100-C 25-C 50-C 75-C C+M 50-C+M C+Y 50-C+Y M+Y 50-M+Y 100-M 25-M 50-M 75-M 100-Y 25-Y 50-Y 75-Y 100-K 25-K 25-19-19 50-K50-40-40 75-K 75-64-64CHAPTER 18
Main-Belt Asteroids
Daniel T. Britt
University of Central Florida
Orlando, FloridaBr. Guy Colsolmagno
Specola Vaticana
Castel Gandolfo, ItalyLarry Lebofsky
Lunar and Planetary Laboratory
University of Arizona
Tucson, Arizona- Introduction to Asteroids 4. Puzzles and Promise
- Locations and Orbits Bibliography
- Physical Characteristics and Composition
1. Introduction to Asteroids
1.1 What Are Asteroids?
Asteroids are small, naturally formed solid bodies that or-
bit the Sun, are airless, and show no detectable outflow of
gas or dust. Shown in Fig. 1 are four asteroids that have
been imaged in detail by spacecraft: 243 Ida, 951 Gaspra,
253 Mathilde, and 433 Eros. The difference between aster-
oids and the other naturally formed Sun-orbiting bodies,
planets and comets, is largely historical and to some extent
arbitrary. To the ancient Greeks and other peoples, there
were three kinds of bright objects populating the heavens.
The first and most important group was the stars, oras-
tronin Greek, which are fixed relative to each other. The
English word “star” is an Old English and Germanic deriva-
tion of the Indo-European base wordstˆer, which provided
the source of the Greekastronand the Latinastralis. The
terms for the study of stars were based on the Greek root
(i.e. astronomy or astrophysics). The second group of ob-
jects is planets, or Greekplanetos,meaning wanderer, be-
cause the planets were not fixed but moved relative to the
background of the stars. For the ancients,planetosincluded
the Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn.
The final group is comets orkometes, meaning long-haired,
because of their long tails or comas and their unpredictable
paths and appearances.
Asteroids were not known to the ancients, and the first
asteroid, 1 Ceres, was discovered in 1801 by the Sicilian
astronomer Giuseppe Piazzi. He was searching in the gap
between Mars and Jupiter for what theorists at the time
speculated would be the location of a “missing planet.” 1
Ceres was thought initially to be this new planet. However,
other astronomers disputed this designation because of
Ceres’ apparently small size. Soon after William Olbers dis-
covered the second such object, Pallas, in 1802, Sir William
Herschel (who had discovered Uranus 20 years earlier) pro-
posed that, because these new objects were planet-like in
their sun-centered orbits, but star-like in that they were un-
resolvable points of light in a telescope, the disused Greek
root for a single starastershould be used to describe this
new addition to the celestial population. However, this term
was not universally adopted at that time. By the mid 1800s,
after several dozen of these bodies had been discovered,
the French and Germans referred to them as “small” (petit
orkleine) planets, while the British Royal Astronomical So-
ciety officially called them “minor planets.” Until modern
times, the term “asteroid” was only used by astronomers in
America.
In 2006, the International Astronomical Union (IAU)
added additional terms to the mix by defining a group of
“dwarf planets.” The IAU was attempting to precisely de-
fine a planet given the increasing evidence that Pluto was
just one of the larger members of theKuiper Beltand