Asteroid Vesta: Is it really an asteroid?
The humongous asteroid Vesta is actually a protoplanet left over from the early days of our solar system, new observations from a NASA space probe suggest.
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"We have just started exploring Vesta’s secrets, and I’m sure other intriguing results will come along shortly," Mr. Marchi added.Skip to next paragraph
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The protoplanet Vesta
The $466 million Dawn spacecraft arrived at the huge asteroid in July 2011 to help unlock its many secrets. One of the probe's main missions, researchers said, is to determine if Vesta is indeed a long-surviving protoplanet — a body left over from the solar system's first few million years, many of which later coalesced to form rocky planets such as Earth and Mars.
Scientists got this idea mainly by examining fallen howardite-eucrite-diogenite (or HED) meteorites, which are thought to come from Vesta. The new Dawn results strongly support the protoplanet notion — by confirming that Vesta is indeed the HED meteorites' parent body, for starters.
Moreover, the huge asteroid isn't just some chunk of uniform rock. Rather, it's now known to be a differentiated object with an iron core about 137 miles (220 km) wide. That's big enough, perhaps, to have once sustained a dynamo like the one that generates Earth's magnetic field, researchers said.
The team figured out the dimensions of Vesta's core in part by carefully tracking Dawn's movements through space, then using this information to calculate Vesta's mass, density, and gravitational pull with unprecedented precision. [Video: Vesta Flyover in 3D]
Other Dawn data also back Vesta's protoplanet status. For instance, its surface composition implies a complex geological history that's more similar to that of terrestrial planets than other asteroids, researchers said. And Vesta boasts color variations unlike anything seen on an asteroid before, further suggesting that the massive object is something special.
"We now know that Vesta is large enough to have had its own internal geologic evolution and is not just a battered lump of rock," said Paul Schenk of the Lunar and Planetary Institute in Houston, lead author of another of the new studies.
Two gigantic (and recent) impacts
Vesta's surface is pocked with craters from countless collisions over the eons. Dawn's observations have allowed scientists to reconstruct the protoplanet's impact history by counting these craters, and noting how many impact features overlie others.
Researchers found a huge difference between Vesta's northern and southern halves. The northern part retains a record of some of the asteroid's earliest impacts, while the south was "reset" by two enormous collisions far more recently.
One of these smashups occurred about 2 billion years ago, creating a 249-mile-wide (400 km) basin called Veneneia. But Veneneia was mostly obliterated about 1 billion years ago by another impact, which created the 314-mile (505 km) Rheasilvia crater.