SAC-B
Satélite de Aplicaciones Científicas-B (SAC-B)

SAC-B, an Argentine / US mission, is designed to study of solar physics and astrophysics through the examination of solar flares, gamma-ray burst sources and the diffuse soft X-ray cosmic background. The mission is organized as a cooperative effort between the National Aeronautics and Space Administration (NASA) and Argentina's National Commission of Space Activities (CONAE). NASA provides two scientific instruments and launch services on a Pegasus XL vehicle. CONAE is responsible for the design and construction of the SAC-B satellite.

Spacecraft
SAC-B is three axis stabilized, using two momentum wheels in a "V" configuration for roll and yaw control. Pitch axis control and momentum unloading is accomplished using magnetic torque coils. Coarse and fine sun sensors, combined with magnetometer readings provide spacecraft attitude knowledge. Four GaAs solar panels produce a total of 256W (beginning of life) to run spacecraft instruments and charge two 10 A-h NiCd batteries. Command and data handling is performed using redundant 80C86 processors with 64 Kbytes of EEPROM, 64 Kbytes of program RAM and 64 Kbytes of data RAM. Telemetry is 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 is located in Buenos Aires, Argentina, with initial orbit and backup support provided by NASA Wallops and DSN stations.

Payload
The SAC-B science objectives are supported by four instrument groups:

The Hard X-Ray Spectrometer (HXRS) provided by the Argentine Institute of Astronomy and Space Physics (IAFE) searches 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 Center (GSFC) has two sets of detectors. One of them, the Soft X-Ray Spectro-meter (SOXS), performs coordinated observations with the HXRS by observing soft X-ray emissions from solar flares. The other, the Gamma Ray Burst Spectrometer (GRaBS) provides 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) is provided by the Pennsylvania State University. CUBIC measures 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), measures 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 is 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 is 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 is so long, it will eventually stabilize 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 will be regained.

Country of Origin	Argentina
Customer/User	NASA, CONAE
Manufacturer(s)	INVAP S.E.
Size	62 x 62 cm wide by 80 cm high rectangular parallelepiped with 4 extended solar panels 62 cm wide by 76 cm long.
Orbit	550 km, 38 deg. inclination
Related Sites	NASA SAC-B fact sheet CONAE SAC-B Info SAC-B site at Penn State

Launch Facts

Name	Int'l Desig.	Date	Site	Vehicle	Orbit	Mass(kg)
Notes
SAC-B	1996-061A	11/4/96	Wallops	Pegasus XL	LEO	181
Was not ejected from launch vehicle rendering spacecraft useless; still attached to HETE and Pegasus upper stage; now being tracked as a single 650kg object.