SAC-B, an Argentine / US mission, was designed to study solar physics and astrophysics through the examination of solar flares, gamma-ray burst sources and the diffuse soft X-ray cosmic background. The mission was organised as a co-operative effort between the National Aeronautics and Space Administration (NASA) and Argentina's National Commission of Space Activities (CONAE). NASA provided two scientific instruments and launch services on a Pegasus XL vehicle. CONAE was responsible for the design and construction of the SAC-B satellite.
Spacecraft: SAC-B was three axis stabilised, using two momentum wheels in a "V" configuration for roll and yaw control. Pitch axis control and momentum unloading was accomplished using magnetic torque coils. Coarse and fine sun sensors, combined with magnetometer readings provided spacecraft attitude knowledge. Four GaAs solar panels produced a total of 256W (beginning of life) to run spacecraft instruments and charged two 10 A-h NiCd batteries. Command and data handling was performed using redundant 80C86 processors with 64 Kbytes of EEPROM, 64 Kbytes of program RAM and 64 Kbytes of data RAM. Telemetry was formatted and stored in a 2 Mbit mass memory for downlink at 50, 100, or 200 kbps using 5W S-band transmitters. The mission control ground station was located in Buenos Aires, Argentina, with initial orbit and backup support provided by NASA Wallops and DSN stations. Payload: The SAC-B science objectives were supported by four instrument groups: The Hard X-Ray Spectrometer (HXRS) provided by the Argentine Institute of Astronomy and Space Physics (IAFE) searched the hard X-ray spectrum between 20 and 320 keV of rapidly varying events on time scales as short as tens of milliseconds. The Goddard X-Ray Experiment (GXRE) provided by NASA/Goddard Space Flight Centre (GSFC) has two sets of detectors. One of them, the Soft X-Ray Spectro-meter (SOXS), performs co-ordinated observations with the HXRS by observing soft X-ray emissions from solar flares. The other, the Gamma Ray Burst Spectrometer (GRaBS) provided time profiles of the X-ray emission from non-solar gamma-ray bursts in the energy range from ~20 keV to > 300 keV. The Cosmic Unresolved X-Ray Background Instrument using CCDs (CUBIC) was provided by the Pennsylvania State University. CUBIC measured the spectrum of the diffuse X-Ray background with unprecedented sensitivity and spectral resolution between 0.1 and 10.0 keV in selected areas of the sky. The Imaging Particle Spectrometer for Energetic Neutral Atoms (ISENA), provided by the Italian Istituto di Fisica dello Spazio Interplanetario (IFSI), measured neutral atoms at spacecraft altitudes. Status as of 11/5/96: SAC-B was launched with HETE on a Pegasus XL on 11/04/96, which achieved a nominal orbit but did not eject the satellite due to a Pegasus transient power bus failure. The system was now flying with SAC-B, HETE and the Pegasus 3rd stage connected together as a single 650 kg object. SAC-B deployed its solar panels successfully and operated for about 10 hours. On-board software was modified to permit operation without a separation indication and the ACS system was placed in safe-hold mode in an attempt to gain control and point the solar panels at the sun. However, the ACS system was not designed to control such a massive tumbling object. With the Pegasus 3rd stage shadowing part or all of the solar array, there was not enough power to charge the batteries, even during the daylight part of the orbit. At the last contact, battery power continued to decrease, and four subsequent passes over Wallops did not produce any signal from the satellite. Because the SAC-B/HETE/Pegasus object was so long, it will eventually stabilise in a gravity-gradient capture mode, although it will probably take a long time for it to lose its existing angular momentum. It could be captured in either orientation: SAC-B up or SAC-B down. In any event, it is unlikely that spacecraft control would be be regained.
Pegasus XL $ 14 million dual launch failed-lost $ 26 million HETE and $ 21.5 million SAC-B when payload failed to separate.
Design Life: 3 years. Total Length: 0.8 m. Maximum Diameter: 0.6 m.