There is a lot to see in the night sky, but there is even more not to see. Our eyes have evolved to detect radiation within a narrow range of wavelengths that we call visible light. In essence, we peer out through a slit—and our view is what we used to think was the universe. Now we know better. The real drama happens in places where matter reaches temperatures of millions of degrees and shines mostly in X-rays our eyes cannot detect. The X-ray sky crackles with previously unimagined action: exploding stars, gas swirling into monster black holes, and pile-driver smashups of whole clusters of galaxies. All of this commotion is finally snapping into focus because of an extraordinary satellite, the Chandra X-ray Observatory, launched by NASA in 1999. What you see on these pages is the universe seen through the eyes of Chandra.
This perspective wouldn’t exist if not for a gung ho young physicist named Riccardo Giacconi, who in 1962 talked the Air Force into letting him launch a Geiger counter into space. NASA, then a brash young agency, had refused to do so. But Giacconi and his team at American Science and Engineering in Cambridge, Massachusetts, already had a contract with the Air Force to monitor atmospheric nuclear tests, and he knew the Air Force was hoping to get in on President Kennedy’s lunar program. He argued that his Geiger counter might detect X-rays from the moon and thus help determine its composition. “It was a good excuse,” he says now.
On June 18, 1962, Giacconi’s Geiger counter lifted off on an Aerobee rocket from the White Sands testing range in New Mexico. During the 350 seconds it spent above Earth’s X-ray-blocking atmosphere, it registered no emission from the moon but picked up an intense, unknown source in the constellation Scorpius. This was the first such source discovered, hence named Scorpius X-1. “Sco X-1 was a boomer,” Giacconi recalls. “We had no trouble detecting it.”
That result was a happy surprise. Researchers at the Naval Research Laboratory had previously detected X-rays from the sun’s hot outer atmosphere, but those rays were only a millionth as intense as the sun’s light. Detecting X-rays from another star light-years away seemed like a long shot. It turned out Sco X-1 was no ordinary star. It was thousands of times as luminous as the sun, and almost all that radiation was X-rays. “The great thing nature did for us is it invented a brand-new class of stars that nobody expected,” Giacconi says. When the Air Force realized he was looking at distant stars rather than at the moon, they ended his program. By then, however, X-ray astronomy had caught on.
The first scans of the X-ray sky were very coarse. Giacconi’s Geiger counter—essentially a box of electrified gas—was fine for recording the passage of X-rays but could not create a picture of the source. Even before Sco X-1, however, he had sketched out a design for a true X-ray-imaging telescope. In 1963 he and his colleague Herbert Gursky proposed to NASA a five-year plan that would culminate with a large, orbiting X-ray telescope. Thirty-six years and innumerable bureaucratic snafus later, NASA launched Chandra.
In that time, numerous simpler X-ray detectors went up. Giacconi, having worked on one of these experiments, dropped out of the project in 1981, before it had really begun. He passed the leadership of his team at the Harvard-Smithsonian Center for Astrophysics to Harvey Tananbaum, who kept the battles going nearly another two decades. The silver lining is that Chandra is a much better telescope than it would have been if it had been launched with 1960s technology.
Chandra’s fundamental design still follows the principles Giacconi sketched in the 1960s. He recognized that you cannot focus X-rays with a lens or reflect them straight off a mirror because they are so energetic they will burrow right in; that’s why such rays are so good for illuminating the insides of the human body. Giacconi’s solution was to direct the X-rays along the sides of a conical mirror, which would cause them to skip along, like pebbles off the surface of a pond. To help pull in faint objects, Chandra contains a series of nested mirrors that each funnel X-rays to a sharp focus at its narrow end, where a camera sits. And to achieve the desired image clarity—equivalent to reading a stop sign 12 miles away—the mirrors are polished to near perfection, with no bumps larger than six atoms high. In a nice twist of redemption, the job was done successfully by Hughes Danbury Optical Systems, the company that had previously botched the mirror for the Hubble Space Telescope.
What Giacconi could not have foreseen in his original plan, drawn up before the digital revolution, was Chandra’s workhorse camera, the ACIS. It detects X-rays using the same kind of silicon chip, called a charge-coupled device, that is in every digital camera. An array of 10 chips in ACIS gives it the unique ability to measure both the position and the energy of the incoming rays with high accuracy. Since each element radiates X-rays at a characteristic set of energies, or wavelengths, ACIS can reveal the composition as well as the appearance of objects. “You can make a picture just of silicon. That’s the power of Chandra—not just that the mirrors are so good,” says astrophysicist Una Hwang of NASA’s Goddard Space Flight Center.