Types of orbit
There are two main types of orbits: open and closed. Closed orbits are the easier to understand, as they can be either circular or elliptical (oval) in shape. A body on a closed orbit constantly travels around another, such as a planet orbiting the Sun or the Moon orbiting the Earth.
An open orbit follows a mathematical shape: either one known as a parabola or another called a hyperbola. Both are sweeping curves that never join up. So, objects on open orbits simply fly by other celestial objects. Some spacecraft and comets follow open orbits.
All four classes of orbit are known as ‘conic sections’ because slicing a cylindrical cone in a different way can make each of their shapes. Closed orbits are achieved by cutting a cone from one side to another at different angles. Open orbits are achieved by slicing the cone from one side down through the base.
Spacecraft can be put into a number of different closed orbits around a planet. These are defined by a number of orbital characteristics, such as the height above the planet’s surface, the inclination to the planet’s equator and the direction in which the spacecraft orbits the planet.
Defined by orbital altitude, these are:
- Low-Earth orbit – As the name implies, this is the lowest altitude a spacecraft must achieve to in order to orbit the Earth. This is around 520 km altitude and spacecraft in these orbits circle the Earth once every ninety minutes or so.
- Geostationary (or geosynchronous) orbit – This is a much higher orbit and so takes a lot more energy to reach. However, once at the altitude of 36,000 km, it takes the spacecraft a full 24 hours to orbit the Earth. Thus, the spacecraft moves at the same speed with which the Earth rotates and therefore appears to ‘hover’ over the same spot on the ground.
All orbits must either follow or cross the equator. With current technology it is impossible for spacecraft to orbit above any line of latitude, except the Equator. Advanced propulsion systems, such as solar sails, will eventually make this possible.
An equatorial orbit is an orbit that lies at any altitude above the Equator. 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, since these always lead from one pole to the other, and then any line of longitude leads back again on the other side of the Earth. Orbital directions are as follows:
- Prograde – Any orbit in which the spacecraft moves from west to east is termed prograde. This is the usual direction of rotation in our Solar System. Only a handful of objects orbit or rotate in the opposite direction.
- Retrograde – Any orbit in which the spacecraft moves from east to west. This is the less usual direction in the Solar System; however, it is not impossible. For example, Venus has retrograde spin and some comets – notably comet Halley, which was encountered by ESA’s Giotto spacecraft in 1986 – also has a retrograde orbit.
Last update: 27 September 2005