The Infrared Space Observatory was Europe's answer to the problem that infrared astronomy from the ground is nearly impossible — the atmosphere both absorbs the wavelengths and glows in them, and the telescope itself glows too unless it is refrigerated.
It carried four instruments cooled by superfluid helium to about three degrees above absolute zero, and it was the first infrared space telescope capable of detailed spectroscopy rather than simple photometry — meaning it could identify specific molecules rather than merely detect warmth.
It found water almost everywhere: in star-forming regions, around dying stars, in planetary atmospheres and in the disks where planets form. It also detected complex carbon molecules in interstellar space, strengthening the case that organic chemistry is widespread rather than exceptional.
Its observations of dusty galaxies revealed that a substantial fraction of star formation in the universe is hidden behind dust and invisible to optical telescopes — a finding that substantially revised estimates of the cosmic star formation history.
The helium ran out after two and a half years, in 1998, as planned. Its archive was designed to be reused and has produced published papers for decades since, and its instruments' design fed directly into Herschel, which flew eleven years later with a mirror four times the area.
