SWAS

Description

SWAS (Submillimeter Wave Astronomy Satellite) is part of NASA's Small Explorer (SMEX) program. The SWAS instrument is a submillimeter wave telescope launched on December 2, 1998 and operated in a 600 km circular orbit with 70° inclination. Scientific objectives of SWAS are to study the chemical composition, energy balance and structure of interstellar clouds, both galactic and extragalactic, and the processes that lead to the formation of stars and planets. SWAS focuses on the following spectral lines: (1) water molecule at 556.936 GHz; (2) oxygen molecule at 487.249 GHz; (3) CI at 492.161 GHz; (4) carbon 13 monoxide molecule at 550.927 GHz; and, (5) oxygen 18 water molecule at 548.676 GHz). Detailed 1 degree x 1 degree maps of giant molecular and dark cloud cores are generated from a grid of measurements taken at 3.7 arc-min 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 the University of Cologne. 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 has a 55 x 71 cm elliptical off-axis Cassegrain telescope with a beam width of 4 arc-min at operating frequencies. SWAS is designed to make pointed observations stabilized on three axes, with a position accuracy of about 38 arc-s, and jitter of about 24 arc-s. Attitude information is obtained from gyros whose drift is corrected via a star tracker. Momentum wheels are used to maneuver the spacecraft. Information on the SMEX program is provided by Baker, D., et al., NASA's Small Explorer program, Phys. Today, 45, No. 12, 44-51, Dec. 1991.