Sharp Focus on the Sun
| Overall height | 10.5 ft (3.2 m) |
|---|---|
| Launch weight | 2300 lb (1042 kg) |
| Sail dimensions | 86 in x 97 in (2.18 m x 2.46 m) |
| Wheel dimensions | 56 in x 21 in (1.42 m x 0.53 m) |
From a 345-mile-high perch above the interference of Earth's atmosphere, the Hughes-built Orbiting Solar Observatory (OSO-I) spacecraft amassed a wealth of data about the sun's radiation.
The spacecraft was launched June 21, 1975, on a two-stage Delta 1910 booster from Kennedy Space Center, Fla. The National Aeronautics and Space Administration (NASA) sponsored the development of the OSO series to help scientists better understand how the sun functions. The OSO-I ("I" as in the ninth letter of the alphabet, not the Roman numeral) spacecraft was the last in a series that began in 1962, and the only one built by Hughes Space and Communications Company. The OSO program was directed by NASA's Goddard Space Flight Center in Greenbelt, Md.
The scientific objectives of the OSO-I mission were to:
- Measure solar ultraviolet and X-ray radiation.
- Study flares and other solar phenomena.
- Continuously study the solar corona.
- Map solar radiation.
- Map the celestial sphere.
- Map zodiacal light.
- Investigate the distribution of proton and energy flux.
Of these, the primary mission objective was to investigate X-ray and ultraviolet radiation in the turbulent, gaseous band between the sun's surface and the upper reaches of the corona, called the chromosphere/corona interface. The corona is the faint outermost layer of the sun's atmosphere, normally seen only during a total solar eclipse. Eight experiments were carried on OSO-I. Two experiments scanned the sun's chromosphere to study ultraviolet radiation, the others made solar and celestial X-ray measurements of the extremely hot plasma and high-energy particles radiated into space.
The experiments and their sources or suppliers were:
- High-resolution ultraviolet spectrometer measurements, University of Colorado, Boulder.
- Chromosphere fine structure study, Centre National de la Recherche Scientifique.
- High-sensitivity graphite crystal spectroscopy of stellar and solar X-radiation, Columbia University, N.Y.
- Mapping X-ray heliometer, Lockheed Missiles & Space Co.
- Soft X-ray background radiation investigation, University of Wisconsin, Madison.
- Cosmic X-ray spectroscopy, Goddard Space Flight Center.
- High-energy celestial X-rays, Goddard.
- Extreme ultraviolet radiation from Earth and space, U.S. Naval Research Laboratory.
The corona constantly sheds high-energy material and high-temperature gases that stream into space. This explosion is called the solar wind.
As solar energy reaches Earth, its lethal ultraviolet and X-rays are absorbed in portions of the atmosphere, while the visible "white light" strikes and warms the lower atmosphere, land, and seas. However, the protection afforded by the atmosphere restricts scientists' ability to observe the sun. Thus the observatory was situated in space.
OSO-I trained its onboard instruments at the sun with a pointing accuracy of 1 arcsecond, or 1/3600 of a degree. This is comparable to a rifle marksman keeping a 10-foot target in his sights over the distance from Boston to Washington, D.C. -- about 400 miles. Its stable platform and finely focused scrutiny enabled OSO-I to scan areas of the sun's rim in 450-mile-wide swaths. Previous OSO satellites had 60 arcseconds (1/60 degree) of pointing accuracy and scanned a solar area 27,000 miles wide.
The observatory was a spin-stabilized vehicle with a despun platform (or sail) and a spinning section (or wheel). The sail section consisted of a solar panel that provided primary power, and two pointing instruments able to scan the sun or search for and point at any location/phenomenon on it, such as flares or sunspots.
The sail was 86 inches (2.18 meters) wide, 97 inches (2.46 meters) high, and generated power of 400 watts. The spinning section of the observatory resembled a large, circular drum 56 inches (1.42 meters) in diameter and 21 inches (0.53 meters) high.
Overall height of the spacecraft (wheel and sail combined) was 10.5 feet (3.2 meters). Spacecraft weight at liftoff was 2300 pounds (1042 kg). The Delta booster placed the observatory into a 345-mile (552 km) circular orbit inclined 33 degrees to the equator. The spacecraft had a mission life of more than a year, but exceeded that by transmitting data until Sept. 26, 1978, outliving all other OSOs by at least six months.