such as the Iridium phone system. Some communication satellites use much higher geostationary orbits and move at the same angular velocity as the Earth as to appear stationary above one location on the planet. Disadvantages Unlike geosynchronous satellite, satellites in LEO have a small field of view and so can observe and communicate with only a fraction of the Earth at a time. That means that a network (or "constellation") of satellites is required to provide continuous coverage. Satellites in lower regions of LEO also suffer from fast orbital decay and require either periodic re-boosting to maintain a stable orbit or launching replacement satellites when old ones re-enter. Examples The International Space Station is in a LEO about to above Earth's surface, and needs re-boosting a few times a year due to orbital decay. The Iridium telecom satellites orbit at about . Earth observation satellites, also known as remote sensing satellites, including spy satellites and other Earth imaging satellites, use LEO as they are able to see the surface of the Earth more clearly by being closer to it. A majority of artificial satellites are placed in LEO. Satellites can also take advantage of consistent lighting of the surface below via Sun-synchronous LEO orbits at an altitude of about and near polar inclination. Envisat (2002–2012) is one example. The Hubble Space Telescope orbits at about above Earth. The Chinese Tiangong space station was launched in April of 2021 and currently orbits between about and . In fiction In the film 2001: A Space Odyssey, Earth's transit station ("Space Station V") "orbited 300 km above Earth." Former The Chinese Tiangong-1 station was in orbit at about , until its de-orbiting in 2018. The Chinese Tiangong-2 station was in orbit at about , until its de-orbiting in 2019. Gravimetry missions such as GOCE orbited at about to measure Earth's gravity field at highest sensitivity. The mission lifetime was limited because of atmospheric drag. GRACE and GRACE-FO were orbiting at about . Space debris The LEO environment is becoming congested with space debris because of the frequency of object launches. This has caused growing concern in recent years, since collisions at orbital velocities can be dangerous or deadly. Collisions can produce additional space debris, creating a domino effect known as Kessler syndrome. The Orbital Debris Program, part of NASA, tracks over 25,000 objects larger than 10 cm in LEO, the estimated number between