Orbital decay is the process of prolonged reduction in the height of a satellite’s orbit due to drag produced by an atmosphere. The drag intensifies during periods of high solar activity and sunspots, due to frequent collisions between the satellite and surrounding air molecules. The drag experienced by the object is proportional to the solar activity at the time. MILSTAR:A communication satellite A satellite is any object that orbits another object (which is known as its primary). ... In physics, an orbit is the path that an object makes, around another object, whilst under the influence of a source of centripetal force, such as gravity. ... Atmosphere is the general name for a layer of gases that may surround a material body of sufficient mass. ... 20 years of solar irradiance data from satellites Solar variation refers to fluctuation in the amount of energy emitted by the Sun. ... 400 year sunspot history A sunspot is a region on the Suns surface (photosphere) that is marked by a lower temperature than its surroundings, and intense magnetic activity. ...

A principal example of orbital decay is the Earth’s atmosphere during solar maxima (sunspots). The Earth's atmosphere causes drag up to a hundred kilometers higher than the drag during solar minima. 400 year sunspot history A sunspot is a region on the Suns surface (photosphere) that is marked by a lower temperature than its surroundings, and intense magnetic activity. ...

Orbital drag resulting in a satellite falling onto a neighboring planet is described by the following sequence: The eight planets and three dwarf planets of the Solar System. ...

Large solar activity -> Increases drag -> Orbital speed decreases -> less orbital height -> Experiences denser atmosphere -> increased heat -> usually burns on entry

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. The positive feedback on unpredictable stimuli makes orbital decay difficult to predict analytically, and makes prediction statistical instead.

Orbital decay exerts a significant effect at the altitudes of space stations, space shuttles and other manned Earth-orbit spacecraft, and satellites with relatively high orbits such as the Hubble Space Telescope. Space stations typically require a regular altitude boost to counteract orbital decay. Uncontrolled orbital decay brought down the Skylab space station, and (relatively) controlled orbital decay was used to de-orbit the Mir space station. Orbital boosts for the International Space Station (ISS) are regularly needed, and are one limiting factor for the length of time the ISS can go between visits from transit spacecraft. 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, and has threatened to destroy the telescope in a few more years given the long-running uncertainty over whether a future Shuttle mission to the Hubble will be carried out. The space station Mir A space station is an artificial structure designed for humans to live in outer space. ... This article is about the NASA Space Shuttle vehicle. ... The Hubble Space Telescope (HST) is a telescope in orbit around the Earth, named after astronomer Edwin Hubble for his discovery of galaxies outside the Milky Way and his creation of Hubbles Law, which calculates the rate at which the universe is expanding. ... Drawing of Skylab with components labelled Skylab was the first space station the United States launched into orbit. ... Mir (Мир, which can mean both world and peace in Russian) was a highly successful Soviet (and later Russian) orbital station. ... International Space Station insignia ISS Statistics Crew: 3 As of July 21, 2006 Perigee: 352. ...