Astronomers study unusual asteroid: A space mission will soon visit an unusual asteroid called Vesta that may turn out to not be an asteroid at all, but a minor planet.

You've heard of Pluto, once a full-scale planet that astronomers now classify as a dwarf planet. Now meet 4 Vesta -- or Vesta for short -- an asteroid that may not be a real asteroid.
The 330-mile diameter object sits in the asteroid belt, a collection of large and small pieces of rubble that circles the sun between the orbits of the planets Mars and Jupiter. But Vesta, numbered 4 because it was the fourth member of the asteroid belt to be discovered, is larger than most of its asteroid companions and also differs from them geologically.   link