astronomy from space
Astronaut F. Story Musgrave,
anchored on the end of the Remote Manipulator System arm,
prepares to be elevated to the top of the Hubble Space Telescope to
install protective covers on the magnetometers.
Astronaut Jeffrey A. Hoffman inside the payload bay assisted Musgrave
with final servicing tasks on the telescope, wrapping up five days of
During 1995, astronomer
Wendy Freedman of the Carnegie Observatories in Pasadena, California,
sought to determine the age of the universe. She had clear new photos from
the Hubble Space Telescope, named for Edwin Hubble, the first person to
propose that the universe had not existed for eternity but indeed had an
age. Freedman reached a disturbing conclusion: The universe appeared to be
younger than the stars it contained. This was somewhat like saying that a
baby might come into the world before its mother was born.
Here was science at its
best, raising deep issues at the foundations of study. Freedman's findings
called into question the basic methods of astronomy, casting doubt on what
it is that scientists truly know and how they can claim to know it. Yet
the telescope she used for her research was to stand at the forefront of
work aimed at resolving this paradox, by sharpening the estimated age of
During the last decade of
the 20th century, space-based telescopes in orbit have become some of the
most important instruments used by astronomers. Large Earth-based
telescopes that are in common use have two disadvantages. They have to
look through the atmosphere, which makes stars twinkle but which also
makes it impossible to see fine detail. In addition, the atmosphere
absorbs many important wavelengths of energy from stars, which therefore
do not reach astronomers' equipment. A ground-based telescope thus
resembles a television set that can only pick up two or three channels.
This is an image of a small
portion of the Cygnus Loop supernova remnant,
which marks the edge of a bubble-like, expanding blast wave from a
colossal stellar explosion,
occurring about 15,000 years ago. The HST image shows the structure behind
the shock waves,
allowing astronomers for the first time to directly compare the actual
structure of the shock with theoretical model calculations.
instruments operate outside the atmosphere. They are like television sets
that receive all 83 standard channels. In addition, stars seen from space
do not twinkle and their images do not blur. Orbiting telescopes such as
Hubble can, therefore, see much finer detail.
As early as 1946, a decade
before the first satellites reached orbit, astronomer Lyman Spitzer
declared that observations made in space could revolutionize the field. In
1958, the National Aeronautics and Space Administration (NASA) was
established. Only a year later, the agency initiated work on spacecraft
called the Orbiting Astronomical Observatories (OAOs), which like their
names, would peer into outer space while in orbit. Although the first
successful mission did not fly until 1968, once in space, the two OAO
satellites gave continuous coverage from that year until 1981.
The OAO program emphasized
observations made in the ultraviolet, at wavelengths shorter than those of
visible light. Carrying telescopes with diameters up to 38 inches, the
spacecraft measured the temperatures of hot young stars, which could not
be studied properly using ground-based instruments. They also observed
enormous clouds of interstellar gas and determined their chemical
Next came the High Energy
Astronomy Observatory (HEAO) program, featuring three satellites that
returned data from 1977 until 1981. They conducted observations at x-ray
wavelengths, which are considerably shorter than ultraviolet. X-rays are
produced by highly energetic celestial events, and the HEAO spacecraft
made complete maps of the sky that showed the locations of the x-ray
sources. A bright x-ray source called Cygnus X-1 proved to be emitting
intense energy from locations smaller than the Moon. Certain galaxies
displayed great jets of matter that were emitting radio waves; these jets
now proved to emit x-rays as well. This gave clues to the violent
processes that had formed them.
In 1977, with HEAO in its
heyday, NASA began work on the Hubble telescope. It was built to gather
light using a curved mirror with a diameter of 94 inches (239 centimetres).
Ground-based telescopes up to four times larger soon were under
construction, but the Hubble was to produce particularly sharp images.
Unfortunately, it didn't. It reached orbit in 1990 and quickly showed that
its mirror had not been shaped to its proper curve. Late in 1993, Shuttle
astronauts visited it and installed a correcting mirror, as if giving it
eyeglasses to improve its faulty vision. With this lens in place, it
fulfilled its promise with a flow of dramatic photos.
A vivid color image showed
stars in the making, forming within pillars of gas six trillion miles in
length. Other photos showed galaxies in collision, forming bright new
stars that lived for a few million years before exploding. Other
explosions, much closer to home, took place during 1994 as fragments of a
comet slammed into the planet Jupiter. Each such impact had the force of
all the world's nuclear weapons detonating together, and the Hubble gave
Disks of gas and dust,
where planets can form, proved to be common around young stars. However,
the Hubble showed that planets are rare in globular clusters, which are
compact groups of hundreds of thousands of stars. Quasars, so bright that
they are easily seen across half the universe, were found to reside within
colliding galaxies. Photos of deep space showed galaxies at a time when
the universe was less than one-tenth of its present age.
Chandra captures image of remnant
of star-shattering explosion.
Dr. Freedman's work gave
an age for the universe of around ten billion years. This made it about
two billion years younger than the oldest known stars. The Hubble
contributed to observations that showed that the universe is pervaded by a
type of anti-gravity, which causes it to expand increasingly rapidly. This
result was truly fundamental, and permitted a re-estimate of its age. The
new value came in between 13 and 14 billion years, which indeed was older
than the stars.
The Hubble Space Telescope
was the first of NASA's Great Observatory spacecraft. The second one, the
Compton Gamma Ray Observatory—named for the physicist Arthur Compton—flew
to orbit in 1991. Gamma rays are produced in the explosion of a nuclear
bomb; they are the most energetic form of radiation that exists.
Astronomers were particularly interested in “gamma-ray bursters,” which
produce brief but highly intense blasts of these rays. A 1979 burst
produced more gamma-ray energy in one-tenth of a second than the sun
generates in all forms of energy for a thousand years.
The Compton showed that
the bursts are not concentrated within our galaxy, but lie at far greater
distances. Work with the Hubble showed in 1997 that the bursters reside in
other galaxies that are forming stars at high rates. A plausible
explanation is that the bursters are colliding neutron stars, which pack
the mass of the sun into a highly compressed object only 20 miles across.
The Compton observed “blazars.”
These were energetic jets of matter ejected from the violent cores of
galaxies, which happened to be pointing in the direction of Earth. Blazars
thus amounted to cosmic searchlights that emitted beams of gamma rays.
Studies of our own galaxy disclosed regions where young bright stars have
been forming and exploding at particularly high rates.
The third Great
Observatory went into orbit in mid-1999. This was the Chandra X-ray
Observatory, named after an astronomer of India, Subrahmanyan
Chandrasekhar. Active in 2002, it continues the work of the Compton, which
ended operation in 2000.
Detected by the Hubble Space
Telescope, this stellar swarm is M80 (NGC 6093),
one of the densest of the 147 known globular star clusters in the Milky
Located about 28,000 light-years from Earth, M80 contains hundreds of
thousands of stars,
all held together by their mutual gravitational attraction.
NASA's Great Observatories
have operated as general-purpose installations, conducting broad observing
programs rather than focusing on specific problems in astronomy. However,
NASA has also developed purpose-built spacecraft that have indeed been
aimed at such problems. COBE, the Cosmic Background Explorer, flew to
orbit in 1989 and stands to this day as an important example.
COBE studied the cosmic
background, which is a weak emission of microwaves or long-wavelength
radio waves from all parts of the sky. This emission comes from the very
early universe, at a time when it was only 300,000 years old. This
background is highly uniform, like the blue sky on a clear day. But COBE
observed faint ripples or fluctuations, which were of the highest
These ripples proved to
have characteristics that had been predicted by a theory called
“inflation.” It gives a compelling scenario for the origin of the universe
and for events that took place during the first few trillionths of a
trillionth of a trillionth of a second of its existence. The inflation
concept asserts that the newly forming universe was filled with an
extremely powerful anti-gravity, which then transformed into other types
Backdropped against the Earth's
cloud-covered surface, the Gamma Ray Observatory (GRO)
with its solar array panels deployed is grappled by the remote manipulator
during STS-37 systems checkout. GRO's four complement instruments are
the Energetic Gamma Ray Experiment Telescope (at the bottom); the Imaging
Compton Telescope (centre);
the Oriented Scintillation Spectrometer Experiment (top); and the Burst
and Transient Source Experiment (on four corners).
suspicion is that the Universe is not only queerer than we suppose, but
queerer than we can suppose.” The
British scientist J.B.S. Haldane wrote those words more than half a
century ago, but today he is only partly right. Using observatories both
in space and on the ground, today's astronomers increasingly are setting
forth theories and supporting them with data, as in any other active field