Esro-2b small.gif
Mission typeAstrophysics
COSPAR ID1968-041A[1]
SATCAT no.03233
Spacecraft properties
Launch mass89.8 kilograms (198 lb)
Start of mission
Launch date17 May 1968, 02:06:00 (1968-05-17UTC02:06Z) UTC[2][1]
RocketScout B
Launch siteVandenberg SLC-5
End of mission
Decay date8 May 1971, shortly after 03:00 UT
Orbital parameters
Reference systemGeocentric
RegimeLow Earth
Perigee altitude326 kilometres (203 mi)[2]
Apogee altitude1,086 kilometres (675 mi)[2]
Inclination97.2 degrees[2]
Period98.9 minutes[2]
Epoch16 May 1968, 22:09:00 UTC[3]

ESRO-2B or Iris (International Radiation Investigation Satellite; sometimes Iris 2[4]) or sometimes ESRO II (or ESRO 2), 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.[5]


ESRO-2B was an 89 kg (196 lb) cylindrical spacecraft with a length of 85 cm and a diameter of 76 cm. In 10 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. Other experiments could still be operated through ground radio links.

ESRO-2B was launched on a Scout B rocket into a highly elliptical near-polar orbit on 17 May 1968. Its predecessor satellite, ESRO-2A (sometimes Iris 1) failed to reach orbit on 29 May 1967,[6] launching on a Scout B rocket from Vandenberg AFB SLC-5. The cause of failure was malfunction of the third stage of the rocket, preventing the satellite from reaching orbit. ESRO-2A was similar to ESRO-2B except it weighed a little less (74 kg).[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[2] 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.[5] While designed for solar observations ESRO-2B is credited with the detection of X-rays from non-solar sources.[2] The instruments were:

  • Monitor of Energetic Particle Flux
  • Solar and Van Allen Belt Protons
  • Solar and Galactic Alpha Particles and Protons
  • Primary Cosmic Ray Electrons
  • Hard Solar X-rays
  • Soft Solar X-rays
  • Flux and Energy Spectra of Solar and Galactic Cosmic Ray Particles


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