INFO 11-1998: ISO completes its observations of the Universe by infrared light
9 April 1998ESA's Infrared Space Observatory, ISO, has ended its observational phase, long after the expiry date of the end of May 1997 foreseen in the specifications for the mission. Instead of the required 18 months, the astronomers have been able to use ISO for more than 28 months, and as a result have gathered a wealth of additional information about the Universe. Altogether ISO has made over 26 000 observations of cosmic objects.
At 07:00 on 8 April 1998, engineers at ESA's ground station at Villafranca near Madrid reported that ISO's telescope was beginning to warm up, above its nominal operating temperature close to absolute zero. This was the sign that ISO had exhausted the superfluid helium used to achieve the very low temperatures necessary for infrared astronomy.
Observations ceased at 23:07 when the temperature of the instruments had risen above -269 °C. At that time, ISO was observing the galaxy NGC 1808 with the camera (ISOCAM) for Prof. J. Hough (UK). The astronomers then handed ISO over to the engineering team for check-outs and decommissioning. The spacecraft will be switched off in about 28 days' time. Infrared rays come from cool places in the sky, and ISO would have been dazzled by its own heat unless its optical system were extremely cold. At its launch in November 1995, ISO carried a supply of 2000 litres of superfluid helium, which boils at -271 °C. Slow venting of the helium into space maintained the low temperature of the optical system.
How did ISO achieve its extended life? Three months came from a prudent safety margin in the engineering calculations of the rate of loss of helium. Two months were the result of favourable circumstances in the launch campaign at Kourou in French Guiana. During a technical check of the Ariane 44P launcher, ISO's engineers seized the chance to recharge the helium, and the quick launch that followed meant that the outer parts of the cryogenic system of the spacecraft had little time to warm up in Kourou's tropical climate.
Finally, the daily loss of helium turned out to be 17 less than expected, at the lower end of a range of possibilities considered by the engineers. That gave ISO an estimated five months of additional life.
The date for the helium to run out remained somewhat uncertain, and astronomers and engineers have been alert to the possibility of its exhaustion for the past few weeks. The huge bonus to astronomers, from ISO's long operating life, is only one of their reasons to be grateful for the highly successful engineering and operations of ISO. The pointing accuracy of ISO's telescope was ten times better than required by the specification, and its jitter was one-fifth of what would have been considered tolerable. Smooth, well-planned operations were extended on a daily basis by a US ground station supporting ESA's Villafranca station. ISO was observing selected targets in the sky for 90-95 of the time available.
Even in between the planned observations a "serendipity" programme with the ISOPHOT instrument charted cool objects as ISO slewed from one target to the next.
Although ISO's observations are now completed, astronomers will be analysing them for years to come, assisted by a team of mission experts who will continue their work at Villafranca and other centres worldwide until 2001.
Commenting on the completion of ISO's observational phase, ESA's director of science, Roger Bonnet, says: "We always knew that ISO's helium would run out one day, and we can be grateful that it has lasted for so long. This is a time for celebration, not sorrow, and I congratulate the VILSPA, ESTEC and ESOC staff who have operated so efficiently the spacecraft as well as European industry which developed such an excellent mission and our scientists who built such an outstanding payload . I wish European astronomers as well as their colleagues from the USA and Japan to now fully exploit the image data acquired by ISO."