4 Vesta (IPA: ) is the second most massive object in the asteroid belt, with a mean diameter of about 530 km (around 330 miles) and an estimated mass 9% the mass of the entire asteroid belt. Its size and unusually bright surface make Vesta the brightest asteroid, and the only one ever visible to the naked eye from Earth, although unverified reports of Ceres and 7 Iris being seen unaided by people with exceptional eyesight do exist. Due to the availability of rock samples in the form of the HED meteorites, it has also been the most studied.