NASA's Hubble Space Telescope is the first major infrared-optical-ultraviolet telescope to be placed into orbit around the Earth. Located high above Earth's obscuring atmosphere, the telescope has provided the clearest views of the universe yet obtained in optical astronomy.

The telescope is named after the American astronomer Edwin P. Hubble, who in the 1920's found galaxies beyond our Milky Way and discovered that the universe is uniformly expanding. The heart of the telescope is the 94.5 inch-diameter (2.4-meter) primary mirror. It is the smoothest optical mirror ever polished, with a surface tolerance of one-millionth of an inch. It is made of fused silica glass and weighs about 1,800 pounds.

Outside the blurring effects of Earth's turbulent atmosphere, the telescope can resolve astronomical objects with an angular size of 0.05 arc seconds, which is like seeing a pair of fireflies in Tokyo from your home in Maryland. This razor-sharp vision is 10 to 20 times better than typical resolution with large ground-based telescopes (depending on atmospheric observing conditions).

The space telescope can detect objects as faint as 31st magnitude, which is slightly better than the sensitivity of much larger earth-based telescopes. (The human eye can see celestial objects as dim as sixth magnitude.) Because generally the fainter an object is the farther away it is, Hubble has been used to probe the limits of the visible universe and uncover never-before-seen objects near the horizon of the cosmos. Because it is outside our atmosphere, the telescope can view astronomical objects across a broad swath of the electromagnetic spectrum, from ultraviolet light, to visible, to near-infrared wavelengths. The telescope can also see faint objects near bright objects. This is an important requirement for studying the environments around stars and the glowing nuclei of active galaxies.

Hubble's crystal-clear vision has triggered a revolution in optical astronomy. It has revealed a whole new level of detail and complexity in a variety of celestial phenomena, from nearby stars to galaxies near the limits of the observable universe. This has provided key new insights into the structure and evolution of our universe across a broad scale.

HISTORY

The Hubble Space Telescope was launched April 24, 1990 by the space shuttle Discovery. Hubble was originally equipped with five science instruments: the Wide-Field Planetary Camera, the Faint Object Camera, the Faint Object Spectrograph, the Goddard High-Resolution Spectrograph, and the High Speed Photometer. In addition, three fine guidance sensors were used for pointing and for precision astrometry, the measure of angles on the sky.

After Hubble was launched, scientists discovered that its primary mirror was misshapen due to a fabrication error. This resulted in spherical aberration: the blurring of starlight because the telescope could not bring all the light to a single focal point. Using image-processing techniques scientists were able to do significant research with Hubble until an optical repair could be developed.

In December 1993 the first Hubble servicing mission carried replacement instruments and supplemental optics aboard the space shuttle Endeavor to restore the telescope to full optical performance. A corrective optical device, called the Corrective Optics Space Telescope Axial Replacement, was installed (requiring removal of the High Speed Photometer) so that it could improve the sharpness of the first generation instruments. The Wide-Field and Planetary Camera was replaced with a second camera, which has a built-in correction for the aberration in the primary mirror.

In February 1997 the space shuttle Discovery returned to Hubble for a second servicing mission. Two advanced instruments: the Near Infrared Camera and Multi-Object Spectrometer and the Space Telescope Imaging Spectrograph were swapped out with the two first-generation spectrographs. The astronauts also replaced or enhanced several electronic subsystems and patched unexpected tears in the telescope's shiny, aluminized thermal insulation blankets, which give the telescope its distinctive foil-wrapped appearance.

In December 1999 the space shuttle Discovery rendezvoused with Hubble for a third servicing mission. Astronauts replaced faulty gyroscopes, which had suspended science observations for nearly a month. The telescope also got a new high-tech computer and a data recorder. The astronauts left the telescope in "better than new" condition.

Two more Hubble servicing missions are planned for 2001 and 2003. The Advanced Camera for Surveys will be installed in 2001. It will yield even sharper pictures and a wider field of view. The Wide Field Camera 3 and the Cosmic Origins Spectrograph will be installed in 2003. The telescope's science operations are expected to end in 2010.

HUBBLE OPERATIONS

Hubble is controlled at the Goddard Space Flight Center in Greenbelt, Md. The science mission is directed by the Space Telescope Science Institute at the Johns Hopkins University in Baltimore, Md. Hubble research and funding engages a significant fraction of the worldwide professional astronomical community.

Astronomers compete annually for observing time on Hubble. The over-subscription is typically four to one. Observing proposals are submitted to peer review committees of astronomer experts. The institute director makes the final acceptance and can use his own discretionary time for special programs.

Accepted proposals must be meticulously planned and scheduled by institute experts to maximize the telescope's efficiency. The telescope is not pointed by direct remote control, but instead automatically carries out a series of preprogrammed commands over the course of a day. A data "pipeline" assembled and maintained by the institute ensures that all observations are stored on optical disk for archival research. The data are sent to research astronomers for analysis and then made available to astronomers worldwide one year after the observation.