LOS ANGELES- The giant asteroid Vesta got clobbered not once but twice, and it has the scars to prove it.
Ever since the Hubble Space Telescope spied a huge depression in the asteroid's south pole, scientists surmised it was carved by a collision with a celestial object, most likely a smaller asteroid. But a recent closer inspection revealed a surprise: There are actually two massive overlapping craters.
"Vesta got whacked twice with large impacts," said Christopher Russell of the University of California, Los Angeles, who heads a team of scientists exploring the asteroid. The double strikes occurred relatively recently - 1 to 2 billion years ago - and came to light only after researchers pored over high-resolution images snapped by the NASA Dawn spacecraft, which slipped into orbit around Vesta last year.
The finding is reported in Friday's issue of Science, which published a series of papers on the $466 million mission. Vesta's surface is pockmarked with pits caused by crashes. Scientists zeroed in on the southern hemisphere, which is dominated by a 310-mile-wide crater. Soon after arriving at Vesta, Dawn spotted a nearby feature that looked like a rim. "It looked kind of weird. We thought, `What the heck is that?"' recalled Paul Schenk of the Lunar and Planetary Institute in Houston who is part of the mission.
Scientists determined the rim belonged to a smaller, older crater gouged by an impact 2 billion years ago. It had been obscured by the larger crater, created by an impact a billion years later. The back-to-back pounding likely would have shattered any other asteroid, but Vesta somehow survived. Even so, the blows scooped out loads of material from Vesta's surface - enough to fill 400 Grand Canyons, estimated team member David O'Brien of the Planetary Science Institute in Tucson, Ariz.
Some of the debris was hurled into space and fell to Earth as meteorites. About 1 out of every 20 meteorites found on our planet came from Vesta. Located in the asteroid belt between Mars and Jupiter, Vesta is not your garden-variety asteroid. Many of the space rocks in the zone resemble potatoes, but Vesta is shaped more like an avocado with its iron core and differentiated layers.
Measuring 330 miles across, it's the second largest object in the asteroid belt. Scientists are intrigued by asteroids because they're leftovers from the solar system's birth some 4.5 billion years ago and studying them can offer clues about how Earth and rocky planets emerged. Dawn will depart Vesta in late August, firing its ion propulsion engines to cruise on to a bigger target - an asteroid named Ceres where it will arrive in 2015.
By ALICIA CHANG AP Science Writer