Hubble Space Telescope

The Hubble Space Telescope is a powerful orbiting telescope that provides sharper images of heavenly bodies than other telescopes do. It is a reflecting telescope with a light-gathering mirror 94 inches (240 centimeters) in diameter. The telescope is named after American astronomer Edwin P. Hubble, who made fundamental contributions to astronomy in the 1920's.

Astronomers have used the Hubble Space Telescope to obtain images of celestial objects and phenomena in detail never before observed. These include pictures of stars surrounded by dusty disks that might someday evolve into planetary systems, images of galaxies on the edge of the observable universe, pictures of galaxies colliding and tearing each other apart, and evidence suggesting that most galaxies have massive black holes in their center.

How the telescope works

In orbit about 380 miles (610 kilometers) above the earth, the Hubble Space Telescope views the heavens without looking through the earth's atmosphere. The atmosphere bends light due to a phenomenon known as diffraction, and the atmosphere is constantly moving. This combination of diffraction and movement causes starlight to jiggle about as it passes through the air, and so stars appear to twinkle. Twinkling blurs images seen through ground-based telescopes. Because an orbiting telescope is above the atmosphere, it can produce pictures in much finer detail than a ground-based telescope can.

The Hubble Space Telescope can also observe ultraviolet and infrared light that is blocked by the atmosphere. These forms of light, like visible light, are electromagnetic radiation. The wavelength (distance between successive wave crests) of ultraviolet light is shorter than that of visible light. Infrared light has longer wavelengths than visible light. Ultraviolet light comes from highly energetic processes, such as the formation of disks around black holes and exploding stars. Infrared light provides information about cooler, calmer events, such as the formation of dust clouds around new stars.

The United States space agency, the National Aeronautics and Space Administration (NASA), operates the Hubble Space Telescope in cooperation with the European Space Agency (ESA). The telescope is controlled by radio commands relayed from NASA's Goddard Space Flight Center in Greenbelt, Maryland. Astronomers tell the telescope where to point, and computer -- driven instruments aboard the telescope record the resulting observations. The telescope transmits the data by radio to astronomers on the ground.

The Hubble Space Telescope has two kinds of instruments: (1) imagers, which take pictures; and (2) spectrographs, which analyze light. Imagers are electronic detectors called charge -- coupled devices (CCD's). The CCD's convert light into electronic signals, which an on -- board computer records and sends to the ground.

A spectrograph, like a prism, spreads light into its component colors, much as water droplets spread sunlight into a rainbow. The resulting band of light is called a spectrum (plural spectra). Using spectrographic data from the Hubble Space Telescope, astronomers can determine the composition of stars and galaxies--measuring, for example, the amounts of hydrogen, carbon, and other chemical elements in them.

History

The space shuttle Discovery launched the telescope into orbit in 1990. Soon after launch, engineers discovered a flaw in the telescope's light -- gathering mirror. The flaw made the images less clear than they otherwise would have been. Engineers designed an optical device to bend light reflected by the mirror in a way that would make up for the error. Astronauts from the space shuttle Endeavour installed the device on the telescope in 1993, and it worked as planned.

During the 1993 mission, astronauts also mounted new instruments on the telescope. As part of a continuing program to upgrade the telescope, astronauts installed additional components in 1997, 1999, and 2002. 

In 2004, NASA administrators canceled a final servicing mission to the telescope that had been scheduled for 2006. The officials based the decision on concern for the safety of astronauts after the loss of the space shuttle Columbia in 2003. Scientists, politicians, and astronomy enthusiasts protested that the decision would bring an early end to the telescope's observations. NASA officials then agreed to study the possibility of sending a robotic craft to perform needed repairs.