National Aeronautics and Space Administration

COS-B

Mission Overview

COS-B, an ESA mission, was launched from NASA’s Western Test Range by a Thor Delta vehicle on 9 August 1975. Its scientific mission was to study in detail the sources of extraterrestrial gamma radiation at energies above about 30 MeV. COS-B operated in a pointing mode with its spin axis directed towards fixed points in the sky. Its operative time period was about 25 hours during a 37-hour orbit. About one quarter of this time was devoted to observations at higher galactic latitudes, especially regions expected to contain extragalactic sources. Pointing periods lasted four to five weeks during the early part of the mission and up to 3 months during later observations. A broad portion of the galactic equator was studied deeply by means of repeatedly overlapping observations. The end of the mission coincided with the end of available attitude gas which had been conserved as much as possible by a careful choice of orbital manoeuvers. Nearly 50% of the celestial sphere was viewed by COS-B.

COS-B carried a single large experiment the design and provision of which have been the responsibility of a group of research laboratories known as the Caravane Collaboration. The characteristics of the instrument are described below. The Gamma-Ray Telescope performed well throughout the mission: the only complication being the occasional erratic performance of the spark-chamber and the inevitable reduction in performance as the spark chamber gas aged. This aging was, however, minimized by means of a gas-replenishment system that permitted emptying and refilling of the spark chamber. As the rate of gas deterioration decreased with time, it became possible to increase the interval between flushing from its initial value of once every 6 weeks to about once every 36 weeks before the final flushing in November 1981. The spark chamber was still performing creditably at the end of the mission in April 1982.

A comparison of data from overlapping observation periods enabled longterm sensitivity changes of COS-B to be estimated. For the first three years of the mission, the sensitivity was virtually stable. However, a slow fall-off began to develop. From careful monitoring, curves for the empirical correction factors were derived. Over time, the level of the background component increased. This increase was due largely to the interaction of cosmic rays with the massive parts of the satellite and surrounding subsystems. Significant reductions were achieved by increasing the modulation of the cosmic rays with the approach of the solar maximum. This effect served to counter-balance the reduction in detector sensitivity.

The originally foreseen duration of the mission was two years, but in fact COS-B was finally switched off on 25th April 1982, having functioned successfully for 6 years and 8 months.

Instrumentation The Gamma-Ray Telescope consisted of a magnetic-core, wire-matrix, spark chamber, triggered by a three-element scintillation counter telescope. A plastic scintillator guard counter surrounding these two units served as an anti-coincidence detector to reject triggers due to incident charged particles and allow only gamma-rays to be detected. Beneath the telescope was an energy calorimeter, which absorbed the secondary particles produced by the incident photons. For on-axis incidence, the effective detector area reached a maximum value of ~50 cm2 at ~400 MeV. The energy resolution had its best value (~40% FWHM) at about 150 MeV and was better than 100% up to at least 3 GeV. The experiment was described in detail by Bignami et al. (1975). Alongside the Gamma-Ray Telescope was mounted a proportional counter sensitive to 2-12 keV X-rays. This detector was intended to provide synchronization of possible pulsed gamma-ray emission from pulsating X-ray sources. The pulsar synchronizer was also used for monitoring the intensity of radiation from X-ray sources.