ESA title
Science & Exploration

Cos-B factsheet

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ESA / Science & Exploration / Space Science

Looking for gamma rays

Name Cos-B is an abbreviation of Cosmic ray Satellite ('option B'.)

Description Cos-B was the first ESA mission dedicated to the study of gamma-ray sources. Its results created a catalogue of these sources, known as the 2CG Catalogue, the first complete map of the gamma-ray emission from the disc of our Galaxy, the Milky Way, and the first detectable emission from an extra-galactic object 3C273.

Launch 9 August 1975 (NASA launcher at Western Test Range, California, United States).

Status Completed (1982).

Journey The 37-hour orbit of Cos-B was an eccentric orbit chosen to ensure that for most of the time the satellite was outside the Earth's radiation belts.

Notes Cos-B is one of the most successful space missions ever. It had no optical telescopes but instead a specially developed spark chamber. It had one objective - pointing in the direction of a star or other object and measuring its gamma-ray emissions.

The major result was the creation of the 2CG Catalogue that contained about 25 gamma-ray sources out of 30 observations made during the first three years. Operations continued for another four years. Additional data allowed astronomers to refine details of these sources. Highly detailed light-curves were measured from the Crab and Vela pulsars and the unknown gamma-ray source Geminga (now known to be a pulsar) was accurately located.

In parallel, a secondary Cos-B instrument was able to spend a significant fraction of its lifetime monitoring X-ray emissions from the Cygnus X-3 X-ray pulsar. Despite devoting about 10% of the entire mission to this object, no gamma variability resembling that at X-rays was detected.

The originally foreseen duration of the mission was two years, but scientists finally switched off Cos-B on 25 April 1982, after the spacecraft had functioned successfully for 6 years and 8 months.

The end of the mission coincided with the end of its fuel supply, which scientists had conserved by careful choice of manoeuvres.

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