III.

Discovery and Study

The American astronomer Charles Kowal discovered the first centaur object in 1977 and originally classified it as a large asteroid. He named the object Chiron after the most famous of the centaurs, half-man, half-horse monsters in Greek mythology. At the time of Chiron’s discovery, astronomers had not yet detected the Kuiper Belt. The first member of the Kuiper Belt was observed in 1992 using a sensitive electronic camera called a charge-coupled device (CCD). With new technologies to see fainter objects, astronomers began finding more such distant small bodies.

In time scientists realized these icy objects were different from rocky asteroids and belonged to new categories of solar system bodies. The distant objects with orbits beyond Neptune were called Kuiper Belt Objects (KBOs). The nearer objects with orbits that had an average distance from the Sun closer than that of Neptune but further than that of Jupiter were called centaurs after their similarity to Chiron.

By 2007 astronomers had cataloged more than 60 centaur objects, and thousands more may exist, many too small to be detected with current equipment. Chiron, the best-known of these objects, has a diameter of about 142 km (88 mi) and a spherical shape. It has a rotation period of 5.9 hours and an orbital period of 51 years.

The second centaur discovered was 5145 Pholus, detected in 1992. It has an average diameter of about 185 km (115 mi) and an elongated shape and reddish color. Its rotation period is about 10 hours and its orbital period lasts 92 years.

Somewhat smaller is 7066 Nessus, found in 1993. Its diameter is about 106 km (66 mi). It orbits the Sun in 123 years. Its rotation rate is not known yet.

The largest known centaur is 10199 Chariklo, discovered in 1997. It has a diameter of about 240 km (148 mi) and an orbit that takes 62.4 years. Its surface apparently has a very uniform appearance and its rotation rate has not been established yet.