ULTRAVIOLET ASTRONOMY

study of the ultraviolet spectra of astronomical objects. It has yielded much important information about chemical abundances and processes in interstellar matter, the Sun, and certain other stellar objects, such as white dwarfs. Ultraviolet astronomy became feasible with the advent of rockets capable of carrying instruments above the Earth's atmosphere, which absorbs most electromagnetic radiation of ultraviolet wavelengths (i.e., roughly 100 to 4,000 angstroms) from celestial sources. Much radiation is lost even at the highest altitudes that balloons can reach. During the 1920s, unsuccessful attempts were made to photograph the Sun's ultraviolet spectrum from balloons; not until 1946 did a rocket-borne camera succeed in doing so. Since the early 1960s the United States and several other countries have placed in Earth orbit unmanned satellite observatories carrying telescopes with optical surfaces specially coated for high ultraviolet reflectivity. These include eight Orbiting Solar Observatories, launched from 1962 to 1975 by the U.S. National Aeronautics and Space Administration (NASA), which enabled astronomers to obtain thousands of ultraviolet spectra of the Sun's corona. Another series of U.S. satellites, known as Orbiting Astronomical Observatories, have permitted the study of the interstellar medium and remote stars in the spectral range of 1,200 to 4,000 angstroms. A telescope carried aboard the International Ultraviolet Explorer spacecraft (launched in 1978 by British and European space agencies in collaboration with NASA) allowed significant ultraviolet observations to be made of objects such as comets and quasars. The high-resolution Hubble Space Telescope, deployed in 1990, also collected ultraviolet-wavelength data about faint objects such as nebulae and distant star clusters. NASA's Extreme Ultraviolet Explorer satellite was launched in 1992.