A moon is a natural body that travels around a planet. Moons are a by-product of planetary formation and can range in size from small asteroid-sized bodies of a few kilometres in diameter to several thousand kilometres, larger even than the planets Mercury and Pluto.
Landing on another moon
One such large moon is Titan, the target for ESA's daring Huygens mission that in 2005 will become the first spacecraft ever to land on a moon of another planet. Titan is slightly bigger than the planet Mercury, and is only called a moon because it orbits the giant planet Saturn rather than the Sun.
Four other large moons can be found around another of our neighbours, Jupiter. These are Io, Europa, Ganymede and Callisto. Europa has captured attention because beneath its icy surface, scientists think that an ocean covers the entire moon. Some scientists have even speculated that microscopic life might be found in that ocean.
In 2008, ESA plans to launch its 'rocky planet' finder Eddington. By detecting the drop in light seen when a world passes in front of its parent star, Eddington will be capable of discovering planets the size of Jupiter, and also those smaller than Mars.
That means, if our own Solar System is anything to go by, it will be capable of detecting moons similar in size to Titan and the four large moons of Jupiter.
It would be particularly exciting if such combinations of planets and moons were found orbiting a star at Earth's distance from the Sun. Perhaps then the surfaces of the moons would be warmed to habitable levels.