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.
"This basin erased at least half the surface and messed up a lot of the rest of it," Schenk told SPACE.com via email.
The Rheasilvia impact also created strange circular troughs around Vesta's equator and raised a central peak more than twice as high as Mount Everest, Russell said. And it excavated approximately 250,000 cubic miles (1.04 million cubic km) of material, much of which was blasted into space.
"Two hundred and fifty thousand cubic miles is enough to fill the Grand Canyon about a thousand times over," said David O'Brien, a Dawn scientist based at the Planetary Science Institute in Tucson, Ariz. "So this is a very large volume of material."
Both giant craters were likely caused by asteroids between 25 and 36 miles (40 to 60 km) wide, Schenk said. And both impacts seem to have occurred surprisingly late, several billion years after the presumed end of the so-called Late Heavy Bombardment that blasted many craters into Earth's moon and other solar system bodies.
More discoveries to come
The new results are based on data that Dawn gathered during the early stages of its stay at Vesta. The spacecraft will continue studying the protoplanet until Aug. 26, so we should expect more discoveries from the mission, researchers said.
Further, Vesta's far northern reaches have been in shadow thus far, so Dawn has been unable to study large chunks of the protoplanet. But that will change before too long.