It is small. It is round.

It was once a planet, but is now cast off as too diminutive.

In March, a NASA spacecraft will arrive there to begin the first close-up examination of a dwarf planet.

It is not Pluto.

It is instead Ceres, 600 miles wide, the largest of the asteroids between Mars and Jupiter. “We’re going to reveal the fascinating details of a giant world of rock and ice,” said Marc Rayman, the chief engineer for NASA’s Dawn spacecraft.

“It’s not like we’re just going out to visit a chunk of rock the size of one of those mountains,” he said, pointing to the San Gabriel Mountains outside the windows at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “Ceres has 38 percent of the area of the continental United States. It’s actually the largest body between the sun and Pluto that a spacecraft has not yet visited.”

A year ago, the Herschel Space Observatory discovered water vapor rising off two spots on Ceres, possibly a sign of ice volcanism. “Ceres may have subsurface ponds or lakes or even oceans of water,” Dr. Rayman said.