 SWAS satellite | |
| Names | Explorer-74 SWAS SMEX-3 |
|---|---|
| Mission type | Submillimetre astronomy |
| Operator | NASA / Goddard |
| COSPAR ID | 1998-071A |
| SATCATno. | 25560 |
| Website | https://www.cfa.harvard.edu/swas/ |
| Mission duration | 2 years (planned) 6 years, 8 months and 26 days (achieved) |
| Spacecraft properties | |
| Spacecraft | Explorer LXXIV |
| Spacecraft type | Submillimeter Wave Astronomy Satellite |
| Bus | SWAS |
| Manufacturer | Goddard Space Flight Center |
| Launch mass | 288 kg (635 lb) |
| Payload mass | 102 kg (225 lb) |
| Dimensions | 1.63 × 1.02 m (5 ft 4 in × 3 ft 4 in) |
| Power | 230watts |
| Start of mission | |
| Launch date | 6 December 1998, 00:57:54UTC |
| Rocket | Pegasus XL (F25) |
| Launch site | Vandenberg,Stargazer |
| Contractor | Orbital Sciences Corporation |
| Entered service | 19 December 1998 |
| End of mission | |
| Deactivated | 1 September 2005 |
| Orbital parameters | |
| Reference system | Geocentric orbit |
| Regime | Low Earth orbit |
| Perigee altitude | 638 km (396 mi) |
| Apogee altitude | 651 km (405 mi) |
| Inclination | 69.90° |
| Period | 97.60 minutes |
| Instruments | |
| Submillimeter Wave Telescope | |
.gif) Submillimeter Wave Astronomy Satellite mission patch ← TRACE (SMEX-4) Wide Field Infrared Explorer (SMEX-5) → ← TRACE (Explorer 73) Wide Field Infrared Explorer (Explorer 75) → | |
Submillimeter Wave Astronomy Satellite (SWAS, alsoExplorer 74 andSMEX-3) is aNASAsubmillimetre astronomy satellite, and is the fourth spacecraft in theSmall Explorer program (SMEX). It was launched on 6 December 1998, at 00:57:54UTC, fromVandenberg Air Force Base aboard aPegasus XLlaunch vehicle.[1] The telescope was designed by theSmithsonian Astrophysical Observatory (SAO) and integrated byBall Aerospace, while the spacecraft was built by NASA'sGoddard Space Flight Center (GSFC).[2] The mission'sprincipal investigator is Gary J. Melnick.[1]
The Submillimeter Wave Astronomy Satellite mission was approved on 1 April 1989. The project began with the Mission Definition Phase, officially starting on 29 September 1989, and running through 31 January 1992. During this time, the mission underwent aconceptual designreview on 8 June 1990, and a demonstration of the Schottky receivers andacousto-optical spectrometer concept was performed on 8 November 1991.[3]
The mission's Development Phase ran from February 1992, through May 1996. The Submillimeter Wave Telescope underwent apreliminary design review on 13 May 1992, and acritical design review (CDR) on 23 February 1993.Ball Aerospace was responsible for the construction of and integration of components into the telescope. TheUniversity of Cologne delivered the acousto-optical spectrometer to Ball for integration into the telescope on 2 December 1993, whileMillitech Corporation delivered the Schottky receivers to Ball on 20 June 1994. Ball delivered the finished telescope to Goddard Space Flight Center on 20 December 1994. GSFC, which was responsible for construction of the spacecraft bus, conducted integration of spacecraft and instruments from January through March 1995. Spacecraft qualification and testing took place between 1 April 1995, and 15 December 1995. After this, SWAS was placed into storage until 1 September 1998, when launch preparation was begun.[3]
SWAS was designed to study the chemical composition, energy balance and structure ofinterstellar clouds, both galactic and extragalactic, and investigate the processes ofstellar and planetary formation.[1] Its sole instrument is a telescope operating in the submillimeter wavelengths offar infrared andmicrowave radiation. The telescope is composed of three main components: a 55 × 71 cm (22 × 28 in) elliptical off-axisCassegrain reflector with a beam width of 4arcminutes at operating frequencies,[1][4] twoSchottky diode receivers, and an acousto-optical spectrometer.[2] The system is sensitive to frequencies between 487–557GHz (538–616μm), which allows it to focus on thespectral lines of molecularoxygen (O2) at 487.249 GHz; neutralcarbon (C i) at 492.161 GHz; isotopicwater (H218O) at 548.676 GHz; isotopiccarbon monoxide (13CO) at 550.927 GHz; and water (H2O) at 556.936 GHz.[1][2] Detailed 1° x 1° maps of giant molecular and dark cloud cores are generated from a grid of measurements taken at 3.7 arcminutes spacings. SWAS's submillimeter radiometers are a pair of passively cooled subharmonic Schottky diode receivers, with receiver noise figures of 2500-3000 K. An acousto-optical spectrometer (AOS) was provided by theUniversity of Cologne, inGermany. Outputs of the two SWAS receivers are combined to form a final intermediate frequency, which extends from 1.4 to 2.8 GHz and is dispersed into 1400 1-MHz channels by the AOS. SWAS is designed to make pointed observations stabilized on three axes, with a position accuracy of about 38 arcseconds, and jitter of about 24 arcseconds. Attitude information is obtained from gyroscopes whose drift is corrected via a star tracker. Momentum wheels are used to maneuver the spacecraft.[1]
| Comparison[5] | |||
| Name | Year | Wavelength | Aperture |
|---|---|---|---|
| Human eye | - | 0.39–0.75 μm | 0.01 m (0.39 in) |
| SWAS | 1998 | 540–610 μm | 0.55 × 0.71 m (1 ft 10 in × 2 ft 4 in) |
| Spitzer | 2003 | 3–180 μm | 0.85 m (2 ft 9 in) |
| HubbleWFC3 | 2009 | 0.2–1.7 μm | 2.4 m (7 ft 10 in) |
| Herschel | 2009 | 60–672 μm | 3.5 m (11 ft) |
The SWAS instrument is a submillimeter-wave telescope that incorporates dual heterodyne radiometers and an acousto-optical spectrometer. SWAS will measure water, molecular oxygen, atomic carbon, and isotopiccarbon monoxide spectral line emissions from galactic interstellar clouds in the wavelength range 540-616 micrometres. Such submillimetre wave radiation cannot be detected from the ground because of atmospheric attenuation. The SWAS measurements will provide new information about the physical conditions (density and temperature) and chemistry in star-forming molecular clouds.[6]
The spacecraft was delivered toOrbital Sciences Corporation at Vandenberg Air Force Base on 2 November 1998, for integration onto their Pegasus XL launch vehicle.[3] Launch occurred on 6 December 1998, at 00:57:54 UTC, from Orbital Sciences'StargazerL-1011 TriStar mothership.[1][7] Its initial orbit was a near-circular 638 × 651 km (396 × 405 mi) with aninclination of 69.90°.[8]
SWAS was originally scheduled to launch in June 1995 but was delayed due to back-to-back launch failures of the Pegasus XL launch vehicle 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.[9]
The commissioning phase of the mission lasted until 19 December 1998, when the telescope began producing useful science data.[10] The SWAS mission had a planned duration of two years and a cost estimate of US$60 million,[9][11] 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.[3] The decision was made to end science and spacecraft operations on 21 July 2004, at which time the spacecraft was placed into hibernation.[12]
To support theDeep Impact mission at comet9P/Tempel, SWAS was brought out of hibernation on 1 June 2005. Vehicle check-out was completed on 5 June 2005 with no discernible degradation of equipment found. SWAS observations of the comet focused on isotopic water output both before and after theDeep Impact impactor struck the comet's nucleus on 4 July 2005. 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 1 September 2005.[13]
As of 2023[update], SWAS remains in Earth orbit on stand-by.
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