Orbital decay in the context of Gravitational wave

A geocentric orbit, Earth-centered orbit, or Earth orbit involves any object orbiting Earth, such as the Moon or artificial satellites. In 1997, NASA estimated there were approximately 2,465 artificial satellite payloads orbiting Earth and 6,216 pieces of space debris as tracked by the Goddard Space Flight Center. More than 16,291 objects previously launched have undergone orbital decay and entered Earth's atmosphere.

A spacecraft enters orbit when its centripetal acceleration due to gravity is less than or equal to the centrifugal acceleration due to the horizontal component of its velocity. For a low Earth orbit, this velocity is about 7.8 km/s (28,100 km/h; 17,400 mph); by contrast, the fastest crewed airplane speed ever achieved (excluding speeds achieved by deorbiting spacecraft) was 2.2 km/s (7,900 km/h; 4,900 mph) in 1967 by the North American X-15. The energy required to reach Earth orbital velocity at an altitude of 600 km (370 mi) is about 36 MJ/kg, which is six times the energy needed merely to climb to the corresponding altitude.

The Kármán line (or von Kármán line /vɒn ˈkɑːrmɑːn/) is a conventional definition of the edge of space; it is widely but not universally accepted. The international record-keeping body FAI (Fédération aéronautique internationale) defines the Kármán line at an altitude of 100 kilometres (54 nautical miles; 62 miles; 330,000 feet) above mean sea level.

While named after Theodore von Kármán, who calculated a theoretical limit of altitude for aeroplane flight at 83.8 km (52.1 mi) above Earth, the later established Kármán line is more general and has no distinct physical significance, in that there is a rather gradual difference between the characteristics of the atmosphere at the line, and experts disagree on defining a distinct boundary where the atmosphere ends and space begins. It lies well above the altitude reachable by conventional airplanes or high-altitude balloons, and is approximately where satellites, even on very eccentric trajectories, will decay before completing a single orbit.

A stellar collision is the coming together of two stars caused by stellar dynamics within a star cluster, or by the orbital decay of a binary star due to stellar mass loss or gravitational radiation, or by other mechanisms not yet well understood.

Any stars in the universe can collide, whether they are "alive", meaning fusion is still active in the star, or "dead", with fusion no longer taking place. White dwarf stars, neutron stars, black holes, main sequence stars, giant stars, and supergiants are very different in type, mass, temperature, and radius, and accordingly produce different types of collisions and remnants.

The Russian space station Mir ended its mission on 23 March 2001, when it was brought out of its orbit, entered the atmosphere and was destroyed. Major components ranged from about 5 to 15 years in age, and included the Mir Core Module, Kvant-1, Kvant-2, Kristall, Spektr, Priroda, and Docking Module. Although Russia was optimistic about Mir's future, the country's commitments to the International Space Station programme left no funding to support Mir.

The deorbit was carried out in three stages. The first stage was waiting for atmospheric drag to decay the orbit to an average of 220 kilometres (140 mi). This began with the docking of Progress M1-5. The second stage was the transfer of the station into a 165-by-220-kilometre (103 mi × 137 mi) orbit. This was achieved with two burns of the Progress M1-5's control engines at 00:32 UTC and 02:01 UTC on 23 March 2001. After a two-orbit pause, the third and final stage of Mir's deorbit began with the firing of Progress M1-5's control engines and main engine at 05:08 UTC, lasting a little over 22 minutes. The atmospheric entry at the altitude of 100 kilometres (62 mi) occurred at 05:44 UTC near Nadi, Fiji.