Death from the Skies! (15 page)

It’s okay to be a little scared of supernovae. But it’s also okay to appreciate them. If supernovae didn’t happen, who would be around to understand them?

There could
be
no warning: the wave front was moving at the velocity of light, the ultimate speed limit in the Universe. Nothing can move faster, so the wave of death brought its own announcement.

By the 1960s, the situation between the United States and the Soviet Union was grim. The USSR had put a base in Cuba, less than a hundred miles off the Florida coast. A failed invasion by the United States hadn’t helped. Both superpowers were testing nuclear weapons on, beneath, and above the surface of the Earth. The USSR had exploded the largest thermonuclear bomb in history, equivalent to the detonation of 50 million tons of TNT.
²⁸

Needless to say, people on both sides were nervous. The end of the world by our own hand was a very real possibility.

So, in August of 1963, the United States, the United Kingdom, and the USSR signed the historic Nuclear Test Ban Treaty, limiting testing of such weapons. The very first article of the treaty states:

 

This was a serious restriction. Even after more than a decade, the results of nuclear testing were often surprising. Weapons were tested not just to increase the explosive yield and improve other engineering issues, but also to see what their effects were on the environment. Just the year before the treaty was signed, in 1962, the United States had exploded a device called “Starfish Prime” 250 miles above a remote location in the Pacific Ocean. This height is essentially in space; the Earth’s atmosphere is extremely tenuous that far above the surface. Starfish Prime had the relatively small yield of 1.4 megatons (that is, equivalent to 1.4 million tons of TNT), yet the effects were profound. A vast pulse of gamma rays, extremely high-energy photons of light, was created in the blast. This wave of gamma rays slammed into the Earth’s atmosphere, blasting electrons off their atoms. Moving charged particles create magnetic fields, and the sudden surge of rapidly moving electrons generated a huge electromagnetic pulse of energy, or EMP. This surge blew out streetlights in Hawaii, fused power lines, and overloaded TVs and radios—
all from over 900 miles away.

Testing in space was dangerous, and the long-term effects were still not understood at the time. It became more and more clear that fallout and other effects made atmospheric and near-space nuclear tests extremely unwise. The Test Ban Treaty was hailed as a major step toward world peace.

Of course, the United States trusted the Soviet Union completely, knowing they wouldn’t dream of violating the treaty . . . yeah,
sure.
While the treaty was an excellent start, no one trusted anyone else at all, and each side was very suspicious of the other. In fact, American scientists pointed out that the USSR could blow up bombs on the far side of the Moon and these would be difficult to detect. The Soviets could break the treaty and the United States would never know. What to do?

Nothing feeds engineering progress like fear. The Americans quickly found a way to check up on those scheming Soviets.

While a bomb blown up behind the Moon might be hard to detect visually, its expanding debris cloud would generate quite a bit of radioactive material in space that could be detected. One such radioactive by-product would be gamma rays. Detection technology for gamma rays was relatively new in the 1960s, but it was sufficient to sniff out any of that radiation from translunar explosions. There was one catch: gamma rays from space cannot penetrate the Earth’s atmosphere, so the detectors would have to be launched on a satellite.

Besides the usual problems involved with lofting detectors into space, there was also the issue of accounting for gamma rays emitted by astronomical sources, and not from Soviet nukes. The Sun emits gamma rays, and high-energy particles from solar flares can be mistaken for them as well. A satellite might see a sudden jump in gamma rays, only to have been fooled by a solar eruption or a random particle hit.

The obvious solution was to launch gamma-ray satellites in pairs. A random particle hitting one satellite would not be seen by the other, providing a check against false detections. The data from each satellite could be compared, and if both saw an event, scientists could assume it potentially came from a noncosmic source. Other, existing satellites tracked solar flares, so those could be consulted as well.

The pairs of satellites were quickly constructed and launched. Named Vela—“watch,” in Spanish—the first set was launched just days after the Test Ban Treaty was signed. They were initially crude, only able to positively detect gamma rays after taking an “exposure” of 32 seconds. But things progressed swiftly, and by 1967 the fourth pair had been launched, with a fifth—highly advanced compared to the earlier missions—ready to go.

Two scientists, Roy Olson and Ray Klebesadel, were assigned the laborious task of comparing the observations of one satellite with those of its mate. As they checked, signal after signal turned out to be negative. But in 1969, they found their first hit. The Vela 4 satellites both registered a gamma-ray event from July 2, 1967, shortly after they were initially launched. A quick look at solar flare data revealed no activity that day. Later, they found that the still-flying Vela 3 satellite pair saw the event as well.

There was one problem—whatever caused the gamma-ray event didn’t look like a nuclear blast. The amount of gamma radiation and how it fades with time are very distinctive for a nuclear weapon, and the July 2 event looked completely different. There was a strong, sharp peak of emission lasting less than a second, followed by a longer, weaker pulse lasting for several more seconds.

What could this be? Unfortunately, the Vela 4 satellites couldn’t tell from what direction the radiation came, so there was no way of determining the source. It may have come from behind the Moon, as feared for a nuclear test, or it may have come from some other spot in the sky entirely. Also, the event began and ended so quickly that there was no prayer of using an optical telescope to find it.

However, the Vela 5 and 6 satellites were more powerful—they were more sensitive to gamma rays, and had better time resolution. If the July 2 event repeated, or something else like it occurred, Velas 5 and 6 had a much better shot at figuring out what was going on. Deciding that discretion was the better part of valor, the scientists waited to release the July 2 event data.

It was a good choice. Over the next few years, several more of these mysterious bursts were detected. Plus, there was an added benefit to having more satellites flying: since they were separated by thousands of miles, a crude direction could be determined for each flash. Even at the speed of light, it takes a finite amount of time for a pulse of radiation to get from one satellite to the next. That time delay, together with the known positions and separations of the satellites, could be used to triangulate on the direction of the event.

As the data built up, the scientists were astonished: the gamma-ray flares were originating from random spots in space! None appeared to come from the Sun or the Moon. It became clear that what Olson and Klebesadel were seeing was some totally unknown but extremely powerful astronomical event that no one had any previous clue about. It seemed ridiculous—how could the Universe hide such a thing from the prying eyes of astronomers?—yet there they were.

By 1973, Klebesadel and Olson had accumulated enough data to go public with the news. Together with another scientist named Ian Strong they presented the results at a meeting of astronomers in Ohio, and published a paper titled “Observations of Gamma-Ray Bursts of Cosmic Origin” in the prestigious
Astrophysical Journal
. The paper outlined the sixteen bursts they had seen up to that time (by 1979, when the Vela missions finally ended, over seventy gamma-ray bursts, or GRBs, had been detected by the satellites).

It should be noted that several other astronomers had found weird gamma-ray emissions in their detectors on various satellites as well, but couldn’t be sure what they were. It took the accumulated high-quality data from the Vela satellites to be able to determine that these events were coming from deep space, or at least from outside the Earth-Moon system.

Not that the scientists had a clue what these things
actually
were. GRBs are confusing today as well as in those early days. When Klebe-sadel’s team released their results, the origin of GRBs was a complete mystery. Gamma rays can only be generated by high-energy events like exploding stars, solar flares, or nuclear weapons. But they had established that none were from the Sun, and none were associated with any supernovae. And they clearly weren’t nuclear tests—the Vela satellites did detect several atmospheric weapons tests (from other countries), but the signals for those were unambiguous.
²⁹

What could the bursts be? To make matters more confusing, the distances to sources of GRBs were completely unknown. It was hard to imagine they were really close by (say, inside the solar system), because it didn’t seem like any object or event could generate gamma rays that we wouldn’t already know about. And again, the data didn’t link the bursts to any observed astronomical events farther away.

As mundane explanations fell by the wayside, odder ideas were proposed. Maybe the bursts were from comets hitting the surfaces of super-dense neutron stars, or maybe they were from some other equally exotic event. No one knew. But one thing most astronomers at the time agreed upon was that GRBs were not
very
far away—that is, from outside the galaxy. The farther away a source is, the brighter it must be for us to detect it. For a GRB to be outside our galaxy meant it had to generate literally unbelievable amounts of energy.

But this didn’t help much. There were still too many unknowns.

There were two fundamental problems with determining the origins of GRBs: the lack of
real-time
information, and the lack of
directional
information.

The former was a significant problem. The time it took for the information from the satellites to be beamed to Earth, recorded, and then interpreted could be measured in days, or even weeks (or, in the case of the first one, two years). The GRBs, however, faded away in mere seconds! By the time the burst was confirmed, it was long gone. There was hope that perhaps GRBs emitted light in other wavelengths—X-rays, or optical light—and that this glow would persist long enough to be seen by other telescopes. Assuming GRBs were some sort of explosion, it would make sense that there would be an afterglow, giving astronomers time to find it. But that leads to the second problem: where to look?