April 19, 1995
PRESS RELEASE NO.: STScI-PR95-20
ASTEROID OR MINI-PLANET?
HUBBLE MAPS THE ANCIENT SURFACE OF VESTA
April 19, 1995
NASA's Hubble Space Telescope images of the asteroid Vesta are
providing astronomers with a glimpse of the oldest terrain ever seen
in the solar system and a peek into a broken off section of the
"mini-planet" that exposes its interior.
Hubble's pictures provide the best view yet of Vesta's complex surface,
with a geology similar to that of terrestrial worlds such as Earth or
Mars. The asteroid's ancient surface, battered by collisions eons ago,
allows astronomers to peer below the asteroid's crust and into the
Astronomers also believe that fragments gouged out of Vesta during
ancient collisions have fallen to Earth as meteorites, making Vesta
only the fourth solar system object, other than Earth, the Moon and
Mars, where scientists have a confirmed laboratory sample. (About
50-60 other meteorite types are suspected to have come from asteroids,
but positive identifications are more difficult to make.)
"The Hubble observations show that Vesta is far more interesting than
simply a chunk of rock in space as most asteroids are," said Ben
Zellner of Georgia Southern University. "This qualifies Vesta as the
'sixth' terrestrial planet."
No bigger than the state of Arizona, Vesta offers new clues to the
origin of the solar system and the interior makeup of the rocky
planets. "Vesta has survived essentially intact since the formation of
the planets," Zellner said. "It provides a record of the long and
complex evolution of our solar system."
Resolving features down to 50 miles across, Hubble reveals a
surprisingly diverse world with an exposed mantle, ancient lava flows
and impact basins. Though only 325 miles (525 kilometers) across, it
once had a molten interior. This contradicts conventional ideas that
asteroids essentially are cold, rocky fragments left behind from the
early days of planetary formation.
Besides providing scientists with direct samples, Vesta's chipped
surface allows Hubble to study the asteroid's rocky mantle, giving
scientists a unique opportunity to see what a planet looks like below
the crust. "Our knowledge of the interior composition of the other
terrestrial worlds, the Moon, Mars, Venus, Mercury and even Earth,
depends heavily on theory and inference," Zellner said. "Vesta allows
us to actually see the mantle and study pristine samples in our
Before these observations, only the smaller and less geologically
diverse asteroids, Ida and Gaspra, have been observed in detail by the
Jupiter-bound Galileo spacecraft. Unlike Vesta, these smaller objects
are pieces torn off larger bodies by collisions that occurred perhaps
only a few hundred million years ago.