Infrared Space Observatory

Orbiting telescope with a 60-cm/24-in diameter mirror. It was launched on 17 November 1995 by the European Space Agency and spent 18 months in an elongated orbit, at a range from the Earth of 1,000–70,500 km/620–43,800 mi, as far as possible outside the radiation belts that swamp its detectors.

The ISO made the first discovery of water vapour from a source beyond the Solar System (in planetary nebula NGC 2027); traced the spiral arms of the Whirlpool galaxy and detected sites of star formation there; and obtained the first comprehensive spectrum of Saturn's atmosphere.