Huge asteroid was hit twice, scientists figure

Thegiant asteroid Vesta, the source of about one in 20 meteorites found on Earth, 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 one billion to two 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 today's issue of Science, which published a series of papers on the $466-million US mission.

'What the heck is that?'

Vesta's surface is pockmarked with pits caused by crashes. Scientists zeroed in on the southern hemisphere, which is dominated by a 500-kilometre-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 two 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.