|Mission duration||3 years|
|Manufacturer||Ball Brothers Research Corporation (BBRC)|
|Launch mass||635 kilograms (1,400 lb)|
|Start of mission|
|Launch date||29 September 1971, 09:50:00UTC|
|Launch site||Cape Kennedy LC-17A|
|End of mission|
|Decay date||9 July 1974|
|Perigee altitude||321.0 kilometers (199.5 mi)|
|Apogee altitude||572.0 kilometers (355.4 mi)|
|Epoch||29 September 1971, 05:50:00 UTC|
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".
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 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:
Among the notable scientific results from OSO 7 were:
The OSO 7 was nearly lost at launch, due to a loss of hydraulic pressure in the second-stage guidance control system ~7 seconds prior to second-stage engine cutoff. The nominal plan was for the spacecraft to be separated from the second stage with the spin axis normal to the Sun direction, so that the sail could be oriented to the Sun, allowing the batteries to be fully charged on orbit. As it was, the orbit was slightly eccentric instead of circular, and the orientation of the spacecraft immediately after launch was unknown, so that the sail could not acquire Sun lock. The spacecraft was launched with its batteries fully charged, giving approximately 12 hours for the controllers, directed by NASA's John Thole, to recover before the spacecraft lost power and command ability. Several hours passed as engineers attempted to interpret the signal strength from the tumbling spacecraft in terms of its transmitting antenna pattern. Finally, an hour or two before the end, Thole decided to abandon caution and "start slewing", and by luck and skill, control was regained.
Because the resulting orbital apogee was ~572 km instead of the planned ~350 km for the nominal circular orbit, several times each day OSO 7 passed fairly deeply into the Van Allen radiation belts, so that bombardment by high energy protons made it somewhat radioactive. The activity then decayed slowly during other times of the day. The complexly varying instrument internal radioactivity complicated the analysis of data from the sensitive X-ray and gamma-ray instruments on board.
The flight spare for OSO H was later acquired by the U.S. Air Force, modified and re-instrumented, and then launched in 1979 as P78-1 (also known as Solwind), the satellite which was shot down by the USAF in a successful anti-satellite missile test in 1985. OSO 7 and P78-1 were not identical in appearance, but more similar to each other than either were to the earlier OSO 1 through OSO 6 spacecraft, or to the final OSO 8.
The content of this article was adapted and expanded from NASA's HEASARC: Observatories OSO 7 [dead link] and NASA's National Space Science Data Center: OSO 7  (Public Domain)