NASA’s Dawn spacecraft has confirmed that one of the largest objects in the Main Asteroid Belt is actually a tiny planet-like body that formed around a molten interior like the Earth, Mars, Venus and Mercury.

Launched in 2007, Dawn has been orbiting Vesta since last July. From its vantage point over the object — believed to be one of the oldest in the Solar System — the Orbital Sciences Corp. spacecraft has returned a batch of data that reveal Vesta as perhaps the sole survivor of the period when everything solid in the proto-Solar System was crashing together as it swirled around the young Sun.

“It formed within 2 million years after the first solids formed in the Solar System, before Ceres formed, before the terrestrial planets formed, and when the concentration of radioactive material with short decay times generated enough heat within Vesta to cause melting,” says Carol Raymond of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., the deputy principal investigator on the Dawn mission. “We now know that Vesta is the only layered planetary building block from the very earliest days of the Solar System.”

Believed to be more than 4.5 billion years old, Vesta didn’t survive the period of “heavy bombardment” a little less than 4 billion years ago unscathed. Its original crust appears to have been pulverized, and its component minerals mixed together. Two huge craters that overlay each other at the body’s south pole bear witness to the force of later collisions — one 2 billion years after the heavy bombardment and another a billion years after that. The one-off collision that created the younger crater — named Rheasilvia — excavated an estimated 250 cubic miles of material from Vesta, some of which has reached Earth in the form of meteorites rich in pyroxene. Before Dawn arrived at Vesta, scientists hypothesized that it was the largest single source of meteorites found on Earth — generating some 6% of the total — and mineral mapping by the spacecraft has confirmed that theory.

Dawn is scheduled to use its solar-electric propulsion system to leave Vesta Aug. 26 for the dwarf planet Ceres, the largest body in the Main Asteroid Belt. As was the case with Vesta, scientists have their theories about Ceres, and Dawn’s instruments will prove or discount them. Either way, the results will be testimony to the advances that planetary science continues to make.

“Vesta appears to be the only intact protoplanet that’s left, because the collisional environment was so intense that many of these were just blasted into small pieces, which is what most of the belt is,” Raymond says. “Ceres is not an igneously differentiated body. It still has a lot of its water, and so it has had a significantly different evolution than Vesta and the other planetary embryos like Vesta. It’s likely that its large size was a big factor in the fact that it still exists today.”