After spiraling outward from Earth for four years, NASA's Dawn probe is set to slide into orbit around the potato-shaped asteroid Vesta early today for a yearlong look at an ancient "minimoon."
Three hundred fifty miles wide and heavily cratered, Vesta formed some 4.5 billion years ago, when the sun was still young. By investigating its secrets, scientists hope to catch a glimpse of how the planets, including Earth, formed out of a swirling disk of gas and dust.
"We are exploring backward in time as far as we can," said lead mission investigator Christopher Russell of the University of California at Los Angeles. "There's going to be a whole bunch of surprises."
Unlike most smaller asteroids — thought to be nearly uniform lumps of rock — Vesta is a "minimoon," Russell said, made up of three layers: an iron core, a rocky mantle and an upper crust. Early in Vesta's existence, Russell said, lava welled up from its interior and cooled to form a crust of volcanic rock.
"Vesta is unique among the large asteroids," said Richard Binzel, professor of planetary sciences at the Massachusetts Institute of Technology. "It's the only one covered with a volcanic surface."
Despite observations from the Hubble Space Telescope in 1997, many mysteries of Vesta's origin and composition remain.
The $466 million Dawn mission launched a year behind schedule. For the next year, Dawn will slowly spiral toward Vesta's surface, coming within 120 miles.
Vesta was discovered by a German astronomer in 1806. The second-most-massive asteroid in the solar system, it circles the sun in the asteroid belt between Mars and Jupiter.