Operated by the European Space Research Organisation, ESRO 2B made astronomical surveys primarily in x-ray and solar particles detectors.

On 10 December 1968 (approx 195 days since mission start) the on-board tape recorder suffered a mechanical failure.

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.

[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.