GOLD WAR, HOT NEWS
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:
Each of the Parties to this Treaty undertakes to prohibit, to prevent, and not to carry out any nuclear weapon test explosion, or any other nuclear explosion, at any place under its jurisdiction or control [ . . . ] in the atmosphere; beyond its limits, including outer space; or under water, including territorial waters or high seas.
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?
The gamma-ray detectors of the time had poor eyesight: early missions simply couldn’t see the direction from which the gamma rays came.
Optical light—the kind we see—has a relatively low energy. Carefully aligned lenses or mirrors inside a telescope bend or reflect the light, bringing it to a focus. This can be used to very accurately measure the position of a source of optical light. Gamma rays, however, are more like bullets zipping around. Changing their paths is much harder, and even today focusing them is beyond our technology.
What this means is that while a gamma ray can be detected and counted, getting a direction from whence it came is very difficult. Only the crudest of directions could be obtained by the Vela satellites (it wasn’t much better than “somewhere over there”30). But the direction is critical to understanding the object. If the gamma-ray source’s position is known, other telescopes can be trained at that spot on the sky to see what’s what. Then any visible source seen there can be compared to known sources like galaxies or stars listed in existing catalogs. But some degree of precision is required: if the position of the burst can only be nailed down to, say, an area on the sky the same size as the full Moon, there are still thousands or even millions of objects detectable by a big optical telescope.
Eventually, technology started catching up to the problem. In 1991, NASA launched the Compton Gamma Ray Observatory satellite, which had GRB detectors on it. Compton’s ability to get the positions of GRBs was still not great—it could only nail them down to an area on the sky the size of a quarter held at arm’s length—but it was a definite improvement. Over the course of the mission, it detected over 2,700 GRBs. And while the directions were not precise, just getting that sheer number of observations was a huge advance; after enough bursts were detected, patterns began to emerge.
For one thing, that large collection of bursts allowed scientists to determine that there appeared to be two kinds of GRBs: short ones, lasting in general less than two seconds; and long ones, which lasted more than two seconds. Some bursts were even found to emit gamma rays for several minutes. As more GRBs were observed, it was found that the shorter bursts tended to give off higher-energy (“harder”) gamma rays, and the longer bursts had lower-energy (“softer”) gamma rays. While it wasn’t understood why this might be, it was an important clue to their origins.
But the big scientific result from Compton’s observations was perhaps far more important in solving the riddle: it saw GRBs spread out evenly across the entire sky. At first glance this may not seem to help, but in fact it eliminates many possibilities for their origins.
Imagine standing in a field, and insects are buzzing around. If you’re in the center of the field, then you’d expect, on average, to see the same number of insects no matter what direction you look. But if you’re close to the eastern edge of the field, you will see far more insects to the west (looking out across the length of the field) than to the east (looking out over the edge). The number of bugs you see in a given direction tells you something about your placement in the bug swarm (assuming the swarm is relatively random and symmetric).
So the information from Compton—that GRBs were spread randomly across the sky—instantly tells us an important fact: we are in the center of the GRB distribution in space.
If GRBs were inside our solar system, we’d expect to see more in one direction than another, because we are not in the center of the solar system—the Sun is. We’re offset from the center by nearly a hundred million miles, and you’d expect to see that reflected in the distribution of GRBs. But there is no offset, so they are not coming from objects in our solar system.
But this also means that GRBs are not coming from sources spread around inside our Milky Way Galaxy. Since the Earth is halfway to the edge of the galaxy, GRBs in that case would be seen preferentially toward the center of the galaxy as viewed from Earth. They aren’t, so they are not galactic in origin either.
That doesn’t leave too many options. They could come from stars very near the Sun, like only a few light-years away, but not from farther stars, say, more than a few hundred light-years away, because then we’d start seeing more toward the galactic center. The other choice is that GRBs are very, very far away, from well outside the galaxy, millions of light-years distant.
If you are in the middle of a field of fireflies (left), you see equal numbers of bugs in every direction you look. But if you are off-center in the cloud of bugs (right), you see more in one direction than in another. This information can be used to determine the shape of the cloud of bugs—or, more practically (to an astronomer), the distribution of GRBs in the Universe.
AURORE SIMONNET AND THE SONOMA STATE UNIVERSITY EDUCATION AND PUBLIC OUTREACH GROUP
Neither of these options is terribly palatable either. Stars shouldn’t be able to make such high-energy bursts, and if they were really far away, the intrinsic energy emitted would be ridiculously high.
Still, astronomers staked their claims on either side of this issue, publishing papers furiously and arguing—sometimes also furiously—over it. They even staged a famous debate about it between two accomplished scientists who took different sides of the debate: one defending the idea that they were from nearby stars, the other saying they were coming from the distant reaches of the Universe. But even by the time the debate was held, preparations were under way to get the real answers.
THE VIEW FROM AFAR
The joint Dutch-Italian satellite BeppoSAX was launched in 1996. While it was not designed specifically to hunt for GRBs, it had that capability. More important, it had on board a revolution waiting to happen: detectors that could actually get a good direction for incoming X-rays (which, like their higher-energy brethren, gamma rays, are diffi
cult to pin down). It also had a wide field of view, which increased the odds of detecting a randomly placed burst, even if the position was not well known at first.
In February 1997, a long GRB was detected by the BeppoSAX monitor. It also happened to lie within the field of view of the X-ray detectors. Observations were made, and then repeated a few days later. Breakthrough! The results were clear—a bright source of X-rays had faded considerably in the interval. Astronomers knew that must be from the fading afterglow of the burst. And better yet, the X-ray detectors were able to get a reasonably good position for the burst, now called GRB 970228 (for the gamma-ray burst seen in 1997 on February 28).
Within a month, the Hubble Space Telescope was pointed at the location of the GRB and the breakthrough got more momentum: a fading glow in visible light was detected, and it appeared to be right next to a dim, distant galaxy. This was too close to be a coincidence.
Then, finally, the clincher. In May of that same year, the mammoth ten-meter Keck telescope in Hawaii obtained spectra31 of a GRB afterglow. This allowed astronomers to determine an accurate distance to GRB 970228, and they were astonished to see that it was located a numbing nine billion light-years away. That’s more than halfway across the Universe!
Finally, after thirty years, thousands of burst observations, and countless arguments, a major question was answered: bursts were not only far away, they were very far away. After this, no one doubted the vast distances to gamma-ray bursts. They were coming from well outside our Milky Way, and in fact close to the visible edge of the Universe.
Death From the Skies!: These Are the Ways the World Will End... Page 11