An asteroid is a small body one mile to 480 miles in diameter. They revolve mostly between the orbits of Mars and Jupiter. They are also called minor planets. There are thousands and thousands of them. Giuseppe Piazzi discovered the first asteroid in January, 1801. He named it Ceres after the Sicilian goddess of grain.
Twenty-six asteroids are larger than 200 kilometers in diameter. It is predicted that there are more than one million asteroids about one kilometer in diameter. Several asteroids have been explored by spacecraft. Galileo made a flyby of Gaspra, Ida, and Ida’s satellite Dactyl. DS-1 made a flyby of Braille. Stardust made a flyby of Annefrank. Dawn will explore the largest asteroids, Ceres and Vesta. It will study Vesta in about October 2011 and Ceres in about February 2015.
Asteroids are classified according to their spectra and albedo. An asteroid is brighter and brighter as the albedo goes from 0.0001 to 1.0000 (in ten thousandths). 75% of them are C-Type, which have an albedo of 0.03 (very dark) and a chemical composition similar to the sun without hydrogen and helium. They are similar to carbonaceous chondrite meteorites. 17% of the asteroids are S-Type, which have an albedo of 0.10—0.22 (brighter than the C-Types) and a composition composed of metallic nickel-iron mixed with iron and magnesium silicates. Most of the rest are M-Type, which have an albedo of 0.10—0.18 and are composed entirely of nickel iron. There are a handful of rare types of asteroids.
Asteroids can also be categorized by their position in the solar system. The Main Belt is the asteroids that are located between Mars and Jupiter. They are about two to four Astronomical Units from the sun. An Astronomical Unit is the average distance from the earth to the sun, or 93 million miles. There are subgroups named after one of the asteroids in the group (Hungarias, Floras, Phocaea, Koronis, Eos, Themis, Cybeles, and Hildas).
The next group is the Near Earth Asteroids. These asteroids orbit close to earth. The group that is closest to earth is the subgroup Aten. The Near Earth group of asteroids is classified by the distance of the elliptical orbit from the Earth. The group that is closest to Earth is the subgroup Aten. It was discovered on January 7, 1976 by Eleanor F. Helin. The semi-major axis of the elliptical orbit is less than one Astronomical Unit and the aphelion is usually greater than 0.983 AU.
The semi-major axes of the subgroup Apollo’s elliptical orbit is greater than one Astronomical Unit and the perihelion is less than 1.017 AU. The Amors subgroup’s perihelion is from 1.017 AU to 1.3 AU. The asteroids of the subgroup Trojan are 60 degrees ahead of and behind the orbit of the planet Jupiter. Several hundred asteroids have been found in the subgroup Trojan area. There might be asteroids similar in orbit to the Trojan subgroup orbiting the Earth, the Moon, or the planet Venus. The asteroid 5261 Eureka is a Mars Trojan.
One group of objects in the outer solar system named the Centaurs might not be true asteroids because their composition is probably similar to comets or objects of the Kuiper Belt. 2060 Chiron’s orbit is between Saturn and Uranus (now considered a comet), 5335 Damocles’ orbit goes from Mars to Uranus, 5145 Pholus orbit is from Saturn to the planet Neptune.