 |
 |
||
 |
|
||
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
||||||||
 |
Space Topics: Asteroids and CometsAsteroid 4 Vesta
Vesta was the fourth asteroid to be discovered and is either the second or
third largest: it competes for that honor with Pallas. (They are very
similar in diameter, but Vesta contains more mass than Pallas.) Vesta
is very different from the largest asteroid, Ceres. Ceres is icy, but
Vesta's surface is silicate rock, indicating that Vesta may have had a geologic
history similar to the terrestrial planets: after it coalesced from the solar
nebula, it heated to the melting point of silicate rocks, separating into
a nickel-iron core and a rocky mantle. This history is very different
from Ceres, and suggests either that Vesta never accreted very much water
or somehow lost its water very early in its formation. Hubble Space Telescope images of Vesta have revealed its surface to be surprisingly diverse and very strangely shaped. At some point in its past, Vesta suffered a huge impact which left a crater nearly as large as the asteroid itself (at 460 kilometers or 290 miles in diameter, it is 80 percent the width of Vesta). The crater is so deep that it exposes materials from deep in Vesta's mantle. If this is true, a visit to Vesta could yield important information that would help to understand the mantles of the terrestrial planets, like Earth and Mars, which are inaccessible to direct observation. The impact events that have left such large scars on Vesta also sent chunks of it flying on their own paths through the solar system. Many smaller asteroids accompany Vesta on similar orbital paths and have spectral similarities to Vesta. About 6 percent of all the meteorites that fall on Earth have compositions that match these spectral properties, indicating that they, too, likely come from Vesta.
Meteorites that fall to Earth are often made entirely of metal or entirely of rock, which indicates that they came from parent bodies that differentiated as Vesta did. But all of these parent bodies have apparently been shattered to bits, except Vesta. Therefore, Vesta preserves a record of the formation of the solar system, and studying it will help scientists understand what the other asteroids and meteorites can tell us about solar system formation. No spacecraft has yet visited Vesta. NASA's Dawn mission is planned to launch in 2007 and enter orbit around the asteroid in 2010. Basic FactsBecause Vesta has not been visited by spacecraft, size estimates depend upon telescopic observations. There has been a lot of variability in published reports as different observers have used different techniques and different instruments to make their estimates; what's worse, Vesta is close enough in size to Pallas that changing estimates over time have caused them to switch places as the second- and third-largest asteroids, more than once. Consequently, numbers published on the Web vary widely. The numbers below come from papers published in 1997 following a series of Hubble observations (see references). Size: Orbit:
|
|||||||||||
 |
 |
|||||||||||
[ what is this? ]