What’s the difference between a planet and an asteroid? Size, mostly. Only 30 asteroids are larger than Belgium, and the mass of all known asteroids put together is probably just a fifth that of Pluto. But Ben Zellner, an astronomer at Georgia Southern University, believes size isn’t everything. If you take geologic history into account, he says, Vesta, the third largest asteroid, with a diameter of about 322 miles, should be called a planet. Early on, it went through the same kind of history that Earth and the other rocky planets went through, says Zellner. He and his colleagues announced last March that they’d used data from the Hubble Space Telescope to create a map, the first of Vesta (below). The yellow green areas show volcanic basalt that cooled at Vesta’s surface. The red areas may show craters: the basalt there is of a type that cools below the surface and presumably could only have been exposed to the Hubble’s view by giant impacts. One such impact, in fact, is believed to have sent pieces of Vesta flying toward Earth--including the meteorite shown at right. The different types of basalt, says Zellner, confirm suspicions that Vesta, now frozen solid, must once have had a structure much like Earth’s, with a crust, mantle, and liquid core.