An engineering triumph
From November 1995 until May 1998, all through its operational life in orbit, ISO was one of the coolest objects in the universe. The large cryostat on board the satellite, filled before launch with more than 2000 litres of superfluid helium, kept the telescope and instruments close to the absolute zero of temperature, minus 273 °C. This was essential for an infrared space telescope that had to sense the heat of a cool object a billion light-years away. It also had to point at objects in the sky as accurately as tracking a man at a distance of 1000 kilometres. ISO was a technological challenge for the 35 companies, headed by Aerospatiale, that built the satellite. ISO's four scientific instruments also pushed technology to its limits. The infrared camera ISOCAM, the photo-polarimeter ISOPHOT and the spectrometers SWS and LWS were built by multinational teams with leaders in France, Germany, the Netherlands and the UK, respectively.
The supply of liquid helium needed to keep ISO chilled set a limit to its operating life. Thanks to excellent engineering and a fortuitous combination of circumstances at launch, the helium lasted over 28 months, much longer than the 18 months required, and made possible about 30 percent more observations.
"It was a very demanding and collaborative effort," said Hans Steinz, ESA's Project Manager for ISO. "But it was a total success as evidenced by the enthusiasm of user scientists, and ISO's technology will be used to build still better space telescopes."
"During the original planned lifetime, the giant star-forming regions in the constellations of Taurus and Orion were inaccessible to ISO," commented Martin Kessler, ISO's Project Scientist. "The extended life brought them into view and gave astronomers an eagerly-awaited chance to unlock some of their secrets."