Their sizes are on the order of the width of a human hair. Meteoroids are small chunks of rock, from the size of sand grains up to hundreds of meters in length, that orbit about the Sun. An asteroid is a large chunk of rock that orbits the Sun. Asteroids are bigger than meteoroids and are found in sizes up to several hundred kilometers. Most asteroids orbit the Sun between the orbits of Mars and Jupiter.
Comets are similar to asteroids, but they are composed of various ices as well as rocky material.
Most comets orbit the Sun on very large orbits in the Kuiper Belt or the Oort Cloud. A planetary satellite is an object which orbits a planet, also called a moon. Since there are 100 cm in 1 m, and 1000 m in 1 km, this is approximately equal to 2 km. 3 km is closest to being how far away the nearest star would be. If the aliens reply inmediately by radio waves, the answer would arrive here on 1988.