The Asteroids occur in many sizes, ranging from several hundred kilometers in diameter,down to bodies that are too small to discern from Earth. There are 26 asteroids larger than 200 km in diameter, but there are probably more than a million with diameters around 1 km. Some asteroids have been photographed by spacecraft in fly-by missions: in 1997 the NEAR-Shoemaker spacecraft orbited and landed on the asteroid Eros. Hubble Space Telescope imagery has revealed details of Ceres (diameter 950 km), Pallas (diameter 830 km) and Vesta (diameter 525 km), which suggest that it may be more appropriate to call these three bodies protoplanets (i.e., still in the process of accretion from planetesimals) rather than asteroids. All three are differentiated and have a layered internal structure like a planet., although the compositions of the internal layers are different. Ceres has an oblate spheroidal shape and a silicate core, and is the most massive asteroid; it has recently been reclassified as a"dwarf planet." Vesta's shape is more irregular and it has an iron core.