Orbital decay

In orbital mechanics, decay is a gradual decrease of the distance between two orbiting bodies at their closest approach (the periapsis) over many orbital periods. These orbiting bodies can be a planet and its satellite, a star and any object orbiting it, or components of any binary system. Orbits do not decay without some friction-like mechanism which transfers energy from the orbital motion. This can be any of a number of mechanical, gravitational, or electromagnetic effects. For bodies in low Earth orbit, the most significant effect is atmospheric drag. In orbital mechanics, decay is a gradual decrease of the distance between two orbiting bodies at their closest approach (the periapsis) over many orbital periods. These orbiting bodies can be a planet and its satellite, a star and any object orbiting it, or components of any binary system. Orbits do not decay without some friction-like mechanism which transfers energy from the orbital motion. This can be any of a number of mechanical, gravitational, or electromagnetic effects. For bodies in low Earth orbit, the most significant effect is atmospheric drag. If left unchecked, the decay eventually results in termination of the orbit when the smaller object strikes the surface of the primary; or for objects where the primary has an atmosphere, the smaller object burns, explodes, or otherwise breaks up in the larger object's atmosphere; or for objects where the primary is a star, ends with incineration by the star's radiation (such as for comets), and so on. Collisions of stellar-mass objects usually produce cataclysmic effects, such as gamma-ray bursts. Due to atmospheric drag, the lowest altitude above the Earth at which an object in a circular orbit can complete at least one full revolution without propulsion is approximately 150 km (90 mi). Atmospheric drag at orbital altitude is caused by frequent collisions of gas molecules with the satellite.It is the major cause of orbital decay for satellites in low Earth orbit. It results in the reduction in the altitude of a satellite's orbit. For the case of Earth, atmospheric drag resulting in satellite re-entry can be described by the following sequence: Orbital decay thus involves a positive feedback effect, where the more the orbit decays, the lower its altitude drops, and the lower the altitude, the faster the decay. Decay is also particularly sensitive to external factors of the space environment such as solar activity, which are not very predictable. During solar maxima the Earth's atmosphere causes significant drag up to a hundred kilometers higher than during solar minima. Atmospheric drag exerts a significant effect at the altitudes of space stations, space shuttles and other manned Earth-orbit spacecraft, and satellites with relatively high 'low earth orbits' such as the Hubble Space Telescope. Space stations typically require a regular altitude boost to counteract orbital decay (see also orbital station-keeping). Uncontrolled orbital decay brought down the Skylab space station, and (relatively) controlled orbital decay was used to de-orbit the Mir space station. Regular orbital boosts are also needed by the Hubble Space Telescope, though on a longer time scale, due to its much higher altitude. However, orbital decay is also a limiting factor to the length of time the Hubble can go without a maintenance rendezvous, the most recent performed successfully by STS-125, with space shuttle Atlantis launching May 11, 2009, though newer telescopes are in much higher orbits, or in some cases in solar orbit, so orbital boosting may not be needed. An orbit can also decay by tidal effects when the orbiting body is large enough to raise a significant tidal bulge on the body it is orbiting and is either in a retrograde orbit or is below the synchronous orbit. The resulting tidal interaction saps momentum from the orbiting body and transfers it to the primary's rotation, lowering the orbit's altitude until frictional effects come into play.