WASHINGTON -- Astronomers looking skyward know that some of the most violent, calamitous occurrences in the universe are happening all around, but no one can see them. Stars are exploding and collapsing, giant clouds of super-hot gases are expanding, and gravity-intense black holes are devouring everything in their vicinity -- all outside the limits of human sight.
Now, after more than 20 years of planning, the United States is in the final stages of opening a new window to the heavens that scientists say will allow a view of some of the most tumultuous and puzzling events in deep space.
Engineers have just finished assembling the world's most powerful X-ray telescope and are putting it through a series of tortuous ground tests before its scheduled launching aboard a space shuttle at the end of the year. The instrument, a 10,500-pound giant that stretches more than 45 feet in length, is the National Aeronautics and Space Administration's Advanced X-ray Astrophysics Facility, also known as AXAF.
AXAF is designed to observe X-ray emissions produced by some of the most violent processes in the universe, providing details invisible to the human eye and conventional optical telescopes. Cosmic X-rays, generated by the tremendous heat and energy of cataclysmic events, are blocked by Earth's thick atmosphere and thus prevented from reaching the ground. They can only be observed by specialized instruments placed in space.
"AXAF is the Hubble Space Telescope of X-ray astronomy," said Dr. Alan N. Bunner, NASA's lead scientist for the project. "We've gone through a long, dry spell in X-ray telescopes in the United States and, fortunately for our scientists, it's about to end."
The last American X-ray telescope, the High Energy Astronomy Observatory-2 or the "Einstein Observatory," operated 350 miles above Earth from 1978 to 1981. Since then, Bunner said, American scientists have managed to stay involved with X-ray astronomy by cooperating in experiments aboard observatories flown by other countries, notably Japan and Germany.
AXAF, however, will be 100 times more sensitive than the previous X-ray telescopes and will produce photograph-like images 10 times sharper, its designers said. The telescope's optics and detectors will capture images from X-rays produced by events that generate temperatures measured in millions of degrees Fahrenheit. In addition, X-rays focused by the telescope, when passed through thin, gold-covered grates moved into their path, will be separated by wavelength to reveal chemical signatures of objects being observed.
AXAF is the third of four of the "Great Observatories" planned by NASA to revolutionize astronomy by observing cosmic phenomenon across the wide span of the electromagnetic spectrum. Different objects and events in the universe produce wavelengths of energy across this spectrum that reveal their nature. This span goes from gamma rays, X-rays and ultraviolet rays into visible light, and continues through infrared, microwaves and radio waves.
The observatory series started with Hubble, launched in 1990 to study and picture visible and ultraviolet light from objects, followed in 1991 by the Compton Gamma Ray Observatory. AXAF will be the next step. And last week, NASA announced that it had authorized construction of the Space Infrared Telescope, the final piece, scheduled for launching in 2001.
The long history of AXAF began shortly after the Einstein Observatory was launched, as X-ray astronomers began to contemplate a more ambitious project, said Dr. Harvey D. Tananbaum, director of the AXAF Science Center, at the Smithsonian Astrophysical Observatory in Cambridge, Mass. The center is responsible for the project's planning and science operations.
After studying ways to make advanced X-ray detectors and produce super smooth mirrors to capture the radiation, NASA planners designated the proposed telescope a Great Observatory and began plotting an ambitious mission that would be launched and maintained by the space shuttle. The observatory was to weigh more than 20,000 pounds and be designed for a 15-year lifetime, much like the Hubble.
NASA formally proposed AXAF as a "new start" in 1988, when the Hubble was running into costly problems, including producing its high-precision mirrors, Tananbaum said. These were the mirrors that were discovered to be flawed after launching, requiring corrective lens to be installed later in space and salvaging the mission.
"Hubble was having technical and cost problems with Congress and NASA was sensitive to comparisons," Tananbaum said. "'Telescope' was a tough word to deal with, so it was decided to use 'facility' instead of telescope to keep a lower profile."
But skepticism about the project lingered. Congress gave NASA conditional approval to continue AXAF, but only if it completed and tested one of six pairs of mirrors planned for the observatory by 1991. Hughes Danbury Optical Systems Inc., formerly Perkin-Elmer, maker of the Hubble mirrors, also made the AXAF mirrors, and they passed tests proving they were made correctly before the deadline was up.
The AXAF mirrors are radically different from the flat, round ones on the Hubble and other optical telescopes. Because of their very short wavelengths, X-rays will be absorbed and not reflected by mirrors if they hit straight on. X-rays will reflect if they hit a slightly angled surface, bouncing off like a stone skipping across a pond.
So the mirrors of X-ray telescopes are finely polished tubes of glass, made by grinding out solid cylinders of the material. After being polished, the mirrors were coated with a vapor-thin layer of the highly reflective rare metal, iridium. The mirrors, which are slightly larger in diameter at one end than the other, are fitted end-to-end in pairs. This allows X-rays to come through the end of the first tube, ricochet off a side into the second, which has a slightly greater angle, and skip again to a focal point where detectors are located.
To increase the collection area of the telescope, for gathering more X-rays and boosting the sensitivity, successively smaller sets of mirrors are nestled inside one another.
But even after the first mirrors were completed, AXAF still faced political problems. To pay for the growing space station program, Congress cut the AXAF budget, and proponents feared delays in launching the craft before the end of the decade. Scientific groups, including the Space Studies Board of the National Academy of Sciences, feared for the program and reminded NASA that AXAF was considered an extremely high priority for astronomy.
So in 1992, scientists and NASA officials decided to redesign AXAF drastically to cut costs and maintain the schedule.
Scientists decided to drop two sets of mirrors, reducing AXAF to four sets weighing 2,100 pounds, significantly lightening the craft, and to put it into a much higher orbit to make up for its diminished sensitivity with greatly increased observation time. The plan called for having a space shuttle release AXAF in low orbit with a booster rocket attached to take it higher. Later, an internal rocket added to the observatory would stabilize it in an exaggerated elliptical orbit.
The mission changes, which mean that AXAF cannot be periodically maintained by the shuttle as originally planned, cut the development cost of the observatory to about $1.4 billion, a savings of $600 million. And without maintenance costs and the expense of developing new instruments for later placement into the observatory, experts said the changes probably saved $1 billion more in operating costs. Tananbaum estimated that the total cost of the program over its lifetime would be about $2 billion.
In a low Earth orbit of 250 miles, as first planned, AXAF would have spent only 60 percent of its time surveying the sky because Earth would block its view at times. The higher orbit, from 6,200 miles at its lowest point out to 86,800 miles, allows sustained observation periods of more than 50 hours.
Ralph Schilling, an engineer with TRW Inc.'s Space and Electronics Group, of Redondo Beach, Calif., the main AXAF contractor, said the weight of the craft was reduced by 60 percent by eliminating two pairs of mirrors, converting the support structure from solid aluminum to aluminum honeycomb with light-weight graphite-epoxy components, making the observatory more compact and dropping some instruments.
"The big change was the choice of orbit," says Schilling, who has worked on AXAF for 18 years, "This meant it could not be serviced by the shuttle, so we had to pay more attention to redundancy in components and making everything more robust since it cannot be repaired on orbit."
AXAF was designed to last at least five years, down from 15 when first proposed, but Schilling said the craft is to carry enough fuel and duplicate equipment to last for 10 years.
"The only problem with this approach is that everything has to work the first time because you can't go up and fix it later," said Tananbaum, "That's why it's going through such extensive testing now."
This test program turns out to be the reason AXAF's launching was postponed until December. TRW will deliver the spacecraft to the Kennedy Space Center in Florida in September, a three-month delay, because of problems developing computer software for the rigorous testing, including checking communications systems and running the assembled craft in a vacuum chamber. "We are now testing around the clock, seven days a week," said Joann MaGuire, a TRW vice president. "We're now on schedule."
When AXAF goes into operation next year, examining energetic stars called pulsars or looking for clouds of invisible dust that represent some of the missing mass of the universe, it will not be alone scanning the X-ray sky. The European Space Agency plans to fly its own large X-ray observatory, a spacecraft called XMM.
The European craft, scheduled to be launched aboard an Ariane 5 rocket in August of 1999, is slightly smaller than AXAF and will fly in a lower elliptical orbit. XMM, which weighs 7,800 pounds, will have three multi-mirror modules at one end of a collecting tube to gather X-rays. Each module has 58 tubular mirrors of different diameters -- the widest being 27 inches -- fitted within one another to collect large quantities of X-rays. AXAF has one large mirror set, the widest 47 inches in diameter.
Bunner of NASA said the two observatories will complement one another more than compete. AXAF is designed to take higher resolution images that can detect faint sources and spot greater detail in structures, he said, while XMM is made to collect huge amounts of X-ray radiation and break it apart to identify more elements and their abundance.
"It's going to be an exciting time for X-ray astronomers," said Dr. Arthur F. Davidsen, professor of physics and astronomy at Johns Hopkins University. "They are going to have two major instruments up at the same time and we should see some very exciting results. It's a way to open up the invisible universe and it could very well excite and fascinate the public as Hubble has."