud. However, objects which formed beyond the orbit of Neptune should still be there, in a region referred to as the Kuiper Belt. The existence of the Oort Cloud has yet to be proved, because the objects in it are too faint and distant to be seen, but a number of Kuiper Belt objects have been found. When we have good enough telescopes, we may find more large objects there than in the asteroid belt.
URL: http://www.randomnotes.co.uk/Astronomy/Rocksprint.htm