A satel (or satellite) is an astronomical body that orbits a larger one, typically a planet or star. Most planets in our solar system have at least one natural satellite, and some, like Earth, have more than one. Artificial satellites are also now commonplace, used for everything from communications to weather monitoring.
Satellites can be very small; the largest known is Jupiter’s Ganymede, which has a diameter of 5262 km (3273 miles). The smallest known is Saturn’s moon Pan, with a diameter of just 9 km (5.6 miles). Satellites can also be very large; the largest artificial satellite currently in orbit is the International Space Station, which has a length of 109m (358 ft) and a width of 73m (240 ft).
Most satellites are spherical or oblate spheroidal in shape due to their formation process and gravitationally relaxed state. However there are many irregularly shaped moons in the solar system, such as Rhea and Titan. Satellites can have surface features ranging from completely smooth to heavily cratered, like our own Moon. They may also have an atmosphere, although this is usually very thin and tenuous.
The term “satellite” comes from the Latin saturnus meaning “attendant”. This referred to the fact that planets seemed to follow fixed paths around stars while orbiting them. In 1610 Galileo Galilei discovered Jupiter’s four largest moons using a telescope: Io, Europa, Ganymede and Callisto. He named them after mythological figures associated with Jupiter because they appeared as bright stars near Jupiter when viewed through his telescope.