Sixteen years ago this Saturday, NASA deployed the Compton Gamma Ray Observatory (CGRO). Astronauts aboard the space shuttle Atlantis, which had roared off the launch pad 2 days earlier, released the observatory from the shuttle's cargo bay. At the time, the 35,000-pound (15,875 kilograms) colossus was the heaviest unmanned spacecraft NASA had launched. The deployment marked the beginning of a 9-year mission that revolutionized our knowledge of the most violent cosmic events.
CGRO was the second in NASA's Great Observatories program. It followed the Hubble Space Telescope (optimized for visible and ultraviolet observations) and preceded the Chandra X-ray Observatory and the Spitzer Space Telescope (an infrared observatory). In the public's eye, CGRO suffered in comparison to its siblings. To my mind, that's largely because it's so hard to focus high-energy gamma rays into sharp pictures. Without the stunning visuals routinely pumped out by the other Great Observatories, CGRO rarely made it into the popular press.
But Compton made up for the lack of publicity with some extraordinary science. The most interesting to me was nailing down the distribution of gamma-ray bursts. These short, intense bursts of gamma rays were discovered by accident. In the 1960s, the United States had launched a series of gamma-ray satellites. These Vela satellites were not looking outward for astronomical purposes, however. They were pointing earthward, searching for the telltale gamma-ray emission that would signal a thermonuclear explosion in violation of the Nuclear Test Ban Treaty.
Fortunately, the satellites never saw what they were looking for. But starting in 1967, they started seeing gamma-ray bursts coming from space. Military secrecy kept the news of gamma-ray bursts from astronomers until 1973. For nearly 2 decades, no one knew where they originated or what caused them. Most astronomers thought they were events in our galaxy, likely linked to some process involving neutron stars. What that process might be kept theorists employed for a while. At one point in the early 1990s, more than 100 ideas had been published - ranging from thermonuclear explosions on neutron stars, to chunks of matter falling on neutron stars, to starquakes wracking a neutron star's surface.
CGRO solved the puzzle of where gamma-ray bursts occur. The observatory's Burst and Transient Source Experiment (BATSE) showed these events happen all over the sky, and are not concentrated along the plane of our galaxy as neutron stars would be. In other words, the bursts had to be located at cosmological distances and be far more energetic than previously thought. In fact, gamma-ray bursts are the most powerful explosions in the universe. Later observations by spacecraft designed to respond quickly to bursts showed many of them arise in exploding stars called hypernovae. Others may arise when neutron stars merge and totally annihilate each other. In any case, CGRO was the first instrument to pin down a vital aspect of gamma-ray bursts.
Compton is gone now. Parts of it lie on the floor of the Pacific Ocean, but most of it burned up in Earth's atmosphere when NASA brought it down June 4, 2000. The controlled reentry came after one of the gyros used to point the observatory had failed. NASA decided an uncontrolled reentry could endanger people on the ground because the most massive spacecraft pieces would survive reentry. Still, CGRO's 9 productive years nearly doubled its planned 5-year mission.