Satellites

A satellite is anything that orbits a celestial body (star, planet, moon etc). Both natural and artificial satellites exist.

Natural satellites

The Moon is the Earth’s natural satellite. Scientists believe that it was formed when a Mars-sized planet collided with the early Earth, throwing some of the crust into orbit. However the Moon was formed, it is locked into the Earth’s gravitation field and circles our planet once every 27.5 days. Many other moons are the natural satellites for other planets in our solar system and beyond.

Artificial satellites

These have been placed into orbit by man. Among other jobs, artificial satellites orbiting the Earth are used for:

  • telecommunications (transmitting information between distant parts of the Earth)
  • satellite navigation systems (‘satnav’)
  • spying on other countries
  • weather forecasts

Communications satellites occupy a geostationary orbit. They are in orbit above the equator at just the right distance so that it takes them one day to complete an orbit. As a result, they always appear in the same position when seen from the ground. This is why satellite television dishes can be bolted into position and do not need to move.

Earth observation and monitoring satellites occupy polar orbits, passing over the North and South poles, and crossing the equator twice during each orbit. The Earth spins beneath the satellite as it moves, so the satellite can scan the whole surface of the Earth.