Mission Overview

The first in a series of 8 successfully launched Orbiting Solar Observatories (OSO 1) was launched on 7 March 1962. The 200 kg spacecraft had a 9-sided spinning wheel section 1.2 m in diameter joined onto a fan- shaped sail section. It was put into a roughly circular orbit (291-nm x 275-nm) at ~ 575 km altitude, 32.8 degrees inclination. The orbital period of 94.7 minutes meant that OSO-1 orbited the Earth 15.2 times each day. It s primary mission objectives were to measure the solar electromagnetic radiation in the UV, X-ray, and gamma-ray regions. Secondarily, it was to investigate dust particle in space. Data transmission ended on 6 August 1963. The satellite reentered the Earth's atmosphere 8 October 1981.

OSO-1 performed well until its second onboard tape recorder gave up the ghost. This anomaly occurred on Tuesday, 15 May 1962. The loss of its last functional data recorder meant that all subsequent measurements had to be transmitted in real-time.

OSO-1 would continue making and transmitting solar physics measurements until May of 1964. At that time, the spacecraft power supply died when its solar cells failed. Although dormant, OSO-1 would continue to orbit the Earth for another seventeen (17) years. The spacecraft reentered the Earth’s atmosphere on Thursday, 08 October 1981.

OSO-1 and all succeeding OSO satellites contributed significantly to progress in the realm of solar physics. The OSO Program laid the foundation for more sophisticated and detailed study of our Sun through the auspices of such solar probes as SOHO, Ulysses and Skylab. Indeed, NASA’s Solar Probe Plus probe, currently scheduled to fly within the Sun’s coronal region sometime in the 2015/2016 period, will continue the legacy begun long ago by OSO-1.


There were a number of X-ray and gamma-ray experiments aboard OSO 1 for performing solar observations. One instrument, however, the University of Minnesota Gamma-ray Experiment, was designed to provide preliminary measurements of the intensity and directional properties of low-energy gamma-rays in space. The detector operated in the 50 keV - 3 MeV range. For the 50-150 keV range, a NaI(Tl) scintillation crystal monitored radiation through a lead shield. The detector operating in the 0.3-1.0 MeV and 1.0-3.0 MeV energy regions used two scintillators connected as a Compton coincidence telescope.


The U. Minnesota gamma-ray experiment on OSO 1 produced a measurement of the extraterrestrial gamma-ray flux between 0.5-3.0 MeV, and an indication of its origin on the celestial sphere. Equally important, this experiment began to define the background problems encountered in gamma-ray astronomy.