Fermi Gamma-ray Space Telescope
The Fermi Gamma-ray Space Telescope, formerly called the Gamma-ray Large Area Space Telescope, is a space observatory being used to perform gamma-ray astronomy observations from low Earth orbit. Its main instrument is the Large Area Telescope, with which astronomers mostly intend to perform an all-sky survey studying astrophysical and cosmological phenomena such as active galactic nuclei, pulsars, other high-energy sources and dark matter. Another instrument aboard Fermi, the Gamma-ray Burst Monitor, is being used to study gamma-ray bursts and solar flares.
Fermi, named for high-energy physics pioneer Enrico Fermi, was launched on 11 June 2008 at 16:05 UTC aboard a Delta II 7920-H rocket. The mission is a joint venture of NASA, the United States Department of Energy, and government agencies in France, Germany, Italy, Japan, and Sweden, becoming the most sensitive gamma-ray telescope on orbit, succeeding INTEGRAL. The project is a recognized CERN experiment.
Overview
Fermi includes two scientific instruments, the Large Area Telescope and the Gamma-ray Burst Monitor.- The LAT is an imaging gamma-ray detector which detects photons with energy from about 20 million to about 300 billion electronvolts, with a field of view of about 20% of the sky; it may be thought of as a sequel to the EGRET instrument on the Compton Gamma Ray Observatory.
- The GBM consists of 14 scintillation detectors, and can detect gamma-ray bursts in that energy range across the whole of the sky not occluded by the Earth.
Both science instruments underwent environmental testing, including vibration, vacuum, and high and low temperatures to ensure that they can withstand the stresses of launch and continue to operate in space. They were integrated with the spacecraft at the General Dynamics ASCENT facility in Gilbert, Arizona.
Data from the instruments are available to the public through the Fermi Science Support Center web site. Software for analyzing the data is also available.
GLAST renamed Fermi Gamma-ray Space Telescope
NASA's Alan Stern, associate administrator for Science at NASA Headquarters, launched a public competition 7 February 2008, closing 31 March 2008, to rename GLAST in a way that would "capture the excitement of GLAST's mission and call attention to gamma-ray and high-energy astronomy ... something memorable to commemorate this spectacular new astronomy mission ... a name that is catchy, easy to say and will help make the satellite and its mission a topic of dinner table and classroom discussion".Fermi gained its new name in 2008: On 26 August 2008, GLAST was renamed the "Fermi Gamma-ray Space Telescope" in honor of Enrico Fermi, a pioneer in high-energy physics.
Mission
NASA designed the mission with a five-year lifetime, with a goal of ten years of operations.The key scientific objectives of the Fermi mission have been described as:
- To understand the mechanisms of particle acceleration in active galactic nuclei, pulsars, and supernova remnants.
- Resolve the gamma-ray sky: unidentified sources and diffuse emission.
- Determine the high-energy behavior of gamma-ray bursts and transients.
- Probe dark matter and early Universe.
- Search for evaporating primordial micro black holes from their presumed gamma burst signatures.
- Blazars and active galaxies
- Gamma-ray bursts
- Neutron stars
- Milky Way galaxy
- Gamma-ray background radiation
- The early universe
- Sun
- Dark matter
- Fundamental physics
- Unknown discoveries
Mission timeline
Prelaunch
On 4 March 2008, the spacecraft arrived at the Astrotech payload processing facility in Titusville, Florida. On 4 June 2008, after several previous delays, launch status was retargeted for 11 June at the earliest, the last delays resulting from the need to replace the Flight Termination System batteries. The launch window extended from 15:45 to 17:40 UTC daily, until 7 August 2008.Launch
Launch occurred successfully on 11 June 2008 at 16:05 UTC aboard a Delta 7920H-10C rocket from Cape Canaveral Air Force Station Space Launch Complex 17-B. Spacecraft separation took place about 75 minutes after launch.Orbit
Fermi resides in a low-Earth circular orbit at an altitude of, and at an inclination of 28.5 degrees.Software modifications
GLAST received some minor modifications to its computer software on 23 June 2008.LAT/GBM computers operational
Computers operating both the LAT and GBM and most of the LAT's components were turned on 24 June 2008. The LAT high voltage was turned on 25 June, and it began detecting high-energy particles from space, but minor adjustments were still needed to calibrate the instrument. The GBM high voltage was also turned on 25 June, but the GBM still required one more week of testing/calibrations before searching for gamma-ray bursts.Sky survey mode
After presenting an overview of the Fermi instrumentation and goals, Jennifer Carson of SLAC National Accelerator Laboratory had concluded that the primary goals were "all achievable with the all-sky scanning mode of observing". Fermi switched to "sky survey mode" on 26 June 2008 so as to begin sweeping its field of view over the entire sky every three hours.Collision avoided
On 30 April 2013, NASA revealed that the telescope had narrowly avoided a collision a year earlier with a defunct Cold War-era Soviet spy satellite, Kosmos 1805, in April 2012. Orbital predictions several days earlier indicated that the two satellites were expected to occupy the same point in space within 30 milliseconds of each other. On 3 April, telescope operators decided to stow the satellite's high-gain parabolic antenna, rotate the solar panels out of the way and to fire Fermi's rocket thrusters for one second to move it out of the way. Even though the thrusters had been idle since the telescope had been placed in orbit nearly five years earlier, they worked correctly and potential disaster was thus avoided.Extended mission 2013–2018
In August 2013 Fermi started its 5-year mission extension.Pass 8 software upgrade
In June 2015, the Fermi LAT Collaboration released "Pass 8 LAT data". Iterations of the analysis framework used by LAT are called "passes" and at launch Fermi LAT data was analyzed using Pass 6. Significant improvements to Pass 6 were included in Pass 7 which debuted in August 2011.Every detection by the Fermi LAT since its launch, was reexamined with the latest tools to learn how the LAT detector responded to both each event and to the background. This improved understanding led to two major improvements: gamma-rays that had been missed by previous analysis were detected and the direction they arrived from was determined with greater accuracy. The impact of the latter is to sharpen Fermi LAT's vision as illustrated in the figure on the right. Pass 8 also delivers better energy measurements and a significantly increased effective area. The entire mission dataset was reprocessed.
These improvements have the greatest impact on both the low and high ends of the range of energy Fermi LAT can detect - in effect expanding the energy range within which LAT can make useful observations. The improvement in the performance of Fermi LAT due to Pass 8 is so dramatic that this software update is sometimes called the cheapest satellite upgrade in history. Among numerous advances, it allowed for a better search for Galactic spectral lines from dark matter interactions, analysis of extended supernova remnants, and to search for extended sources in the Galactic plane.
For almost all event classes, Version P8R2 had a residual background that was not fully isotropic. This anisotropy was traced to cosmic-ray electrons leaking through the ribbons of the Anti-Coincidence Detector and a set of cuts allowed rejection of these events while minimally impacting acceptance. This selection was used to create the P8R3 version of LAT data.