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Esro-2b small.gif
Mission type Astrophysics
Operator ESRO
COSPAR ID 1968-041A
SATCAT no. 03233Edit this on Wikidata
Spacecraft properties
Launch mass 89 kilograms (196 lb)
Start of mission
Launch date 6 March 1968, 01:55:08 (1968-03-06UTC01:55:08Z) UTC[1]
Rocket Scout B
Launch site Vandenberg SLC-5
End of mission
Decay date 8 May 1971
Orbital parameters
Reference system Geocentric
Regime Low Earth
Perigee 326 kilometres (203 mi)[1]
Apogee 1,086 kilometres (675 mi)[1]
Inclination 97.2 degrees[1]
Period 98.9 minutes[1]
Epoch 16 May 1968, 22:09:00 UTC[2]

ESRO-2B or Iris (International Radiation Investigation Satellite) was a European astrophysical spin-stabilised research satellite which was launched in 1968. Operated by the European Space Research Organisation, ESRO 2B made astronomical surveys primarily in x-ray and solar particles detectors.[3]


ESRO-2B was an 89 kg (196 lb) cylindrical spacecraft with a length of 85 cm and a diameter of 76 cm. In December 1968 (approx 195 days since mission start) the on-board tape recorder suffered a mechanical failure. This effectively ended the two X-ray experiments as they did not provide any significant data return from then on.

ESRO-2B was launched on a Scout B rocket into a highly elliptical near-polar orbit on 16 May 1968 after the ESRO-2A failed to reach orbit.[4]

Spin-stabilised, ESRO-2B had a spin rate of approximately 40 rpm and re-entered Earth's atmosphere on 8 May 1971 after completing 16,282 orbits.[4]


Seven instruments were carried aboard EROS 2B[1] designed to detect high energy cosmic rays, determine the total flux of solar X-rays and to measure Van Allen belt protons and cosmic ray protons.[3] While designed for solar observations ESRO-2B is credited with the detection of X-rays from non-solar sources.[1]


In September 2018, Belgium launched a 2 euro commemorative coin commemorating the 50th anniversary of ESRO-2B's launch.[5]


  1. ^ a b c d e f g "ESRO 2B". NASA. Retrieved 6 March 2013.
  2. ^ "NASA - NSSDCA - Spacecraft - Trajectory Details". Retrieved 2018-05-01.
  3. ^ a b "ESRO 2B: May - December 1968". University of Indiana. Retrieved 6 March 2013.
  4. ^ a b "ESRO 2A, 2B (Iris 1, 2)". Gunters Space Page. Retrieved 6 March 2013.
  5. ^ "2 euro Belgium 2018 - Satellite ESRO-2B Launch UNC". Retrieved 19 September 2018.