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
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
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.
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
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
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 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