Comets are small bodies that orbit the Sun and, when close enough to the Sun, exhibit a visible coma (atmosphere seen as a fuzzy outline. The word comes from the Latin word for "hair" - hence our word comb!) and/or a tail. These effects come from the action of solar radiation upon the comet's nucleus. Comet nuclei are themselves loose collections of ice, dust and small rocky particles, measuring a few kilometres or tens of kilometres across - held together by gravity.

Comets have a variety of orbital periods, ranging from a few years to hundreds of thousands of years - and some are believed to pass through the inner Solar System only once before being thrown out into interstellar space!

Asteroids are also sometimes called called minor planets or planetoids. The term asteroid is generally used to indicate a diverse group of small celestial bodies in the solar system that orbit around the Sun that can't be called a planet or a comet.

"Asteroid" is Greek for "star-like". It is the most commonly used word in the English literature for minor planets while other languages prefer the term "Planetoid" which is Greek for "planet-like". That is a better term because it more or less describes what they are! (Stars they ain't!).

The first asteroid to be discovered, Ceres, is the largest asteroid yet discovered and that is now classified as a dwarf planet. All others are currently classified as 'small solar system bodies'. The vast majority of asteroids are found within the main asteroid belt. They follow elliptical orbits between Mars and Jupiter. It is thought that these asteroids are remnants of the protoplanetary disc, and in this region their pulling together into a larger planet or planets during the formative period of the solar system was prevented by large intermittentt gravitational tugs by Jupiter.

Some asteroids have their own moons (natural satellites that orbit them) or are found in co-orbiting pairs known as binary systems.