X-Raying the Universe
What would the universe look like if, instead of seeing colors, we saw X-rays?
Scientists hope to see space a little differently after the Monday launching of the third of NASA’s “Great Observatories.”
The Chandra X-Ray Observatory will view things invisible to the Hubble Telescope and the Compton Gamma Ray Observatory, the first two orbiting observatories. Scientists have submitted more than 800 proposals for Chandra, with targets ranging from black holes to newborn stars.
Since almost every object in the universe emits X-rays and some objects emit nothing else, Chandra will see objects that other telescopes can’t, according to scientists.
“Chandra will study the most extreme environments in the universe. It will allow us to test the laws of physics in ways we can’t on Earth,” said Ed Weiler, associate administrator of NASA’s Office of Space Science.
Though there have been a few other orbiting X-ray telescopes, Chandra will have resolution as much as 100 times better than any of its predecessors. It will be able to discern fine details in pictures that were previously blurred. (There are no X-ray telescopes on the ground; the Earth’s atmosphere absorbs X-rays, leaving nothing for the telescopes to detect.)
The improved resolution will be made possible in part by Chandra’s unusually high elliptical orbit, which places it well beyond the shielding effect of the Earth’s atmosphere. At the farthest point in its orbit, it will be 86,800 miles from Earth--more than a third of the way to the moon.
Chandra must be problem-free before launch because, unlike the Hubble--already scheduled for its third overhaul since its launch in 1990--Chandra will be orbiting Earth too far away to be reached for repairs.
“It means we must have faith that we’ve designed, built and tested the telescope well enough to ensure that it performs as intended throughout its five-year mission,” wrote Martin Elvis, head of the science data systems group at the Chandra Observatory Center, in Sky and Telescope magazine.
Everyone involved with the mission seems to have that faith. Harvey Tananbaum, director of the Chandra Center, said project scientists had “a learned debate on how many tests” were necessary to prove that all of the parts worked. Included were extensive simulations by the crew of the space shuttle Columbia, which will deliver Chandra to an intermediate point in space; from there, the craft will boost itself into its orbit.
The elongated orbit will allow Chandra to spend most of its time collecting X-ray pictures. Telescopes with smaller orbits, such as the Hubble, have Earth in their way as much as half the time.
“In five years, we can do about seven years of science, compared to a lower orbit,” said Chandra program director Fred Wojtalik. Among the project proposals approved so far are further pinpointing the age of the universe, measuring the temperature of the Crab nebula, observing the ultra-hot gases in far-off clusters of galaxies and studying the edges of black holes.
“For many X-ray astronomers, Chandra represents the culmination of our careers,” Elvis wrote.
Kimberly Weaver, an astrophysicist at the Goddard Space Flight Center in Greenbelt, Md., is among those who want to study black holes. “We can see that [black holes] do a lot of violent things. What we haven’t been able to do is probe down into the center,” Weaver said. “I’ll be able to see for the first time the objects I’ve been studying.”
Telescopes like Chandra must work differently than optical telescopes because X-rays have higher energy than visible light. Instead of reflecting off mirrors placed in their path, X-rays would just be absorbed. Chandra’s mirrors are placed along the sides of the telescope, and they gradually focus the beam into a point. It’s like skipping a stone across the surface of a stream, said project scientist Martin Weisskopf of the Marshall Space Flight Center in Huntsville, Ala.
Chandra’s ability to perceive X-rays will complement what is known about space from Compton and Hubble. The Compton Observatory tracks very high-energy light known as gamma rays, and Hubble can see visible and some ultraviolet light. X-rays have less energy than gamma rays, but more than visible light.
NASA’s planned fourth observatory, the Space Infrared Telescope Facility, is scheduled to be launched in 2001 and will record light of lower energy.
According to mission specialist Steven Hawley, “The goal of the Great Observatories was to have major, world-class facilities in four wavelength regimes. Chandra is that major facility for X-rays. It is a bright new star in the constellation of NASA observatories.”
*
Chandra--previously known as AXAF for Advanced X-Ray Astrophysics Facility--was named in honor of Nobel laureate Subrahmanyan Chandrasekhar, called Chandra by his friends. Jatila Van der Veen, the high school teacher who suggested the name, felt it was appropriate to name the observatory after the astrophysicist, who died in 1995: Chandra means “moon” in Sanskrit, giving the name a double connection to space.
Though deploying Chandra is the most talked-about part of the shuttle mission, the astronauts will be involved in a number of other, smaller experiments. Among the studies are an investigation of plant behavior in space--whether a plant knows to grow “up” when there is no up and whether growing in space is difficult--and work on high-definition TV, which may eventually give ground-bound folks a clear view of activities on the space shuttle.
First proposed in 1976, Chandra has encountered many obstacles along the way to becoming reality: delays because of software problems, faulty circuit boards, sweeping budget cuts, potential defects in the rockets needed to lift it into orbit and a defective roller bearing.
Scientists and astronauts expressed some frustration with the delays, which added about $58 million to Chandra’s final price tag, now about $2.78 billion, including shuttle costs. “We’ve been within six months of the launch date for a very long time,” said mission commander Eileen Collins, the first woman to command a NASA shuttle mission.
(BEGIN TEXT OF INFOBOX / INFOGRAPHIC)
NASA’s Newest Observatory
The Chandra X-Ray Observatory will be launched from the space shuttle Columbia at 9:36 p.m. Monday. The mission was planned to coincide with the 30th anniversary of Apollo’s lunar landing.
With 100 times the resolving power of previous X-ray scopes, Chandra will be able to view the fine details of such high-energy phenomena as exploding stars and clusters of hot gas. Chandra’s improved resolution is made possible in part by its unusually high elliptical orbit, which will take it a third of the way to the moon.
*
THE OBSERVATORY
Chandra’s communication system relays data to astronomers on Earth and receives their directions for locating new targets. When Chandra turns in space, it will move more slowly than the minute hand on a clock.
A thermal system provides the constant temperature necessary for the telescope to work as an observatory. The observatory measures the number, position and energy of incoming X-rays to create an X-ray image and to study the temperature and other properties of the source. Chandra was designed and built by TRW in Redondo Beach.
*
INSIDE CHANDRA
The telescope system consists of four pairs of barrel-shaped mirrors and their support structures. The mirrors are placed along the sides of the telescope, gradually focusing X-ray beams to a point. Mirror smoothness is very important; according to NASA officials, if Colorado were as smooth as Chandra’s mirrors, the highest mountain would be less than an inch tall. Chandra’s ability to distinguish objects is equal to reading a newspaper from half a mile away.
*
WHAT ARE X-RAYS?
X-rays are high-energy light, more energetic than visible light. Doctors and dentists use X-rays to look at the shadow cast by a tooth or a bone blocking a beam of X-ray light produced by a machine. Chandra will record the natural X-rays that are emitted by almost every object in the universe, filling in the gap left by the previous “Great Observatories.” The first of these, the Hubble Telescope, measures visible and some ultraviolet light; the Compton Gamma Ray Observatory views high-energy gamma rays. A proposed fourth observatory, the Space Infrared Telescope Facility, to be launched in 2001, will observe low-energy light.
*
Sources: NASA; Sky & Telescope; Chandra X-ray Center, Smithsonian Astrophysical Observatory
