Vesta is one of the most diverse asteroids ever studied. It has many features that resemble our own Moon, including ancient lava flows that indicate the rock once had a molten interior, leading scientists to believe it used to be larger.

Maybe, some suggest, Vesta was once a contender to be a planet, back when the solar system was young and chaotic and everything could change in one catastrophic collision.

Vesta is about 334 miles (538 kilometers) in diameter and was the fourth asteroid to be discovered. It is also the fourth largest. Ceres, the largest known asteroid, is nearly twice as big.

Other than a few impacts from other asteroids, Vesta has probably remained largely unchanged for about 4 billion years. At least one of the impacts may have torn large parts of Vesta away, which would explain the exposed subsurface that scientists have seen.

And at least one meteorite found on Earth is likely a chip off Vesta, helping researchers further understand the asteroid and its chemical makeup. Much of what's known about Vesta, however, came in a 1995 Hubble Space Telescope study.

"The Hubble observations show that Vesta is far more interesting than simply a chunk of rock in space as most asteroids are," said Ben Zellner, a Georgia Southern University researcher who worked on the study. "This qualifies Vesta as the 'sixth' terrestrial planet."

Zellner said those words back in 1995. And while Vesta has never formally been adopted as a planet, researchers agree that it does share many characteristics of the rocky, so-called terrestrial planets like Earth.

Soon, researchers hope to learn more about Vesta. In December 2001, NASA gave the final green light to a mission to study Vesta and Ceres.

The Dawn mission will make a nine-year journey to rendezvous with the rocks. The robot will orbit from as high as 800 kilometers (500 miles) to as low as 100 kilometers (about 62 miles) above the surfaces.

Both asteroids are in the main asteroid belt, between Mars and Jupiter, but scientists say they are very different from each other.

Ceres has a surface that contains water-bearing minerals. It may hold a very weak atmosphere and frost, scientists say. Vesta, on the other hand, is thought to be dry, having been resurfaced by lava flows.

Dawn will weigh and measure the asteroids and examine their craters. It will also work to determine what they are made of and how magnetic they are. All this information should help scientists better understand how our solar system formed and evolved.

The mission, set to cost $299 million or less, will be managed by NASA's Jet Propulsion Laboratory.