Space debris

Initially, the term space debris referred to the natural debris found in the solar system: asteroids, comets, and meteoroids. However, with the 1979 beginning of the NASA Orbital Debris Program, the term also refers to the debris (alt. space waste or space garbage) from the mass of defunct, artificially created objects in space, especially Earth orbit. These include old satellites and spent rocket stages, as well as the fragments from their disintegration and collisions. As of December 2016, five satellite collisions have generated space debris. Space debris is also known as orbital debris, space junk, space waste, space trash, space litter or space garbage. As of 5 July 2016, the United States Strategic Command tracked a total of 17,852 artificial objects in orbit above the Earth, including 1,419 operational satellites. However, these are just objects large enough to be tracked. As of January 2019, more than 128 million bits of debris smaller than 1 cm (0.4 in), about 900,000 pieces of debris 1–10 cm, and around 34,000 of pieces larger than 10 cm were estimated to be in orbit around the Earth. Collisions with debris have become a hazard to spacecraft; they cause damage akin to sandblasting, especially to solar panels and optics like telescopes or star trackers that cannot be covered with a ballistic Whipple shield (unless it is transparent). Below 2,000 km (1,200 mi) Earth-altitude, pieces of debris are denser than meteoroids; most are dust from solid rocket motors, surface erosion debris like paint flakes, and frozen coolant from RORSAT (nuclear-powered satellites). For comparison, the International Space Station orbits in the 300–400 kilometres (190–250 mi) range, and the 2009 satellite collision and 2007 antisat test occurred at 800 to 900 kilometres (500 to 560 mi) altitude. The ISS has Whipple shielding; however, known debris with a collision chance over 1/10,000 are avoided by maneuvering the station. The Kessler syndrome, a runaway chain reaction of collisions exponentially increasing the amount of debris, has been hypothesized to ensue beyond a critical density. This could affect useful polar-orbiting bands, increases the cost of protection for spacecraft missions and could destroy live satellites. Whether Kessler syndrome is already underway has been debated. The measurement, mitigation, and potential removal of debris are conducted by some participants in the space industry. There are estimated to be over 128 million pieces of debris smaller than 1 cm (0.39 in) as of January 2019. There are approximately 900,000 pieces from one to ten cm. The current count of large debris (defined as 10 cm across or larger) is 34,000. The technical measurement cutoff is c. 3 mm (0.12 in). Over 98 percent of the 1,900 tons of debris in low Earth orbit (as of 2002) was accounted for by about 1,500 objects, each over 100 kg (220 lb). Total mass is mostly constant despite addition of many smaller objects, since they reenter the atmosphere sooner. Using a 2008 figure of 8,500 known items, it is estimated at 5,500 t (12,100,000 lb). In LEO there are few 'universal orbits' which keep spacecraft in particular rings (in contrast to GEO, a single widely used orbit). The closest are sun-synchronous orbits that keep a constant angle between the Sun and the orbital plane; they are polar, meaning they cross over the polar regions. LEO satellites orbit in many planes, up to 15 times a day, causing frequent approaches between objects (the density of objects is much higher in LEO). Orbits are further changed by perturbations (which in LEO include unevenness of the Earth's gravitational field), and collisions can occur from any direction. For these reasons, the Kessler syndrome applies mostly to the LEO region; impacts occur at up to 16 km/s (twice the orbital speed) if head-on – the 2009 satellite collision occurred at 11.7 km/s, creating much spall in the critical size range. These can cross other orbits and lead to a cascade effect. A large-enough collision (e.g. between a space station and a defunct satellite) could make low Earth orbit impassable.