''ESRO-2B'' or ''Iris'' (''International Radiation Investigation Satellite''; sometimes ''Iris 2'') or sometimes ''ESRO II'' (or ''ESRO 2''), was a European

astrophysical Astrophysics is a science that employs the methods and principles of physics and chemistry in the study of astronomical objects and phenomena. As one of the founders of the discipline, James Keeler, said, astrophysics "seeks to ascertain the ...

spin-stabilised research

satellite A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scient ...

which was launched in 1968. Operated by the

European Space Research Organisation The European Space Research Organisation (ESRO) was an international organisation founded by 10 European nations with the intention of jointly pursuing scientific research in space. It was founded in 1964. As an organisation ESRO was based on a ...

, ESRO 2B made

astronomical survey An astronomical survey is a general celestial cartography, map or astrophotography, image of a region of the sky (or of the whole sky) that lacks a specific observational target. Alternatively, an astronomical survey may comprise a set of image ...

s primarily in

x-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

and solar particles detectors.

Spacecraft

ESRO-2B was an cylindrical

spacecraft A spacecraft is a vehicle that is designed spaceflight, to fly and operate in outer space. Spacecraft are used for a variety of purposes, including Telecommunications, communications, Earth observation satellite, Earth observation, Weather s ...

with a length of 85 cm and a diameter of 76 cm. On 10 December 1968 (approx 195 days since mission start) the on-board

tape recorder An audio tape recorder, also known as a tape deck, tape player or tape machine or simply a tape recorder, is a sound recording and reproduction device that records and plays back sounds usually using magnetic tape for storage. In its present ...

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, launching on a Scout B rocket from

Vandenberg AFB Vandenberg Space Force Base , previously Vandenberg Air Force Base, is a United States Space Force Base in Santa Barbara County, California. Established in 1941, Vandenberg Space Force Base is a space launch base, launching spacecraft from the ...

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). Spin-stabilised, ESRO-2B had a spin rate of approximately 40

rpm Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or r⋅min−1) is a unit of rotational speed (or rotational frequency) for rotating machines. One revolution per minute is equivalent to hertz. Standards ISO 80000-3:2019 def ...

and re-entered Earth's atmosphere on 8 May 1971 after completing 16,282 orbits.

Instruments

Seven instruments were carried aboard EROS 2B designed to detect high energy cosmic rays, determine the total flux of solar X-rays and to measure

Van Allen belt The Van Allen radiation belt is a zone of energetic charged particles, most of which originate from the solar wind, that are captured by and held around a planet by that planet's magnetosphere. Earth has two such belts, and sometimes others ma ...

protons and

cosmic ray Cosmic rays or astroparticles are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the ...

protons. While designed for solar observations ESRO-2B is credited with the detection of X-rays from non-solar sources. 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

References

{{reflist 1972 in spaceflight European Space Agency satellites