The practical uses of satellites in orbit around the Earth are described in many separate articles and in more general entries such as communications satellites. The scientific exploration of space and the Earth has been advanced by many other orbital systems. NASA, for example, has successfully launched eight Orbiting Solar Observatories and the Solar Maximum Mission for studying the Sun. To study regions beyond the solar system, NASA launched three Small Astronomical Satellites, two Orbiting Astronomical Observatories, and three High Energy Astronomical Observatories. After long delays it also launched the Hubble Space Telescope by means of the Space Shuttle. With the European Space Agency (ESA) and the United Kingdom it has launched the International Ultraviolet Explorer and, with the Netherlands, the Infrared Astronomy Satellite. ESA, for its part, has launched the Exosat astronomy satellite for exploring the X-ray region of the electromagnetic spectrum. Earth-observing satellites have included NASA's six Orbiting Geophysical Observatories and many of its Explorer series.
Geodetic surveys and accurate navigational data may be obtained from precise tracking of the positions of the U.S. Navy's Transit satellites and the more recent NAVSTAR series. The most accurate geodetic data can be obtained from pulsed laser beams. The Laser Geodynamic Satellite, or LAGEOS, placed in orbit in 1976, permits measurements of the movement of the Earth's crust to an accuracy of within 2 cm (0.8 in), thereby providing information vital to earthquake-prediction research.
Geosynchronous orbits provide ideal observational positions for many applications satellites, but another highly useful location for Earth observation is a Sun-synchronous orbit, which is slightly tilted off true polar orbit so that the orbital plane shifts to keep pace with the Sun's annual motion through the ecliptic.