 With an estimated 7% of the mass of the asteroid belt, it is the third-most-massive (and third-largest) asteroid, being three quarters the mass of 4 Vesta and one quarter the mass of Ceres.It is about in diameter, slightly smaller than Vesta.When Pallas was discovered by the German astronomer Heinrich Wilhelm Matthäus Olbers on 28 March 1802, it was counted as a planet, as were other asteroids in the early 19th century.