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 the cosmos.

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.

But space-based 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 composition.

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 close-up views.

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 Way galaxy. 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 importance.

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 of energy.

 
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 system during STS-37 systems checkout. GRO's four complement instruments are visible: 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).