Communications satellites

On 11 June 2003, Arianespace orbited two geostationary communications satellites. One of them was the Optus and Defence C1 for the Australian operator Singtel-Optus and the Australian Department of Defence.
Source: Photo from ESA, CNES, ARIANESPACE

Satellites are any man-made objects launched into space and set to orbit the Earth. The term 'communications satellite' is used to refer the satellites we use to transmit information.

There are currently over 2,600 communications satellites in orbit and over 9,000 satellites in total including 'space junk' - objects left in orbit by astronauts by accident, parts of obsolete communications satellites and other man-made but useless items.

Communications satellites range from the size of a bowling ball up to the size of a bus. They are used for many different purposes: relaying television and phone signals, for ship, plane and car navigation; tracking weather patterns; and for military and intelligence uses such as spy photography, and tracking missile launches.

Many communications satellites are in geosynchronous orbits. This means that they stay in the same place in the sky relative to the Earth. As the Earth turns, the satellites move at the same pace.

To make this possible, a satellite has to be launch into space at about 11,000 kilometres per hour to reach a height of approximately 35,700 km above the Earth's surface. The satellite will then revolve around the Earth once every 24 hours - the same number of hours it takes the Earth to make one full rotation.