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 past.
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 laboratories."
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.
NASA Headquarters, Washington, DC
Goddard Space Flight Center, Greenbelt, MD
Space Telescope Science Institute