OSO 7 or Orbiting Solar Observatory 7 (NSSDC ID: 1971-083A), before launch known as OSO H is the seventh in the series of American Orbiting Solar Observatory satellites launched by NASA between 1962 and 1975. OSO 7 was launched from Cape Kennedy (now Cape Canaveral) on 29 September 1971 by a Delta N rocket into a 33.1° inclination, low-Earth (initially 321 by 572 km) orbit, and re-entered the Earth's atmosphere on 9 July 1974. It was built by the Ball Brothers Research Corporation (BBRC), now known as Ball Aerospace, in Boulder Colorado.
While the basic design of all the OSO satellites was similar, the OSO 7 was larger [total spacecraft mass was 635 kg (1397 lb)] than the OSO 1 through OSO 6, with a larger squared-off solar array in the non-rotating "Sail", and a deeper rotating section, the "Wheel".
Sail instruments
The "Sail" portion of the spacecraft, which was stabilized to face the Sun in all the OSO series satellites, carried two instruments on OSO 7, which continuously viewed the Sun during orbit day. These were:
The GSFC X-Ray and EUV Spectroheliograph (covering the wavelength range 2 to 400 Å), under the direction of P.I. Dr. Werner M. Neupert of the NASA GSFC which imaged the Sun in the Extreme Ultraviolet and soft X-ray bands, to determine the temperature and distribution of matter in the corona above active regions and during solar flares. The NRL White-Light Coronagraph and Extreme Ultraviolet Corona Experiment, directed by Dr. Richard Tousey of the US Naval Research Laboratory, which imaged the while light corona, using an occulting disk, allowing comparison between the structure of the corona and the active regions on the solar surface.
Wheel instruments
The rotating "Wheel" component of the spacecraft, which provided overall gyroscopic stability to the satellite, carried four instruments, which looked radially outwards and scanned across the Sun every 2 seconds. Two of these were solar-observing instruments, and the other two were cosmic X-ray instruments: UCSD Hard Solar X-Ray Monitoring instrument, P.I. Prof. Laurence E. Peterson. covered the 2–300 keV energy range using proportional counter and NaI scintillator detectors, plus three small charged-particle detectors for monitoring the local radiation environment. UNH Solar Gamma-Ray Monitor. P.I. Prof. Edward Chupp, observed 0.3–10 MeV solar flare gamma rays with a NaI(Tl) scintillation spectrometer in a CsI(Na) active anti-coincidence shield. MIT Cosmic X-Ray Experiment, P.I. Prof. George W. Clarke, observed cosmic X-ray sources in the range 1.5 to 9 Å. This instrument used proportional counters to observe cosmic X-ray sources in the 1 to 60 keV range, in five broad logarithmically spaced energy bands, with about 1° angular resolution. UCSD Cosmic X-ray Experiment, P.I. Prof. Laurence E. Peterson. This instrument, which had a field of view (FWHM) about 6°, looked out perpendicular to the Wheel spin axis, sweeping a great circle on the sky every 2 seconds. As the Wheel spin axis moved to keep the Sail instruments pointed at the Sun, it scanned the whole sky every 6 months. It featured a 1 cm thick NaI(Tl) scintillation detector that covered the energy range from ~7 keV to ~500 keV in 126 PHA channels, with an effective area of 100 cm2 at the lower energies. The detector was enclosed in a thick CsI(Na) anti-coincidence scintillation shield with 10 holes bored through it, which defined the optical field of view of the detector. Events were individually recorded and telemetered, with time and pulse height tagged for each, at a maximal rate of 3.2 per second.
Scientific results
Among the notable scientific results from OSO 7 were:
All-sky hard X-ray surveys by the MIT and UCSD cosmic instruments. The first observation of solar gamma ray (γ) line emission, due to electron/positron annihilation at 511 keV, from solar flares in early August 1972, by the UNH spectrometer. Long legendary in NASA due to the hazard to human spaceflight, it would have incurred a potentially fatal radiation dose had astronauts been in space at time and outside the Earth's protective magnetosphere (as is the case during much of a lunar Apollo mission). The first clear detection of a coronal mass ejection (CME), by the NRL instrument. Observations of the hard X-ray spectra of the AGN NGC 4151 and Cen A Position and spectral variability of the 14 May 1972 cosmic gamma-ray burst