Submillimeter Wave Astronomy Satellite

TRACE High Energy Transient Explorer Small Explorer program
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Submillimeter Wave Astronomy Satellite
Swas 1.jpg
Illustration of SWAS
NamesExplorer-74, SMEX-3
Mission typeSubmillimeter astronomy
OperatorNASA / Goddard
COSPAR ID1998-071A
SATCAT no.25560
Mission durationPlanned: 2 years
Final: 6 years, 8 months and 26 days[1]
Spacecraft properties
ManufacturerNASA / Goddard
Launch mass288 kg (635 lb)[2]
Payload mass102 kg (225 lb)[2]
Dimensions1.63 × 1.02 m (5.3 × 3.3 ft)[3]
Power230 W[2]
Start of mission
Launch dateDecember 6, 1998, 00:57 (1998-12-06UTC00:57Z) UTC[4]
RocketPegasus XL
Launch siteStargazer, Vandenberg AFB
ContractorOrbital Sciences
Orbital parameters
Reference systemGeocentric
RegimeLow Earth
Semi-major axis6,968.1 km (4,329.8 mi)
Perigee altitude586.2 km (364.2 mi)
Apogee altitude593.8 km (369.0 mi)
Period96.4833 min
Argument of perigee74.6613°
Mean anomaly285.5145°
Mean motion14.9254 rev/day
EpochSeptember 9, 2015, 13:11:43 UTC[5]
Revolution no.90902
Main telescope
NameSubmillimeter Wave Telescope
Diameter55 × 71 cm (22 × 28 in)[6]
Wavelengths540-610 μm

The Submillimeter Wave Astronomy Satellite (SWAS) is a NASA submillimeter astronomy satellite, and is the third spacecraft in the Small Explorer program. It was launched on December 6, 1998 (UTC), from Vandenberg Air Force Base aboard a Pegasus XL rocket.[6] The telescope was designed by the Smithsonian Astrophysical Observatory and integrated by Ball Aerospace, while the spacecraft was built by NASA's Goddard Space Flight Center.[7] The mission's principal investigator is Gary J. Melnick.[6]


SWAS was designed to study the composition and structure of interstellar clouds and investigate the processes of stellar and planetary formation.[6] Its sole instrument is a telescope operating in the submillimeter wavelengths of far infrared and microwave radiation. The telescope is composed of three main components: a 55 × 71 cm (22 × 28 in) elliptical off-axis Cassegrain reflector,[6][8] two Schottky diode receivers, and an acousto-optical spectrometer.[7] The system is sensitive to frequencies between 487–557 GHz (538–616 μm), which allows it to focus on the spectral lines of molecular oxygen (O2) at 487.249 GHz; neutral carbon (C i) at 492.161 GHz; isotopic water (H218O) at 548.676 GHz; isotopic carbon monoxide (13CO) at 550.927 GHz; and water (H2O) at 556.936 GHz.[6][7]

Name Year Wavelength Aperture
Human eye - 0.39–0.75 μm 0.01 m
SWAS 1998 540–610 μm 0.55×0.71 m
Spitzer 2003 3–180 μm 0.85 m
Hubble WFC3 2009 0.2–1.7 μm 2.4 m
Herschel 2009 60–672 μm 3.5 m


The Submillimeter Wave Astronomy Satellite mission was approved on April 1, 1989. The project began with the Mission Definition Phase, officially starting on September 29, 1989, and running through January 31, 1992. During this time, the mission underwent a conceptual design review on June 8, 1990, and a demonstration of the Schottky receivers and acousto-optical spectrometer concept was performed on November 8, 1991.[10]

The mission's Development Phase ran from February 1992, through May 1996. The Submillimeter Wave Telescope underwent a preliminary design review on May 13, 1992, and a critical design review on February 23, 1993. Ball Aerospace was responsible for the construction of and integration of components into the telescope. The University of Cologne delivered the acousto-optical spectrometer to Ball for integration into the telescope on December 2, 1993, while Millitech Corporation delivered the Schottky receivers to Ball on June 20, 1994. Ball delivered the finished telescope to Goddard Space Flight Center on December 20, 1994. Goddard, which was responsible for construction of the spacecraft bus, conducted integration of spacecraft and instrument from January through March 1995. Spacecraft qualification and testing took place between April 1, 1995, and December 15, 1995. After this, SWAS was placed into storage until September 1, 1998, when launch preparation was begun.[10]

The spacecraft was delivered to Orbital Sciences Corporation at Vandenberg Air Force Base on November 2, 1998, for integration onto their Pegasus XL rocket.[10] Launch occurred on December 6, 1998, at 00:57 UTC, from Orbital Sciences' Stargazer L-1011 TriStar mothership.[4][11] Its initial orbit was a near-circular 638 × 651 km (396 × 405 mi) with an inclination of 69.9 degrees.[4]

SWAS was originally scheduled to launch in June 1995, but was delayed due to back-to-back launch failures of the Pegasus XL rocket in June 1994 and June 1995. A launch opportunity in January 1997 was again canceled due to a Pegasus XL launch failure in November 1996.[12]

The commissioning phase of the mission lasted until December 19, 1998, when the telescope began producing useful science data.[13] The SWAS mission had a planned duration of two years and a cost estimate of US$60 million,[12][14] but mission extensions allowed for five and a half years of continuous science operations. During this time, data was taken on more than 200 astronomical objects.[10] The decision was made to end science and spacecraft operations on July 21, 2004, at which time the spacecraft was placed into hibernation.[3]

To support the Deep Impact mission at comet 9P/Tempel, SWAS was brought out of hibernation on June 1, 2005. Vehicle check-out was completed on June 5 with no discernible degradation of equipment found. SWAS observations of the comet focused on isotopic water output both before and after the Deep Impact impactor struck the comet's nucleus on July 4. While water output was found to naturally vary by more than a factor of three during the observation campaign, SWAS data showed that there was no excessive release of water due to the impact event. After three months of observation, SWAS was once again placed into hibernation on September 1, 2005.[1]

As of 2019, SWAS remains in Earth orbit on stand-by.


  1. ^ a b Bensch, Frank; Melnick, Gary J.; Neufeld, David A.; Harwit, Martin; Snell, Ronald L.; et al. (October 2006). "Submillimeter Wave Astronomy Satellite observations of Comet 9P/Tempel 1 and Deep Impact". Icarus. 184 (2): 602–610. arXiv:astro-ph/0606045. Bibcode:2006Icar..184..602B. doi:10.1016/j.icarus.2006.05.016.
  2. ^ a b c "The SWAS Spacecraft". Harvard–Smithsonian Center for Astrophysics. Retrieved September 5, 2015.
  3. ^ a b Rumerman, Judy A. (2009). NASA Historical Data Book, Volume VII: NASA Launch Systems, Space Transportation/Human Spaceflight, and Space Science 1989-1998 (PDF). NASA History Series. NASA. p. 814-815. ISBN 978-0-16-080501-1. SP-2009-4012.
  4. ^ a b c "SWAS - Trajectory Details". National Space Science Data Center. NASA. Retrieved September 8, 2015.
  5. ^ "SWAS - Orbit". Heavens Above. September 9, 2015. Retrieved September 9, 2015.
  6. ^ a b c d e f "SWAS". National Space Science Data Center. NASA. Retrieved September 5, 2015.
  7. ^ a b c Melnick, Gary J.; Stauffer, John R.; Ashby, Matthew L. N.; Bergin, Edwin A.; Chin, Gordon; et al. (August 2000). "The Submillimeter Wave Astronomy Satellite: Science Objectives and Instrument Description". The Astrophysical Journal. 539 (2): L77–L85. Bibcode:2000ApJ...539L..77M. doi:10.1086/312856.
  8. ^ "The SWAS Instrument". Harvard–Smithsonian Center for Astrophysics. Retrieved September 5, 2015.
  9. ^ "Herschel Space Observatory: Related Missions". NASA / Jet Propulsion Laboratory. Retrieved August 9, 2011.
  10. ^ a b c d Tolls, V.; Melnick, G. J.; Ashby, M. L. N.; Bergin, E. A.; Gurwell, M. A.; et al. (May 2004). "Submillimeter Wave Astronomy Satellite Performance on the ground and in orbit". The Astrophysical Journal Supplement Series. 152 (1): 137–162. Bibcode:2004ApJS..152..137T. doi:10.1086/382507.
  11. ^ "The Launch". Harvard–Smithsonian Center for Astrophysics. Retrieved September 8, 2015.
  12. ^ a b Cowen, Ron (January 18, 1997). "Space 1997: The New Year Brings an Uncertain Launch Schedule". Science News. 151 (3). Archived from the original on March 4, 2016. Alt URL
  13. ^ "SWAS Data Online". Legacy Archive for Microwave Background Data Analysis. NASA. April 18, 2008. Retrieved September 9, 2015.
  14. ^ "SWAS". NASA / Science Mission Directorate. Retrieved September 9, 2015.