photo of SAS-2


* Mission Overview

SAS-2 (also refered to as SAS-B and Explorer 48) was launched on 19 November 1972. To minimize the background flux from cosmic-rays, SAS-2 was placed in a low Earth equatorial orbit having a 2 degree orbital inclination. Its apogee and perigee were 610 km and 440 km respectively, with an orbital period of about 95 minutes.
During the ~6 months of the mission, 27 pointed observations (typically a week in duration) were made, resulting in about 55 percent of the sky being observed, including most of the galactic plane.
On 1973 June 8, a failure of the low-voltage power supply ended the collection of data.

* Instrumentation

The SAS-2 satellite carried a single instrument: a gamma-ray telescope that used a 32-level wire spark-chamber. The telescope covered the energy range 20 MeV - 1 GeV. The instrument 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 out on the gamma-ray telescope before SAS-2 was launched. The National Bureau of Standards (NBS) Synchrotron accelerator in Gaithersburg, Maryland was used to study the performance of the telescope in the 20 - 114 MeV range. The performance between 200 - 1000 MeV was studied at the Deutsches Elektronen-Synchrotron (DESY) accelerator in Hamburg, West Germany.

* Science

It is generally acknowledged that SAS-2 provided the first detailed information about the gamma-ray sky and demonstrated the ultimate promise of gamma-ray astronomy.
SAS-2 revealed that the galactic plane gamma-radiation was strongly correlated with galactic structural features, especially when the known strong discrete sources of gamma-radiation were 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.
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