Table of Contents

• Asteroid Introduction (This Page)
  • Introduction
  • Asteroid Summary
• Asteroids
  • Gaspra, Ida & Dactyl, Toutatis, Castalia, Vesta, Geographos, Mathilde
• Chronology of Asteroid Exploration
• Asteroid Image and Animation Index

See Also

• Terrestrial Impact Craters
• Asteroid Data On-Line at the SBN
• NEAR Spacecraft Homepage
• Near-Earth Asteroid 3753 - Earth's Curious Companion

Introduction

Asteroids are rocky and metallic objects that orbit the Sun but are too small to be considered planets. They are known as minor planets. Asteroids range in size from Ceres, which has a diameter of about 1000 km, down to the size of pebbles. Sixteen asteroids have a diameter of 240 km or greater. They have been found inside Earth's orbit to beyond Saturn's orbit. Most, however, are contained within a main belt that exists between the orbits of Mars and Jupiter. Some have orbits that cross Earth's path and some have even hit the Earth in times past. One of the best preserved examples is Barringer Meteor Crater near Winslow, Arizona.

Asteroids are material left over from the formation of the solar system. One theory suggests that they are the remains of a planet that was destroyed in a massive collision long ago. More likely, asteroids are material that never coalesced into a planet. In fact, if the estimated total mass of all asteroids was gathered into a single object, the object would be less than 1,500 kilometers (932 miles) across -- less than half the diameter of our Moon.

Much of our understanding about asteroids comes from examining pieces of space debris that fall to the surface of Earth. Asteroids that are on a collision course with Earth are called meteoroids. When a meteoroid strikes our atmosphere at high velocity, friction causes this chunk of space matter to incinerate in a streak of light known as a meteor. If the meteoroid does not burn up completely, what's left strikes Earth's surface and is called a meteorite.

Of all the meteorites examined, 92.8 percent are composed of silicate (stone), and 5.7 percent are composed of iron and nickel; the rest are a mixture of the three materials. Stony meteorites are the hardest to identify since they look very much like terrestrial rocks.

Because asteroids are material from the very early solar system, scientists are interested in their composition. Spacecraft that have flown through the asteroid belt have found that the belt is really quite empty and that asteroids are separated by very large distances. Before 1991 the only information obtained on asteroids was though Earth based observations. Then on October 1991 asteroid 951 Gaspra was visited by the Galileo spacecraft and became the first asteroid to have hi-resolution images taken of it. Again on August 1993 Galileo made a close encounter with asteroid 243 Ida. This was the second asteroid to be visited by spacecraft. Both Gaspra and Ida are classified as S-type asteroids composed of metal-rich silicates.

On June 27, 1997 the spacecraft NEAR made a high-speed close encounter with asteroid 253 Mathilde. This encounter gave scientists the first close-up look of a carbon rich C-type asteroid. This visit was unique because NEAR was not designed for flyby encounters. NEAR is an orbiter destined for asteroid Eros in January of 1999.

Astronomers have studied a number of asteroids through Earth-based observations. Several notable asteroids are Toutatis, Castalia, Geographos and Vesta. Astronomers studied Toutatis, Geographos and Castalia using Earth-based radar observations during close approaches to the Earth. Vesta was observed by the Hubble Space Telescope.

Asteroid Summary

Num	Name	Radius (km)	Distance* (10^6km)	Albedo	Discoverer	Date
1	Ceres	457	413.9	0.10	G. Piazzi	1801
511	Davida	168	475.4	0.05	R. Dugan	1903
15	Eunomia	136	395.5	0.19	De Gasparis	1851
52	Europa	156	463.3	0.06	Goldschmidt	1858
951	Gaspra	17x10	205.0	0.20	Neujmin	1916
10	Hygiea	215	470.3	0.08	De Gasparis	1849
243	Ida	58x23	270.0	?	J. Palisa	29 Sep 1884
704	Interamnia	167	458.1	0.06	V. Cerulli	1910
2	Pallas	261	414.5	0.14	H. Olbers	1802
16	Psyche	132	437.1	0.10	De Gasparis	1852
87	Sylvia	136	521.5	0.04	N. Pogson	1866
4	Vesta	262.5	353.4	0.38	H. Olbers	1807

* Mean distance from the Sun.

Return to Pluto Visit the Comets

Copyright © 1997 by Calvin J. Hamilton. All rights reserved.