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Beginner's Guide

1.   What is an asteroid?

Unlike comets, which were observed by ancient astronomers, asteroids were only recently discovered, so there are no myths or folk tales associated with them as with comets. Asteroids are essentially small rocky objects in orbit around the sun. They along with comets are classified as the small bodies of the Solar System. Asteroids are also known as minor planets, not to be confused with dwarf planets.

In 1801 Giuseppe Piazzi discovered an object that he thought was a comet. After the orbit was better known, it was realized that the object was not a comet, as it did not exhibit the characteristics of comets, and that it might be the "missing planet" that was believed to exist between Mars and Jupiter as predicted by Bode's Law. In keeping with the mythological naming scheme for the planets, Piazzi named his discovery Ceres after the Roman Goddess of Grain. Unfortunately, over the next several years, several other objects were discovered in the region. Astronomers quickly realized that the objects were much smaller than the traditional planets and so because of their starlike appearance in the modest telescopes of the time, they were called asteroids, aster being the Greek word for star.

Today, over one hundred thousand objects are known to exist between Mars and Jupiter, in the region known as the Main Belt. But they are also found elsewhere in the Solar System crisscrossing the orbits of the inner planets (NEOs), in resonances with Jupiter (Trojans) and tossed out amongst the out planets (Centaurs). In addition, photometric observations through a large variety of filters spanning the visible and infrared portions of the electromagnetic spectrum show that asteroids fall into different classes. So, asteroids are classified by location and by their color and albedo.

from the 8 Planets page on Asteroids:

Location

  • Main Belt: located between Mars and Jupiter roughly 2 - 4 AU from the Sun; further divided into subgroups: Hungarias, Floras, Phocaea, Koronis, Eos, Themis, Cybeles and Hildas (which are named after the main asteroid in the group).
  • Near-Earth Asteroids or Objects (NEAs, NEOs): ones that closely approach the Earth
    • Atens: semimajor axes less than 1.0 AU and aphelion distances greater than 0.983 AU;
    • Apollos: semimajor axes greater than 1.0 AU and perihelion distances less than 1.017 AU
    • Amors: perihelion distances between 1.017 and 1.3 AU;
  • Trojans: located near Jupiter's Lagrange points (60 degrees ahead and behind Jupiter in its orbit). (There may also be a few small asteroids in the Lagrange points of Venus and Earth that are also sometimes known as Trojans; 5261 Eureka is a "Mars Trojan".)

Color, Albedo

  • C-type, includes more than 75% of known asteroids: extremely dark (albedo 0.03); similar to carbonaceous chondrite meteorites; approximately the same chemical composition as the Sun minus hydrogen, helium and other volatiles;
  • S-type, 17%: relatively bright (albedo .10-.22); metallic nickel-iron mixed with iron- and magnesium-silicates;
  • M-type, most of the rest: bright (albedo .10-.18); pure nickel-iron.
  • There are also a dozen or so other rare types.

Ceres

Largest of the minor planets and yet smallest of the dwarf planets, Ceres currently holds dual citizenship as the IAU attempts to sort out the planetary classification schemes, which admittedly was much simpler many years ago!

More about Ceres from:

Vesta

Fourth asteroid to be discovered, Vesta is the first target of the Dawn mission.

More about Vesta from:

What are meteors?
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Updated: 30-Jul-2013