Far Ultraviolet Spectroscopic Explorer
FUSE in a pre-launch cleanroom
|Organization||Johns Hopkins APL / NASA|
|Launch date||June 24, 1999 at 15:44 UTC|
|Launched from||Cape Canaveral
|Launch vehicle||Delta II 7320|
|Mission length||Three years (design)
Eight years (actual)
|Mass||1400 kg (3086.5 lb)|
|Type of orbit||Circular low Earth orbit|
|Orbit height||746 km (402.8 nmi) to 760 km (410.4 nmi)|
|Orbit period||99.9 minutes|
|Telescope style||Far ultraviolet|
|Wavelength||90.5 to 119.5 nm|
The Far Ultraviolet Spectroscopic Explorer (FUSE) is a space-based telescope operated by the Johns Hopkins University Applied Physics Laboratory. FUSE was launched on a Delta II rocket on June 24, 1999, as a part of NASA's Origins program. FUSE detected light in the far ultraviolet portion of the electromagnetic spectrum, between 90.5-119.5 nanometres, which is mostly unobservable by other telescopes. Its primary mission was to characterize universal deuterium in an effort to learn about the stellar processing times of deuterium left over from the Big Bang. FUSE resides in a low Earth orbit, approximately 760 km (410 nmi) in altitude, with an inclination of 25 degrees and just less than a 100 minute orbital period. Its Explorer designation is Explorer 77.
On July 12, 2007, FUSE's final reaction wheel, which is required for accurately pointing a spacecraft, failed and efforts to restart it were unsuccessful. An announcement was made on September 6 that because the fine control needed to perform its mission had been lost, the FUSE mission would be terminated.
Optical design 
Although the original specification was to have a Wolter-type grazing incidence telescope, the final design of the FUSE telescope comprises four individual mirrors. Each of the four mirrors is a 39-by-35 cm (15.4-by-13.8 in) off-axis parabola. Two mirror segments are coated with silicon carbide for reflectivity at the shortest ultraviolet wavelengths, and two mirror segments are coated with lithium fluoride over aluminum that reflects better at longer wavelengths. This optimizes performance over the entire spectral range.
Each mirror has a corresponding astigmatism-corrected, holographically-ruled diffraction grating, each one on a curved substrate so as to produce four 1.65 m (5.4 ft) Rowland circle spectrographs. The dispersed ultraviolet light is detected by two microchannel plate intensified double delay-line detectors, whose surfaces are curved to match the curvature of the focal plane.
Science results 
Over 400 scientific papers have been written using data from FUSE, with subjects ranging from cool stars to the intergalactic medium. One of the primary science goals of FUSE was to study the abundance of deuterium, an isotope of hydrogen. Because of the large number of atomic absorption and emission lines in the far-ultraviolet, FUSE enabled many studies of galactic, extragalactic and intergalactic chemistry and chemical evolution.
- "NSSDC Master Catalog Display: Spacecraft". NASA.gov. 2007-05-18. Retrieved 2007-09-07.
- "NSSDC Master Catalog Display: Spacecraft Launch/Orbital Information". NASA.gov. 2007-05-18. Retrieved 2007-09-07.[dead link]
- NASA HQ (2007-09-06). "ROSES-07 Amendment 20: Cancel FUSE Legacy Science Program". Spaceref.com. Retrieved 2007-09-07.
- "FUSE 1 Satellite details". N2YO.com. Retrieved 2007-09-07.
- Content, David A.; et al. (July 1990). "Optical design of Lyman/FUSE". Instrumentation in Astronomy VII 1235: pp. 943–952. Bibcode:1994SPIE.2011...34K. doi:10.1117/12.19157.
- Sahnow, D.J.; et al. (1995-07-03). "The Far Ultraviolet Spectroscopic Explorer Mission". JHU.edu. Retrieved 2007-09-07.
- "FUSE Publications". JHU.edu. Retrieved 2007-09-07.
- FUSE website at Johns Hopkins University
- FUSE website at Institut d'Astrophysique de Paris
- FUSE archive at Multimission Archive at STScI (MAST)