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Satellite Showcase

The Second Small Astronomical Satellite (SAS-2)

Credit: NASA

The Mission

The second NASA Small Astronomy Satellite (SAS-2) was dedicated to gamma-ray astronomy in the energy range above 35 MeV. The satellite carried a single experiment. It spun so that the spin axis was the same as the telescope axis. SAS-2 was launched on 15 November 1972. Data collection stopped on 8 June 1873 when a power supply failed.

The low fluxes involved in the study of gamma-ray sources make it desirable to minimize the background flux from cosmic-rays. Therefore a low Earth, equatorial orbit was chosen having a 2 degree inclination; an apogee and perigee of 610 km and 440 km, respectively, and an orbital period of about 95 minutes.

During the approximately six months of the mission, 27 pointed observations, typically of a week duration, were made resulting in about 55 percent of the sky being observed, including most of the galactic plane.

The Instrumentation

SAS-2 spark chamber
Credit: NASA

The SAS-2 satellite carried a single telescope using a 32-level wire spark-chamber covering the energy range from 20 MeV to 1 GeV. The experiment was the work of Fichtel et. al. at NASA-GSFC. During the short lifetime of the mission, there was some noticeable decrease in sensitivity due to deterioration of the spark-chamber gas.

An extensive calibration program was carried for the SAS-2 gamma-ray experiment. The National Bureau of Standards (NBS) Synchrotron, Gaithersburg, Maryland, studied data in the 20 to 114 MeV range. The energy range between 200 to 1000 MeV was studied at the Deutsches Elektronen-Synchrotron (DESY), Hamburg, West Germany.

The Science Results

It is generally acknowledged that SAS-2 provided the first detailed information about the gamma-ray sky and indicated the ultimate promise of gamma-ray astronomy.

SAS-2 revealed that the galactic plane gamma-radiation is strongly correlated with galactic structural features, especially when the known strong discrete sources of gamma-radiation are subtracted from the total observed radiation. The SAS-2 results clearly established a high energy (> 35 MeV) component to the diffuse celestial radiation. High-energy gamma-ray emission was also seen from discrete sources such as the Crab and Vela pulsars.


Fichtel, C.E., Hartman, R.C., Kniffen, D.A., Thompson, D.J., Bignami, G.F., Ogelman, H., Ozel, M.E., & Tumer, T. 1975, ApJ, 198, 163.

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