The Fast Auroral SnapshoT Explorer (FAST), the second mission in NASA's Small
Explorer Satellite Program (SMEX), is a satellite designed to study Earth's
aurora. This highly successful spacecraft has ... helped scientists answer
fundamental questions about the causes and makeup of the aurora.
FAST was launched on August 21, 1996 from a Pegasus rocket into a highly
elliptical orbit. It crosses Earth's auroral zones (donut shaped regions
centered on the poles) four times each orbit, and only collects high-resolution
data ("snapshots") while in those zones. It ventures high into the charged
particle environment of the aurora to measure the electric and magnetic fields,
plasma waves, energetic electrons and ions, ion mass composition, and thermal
plasma density and temperature.
The FAST instrument set consists of sixteen electrostatic analyzers, four
electric field langmuir probes suspended on 30 m wire booms, two electric field
langmuir probes on 3 m extendible booms, searchcoil and fluxgate magnetometers
and a time-of-flight mass spectrometer. The science investigation makes
extremely temporal and spatial resolution measurements of the auroral plasma at
apogee altitude. The instrument hardware consists of the sensor assemblies and
an instrument data processor. The instrument electronics include a 32-bit data
processing unit that performs the science data processing and recording in a
one gigabit, solid-state memory. The stored data are transferred to the ground
at one of three selectable high data rates of 900 Kbps, 1.5 Mbps or 2.25 Mbps.
The instruments weigh 51 kg; the total observatory mass is 191 kg. The FAST
mission is in a 351 x 4175 km orbit with an 83? inclination.
The FAST observatory is a 12 rpm, spin-stabilized spacecraft with its spin axis
oriented parallel to the orbit axis. Spin rate and spin-axis orientation are
maintained by two magnetic torquer coils, one spinning Sun sensor, one horizon
crossing indicator and a spacecraft magnetometer. The Attitude Control System
(ACS) provides closed-loop spin-rate control. Spin-axis precession is performed
open loop and is closed via ground commands. After computation on the ground,
attitude knowledge is accurate to within one degree.
The body-mounted solar array contains 5.6 m2 of solar cells that can distribute
52 W of orbit average power to the spacecraft and instruments. The orbit
average power consumption of the spacecraft hardware is 33 W. The instruments
consume 19 W orbit average power, 39 W when operating. Instruments are
frequently powered off in order to maintain a positive energy balance.
The data system for the FAST mission consists of dual 8085, 8-bit spacecraft
computers. The spacecraft computers perform health and safety functions, power
distribution, data encoding/decoding and launch vehicle interface. A
multi-element micropatch antenna mounted on a boom above the spacecraft
supports ground communications. Commands are uplinked at 2 Kbps. Health and
safety data is telemetered to the ground at 4 Kbps. A Transportable Orbital
Tracking Station (TOTS) was placed in Alaska to collect real-time science
telemetry while the spacecraft is passing through the northern aurora. TOTS is
highly automated and portable; it has an 8 m antenna with 200 W of uplink power
and can be packed for shipment in three ISO containers.
The FAST instruments consist of:
Electric Field Experiment: The electric field experiment is composed of three
orthogonal boom pairs. Spherical sensors deployed on radial wire and axial
stacer booms will provide information on the plasma density and electron
Magnetic Field Experiment: The magnetic field experiment consists of two
magnetometers mounted 180? apart on deployable graphite epoxy booms. The search
coil magnetometer uses a three-axis sensor system to provide magnetic field
data over the frequency range of 10 Hz to 2.5 kHz. The flux gate magnetometer
is a three-axis system using high, stable, low noise, ring core sensors to
provide magnetic field information for DC to 100 Hz.
Time-of-Flight Energy Angle Mass Spectrograph (TEAMS): The TEAMS instrument is
a high sensitivity, mass-resolving spectrometer that measures full
three-dimension distribution functions of the major ion species with one spin
of the spacecraft. The TEAMS experiment covers the core of all plasma
distributions of importance in the auroral region.
Electrostatic Analyzers (ESA): Sixteen ESAs configured in four stacks will be
used for both electron and ion measurements. The four stacks are placed around
the spacecraft such that the entire package is provided a full 360? field of
view. The ESAs can provide a 64-step energy sweep, covering approximately 3 eV
to 30 KeV up to 16 times per second.
For more information, see: