Centaur (astronomy)

I

Introduction

Centaur (astronomy), small comet-like body that orbits the Sun in the outer solar system between the giant planets Jupiter and Neptune. Centaur objects such as Chiron typically have highly elliptical orbits that may cross the orbits of one or more of the outer planets (Jupiter, Saturn, Uranus, and Neptune).

Centaurs are made of ice, dust, and rock, and range in shape from nearly spherical to elongated. They show a range of colors from dark gray to reddish. Astronomers have detected water ice and carbon compounds on their porous surfaces. A few centaurs, including Chiron, sometimes release gas and dust like comets. Other centaurs, however, show no such cometary activity.

II

Origin and Fate of Centaur Objects

Centaurs are thought to originate in the Kuiper Belt, a collection of small frozen objects that form a disk in the outer solar system beyond the orbit of Neptune. Over time the gravitational pull of Neptune may disturb the orbits of certain Kuiper Belt Objects (KBOs), causing them to leave the main region of the Kuiper Belt. Some objects are scattered into highly elliptical orbits (so-called scattered disk objects (SDOs)). Centaur objects are thought to begin as SDOs or, less often, to come directly from the Kuiper Belt.

The orbits of centaurs are unstable and they apparently only remain in the region of the outer planets for a few tens of millions of years. Many centaurs are eventually thrown out of the solar system. Others may impact on the giant planets. Some centaurs, however, enter the inner solar system to become short-period comets that are influenced by Jupiter’s gravity. These so-called Jupiter-family comets have orbital periods around the Sun of 20 years or less, with most orbits taking about 8 years. Occasionally such comets may return to the outer solar system to become centaurs again.

---

---

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.

	More from Encarta
	•  7 tips for funding an online degree •  How to succeed in the fashion industry without being a top designer •  Presidential Myths Quiz •  Coffee break: Recharge your brain