Spaceflight Insider

Our spaceflight heritage: Flight of COS-B marks 40 years of ESA on orbit

COS-B_1975-1982_Europe_s_pioneer_gamma-ray_observatory ESA photo posted on SpaceFlight Insider

Photo Credit: European Space Agency

Forty years ago, on Aug. 9, 1975, the European Space Agency (ESA) launched its first satellite, named COS-B (Cosmic ray Satellite – option B). The spacecraft, dedicated to studying gamma-ray sources, was launched into orbit by NASA on a Thor Delta 2913 launch vehicle from the Western Test Range in California.

COS-B is considered by ESA to be one of the most successful space missions it has ever conducted. It had no optical telescopes but was equipped with a Gamma-Ray Telescope consisting of a magnetic-core, wire-matrix, spark chamber which was triggered by a three-element scintillation counter telescope.

COS-B was launched atop a Delta II rocket from Vandenberg Air Force Base in California. Photo Credit: NASA

COS-B was launched atop a Delta II rocket from the Western Test Range in California. Photo Credit: NASA

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. It had one objective – to measure the gamma-ray emissions from a star or other object.

Beneath the telescope was an energy calorimeter, which absorbed the secondary particles produced by the incident photons.

Power was supplied by some 9,480 solar cells mounted on 12 subpanels on the cylindrical body of the spacecraft.

While it was on orbit, communications, command, and control of the COS-B satellite was provided by the ESA Estrack network.

The scientific results provided by the satellite helped create a catalog of gamma-ray sources. What is known as the 2CG Catalogue, it is the first complete map of gamma-ray emissions from the disc of the Milky Way galaxy.

“Its mission was to map the sky, in particular, the Milky Way, in the light of gamma rays with energy greater than 50 MeV. Such gamma rays may be produced by cosmic rays interacting with the interstellar medium, starlight, and magnetic fields,” said Brian Taylor, the former ESA COS-B Project Scientist.

The 37-hour eccentric orbit of COS-B was chosen to ensure that satellite would spend most of its time outside of the Earth’s radiation belts. About one-quarter of this time was devoted to observations at higher galactic latitudes, especially regions expected to contain extra-galactic sources. Pointing periods lasted four to five weeks during the early part of the mission and up to 3 months during later observations.

The satellite measured highly-detailed light curves from the Crab and Vela pulsars as well as to locate the unknown gamma-ray source Geminga (which is now known to be a pulsar).

COS-B_1975-1982_Europe_s_pioneer_gamma-ray_observatory ESA photo posted on SpaceFlight Insider

COS-B prior to its launch in 1975. Photo Credit: ESA

“Finding gamma rays among the 10,000 times more numerous charged-particle cosmic rays was akin to finding the needle in the haystack,” Taylor said.

COS-B was also 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 percent of the entire mission to this object, no gamma variability resembling that at X-rays was detected.

The satellite’s main development started in February of 1972, with the prime contractor being Germany’s MBB Ottobrunn. COS-B also had co- and sub-contractors in Belgium, Denmark, Spain, France, Italy, the Netherlands, and the UK. The gamma-ray instrument was designed and built by a consortium of five institutes from Germany, the Netherlands, France and Italy.

The spacecraft was initially planned to be launched on the Europa-2 launch vehicle, but due to the unavailability after it had encountered several launch failures, it was finally launched on the Delta 2913 rocket instead.

COS-B’s mission was originally scheduled to last for a period of two years. However, the satellite was not turned off until April 25, 1982 – completing a successful mission that lasted for 6 years and 8 months. The end of the spacecraft’s operational life coincided with the end of available attitude gas supply, which had been conserved as much as possible by a careful choice of orbital maneuvers.

Some ten years after it had been launched, the scientific data from COS-B had been analyzed. When all was said and done, it is thought that COS-B increased the understanding about gamma rays by a factor of 25.

“Uniquely, COS-B carried a single instrument, conceived in the mid-1960s and approved for flight by the ESRO Council in 1969. Up to that point, ESRO’s early satellites carried typically seven separate instruments, so COS-B was really a case of all eggs in one basket,” Taylor said. “The objective was to search for cosmic sites where these particles might be accelerated to relativistic speeds. Being charged, their arrival directions at Earth bear no relation to their point of origin, since their trajectories are ‘scrambled’ by the interstellar magnetic fields whereas gamma rays come straight to us.”


Tomasz Nowakowski is the owner of Astro Watch, one of the premier astronomy and science-related blogs on the internet. Nowakowski reached out to SpaceFlight Insider in an effort to have the two space-related websites collaborate. Nowakowski's generous offer was gratefully received with the two organizations now working to better relay important developments as they pertain to space exploration.

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