Far Ultraviolet Spectroscopic Explorer, also called FUSE, is a satellite designed to observe heavenly objects in a type of light called far ultraviolet light. Scientists often describe light in terms of its wavelength, the distance between two successive crests of a wave. Visible light ranges in wavelength from about 400 to 700 nanometers. One nanometer equals 1 billionth of a meter, or 1/25,400,000 inch. The light that FUSE detects ranges in wavelength from 90 to 120 nanometers. The National Aeronautics and Space Administration (NASA) launched FUSE on June 24, 1999. The satellite orbits 500 miles (800 kilometers) above Earth’s surface, circling the planet once every 100 minutes. Due to mechanical failure, FUSE was deactivated on Oct. 18, 2007.
The Far Ultraviolet Spectroscopic Explorer’s primary scientific instrument is a telescope equipped with a spectrograph. A spectrograph is a device that spreads light into its various wavelengths. The resulting band of light is called a spectrum (plural spectra). The telescope’s mirrors focus far ultraviolet light into the spectrograph, and electronic detectors record the spectrum. At regular intervals, the satellite transmits these data to radio antennas on the ground. The information is relayed to a control center at Johns Hopkins University in Baltimore, Maryland. There, computers convert the data into a form that astronomers can use for analysis.
The far-ultraviolet spectra FUSE produces can reveal information about an object’s chemical composition, temperature, density, and movement. In addition, each FUSE spectrum can provide information on clouds of gas and dust that lie between Earth and the object being observed. Such clouds occupy vast regions of space that appear empty when observed in visible light. The sensitivity of FUSE’s primary instrument enables astronomers to detect such faint material and study its characteristics in detail.
One of FUSE’s most interesting discoveries is a large region of extremely hot, low density gas surrounding our galaxy, the Milky Way. Some astronomers had predicted the existence of this region, often called the galaxy’s corona. However, no telescopes on the ground or in space had detected it. Astronomers identified the corona by combining data from many separate FUSE observations. Other FUSE observations confirmed the presence of similar structures around nearby galaxies.
Astronomers from around the world have used FUSE to observe thousands of objects, ranging from nearby stars and planets to faraway galaxies and extremely luminous, distant objects called quasars. The data gathered by FUSE contain so much information that they will remain available for astronomers to analyze even though the mission has ended.
