brings the light to a focus; a smaller secondary mir-
ror, which bounces the light back toward the pri-
mary, where it passes through a central hole; and a
set of 45-degree mirrors that can reflect the image to
a choice of cameras and spectrographs.
A thin aluminum shell covers the entire telescope
to protect it from solar radiation, and many layers of
insulation help to keep temperatures low, as the in-
struments must be very cold to operate most effi-
ciently. Inside there are four heavy flywheels that are
used to orient the telescope toward the place in the
sky that is to be observed. The HST is remarkably ac-
curate at pointing at celestial targets: It can hold on a
star or galaxy for a few thousandths of an arc second.
The telescope is powered by two 8-foot-long solar
panels, which are mounted to the sides of the tele-
scope and rotate to face the Sun for maximum
power. Batteries provide backup power during the
times when Earth eclipses the Sun. Since the tele-
scope’s launch in 1990, the panels have been re-
placed twice by newer models, which can generate
about 5,700 watts of electrical power.
The HST remains in a low orbit, about
610 kilometers (380 miles) above Earth, so
that it can be serviced by astronauts, but
the result is that the planet blocks about
half of the sky during the 180-minute or-
bit. Therefore, exposures are limited to
less than half that time, and targets are
usually visited many times. Very long expo-
sures can be built up in this way to allow
Hubble scientists to detect extremely faint
objects, a hundred times fainter than
those objects detectable from ground-
based telescopes.
The telescope was designed to be up-
graded as needed, and provisions were
made for the easy removal and exchange
of various instruments by astronauts visit-
ing the space telescope. This feature re-
sulted in keeping the telescope up-to-date
with the most recent technological ad-
vances throughout the 1990’s and in the
early twenty-first century. There were
three such shuttle missions in the 1990’s.
Disaster,Recovery, and Triumph Shortly
after the launch of the Hubble Space Tele-
scope, the worldwide astronomy commu-
nity, which had waited forty years to have
access to the wonders expected to be revealed by the
HST, was dismayed to learn that a colossal error had
been made by one of the National Aeronautics and
Space Administration (NASA) contractors who pro-
duced the primary mirror. Instead of sharp images,
stars appeared fuzzy and indistinct. The mirror suf-
fered from a defect called spherical aberration: It
had the wrong shape.
NASA and the European Space Agency (ESA) im-
mediately appointed a panel of astronomers, engi-
neers, and technicians to design a solution to the
problem. It took three years to produce a compli-
cated instrument, the Corrective Optics Space Tele-
scope Axial Replacement (COSTAR), and to have
astronauts install it in the HST’s interior. This oc-
curred in December, 1993, and the COSTAR worked
perfectly. The telescope produced images that were
basically as good as the original specifications.
Throughout the rest of the decade, the HST
made an astounding number of dramatic discover-
ies in virtually all fields of astronomy, ranging from434 Hubble Space Telescope The Nineties in America
A long view of the Hubble Space Telescope over Shark Bay, Australia. The
photo was taken from the space shuttleDiscovery.(NASA)