Asteroid or Planet?

Scientists still aren't sure what to make of Vesta, a small body that orbits the sun. Is it an asteroid or a planet? NASA's Dawn spacecraft could settle the matter. Vesta was spotted 200 years ago and is officially a "minor planet" — a body that orbits the sun but is not a proper planet or comet. Yet, many astronomers call Vesta an asteroid because it lies in the main asteroid belt between Mars and Jupiter.

But Vesta is not a typical member of that orbiting rubble patch. The vast majority of objects in the main belt are relative lightweights, 62 miles(100 kilometers) wide or smaller, compared with Vesta, which is 329 miles(530 km) wide. If Vesta is an asteroid, it would be the second-largest in the solar system. Some scientists, however, are skeptical about that designation. "I don't think Vesta should be called an asteroid," said Tom McCord, a Dawn team member at the Bear Fight Institute in Winthrop, Wash. "Not only is Vesta so much larger, but it's an evolved object, unlike most things we call asteroids."

The evolution of Vesta

The onion-like structure of Vesta (core, mantle and crust) is the key trait that makes Vesta more like planets such as Earth, Venus and Mars than the other asteroids, McCord said. Like the planets, Vesta had sufficient radioactive material inside when it formed, releasing heat that melted rock and enabled lighter layers to float to the outside. Signatures of a type of volcanic rock called basalt were detected in 1972, which meant that the body had to have melted at one time.