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A Stellar Imagemaker

Smithsonian and NASA's Chandra x-ray observatory sheds new light on the mysteries of the universe

August 19, 1999, Smithsonian’s Chandra X-ray Observatory Center in Cambridge, Massachusetts: a large room full of computers, monitoring equipment and anxious scientists. They were anxious because after many years of hard work, after two scrubbed launches and a near-abort, after seven booster rocket firings joggling the delicate machinery this way and that, their x-ray telescope was finally in orbit and about to open up for business.

“It was quite a scene,” recalled Leon van Speybroeck, one of the men who put it there. “The launch was on the Columbia space shuttle, carrying its biggest payload ever. Now, a month later, we were ready. So, we sent the computer commands, and waited. Astonishingly, 80,000 miles away, our pyrotechnic device exploded — it was like an M-80 firecracker. It swung open the 120-pound door on the spacecraft — just as planned.”

Cosmic x rays shone on the delicate mirrors of the precious telescope for the first time. The scientists back on Earth monitoring the event pulled off their headphones and rushed into the imaging room. For 45 long minutes everyone waited to see if they would get an image from the telescope or if the whole project would end up with “a bucket of broken glass,” as van Speybroeck put it.

Then, in the classic grave space-age monotone, a scientist announced: “We’re getting photons.”

First just a dot on the screen — photons being tiny units of light — then another, and another. Gradually a picture of a distant galaxy emerged.

More than 23 years in the making, mainly at the Smithsonian Astrophysical Observatory in Cambridge, which is part of the Harvard-Smithsonian Center for Astrophysics, and named for the late Nobel laureate Subrahmanyan Chandrasekhar, the Chandra telescope’s first images astounded the sophisticated spacewatchers.

The first official Chandra image shows the aftermath of a vast stellar explosion in Cassiopeia A, a supernova remnant 10,000 light-years away, with such clarity that a neutron star or black hole appears to be visible at its center.

“We see the collision of the debris from the exploded star with the matter around it,” said center director Harvey Tananbaum, describing the image. “We see shock waves rushing into interstellar space at millions of miles per hour, and for the first time a bright point near the center of the remnant that could possibly be a collapsed star.”

Another early x-ray image attesting to Chandra’s power and potential came all the way from a quasar six billion light-years away. Dubbed PKS 0637-752 by scientists, it radiates with the power of ten trillion suns. Complementing the Hubble Space Telescope, another large space observatory now orbiting Earth, Chandra should allow scientists to analyze some of the great mysteries of the universe. For more than a year now, the x-ray telescope has been transmitting a stream of images that have thrilled and challenged the scientific community.

For instance, Chandra’s observation of Sagittarius A*, a source of radio waves at the core of the Milky Way that scientists surmise is powered by a black hole 2.6 million times the mass of our sun, created a stir last winter. With the remarkable detection of an x-ray source from Sag A*, astronomers are closer than ever to clearing up the mystery of the supermassive black hole.

Chandra’s high-resolution images surely will give us new insights into black holes, which are space entities so dense that nothing that ventures close can escape their gravity, not even light. Chandra’s ability to examine particles up to the last millisecond before they are sucked out of sight will enable astronomers to study the theory of gravity under the most extreme conditions.

Smithsonian’s Chandra X-ray Center operates the space-based observatory under contract with NASA’s Marshall Space Flight Center in Alabama. On my visit to the Smithsonian center in Cambridge, I needed a lot of help. (Got a D in physics in prep school.) Wallace Tucker, astrophysicist and Chandra spokesman, was able to talk me in as much as anyone could.

X rays are at the short end of the light-wave spectrum. Optical telescopes can deal with stars radiating tens of thousands of degrees of heat, but x-ray telescopes (Smithsonian, July 1998) can observe gaseous objects up to several hundred million degrees.

A wave with such fantastically high energy is extremely difficult to focus or direct. If you put a conventional telescope in front of it, the wave is simply absorbed.

But, I interrupted, what about my x-rays at the hospital? Ah, replied Tucker, those pictures are just shadows. The bones being denser than the flesh, they make a deeper shadow as the x rays pass through your whole body.

“Besides,” he added, “we’re talking about much longer distances and finer images. Like looking at a dime from four miles away.”

The solution to directing the waves was to design a mirror that would reflect the rays at an extremely shallow angle so that they would bounce off, like skipping stones on water, instead of being absorbed. Then they could be directed onto an electronic detector, stored and later transmitted to the Chandra center.

Whereas optical telescope mirrors are dishes that focus the faint beams from space, Chandra’s mirrors are barrel-shaped. Four pairs of them are nested like Russian dolls to provide a larger area for the x rays to hit.

It was not a new idea. Hans Wolter did the basic design work, a geometrical invention on paper, in Germany in 1952. In the 1970s Riccardo Giacconi successfully adapted the principle to x-ray astronomy. Giacconi moved on to other conquests in the 1980s, notably to direct work on the Hubble Space Telescope, but his team carried on here. Of course a large number of brilliant people created Chandra, but I don’t think it is too much to say that the person responsible for the unique mirrors, the world’s great expert on their design, is Leon van Speybroeck, officially the Chandra Telescope Scientist, an MIT graduate from Wichita, Kansas, who has been with the Smithsonian since the early 1970s.

“Giacconi had the idea in the 1960s,” noted Tucker, “but NASA was skeptical. The Chandra mirrors are a high point of Leon’s career.” We are talking about a mirror so smooth that if it were the state of Colorado, Pikes Peak would be less than an inch high. We are talking about smoothness to within a few atoms, smoothness that is virtually mathematical in its perfection. The mirrors are two to four feet in diameter, nearly three feet long and weigh more than a ton.

“They had to make special structures just to build these mirrors,” Tucker told me. “They searched the world for grinding powders. Finally a guy in Tennessee developed a cerium oxide compound that was mixed with a tree sap extract from Switzerland.”

And delicate: touch the surface and grease from your fingertips could ruin it. Imagine not only building these mirrors but getting them fixed precisely in line, and so firmly that the shock of being hurled into space wouldn’t knock them a hair off kilter.

I studied a color photograph of Cassiopeia A, and it was hard to relate the picture to the first dots that appeared on the plate. Building the portrait is a laborious process, the ultimate pointillist art.

“We detect the photons one at a time and keep track of when they were found, where and how much energy was in them,” Tucker told me.

And what about the camera that records these amazing sights? There are two of them, a high resolution one, designed by Smithsonian scientists, with 69 million glass tubes in a grid for determining the exact position and arrival time of each x ray, and an imaging spectrometer, a special digital-like camera whose ten x-ray-sensitive chips contain a million pixels each to record the position and energy of the rays. Two special screening devices disperse the rays into a high-energy rainbow, like a spectroscope with thousands of distinct colors, to allow study of the chemistry of their celestial source.

“NASA’s Deep Space Network stations in Australia, Spain and California send us the data,” Tucker continued. “And we send back information saying where we want Chandra to look next, every 72 hours or so. The targets are selected by a peer review process.”

The flying observatory travels almost one-third of the way to the moon in an elliptical orbit ranging from 6,000 to 86,400 miles up as it orbits the Earth every 64 hours. On average its orbit is 200 times higher than the Hubble telescope’s.

There have been other x-ray telescopes, but Chandra can see objects that are 20 times fainter than anything they could detect.

Chandra’s resolving power is 0.5 arc seconds, which means that it could read the letters of a stop sign from 12 miles away. Or a newspaper headline one centimeter high at a distance of half a mile. On the other hand, it can observe x rays in gas clouds so wide that it takes light five million years to cross them. And it can study quasars whose light has taken ten billion years to get to us, so that we are seeing that many years into the past. I love stats.

As Edward Weiler, a top NASA administrator, put it: “History teaches us that whenever you develop a telescope ten times better than what came before, you will revolutionize astronomy. Chandra is poised to do just that.”


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