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Word of the Week: Activation

(Image - Fermi Gamma-ray Space Telescope)
An artist's conception of the Fermi Gamma-ray Space Telescope. (Image: NASA.)

In space instrument parlance, activation refers to the detailed sequence of commissioning and checkout procedures needed to verify that a spacecraft and its instruments are working as intended after launch. Two years ago today, the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope spacecraft successfully completed activation. In the LAT's case, activation included methodically turning on and configuring each of the instrument's many sub-systems, from power supplies and detector elements to onboard data processing computers, and verifying at each step that the hardware and software were working as expected.

LAT team members working at the Mission Operations Center in the Goddard Space Flight Center carefully executed the command sequences and inspected data returned by the instrument—from basic voltages, currents and temperatures to detailed message streams from the instrument's flight software. Teams of scientists at the LAT Instrument Science Operations Center at SLAC followed up with thorough inspection of the science data, looking for the characteristic signatures of gamma rays collected by the telescope.

LAT activation began at 10:59 UT on June 24, 2008. Thanks to a well-built and exhaustively tested instrument, a seasoned team that had practiced the procedures many times, and strong support from spacecraft controllers at Goddard, the process took just under 24 hours. By 10:57 UT on June 25, 2008, the LAT was up and taking its very first data.

In the subsequent weeks, the LAT team conducted a detailed series of checks on the functionality and calibration of the instrument—which, according to Fermi LAT Commissioner Eric Grove of the Naval Research Lab, went flawlessly—and in the following two years the LAT has delivered an avalanche of results. Thanks to its ability to quickly and accurately scan the gamma-ray universe, viewing the entire sky every three hours, it's caught a huge number of variable sources as they flare and fade. In this manner, the LAT discovered a number of objects never before seen in gamma rays, including "starburst" galaxies that serve as stellar nurseries. It's seen more than 600 blazars—galaxies with very bright gamma-ray emission that eject a jet of plasma at nearly the speed of light—and more than 60 pulsars, including 24 found in blind searches and 18 "millisecond pulsars" that spin up to 700 times a second. It's observed half a dozen supernova remnants, offering insight into how cosmic rays are accelerated by these relics, and possibly detected the first high-energy gamma rays streaming from a nova. It offered high-precision, very broadband measurements of the background of electron and positron cosmic rays in our galaxy, and appraised the diffuse gamma-ray emission from our Galaxy, which is created by cosmic rays striking gas and photons in the Milky Way and from unknown sources beyond.

In addition, when the other instrument onboard Fermi, the Gamma-ray Burst Monitor, sees a quick, unpredictable flash called a gamma-ray burst, the LAT can pause its scan of the whole sky to point in the direction of the burst for several hours. From these measurements, the LAT has detected high-energy gamma rays from 17 bursts, including those with the greatest total energy, the fastest motions, and the highest-energy photons ever seen from such phenomena.

—Kelen Tuttle
SLAC Today, June 25, 2010