On October 7, an MIT-built satellite roughly the size and shape of a dishwasher will be launched into near-Earth orbit to detect the largest known explosions in the universe. These occurrences, called gamma-ray bursts (GRBs), signal the extragalactic release of as much power as a billion trillion suns, but no one is sure what causes them or exactly where they originate.
The High-Energy Transient Explorer (HETE-2) -- the first satellite dedicated to the study of GRBs -- will help scientists understand these perplexing explosions. HETE-2 is the result of an international collaboration of scientists and engineers at MIT and other institutions in the US, France and Japan. HETE-2 will serve the world's astronomers as the premiere burst spotter until the end of its extended mission in 2004.
HETE-2 is the replacement mission for the original HETE satellite, lost during its launch in 1996 because of a rocket malfunction.
A BEACON TO THE PAST
Gamma-ray bursts are one of the hottest topics in astronomy. Like beacons from the early universe, these bursts are thought to originate billions of light years away, at the limit of the Hubble Space Telescope's vision. Because the speed of light is finite, looking far away is like looking back in time.
Gamma-ray bursts may be the product of a hypernova, a giant star explosion up to 1,000 times more powerful than a supernova, or they may be caused by an orbiting pair of neutron stars coalescing, or even a neutron star being sucked into a black hole.
"Gamma ray bursts are colossal explosions. They are the most energetic events since the Big Bang, yet one occurs about once a day in the sky," said George R. Ricker, senior research scientist at the Center for Space Research and principal investigator for the 20-person international team. "The magic of HETE-2 is that it not only detects a large sample of these bursts, but it also will relay the accurate location of each burst in real time to ground-based optical and radio observatories."
The Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory, deorbited byNASA in June, detected nearly 3,000 GRBs over its 10-year lifetime, yet no two were ever seen in exactly the same place in the sky. Astronomers have been able to pinpoint the exact distances of only a dozen GRBs, mainly with the help of the Italian BeppoSAX mission.
GRBs can last from 10 milliseconds to more than 15 minutes. They are followed by afterglows that are visible at X-ray and optical wavelengths for several days. HETE-2 was designed to facilitate observations of these afterglows.
The HETE-2 satellite is being prepared for launch from Kwajalein Atoll in the Republic of the Marshall Islands. It will be deployed to an orbit around 600 kilometers above the Earth by an expendable Pegasus rocket launched with the aid of an L-1011 aircraft. The chosen orbit allows the satellite to race above the equator, circling the Earth 15 times per day along the exact same path.
DATA IN SECONDS
Within seconds of detecting a burst, HETE-2 will calculate the precise coordinates of the event and transmit its calculations to the nearest of 12 receiving stations girdling the planet, immediately allowing ground-based observers to gather detailed observations of the initial phases of GRBs. The satellite uses a low-rate VHF transmitter to continuously broadcast the burst information; on the ground, an array of listen-only burst alert stations (BAS) receive the data and transmit them to the MIT Control Center. Once received at MIT, burst information is immediately relayed to the GRB Coordinate Distribution Network (GCN) at Goddard Space Flight Center in Greenbelt, MD, for wide distribution through the Internet. Ground-based optical and radio observatories, as well as space telescopes such as Chandra and Hubble, can then follow up with a closer look.
News of a burst will reach the astronomy community in approximately 10-20 seconds, as opposed to hours or days in the past. "Routinely, HETE-2 will provide astronomers with a good chance of seeing a burst while it is still going on," Ricker said. "Using HETE-2's localizations, observatories worldwide will be able to rapidly acquire the burst and study its evolution at all wavelengths." In addition, HETE-2 will determine the location and environment of short bursts, a class of bursts about which little is known.
In addition to Dr. Ricker, the MIT HETE-2 team includes the following scientists in the Center for Space Research: David Breslau, Geoffrey Crew, Janice Crisafulli, Robert Dill, John Doty, Mike Doucette, Richard Foster, James Francis, Eugene Galton, Steve Kissel, Alan Levine, Francois Martel, Frederick Miller, Gregory Prigozhin, Roland K. Vanderspek, Joel Villasenor, Michael Vezie and Peter Young; as well as graduate students Tye Brady, Nathaniel Butler and Glen Monnelly.
In the US, the HETE mission is supported by NASA.
A version of this article appeared in MIT Tech Talk on October 4, 2000.