It is 20 years ago this year that Europe, in collaboration with the United States, launched the first infrared observatory into space. Its infrared powers revealed a secret universe that, to this day, continues to fascinate. The more astronomers look, the better the picture gets...

Everything started when the German-born British astronomer William Herschel, famous for discovering the planet Uranus, discovered infrared radiation in 1800. He used a thermometer to measure the heating power of the Sun's light, having split the light into a rainbow of colours. He noticed that the temperature increased towards the red end of the spectrum and continued to increase beyond, even though no light was visible. Scientists eventually called these invisible 'heat' rays infrared.

In 1856, the Astronomer Royal for Scotland, Charles Piazzi Smythe, invented infrared astronomy by climbing Mount Teide on Tenerife and detecting infrared radiation coming from the Moon. However, the instrumentation was crude by modern standards and little improved for the next 100 years.

In the latter half of the 20th century, came the next major step in infrared astronomy. By 1965, astronomers Gerry Neugebauer and Robert Leighton made the first infrared survey of the cosmos. To do so, they used a declassified version of the infrared technology developed by military organisations to build 'night vision' tools. They found ten objects that were only visible at infrared wavelengths, but four years later, the list had grown to thousands. Clearly, infrared astronomy could provide an entirely new insight of a hidden universe, one that is invisible at optical wavelengths.

The desire to see more triggered the Infrared Astronomical Satellite (IRAS). A joint project between The Netherlands, United Kingdom, and United States, it surveyed the Universe for 11 months during 1983 and detected about 500 000 infrared sources.

Even before the launch of IRAS, ESA was already working on its successor, the Infrared Space Observatory (ISO). Designed to look more closely at the objects discovered by IRAS, ISO did that and a lot more. It proved to be the most successful infrared space telescope to date. Its instruments showed how similar the chemical composition of our Solar System is to that of other star systems. It stunned astronomers by revealing that our Solar System contains twice as many asteroids as previously counted, and that water and organic molecules are widespread in space. Visible light telescopes would never have been able to see into dusty regions of space in this way.

This month, NASA continues the tradition by launching the Space Infrared Telescope Facility (SIRTF). However, as with IRAS before, ESA is already preparing to build a successor, Herschel. This new spacecraft will have the most sophisticated infrared telescope ever built, with a mirror 1.5 times larger than NASA-ESA's famous Hubble Space Telescope. What dark secrets will it find?

Herschel will reveal the birth of stars and whole galaxies in details that would astonish early space infrared pioneers. The joint NASA-ESA James Webb Space Telescope (JWST), an infrared space telescope to replace the (largely optical) Hubble Space Telescope, will look at the sky in infrared also. Even ESA's mission to find Earth-like planets, the Darwin mission, will consist of infrared instruments. Why? You can detect a planet's atmosphere and the possible giveaway signs of life most easily in the infrared region of the spectrum.

We have come a long way since the birth of infrared satellite astronomy 20 years ago. What surprises will the next 20 years bring?