Introduction
A large number of observatories have been launched into orbit, and most of them have greatly enhanced our knowledge of the extraterrestrial universe.
Performing astronomy from the Earth's surface is limited by the filtering and distortion of electromagnetic radiation (scintillation or twinkling) due to the atmosphere. Some terrestrial telescopes (such as the Very Large Telescope) can reduce atmospheric effects with adaptive optics. A telescope orbiting the Earth outside the atmosphere is subject neither to twinkling nor to light pollution from artificial light sources on the Earth.
Space-based astronomy is even more important for frequency ranges which are outside of the optical window and the radio window, the only two wavelength ranges of the electromagnetic spectrum that are not severely attenuated by the atmosphere. For example, X-ray astronomy is nearly impossible when done from the Earth, and has reached its current importance in astronomy only due to orbiting X-ray telescopes such as the Chandra observatory and the XMM-Newton observatory. Infrared and ultraviolet are also greatly blocked.
Space observatories can generally be divided into two classes: missions which map the entire sky (surveys), and observatories which make observations of chosen parts of the sky.
Many space observatories have already completed their missions, while others continue operating, and still others are planned for the future. Satellites have been launched and operated by NASA, ESA, Japanese Space Agency and the Soviet space program later succeeded by Roskosmos of Russia.
Read more about this topic: Space Telescopes
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