Astronomers are enthralled by results from the European Space Agency's Infrared Space Observatory, ISO, which reveal the chemistry of our Galaxy in unprecedented detail. Surprisingly conspicuous in the neighbourhood of stars at the end of their lives is water vapour made by the combination of primordial hydrogen with oxygen atoms newly manufactured by the stars themselves. Water then reappears during the formation of new stars and planets from the interstellar medium. This happened at the origin of tbe Solar System, and incidentally supplied the water which accounts for more than half of a human being's body weight. In retracing this history, ISO also observes water in the form of ice in cooler regions around the stars, and in the dust surrounding young stars, from which planets could evolve.
Comets represent an intermediate stage in planet-building, and they contain much water ice.
According to one hypothesis the newly formed Earth received some of its water directly from impacting comets. Water vapour in the Earth's atmosphere has prevented telescopes on the ground from detecting the water vapour among the stars, except in very unusual circumstances. ISO orbiting in space escapes the impediment of the atmosphere. Excellent onboard instruments register the characteristic infrared signatures of water vapour, water ice and many other materials. So has the Short Wavelength Spectrometer, which also detects water ice.