Sep 18, 1997

Astronomythat Almost Didn't Happen





September 19, 1997
more gamma-ray discoveries


Vela 5b spacecraft
The discovery that has ultimately sent so many astrophysicists chasing after the most elusive of cosmic critters came from an effort to make sure that no one cheated on a treaty banning the testing of nuclear weapons in space.


For the many attendees at the Fourth Huntsville Gamma Ray Burst Symposium, Ray Klebasabel described how he discovered gamma ray bursts as he was making sure that his detectors would not be fooled by mother nature.

As nuclear test ban treaties were negotiated in the late 1950s, President Eisenhower's science advisors cautioned that USSR could not be trusted not to try secret nuclear tests in space. They suggested building satellites carrying detectors like those used to analyze nuclear blasts on Earth and in other nuclear studies. The Air Force would be in charge, and the detector task was assigned to the Los Alamos National Laboratory where Klebasabel was working. He joined the team in July 1960.

The project was code-named Vela and although an aura of secrecy has grown about it, Klebasabel said that only the exact capabilities were classified. Stirling Colgate, who also works at Los Alamos, recalled that the Soviets were briefed on the project and consented (reluctantly).


The first spacecraft was launched in October 1963 and carried six gamma ray detectors along with other instruments. It was orbited at an altitude of 120,000 km (74,400 miles). Three more soon followed, and starting with Vela 3, the gamma ray detectors were made of cesium iodide which scintillates - flashes with visible light - when gamma rays pass through it. Improved electronics were added, too.

After Vela 4 was launched, Klebasabel said he felt the need to test the anticoincidence electronics designed to keep the detectors from sounding the alarm when cosmic radiation passed through.

This was a horrendous task compared to the automated data analyses now performed on desktop computers: "We had to plow through old fashioned stack of computer listings which we stacked into books and went through by hand." Instead of graphs that would quickly show what happened, Klebasabel's people had to examine columns of numbers and look for significant changes.

About the Symposium:

More than 200 astrophysicists are meeting in Huntsville this week for the Fourth Biennial Huntsville Gamma Ray Burst Symposium hosted by NASA's Marshall Space Flight Center.

For additional information during the symposium, call Kelly McFalls at the symposium newsroom at 205-518-9424 or 518-2446 through Friday, September 19, or e-mail Dr. John Horack.

"We had not progressed very deeply into the book when we found, much to my surprise, events that could not be explained as nuclear events," he continued.

It was now mid-1969, and in data from July 2, 1967, Klebasabel found a spike in the data, a dip, a second spike, and a long, gradual tail off.

"One thing that was immediately apparent was that this was not a response to a clandestine nuclear test," Klebasabel said. His team checked for possible solar flares and supernovae and found none.

Luck intervened when the detectors aboard the Vela 5 satellite (the Vela 5b is pictured above) were incorrectly calibrated. They were more sensitive than planned, and Vela 5 recorded so much data that Klebasabel had to learn programming himself when no staff was available to help with programming computers to sort through the data.

"Rather rapidly we found a number of events," he said. With the timing between Vela 5 and 6 synchronized to within 1/64th of a second, the Vela team was able to triangulate the locations of the bursts by comparing differences in arrival times at widely separated satellites. What they found was that the bursts (as suspected) came from outside the solar system. Already, by their random scatter across the sky, the data hinted that the sources were out in the universe rather than in the galaxy.

That he was looking at a cosmic phenomenon was not obvious.

"It developed slowly and only became apparent by the time we developed the capability to get the location," Klebasabel said.

By 1973, when he was ready to publish his results in Nature and present them at the American Astronomical Society meeting, he had at least 16 confirmed bursts.

In the audience at the meeting was a young astrophysicist named Jerry Fishman.

"Right away I realized that if the detectors, which were so small (only an inch across), could see dozens of bursts per year, then by scaling up the size we could see a dozen a day," Fishman said. Balloon flights of larger detectors (equipped with beer cans to shield sensitive parts of the detectors) showed that something more was out there to be seen.

Encouraged by Dr. Thomas Cline of Goddard Space Flight Center, Fishman proposed building an all-sky monitor that would serve the other instruments on the Gamma Ray Observatory that were being planned. Renamed the Compton Gamma Ray Observatory, it was launched in 1991 and joined a wide array of Earth satellites and deep space probes that carried much smaller detectors. Over the last six years, this armada has opened the window on a new field of astronomy.

Fishman's all-sky monitor became the Burst and Transient Source Detector. Although its data are far more detailed than what Vela returned, the nearly 2,000 bursts seen by BATSE follow the same pattern observed in 1969.

The Vela satellites have long since passed into history (their monitoring role is handled by sensors aboard other military satellites), but they have left a legacy far greater than the night watchman role they were designed to fill.

more on the history of gamma-ray bursts

more gamma-ray discoveries


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Author: Dave Dooling
Curator: Bryan Walls
NASA Official: John M. Horack