The only way to see water in the Milky Way's clouds

Space · Published 18 October 2026 Swipe more like this →

In short: Earth's atmosphere acts as a shield, blocking the very signals needed to map our galaxy's chemistry. To study the birth of stars and planets, NASA had to send a specialized telescope into orbit to capture submillimeter waves—the invisible signatures of water and oxygen.

The Submillimeter Wave Astronomy Satellite (SWAS) was designed to overcome a fundamental terrestrial limitation: atmospheric attenuation. Because our atmosphere absorbs submillimeter radiation, scientists cannot detect these specific wavelengths from the ground. By placing the telescope in space, SWAS could observe frequencies between 487–557 GHz, allowing it to identify spectral lines of critical molecules like molecular oxygen (O2), neutral carbon (C i), isotopic water (H218O), and water (H2O).

The mission's primary objective was to decode the chemical evolution of interstellar clouds. By measuring these molecules, astronomers could determine the physical conditions—specifically density and temperature—of the environments where new stars and planets are born. SWAS provided the first measurement of the distribution of water across the Milky Way, offering a blueprint of the galaxy's chemical makeup.

The project faced significant hurdles before launch, including three separate Pegasus XL launch vehicle failures between 1994 and 1996 that delayed its debut until December 1998. Despite these setbacks, SWAS exceeded its two-year planned duration, operating for five and a half years. It even performed a unique 'interruption' in 2005 to support the Deep Impact mission, observing comet 9P/Tempel to see if the impact caused an excessive release of water—data which showed no such spike occurred.

Technologically, SWAS utilized a 55 × 71 cm elliptical off-axis Cassegrain reflector and a unique acousto-optical spectrometer provided by the University of Cologne. These components allowed for high-precision mapping of giant molecular and dark cloud cores with a grid of measurements taken at 3.7 arcminute spacings.

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