Fermi Gamma-ray Space Telescope

Overview of the Mission

Fermi continues to perform gamma-ray measurements over the entire celestial sphere, with a sensitivity factor of 30 or more greater than obtained by earlier gamma-ray missions. Fermi has accomplished the next major advance in high-energy gamma-ray astrophysics by providing significant improvements in angular resolution, effective area, field-of-view (FOV), energy resolution and range, and time resolution.

Fermi observatory being prepared for launch

Fermi observatory being prepared for launch

Fermi's scientific objectives are satisfied by two instruments. Covering the 20 MeV to >300 GeV energy range, the Large Area Telescope (LAT) has a large collecting area, an imaging capability over a large FOV, and the time resolution and low deadtime sufficient to study transient phenomena. The LAT also provides active background discrimination and rejection against the large fluxes of cosmic rays, earth albedo gamma rays, and trapped radiation that are encountered in orbit. The Fermi Gamma-ray Burst Monitor (GBM) provides the spectral and temporal context in the classical 10 keV to 25 MeV energy band for gamma-ray bursts (GRBs) observed by the LAT, detects and localizes bursts, and alerts the LAT that a burst is in progress. For events occurring within the field of view, Fermi provide rapid notification to the science community. The next sections describe the instruments in greater detail.

The primary communication between the spacecraft and the ground is through the Tracking and Data Relay Satellite System (TDRSS). Science and housekeeping data are downlinked at Ku-band frequencies ~10-11 times per day at 40 Mbps during seven to eight minute real time telemetry contacts. During these downlinks there is a direct 4 kbps uplink rate available at S-band frequencies. Burst alerts and spacecraft alarms can be downlinked in the S-band (variable data rates) at all times. S-band uplink and downlink (no science data) directly to ground stations is possible as a backup to TDRSS. Time and spacecraft position are provided by an onboard GPS system.

Mission Timeline

Fermi was launched in June 11, 2008, from Cape Canaveral by a Delta 2920H-10 (also known as a Delta II 'Heavy') into an initial orbit of ~565 km altitude at an 25.6 degree inclination with an eccentricity <0.01. The orbital period is 96.5 minutes, and has a precession period of 53.4 days (so the RA and Dec of the orbit poles trace a 25.6 degree circle on the sky every 53.4 days). The mission design lifetime was a minimum of 5 years, with a goal of 10 years, which it has now exceeded.

After launch the mission has consisted of three phases: a ~2 month on-orbit initial checkout (Phase 0), a one year science verification period during which a full sky survey will be performed (Phase 1), and then at least four years of operations determined by the scientific goals and requirements of guest investigations (Phase 2). Subsequent to that 5-year prime-mission period the mission has been granted multiple extensions through a competitive review process: the NASA Senior Review of astrophysics missions. There was one cycle of guest investigations during the verification and sky survey phase, and annual guest investigation cycles during Phase 2. The GBM data will be publicly released during Phase 1 while the LAT data to which the tools described here can be applied will be released only in Phase 2, i.e., about 14 months after Fermi's launch.

Observing Modes

The LAT and GBM have very wide FOVs, and the observatory was designed to be very flexible in the direction in which it can point, but it is currently much more limited following the failure of a solar-array drive motor in 2018. An observational constraint is to avoid pointing at or near the Earth to maximize the detection of astrophysical photons. However, the LAT may occasionally observe the Earth's limb to detect albedo gamma rays for instrument calibration. Orientation requirements for the LAT's cooling radiators, the battery radiators, and the observatory solar panels also impose engineering constraints, particularly during slewing maneuvers. No science data is taken while the observatory is transiting the South Atlantic Anomaly (SAA) since the instruments lower the voltage on their photomultiplier tubes (PMTs). The SAA is a region over the South Atlantic with a high density of charged particles that are trapped by the configuration of the Earth's magnetic field. In a 25.6 degree inclination orbit and at Fermi's altitude, SAA outages cost ~15% of the LAT's and GBM's potential observing time.

The Fermi spacecraft operates in a number of observing modes. Transitions between modes may be commanded from the ground or by the spacecraft. Prior to 2018, based on data from the LAT or the GBM, the LAT could request autonomous repointing of the spacecraft and change the observing mode to monitor the location of a GRB (or other short timescale transient) in or near the LAT's FOV. This capability however has become extremely limited and is rarely employed since the aforementioned solar-array drive anomaly in 2018. After a pre-determined time the spacecraft returned to the scheduled mode. Currently the dwell time for such autonomous repoints was five hours. The pointing accuracy was <2 degrees (1σ, goal of <0.5 degrees), with a pointing knowledge of <10 arcsec (goal <5 arcsec).

In survey mode, which now comprises essentially all of the observing time, the LAT's pointing is relative to the zenith (the direction away from the Earth), and therefore changes constantly relative to the sky. Uniformity of exposure is achieved by "rocking" the pointing perpendicular to the orbital motion. The default profile rocks the instrument axis ~50 degrees north for one orbit, then ~50 degrees south for one orbit, resulting in a two-orbit periodicity. Following the solar-array drive anomaly asymmetric strategies may be employed. The maximum rocking angle is 60 degrees. The figure-of-merit to be optimized by a particular rocking profile is nominally uniformity of sky coverage, but may change as the mission progresses.

When justified by the demands of a particular investigation, the LAT can in principle still be pointed at (or near) a target. A pointed observation may be optimum for pulsar timing studies (to reduce the effect of variations in a pulsar's period) or for other studies where building up exposure over a short time will be useful. This mode keeps the earth out of the FOV; the default Earth Avoidance Angle (defined as the minimum angle between the LAT axis and the Earth's limb) is 30 degrees. When the target is unocculted but within the Earth Avoidance Angle of the Earth's limb, the spacecraft will keep the target in the LAT's FOV while keeping the Earth out of the LAT's FOV. The observatory may observe a secondary target when the Earth occults the primary target.

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