GLAST (Gamma-ray Large Area Space Telescope) is an American (NASA) astrophysics craft that was launched by a Delta 2 rocket from Cape Canaveral at 16:05 UT on 2008 June 11. It carries two major instruments: LAT to monitor gamma-rays from many distributed sources, and GBM for the capture of large, random bursts of gamma-rays. The initial orbital parameters were period 95.7 min, apogee 562 km, perigee 542 km, and inclination 25.6 degrees.
The scientific objectives are to: (1) explore the most extreme environments in the universe; (2) search for signs of new laws of physics and understand the composition of dark matter; (3) study the acceleration of relativistic velocity jets of material by black holes; (4) detect and collect data on gamma-ray bursts; and, (5) help gain a better understanding of other cosmic phenomena, such as solar flares, pulsars, and the origin of cosmic rays.
Fermi carries two primary instruments, the Large Area Telescope (LAT) designed to observe gamma-ray sources over the energy range from 20 MeV to over 300 GeV, and the GLAST Burst Monitor (GBM), which will view the entire sky not occulted by Earth and detect gamma-ray bursts with an energy coverage from about 10 keV to 30 MeV.
LAT (Large Area Telescope) is an array of 4 x 4 "towers." Each tower carries an anti-coincidence screen, backed by 19 layers of thin tungsten sheets and 19 layers of position-sensitive silicon strips, with one set of strips along the "x-direction" and the other in the "y-direction." The strip layers and the tungsten sheets alternate. Finally, there is the cesium iodide calorimeter to measure the total energy by means of its light flash. The anti-coincidence sheet is made of a plastic that does not flash when a gamma-ray enters, but does when a charged cosmic ray enters. These cosmic rays are a million-fold more numerous than the gamma-rays, so must be rejected thoroughly, by the coincidence of the light flashes from that screen and the calorimeters. The purpose of the tungsten layer is to convert the gamma-rays into an electron-positron pair, so that each of them can be tracked by the position sensitive x-y strips. The instrument covers gamma-rays in the energy band 30 MeV to 300 GeV, and can pinpoint the direction of arrival with a precision of an arc-minute.
GBM is a Gamma-ray Burst Monitor that detects the random and episodic giant bursts in the x-rays and low-energy gamma rays in the energy band of 8 keV to 30 MeV. These bursts last from a millisecond to a few minutes. It deploys 12 thin disks of sodium iodide oriented in 12 different directions. The direction of the burst is inferred from the direction of the disk that gave the largest flash. This inferred direction is passed on to the LAT instrument which can rapidly reorient to look at the source with much greater positional accuracy.
Alternate Names
GLAST
Gamma-ray Large Area Space Telescope
Facts in Brief
Launch Date: 2008-06-11
Launch Vehicle: Delta II
Launch Site: Cape Canaveral, United States
Mass: 4303.0 kg
Nominal Power: 3122.0 W