| 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.
| 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.
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