Original: 4 Vesta is the second most massive object in the asteroid belt, with a mean diameter of about 530 km and an estimated mass of 9% of the mass of the entire asteroid belt.
Simplified: 4 Vesta is the second most massive asteroid in the solar system, with an average diameter of about 530 km (around 330 miles) and an estimated mass 9% the mass of the entire asteroid belt.