Fermi Gamma-ray Space Telescope
What Is FGST?

The newest major space observatory, the Fermi Gamma-ray Space Telescope (FGST), is working to unveil the mysteries of the high-energy universe. Launched into orbit on June 11, FGST studies the most energetic particles of light, observing physical processes far beyond the capabilities of earthbound laboratories.

FGST's main instrument, the Large Area Telescope (LAT), operates more like a particle detector than a conventional telescope. From within its 1.8-meter cube housing, the LAT uses 880,000 silicon strips to detect high-energy gamma rays with unprecedented resolution and sensitivity, filling in gaps in understanding left by previous missions, and pushing new boundaries in particle physics and astrophysics.

The SLAC National Accelerator Laboratory, a U.S. Department of Energy (DOE) laboratory operated by Stanford University, managed the development of the LAT and integrated the instrument from hardware fabricated at laboratories around the world. SLAC also runs the Instrument Science Operations Center (ISOC), which processes the LAT data.

The total U.S. cost of the LAT is $196 million, of which the DOE contributed $45 million for LAT fabrication; the DOE also supported LAT researchers and the ISOC facilities and staff.

As FGST orbits Earth, gamma rays—emanating from jets of plasma streaming from enormous black holes, pulsars, and other astronomical sources—first encounter several layers of tungsten metal in the LAT. The high-energy gamma rays interact with tungsten's massive and highly charged atomic nuclei in a way that creates pairs of charged particles: one electron and one positron. These particles are then detected by silicon-strip sensors positioned just below each tungsten layer. Later, these signals are used to reconstruct the direction and arrival time of the original gamma-ray photon.

After traversing through the LAT's tracking layers, the particles pass into a cesium iodide imaging calorimeter, where they generate tiny amounts of light—flashes with brightness proportional to the particles' energies.

This multi-step process makes the LAT at least 30 times more sensitive than any previous satellite detector and allows it to survey the entire sky several times per day. Physicists and astronomers expect that this unprecedented look at the gamma-ray sky will reveal vital information about subatomic particles at energies far greater than those seen in ground-based particle accelerators, about the accelerating powers of supermassive black holes, and about the birth and evolution of the universe.

FGST also carries a smaller instrument, called the Gamma-ray Burst Monitor (GBM), to detect gamma-ray bursts and other transient phenomena. Together with the LAT, the GBM enables FGST to make gamma-ray burst observations spanning a factor of ten million in energy. FGST is planned for a five-year primary mission operating phase, which may be extended for up to ten years. 

NASA’s FGST mission is an astrophysics and particle physics partnership, developed in collaboration with the U.S. Department of Energy, along with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden, and the United States.

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Rendering of FGST in space
Rendering of the FGST spacecraft — image courtesy of NASA
FGST in the General Dynamics clean room
Bill Atwood (UCSC), Peter Michelson (Stanford), Steve Ritz (NASA-GSFC) with FGST in the General Dynamics clean room — image courtesy of NASA