1 November 2004--NASA plans to launch a satellite this month that can peer back in time billions of years to a point in the history of the universe about which almost nothing is known. The only evidence from that period comes from the most violent explosions ever observed, called gamma-ray bursts (GRBs). Astronomers do not know for sure what causes them. They could be caused by colliding neutron stars or black holes forming in the death throes of the very first stars in the universe.
With the new satellite, the Swift Gamma Ray Burst Explorer, a collaboration between NASA and scientific institutions in the United Kingdom and Italy, astronomers are hoping to pin down definitively the origins of GRBs. Swift was originally supposed to be launched in 2003, but a number of set backs, the latest being hurricane damage to NASA's Kennedy Space Center, in Florida, have delayed the mission. At press time, Swift's launch is scheduled for 11 November, and scientists are keeping their fingers crossed.
" We are really looking forward to having it up and working," says Neil Gehrels, the principal investigator of the Swift mission and the head of the gamma-ray group at NASA's Goddard Space Flight Center, in Greenbelt, Md., who shepherded the US $250 million project from the beginning. "Once the satellite is launched, we expect to turn on all the instruments within days."
Swift has three instruments to use in its hunt for gamma rays: an X-ray telescope; an optical and ultraviolet one; and the key to the satellite's mission, the Burst Alert Telescope (BAT). The BAT is a sophisticated, wide-angle gamma-ray detector that covers a full one-sixth of the sky. It will give scientists a better chance to catch a glimpse of bursts, which can occur at any point in the sky. Apart from occurring unpredictably in any part of the sky, what makes GRBs tricky to track is that, although they happen on average once per day, they last from only a few milliseconds to a few hundred seconds each. So often, even if someone is lucky enough to locate a GRB as it is happening it takes too long to move a telescope into position, and all that is left is a rapidly fading afterglow.
Swift was designed to overcome this problem. Its BAT instrument surveys a wide swath of the sky. When the BAT detects a GRB, the craft will quickly swivel to center the burst on the BAT and train its other instruments on it--moving into position in 20 to 75 seconds. BAT itself is a sensitive gamma-ray camera comprised of almost 33 000 detectors made of cadmium-zinc-telluride, a compound used in certain types of medical imagers.
On top of making its own observations, within seconds of detecting gamma rays, Swift will relay their coordinates to ground stations, allowing both ground-based and space-based telescopes around the world the opportunity to observe the burst's afterglow--a phenomenon that lasts hours to days. For example, the Peters Automated Infrared Imaging Telescope (PAIRITEL) at Mount Hopkins, Ariz., will conduct follow-up observations of the objects Swift finds. The robotic PAIRITEL can move to any area of the sky that Swift pinpoints in less than 2 minutes. By comparison, turning the Hubble Space Telescope to focus on a newly-sighted GRB can take almost a day.
Observing GRBs requires an infrared telescope like PAIRITEL. "Afterglows cannot be seen at optical wavelengths due to absorption by hydrogen along the line of sight," explained Joshua Bloom of the Harvard-Smithsonian Center for Astrophysics, in Cambridge, Mass., who works with PAIRITEL. "If you want to catch an ancient gamma-ray burst afterglow you must do so looking as red as possible and as quickly as possible."
Owing to the enormous distance between Earth and most gamma-ray sources, the gamma rays Swift catches are likely to be many billions of years old.
Looking back to the period when the universe was younger than 1 billion years is one of the most difficult goals of observational astronomy. Astrophysicists have a fair idea of what happened right after the Big Bang, but they know little about the period between when the universe was a few hundred million years old to when it was about a billion years old. These are the so-called dark ages of astronomy, when the first stars were being formed.
For years, scientists have realized that to look into the dark ages and see the first stars, they needed gamma rays. The intense gamma rays penetrate gas and dust and so can be seen despite their distance.
"The universe is transparent to gamma rays," says Stan Woosley of the University of California, Santa Cruz, who was one of the scientists who first drew the connection between exploding first-generation stars and GRBs.
"We have no data on the first generation of stars," says Gehrels. "With Swift, we can see gamma-ray bursts all the way across the universe, and learn how the first stars formed."