3753 Cruithne It was not until 1997 that its unusual orbit was determined by Paul Wiegert and Kimmo Innanen, working at York University in Toronto, and Seppo Mikkola, working at the University of Turku in Finland. Cruithne is approximately in diameter, and its closest approach to Earth is , approximately thirty times the separation between Earth and the Moon. From 1994 through 2015, Cruithne made its annual closest approach to Earth every November. Although Cruithne's orbit is not thought to be stable over the long term, calculations by Wiegert and Innanen showed that it has probably been synchronized with Earth's orbit for
Earth observation satellite to get the desired global coverage while the relatively constant geometry to the Sun mostly is an advantage for the instruments. The "frozen" orbit is selected as this is the closest to a circular orbit that is possible in the gravitational field of the Earth. Terrain can be mapped from space with the use of satellites, such as Radarsat-1 and TerraSAR-X. Earth observation satellite Earth observation satellites or Earth remote sensing satellites are satellites specifically designed for Earth observation from orbit, similar to spy satellites but intended for non-military uses such as environmental monitoring, meteorology, map making etc. The first