Asteroids can reach as large as Ceres, which is 940 kilometers (about 583 miles) across and is also considered a dwarf planet. On the other hand, one of the smallest, discovered in 1991 and named 1991 BA, is only about 20 feet (6 meters) across.
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| Fig 1.1 Asteroids are irregularly shaped. |
Asteroids are leftovers from the formation of our solar system about 4.6 billion years ago. Early on, the birth of Jupiter prevented any planetary bodies from forming in the gap between Mars and Jupiter, causing the small objects that were there to collide with each other and fragment into the asteroids seen today.
Nearly all asteroids are irregularly shaped, although a few are nearly spherical, such as Ceres. They are often pitted or cratered — for instance, Vesta has a giant crater some 285 miles (460 km) in diameter.
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| Fig 1.2 Mars' moons, Phobos and Deimos (Phobos is the one on the left) |
As asteroids revolve around the Sun in elliptical orbits, they rotate, sometimes tumbling quite erratically. More than 150 asteroids are also known to have a small companion moon, with some having two moons. Binary or double asteroids also exist, in which two asteroids of roughly equal size orbit each other, and triple asteroid systems are known as well. Many asteroids seemingly have been captured by a planet's gravity and become moons — likely candidates include among Mars' moons Phobos and Deimos and most of the distant outer moons of Jupiter, Saturn, Uranus and Neptune.
The average temperature of the surface of a typical asteroid is minus 100 degrees F (minus 73 degrees C). Asteroids have stayed mostly unchanged for billions of years — as such, research into them could reveal a great deal about the early solar system.
The asteroid belt is a band of debris orbiting about 2 to 3 A.U.'s from the Sun. This orbit is between Mars and Jupiter, and should be a planet. The tremendous gravity from Jupiter has perturbed the belt so that the debris did not form a proto-planet during our Solar System formation.
If the asteroids are brought together to form a single body, they would make a planet about as large as our Moon.
As far as categories, there are three types of asteroids:
- C-type (carbonaceous), making up 75% of the asteroid population and reside in the outer (away from the Sun) portion of the belt
- M-type (metallic), making up only 8% of the population and reside in the middle portion of the belt
- S-type (silicaceous), making up 17% of the population and residing in the inner belt
While the movies depict asteroid belts as very dense regions, in actuality there are wide spaces between each body - so much so that every probe we have sent through the belt had to be programmed to come close to an asteroid if we wanted an image. As far an naming the asteroid, that is left up to the International Astronomical Union. Names of asteroids are usually numbers associated with a name and/or date of discovery. The first asteroid discovered was Ceres, so the official name is 1 Ceres. It is also the first, being accidentally discovered in 1801 by Italian priest and astronomer Giuseppe Piazzi while making a star map
Asteroids are very small, so imaging them is very difficult. But radar imagery has been used, and they show the asteroids in good detail.
