Spacecraft can enter orbit around Earth, its Moon, other planets in the Solar System, or the Sun, to provide in-depth studies of the object of interest. Most spacecraft have prograde orbits, that is they move from west to east, which is the usual direction of rotation of objects in our Solar System (although there are exceptions, such as planet Venus).
A low-Earth orbit (LEO) is the lowest altitude a spacecraft must achieve to orbit the Earth – at least 160 km. Spacecraft in these orbits circle our planet once every ninety minutes or so. The International Space Station and the Hubble Space Telescope are both in LEO.
Geostationary and geosynchronous orbits
A geostationary or geosynchronous orbit is located at an altitude of 36,000 km, and takes a lot more energy to reach than LEO. At this higher altitude it takes the satellite a full 24 hours to orbit the Earth. Thus, the satellite moves at the same speed as the Earth rotates. Geostationary satellites are those orbiting above the equator in a circular orbit – they appear to ‘hover’ in the sky over the same spot on the ground. Geosynchronous satellites are not positioned over the equator or have an elliptical orbit and so appear to move across the sky. These types of satellites are typically communications or weather satellites, for example ESA’s Meteosat family of weather satellites operate in geostationary orbit.
A polar orbit is any orbit in which the spacecraft passes over the rotation poles of the planet. In a polar orbit, the spacecraft can be made to follow any line of longitude. As the Earth rotates below the satellite, the satellite passes over a different region of the planet with each orbit. Polar orbits are used to map a planet – ESA’s Mars Express and Venus Express both have elliptical, polar orbits – or to observe specific effects relating to polar regions, such as Cluster observing the Sun-Earth interaction.
An orbit that passes over the same part of Earth at the same local time each day is called Sun-synchronous. ESA’s Proba-2 is situated in a Sun-synchronous orbit.