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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.

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.

Orbital drag resulting in a satellite falling onto a neighbouring planet is described by the following sequence:

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.

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