The largest asteroid by far is 1 Ceres. All other known asteroids are less than 340 km across. In 1993, Galileo flew by Ida on the way to Jupiter. It is the first natural satellite of an asteroid ever discovered. This makes them very interesting as samples of the early history of the solar system.
Comets are invisible except when they are near the Sun.
Most comets have highly eccentric orbits which take them far beyond the orbit of Pluto, into the Oort Cloud. After 500 or so passes near the Sun, most of a comet's ice and gas is lost leaving a rocky object very much like an asteroid in appearance. Larger meteoroids whose cores don't entirely burn up before they hit the ground. The eighth largest planet. Mariner 10 flew by three times in 1973 and 1974.