Orbiting astronomical observatories permit observations of distant cosmic objects high above the interference and distorting effects of the Earth's atmosphere. Astronomical satellites have studied the sun's radiation, cosmic radiation, the solar wind, the electromagnetic characteristics of space, stellar radiation, and other information not available through the earth's atmosphere.

The unmanned Infrared Astronomy Satellite (IRAS), launched in 1983 by the United States in collaboration with the United Kingdom and The Netherlands, has discovered much about infrared sources far beyond the limits of the solar system. That same year, the Soviet Union placed in Earth orbit another advanced observational satellite, called Astron. Equipped with an ultraviolet telescope developed by France, Astron has studied cosmic radiation from galactic and extragalactic sources.

HST - a joint ESA/NASA project - is a 2.4-meter reflecting telescope which was deployed in low- Earth orbit (600 kilometers) by the crew of the space shuttle Discovery (STS-31) on 25 April 1990. It is a long-term spacebased observatory. The observations are carried out in visible, infrared and ultraviolet light. During its years of operation HST has managed to become one of the most important science projects ever. HST has enabled scientists to make many astronomical discoveries, including the first observations of a black hole.

HST has in many ways revolutionised modern astronomy, being a highly efficient tool for making new discoveries, but also by driving astronomical research in general. HST was designed to take advantage of being above the Earth's disturbing atmosphere, and thereby providing astronomers with observations of very high resolution - essentially opening new windows to planets, stars and galaxies. HST was designed as a flagship mission of high standard, and has served to pave the way for other space-based observatories. Hubble Space Telescope is named after Edwin Powell Hubble (1889-1953) who was one of the great pioneers of modern astronomy.

   HST is an observatory first dreamt of in the 1940s, designed and built in the 1970s and 80s, and operational only in the 1990s. Since its preliminary inception, HST was designed to be a different type of mission for NASA -- a long term space- based observatory. To accomplish this goal and protect the spacecraft against instrument and equipment failures, NASA had always planned on regular servicing missions.

HST was designed with modular components so that on subsequent Shuttle missions it could be recovered and have faulty or obsolete parts replaced with new or improved instruments before being re-released into orbit.

   HST is as large as a school bus and looks like a five-story tower of stacked silver canisters. Each canister houses important telescope equipment: the focusing mirrors, computers, imaging instruments, and pointing and control mechanisms. Extending from the telescope are solar panels for generating electricity and antennas for communicating with operators on the ground.

Power for the two on-board computers and the scientific instruments internal link is provided by two 2.4 x 12.1 m solar panels The power generated by the arrays is also used to charge six nickel- hydrogen batteries which provide power to the spacecraft during the roughly 25 minutes per orbit in which HST flies through the Earth's shadow.

The 12-ton telescope collects faint starlight with an 8-foot-diameter mirror. The mirror - tucked inside a long, hollow tube that blocks the glare from the sun, Earth, and moon - is slightly curved to focus and magnify light.

Unlike ground-based telescopes, astronomers cannot look through Hubble’s lens to see the universe. Instead, Hubble’s scientific instruments are the astronomers’ electronic eyes. The telescope’s instruments include cameras and spectrographs. The cameras don’t use photographic film, but rather electronic detectors similar to those used in home video cameras. The spectrographs collect data by separating starlight into its rainbow of colors, just as a prism does to sunlight. By closely studying the colors of light from a star, astronomers can decode the star’s temperature, motion, composition, and age.

HST Hubble must maintain a steady position to take long exposures— sometimes hours— of the same subject to produce images of distant or faint objects. Otherwise the images will be blurred. To accomplish this mission, the telescope must battle such celestial elements as air drag, the sun’s radiation, and the gravitational pull of objects.

To improve its stability during observations, the telescope uses an elaborate system for attitude control. For Hubble, maintaining proper direction is similar to a sailor fighting the wind and water to keep his sailboat on course. Manoeuvring is performed by reaction wheels and its position in space monitored by four of six gyros. Pointing maintained in this way is known as 'coarse track mode'. Hubble is successful because of its sophisticated pointing control system, which includes gyroscopes and Fine Guidance Sensors (FGSs), which can be used to lock onto guide stars (fine lock) to reduce spacecraft drift and increase pointing accuracy.

Once the telescope locks onto an object, its sensors check for movement 40 times a second. If movement occurs, the wheels, which are constantly rotating, change speeds to smoothly move the telescope back into position.

Once Hubble gathers pictures and data on celestial objects, its computers turn the information into long strings of numbers that are beamed to Earth as radio signals. This information streams through a series of satellite relays to the Goddard Space Flight Center and then by telephone line to the Space Telescope Science Institute, where the numbers are turned back into pictures and data.

The information collected daily by Hubble is stored on optical computer disks. A single day’s worth of observations would fill an encyclopedia. The constantly growing collection of Hubble pictures and data are a unique scientific resource for current and future astronomers.