Asteroids are minor planets located mainly in the inner Solar System between Mars and Jupiter. They range greatly in size, from almost 1000 km for the largest down to rocks just tens of meters across. Asteroids are composed of various materials like rock, metal, and ice, with some like Ceres having a rocky core and icy mantle. They become darker and redder over time due to space weathering. Exploration of asteroids began with early probes imaging Phobos and Deimos in 1971, but their shapes and terrain were unknown until the age of space travel.