Asteroid
The dwarf planet Ceres is by far the largest asteroid, with a diameter of . The next largest are 4 Vesta and 2 Pallas, both with diameters of just over . Vesta is the only main-belt asteroid that can, on occasion, be visible to the naked eye. On some rare occasions, a near-Earth asteroid may briefly become visible without technical aid; see 99942 Apophis.
Space exploration
Until the advent of space travel, objects in the asteroid belt were merely pinpricks of light in even the largest telescopes, their shapes and terrain remaining a mystery.
Several asteroids have now been visited by probes, the first of which was "Galileo", which flew past two: 951 Gaspra in 1991, followed by 243 Ida in 1993. Both of these lay near enough to "Galileo"'s planned trajectory to Jupiter that they could be visited at acceptable cost. The first landing on an asteroid was performed by the "NEAR Shoemaker" probe in 2000, following an orbital survey of the object. The dwarf planet Ceres and the asteroid 4 Vesta, two of the three largest asteroids, were visited by NASA's "Dawn" spacecraft, launched in 2007.