Mission Overview

SAS-C was the third in the series of small spacecraft whose objectives were to survey the celestial sphere for sources radiating in the X-ray, gamma-ray, UV, and other spectral regions. The primary missions of SAS-C were to measure the X-ray emission of discrete extragalactic sources, to monitor the intensity and spectra of galactic X-ray sources from 0.2 to 60 keV, and to monitor the X-ray intensity of Scorpio X-1. The spacecraft was launched from the San Marco platform off the coast of Kenya, Africa, into a near-circular, equatorial orbit. This spacecraft contained four instruments: the Extragalactic Experiment, the Galactic Monitor Experiment, the Scorpio Monitor Experiment, and the Galactic Absorption Experiment. In the orbital configuration, the spacecraft was 145.2 cm high and the tip-to-tip dimension was 470.3 cm. Four solar paddles were used in conjunction with a 12-cell nickel-cadmium battery to provide power over the entire orbit. The spacecraft was stabilized along the Z axis and rotated at about 0.1 deg/s. Changes to the spin-axis orientation were by ground command, either delayed or in real time. The spacecraft could be made to move back and forth plus or minus 2.5 deg across a selected source along the X axis at 0.01 deg/s. The experiments looked along the Z axis of the spacecraft, perpendicular to it, and at an angle.

Launch Date: 1975-05-07 at 22:45:01 UTC
Launch Vehicle: Scout
Launch Site: San Marco Platform, Kenya
Decay Date: 1979-04-09

Trajectory Description

Type: Orbiter
Central Body: Earth
Epoch start: 1975-05-07 00:00:00 UTC

Orbital Parameters
Periapsis 509.0 km
Apoapsis 516.0 km Period 94.9000015258789 minutes
Inclination 3.0°
Eccentricity 5.080000264570117E-4

Instrumentation

Extragalactic Experiment (EGE)

This experiment determined the positions of very weak extragalactic X-ray sources. The instrument viewed a 100-sq deg region of the sky around the direction of the spin axis of the satellite. The nominal targets for a 1-year study were (1) the Virgo cluster of galaxies for 4 months, (2) the galactic equator for 2 months, (3) the Andromeda Nebula for 3 months, and (4) the Magellanic Clouds for 3 months. The instrumentation consisted of one 2.5-arc-min and one 4.5-arc-min FWHM modulation collimator, as well as proportional counters sensitive over the energy range from 1.5 to 10 keV. The effective area of each collimator was about 225 sq cm. The aspect system provided information on the orientation of the collimators to an accuracy of 15 arc-s.

Galactic Monitor Experiment (GME)

The objectives of this experiment were to locate galactic X-ray sources to 15 arc-s and to monitor these sources for intensity variations. The source positions were determined with the use of the modulation collimators of the Extragalactic Experiment during the nominal 2-month observation of the galactic equator. The monitoring of the X-ray sky was accomplished by the use of three slat collimators. One collimator, 1- by 70-deg FWHM, was oriented perpendicular to the equatorial plane of the satellite, while the other two, each 0.5- by 45-deg FWHM, were oriented 30 deg above and 30 deg below the first. The detector behind each collimator was a proportional counter, sensitive from 1.5 to 13 keV, with an effective area of about 100 sq cm. The 1.0-deg collimator had an additional counter of the same area, sensitive from 8 to 50 keV. Three lines of position were obtained for any given source when the satellite was being spun at a steady rotation of 4 arc-min/s about the Z axis.

Scorpio Monitor Experiment (SME)

A 12- by 50-deg FWHM slat collimator was oriented with its long axis perpendicular to the satellite spin axis such that a given point on the sky could be monitored for about 25% of a rotation. This collimator was inclined by 31 deg with respect to the equatorial plane of the satellite, so that Scorpio X-1 was observed while the Z axis was oriented to the Virgo cluster of galaxies. The detectors used in this experiment were proportional counters with 1-mm beryllium windows. The energy range was from 1.0 to 60 keV, and the total effective area was about 40 sq cm.

Galactic Absorption Experiment (GAE)

The density and distribution of interstellar matter was determined by measuring the variation in the intensity of the low-energy diffuse X-ray background as a function of galactic latitude. A 1-micrometer polypropylene window proportional counter was used for the 0.1- to 0.4-keV and 0.4- to 1.0-keV energy ranges, while a 2-micrometer titanium window counter covered the energy range from 0.3 to 0.5 keV. In addition, two 1-mm beryllium window counters were used for the 1.0- to 10-keV energy range. The collimators in this experiment had fields of view of 3 deg for the 1-micrometer counter, 2 deg for the 2-micrometer counter, and 2 deg for the 1-mm counters.

Science
Summary