Hubble Space Telescope

The Hubble Space Telescope is a low-earth-orbiting telescope designed to observe and photograph objects in space, at the highest resolution, using any and all wavelengths of electromagnetic radiation. Launched on Launch date::April 25, 1990 by the Launch authority::National Aeronautics and Space Administration, it functions under the control of the Control authority::Goddard Space Flight Center and the Control authority::Space Telescope Science Institute. It is the only one of the four Great Observatories of NASA that is servicable by the Space Transportation System. For the eighteen years of its operation (as of October 14, 2008), it has been one of the most valuable resources available to astronomers seeking to understand everything from the origin of the universe to the history and composition of the many objects in the solar system.

Spacecraft
Two seven-meter arrays of photovoltaic cells provide Probe power::5680 watts of electricity to run the telescope's systems. Two antennae transmit the telescope's data and telemetry information to the Tracking and Data Relay Satellite system for relay to the ground controllers and observers.

A hinged Aperture Door, like a giant lens cap, protects that telescope's optical systems from overexposure and during servicing missions. A long sleeve provides a recess for the incident light and shields the telescope optics from stray light.

Optics
The Hubble Space Telescope is a reflecting telescope. Light passes through the light shield and then through the aperture and along the main baffle, to reflect from the primary mirror. This mirror, which measures 2.4 meters in diameter, converges the light to a secondary mirror that has its own baffle. The secondary mirror, which measures 0.3 meter in diameter, converges the light again and directs it through a third or central baffle to a second aperture in the center of the primary mirror. Beyond this aperture is the focal plane, where the final image projects itself for examination by the telescope's scientific instruments.

Aiming system
The telescope uses four flywheels, each with its own magnetic regulator, to direct it at any target. Six gyroscopes hold the telescope in position. Various coarse and fine sensors provide guidance and also warn when the incident light is too bright. In that case, the aperture door closes.

Scientific instruments
These instruments are all located aft of the primary mirror. Five of these are mounted along the telescope's optical axis and receive light directly from the optical assembly. Four radial bays, each forward and to the side of the focal plane, house more scientific instruments and three fine-guidance sensors that assist in keeping the telescope on target.

History
In 1969, the National Academy of Sciences, at the suggestion of the astronomer Lyman Spitzer and his colleagues, approved the plan to build a large telescope in space, far above the atmosphere and able to gather light free of distortion and achieve far higher resolution that a ground-based telescope can provide. Congress provided funding in 1977, and in that same year NASA named the project after Edwin Hubble, the astronomer who first realized that the universe was expanding. In 1981 the Space Telescope Science Institute opened its doors in anticipation of the launch.

On April 24, 1990, the United States space transport Discovery carried the Hubble Space Telescope into orbit. Discovery released the telescope into orbit on the following day.

On June 25, the controllers were dismayed to realize that the primary mirror had been shaped to the wrong radius, and all the telescope's images were out of focus. NASA received approval to build an instrument (the Corrective Optics Space Telescope Axial Replacement, or COSTAR) to replace one of Hubble's original instruments and correct for the faulty mirror. On December 2, 1993, the space transport Endeavor embarked on the first of four Servicing Missions to install the new optical assembly and replace or install several other instruments.

Discovery flew the second servicing mission on February 11, 1997 to replace more of Hubble's systems. In some cases, the replacements were of a significantly improved design. Discovery then flew another mission, on December 19, 1999, to install a new on-board computer system, replace the six gyroscopes, and install better recording equipment and power subsystems.

The transport Columbia flew the most recent servicing mission, on March 1, 2002, to install a new camera for photographing the most-distant objects, new and more efficient solar arrays, and a new power system for taking full advantage of the power that the solar arrays provide. This mission also included replacement of some of the telescope's guidance systems.

NASA announced shortly thereafter that they could not keep the Hubble Telescope functioning for long past 2008. After much outcry from the astronomical community, NASA announced on October 31, 2006, that they would mount another servicing mission intended to keep the telescope functioning for another ten years. However, the Science Data Formatter suffered a significant malfunction on September 27, 2008, and the main computer ordered the formatter to place itself in safe mode. NASA switched all data flow to the system's redundant counterpart, and announced that the fifth servicing mission would not fly until 2009.