29 May 2008—Next week, NASA will launch a new US $690 million gamma-ray space telescope, designed to explore the most energetic regions of the universe, where photons some billion times as energetic as visible light originate. The Gamma-ray Large Area Space Telescope (GLAST), a 4.3-metric-ton observatory, is set for launch on 3 June. A joint effort of NASA and the U.S. Department of Energy, the project also has electronic hardware contributions from international partners in France, Italy, Japan, and Sweden. Because Earth’s atmosphere provides a natural shield against gamma rays, GLAST must be put into low Earth orbit to observe them.
GLAST’s main mission during its expected five-year life is the study of gamma-ray bursts from stars collapsing to form black holes and other celestial sources. These brief events, lasting from a few milliseconds to several minutes, radiate the most energetic form of light in the universe—8000 electron volts to more than 300 billion electron volts. GLAST will be 30 times as sensitive at detecting these elusive bursts than its predecessor, the NASA Compton Gamma Ray Observatory, which operated from 1991 to 1999.
To hunt its quarry, GLAST is equipped with two unique instruments: the Large Area Telescope (LAT) and the GLAST Burst Monitor (GBM). The LAT is a 3-ton particle detector and tracker that can observe one-fifth of the sky at any given moment—it can cover the entire sky every three hours when in orbit. The detector is made up of 70 square meters of silicon-strip detectors—similar to those that detect particles in high-energy physics experiments. When a gamma ray strikes one of 16 thin tungsten sheets in the LAT, it is converted to an electron and a positron. A tracker made from silicon strips then measures the path of the electron and the positron, which in turn reveals the direction of the gamma-ray source. (The LAT is designed to reject cosmic rays from outside the solar system and those gamma rays that originate from the sun.)