ISO INFO Newsletters: No. 13-May 1998

Liquid Helium depleted on April 8 1998: Satellite switched off on May 16

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So ISO has finally run out of Liquid Helium and, after a one month Technology Test Phase (TTP), was switched off. Information on these events can be found on the front cover and in the `Project Status' article.

Even though ISO ran out of Helium, observations with SWS continued through the TTP! The band 1 detectors (2.36 - 4.08 microns) could still operate at the enhanced temperatures encountered, and a special observing programme was designed prior to Helium depletion to take advantage of any spare time in the TTP schedule. Information on how SWS coped with the new temperature regime and of the observing programme can be found in the articles `SWS operations post-Helium' and `The SWS stellar classification programme'.

This issue also contains observing statistics for the routine phase and the final list of successful discretionary time proposals.

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Figures 1 to 6 give details of the number, type and length of observations carried out by ISO. This list is close to final; however the quality control process is continuing for the last few revolutions. Note that the given numbers do not include observations made during the Performance Verification phase, or calibration observations (which also includes a few polarimetry observations).

Shown in figures 1 to 4 are the distribution of observing time per instrument and per observing mode, expressed both in terms of number of observations, and also by observing time.

By the time ISO's Helium had been depleted, 26200 successful science observations had been performed, along with approximately 4000 calibration observations performed using the normal observing modes. Each of these 30000 observations lasted on average just over 23 minutes. The facility to concatenate two or more observations was used rather extensively however, and therefore each observation chain lasted on average 40 minutes. Due to the skewness of the observation duration distribution, the median length of an observing block lies between 10 and 20 minutes. Distributions are shown in figures 5 and 6.

  
Figure 1: ISO Instrument usage by number of observations

  
Figure 2: ISO Instrument usage by time

 

 

Figure 3: Number of observations per observing mode

 

 

Figure 4: Total time used per observing mode

 

 

Time (hours)

Figure 5: Log distribution of the number of observations against time for all observations

 

 

Time (minutes)

Figure 6: Linear distribution of number of observations against time for observations of less than 1 hour

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ISO's supply of liquid helium ran out on 8 April, thus bringing to an end the highly successful routine operations phase of the world's first true orbiting infrared observatory. This phase lasted nearly 1 year longer than specified. Activities then turned to a 1-month period of calibrations and technology tests prior to the de-commissioning of the satellite on 16 May. During the next 3.5 years, astronomers all over the globe will collaborate to re-analyse and re-calibrate all of ISO's 30000 individual observations to extract the maximum scientific return from this unique mission and to leave behind a legacy archive to serve the community for the next decades.

Some of the main activities during the technology test programme were testing the operation of the star trackers at low altitudes, i.e. in the radiation belts, use of the on-board redundant units that were not needed during the routine operations due to the superb performance of the satellite, and evaluation of the software intended to overcome multiple gyro failures. The results of these test will be of great benefit to other ESA spacecraft, e.g. XMM and Integral, which use some of the same components. During gaps in this programme, the shortest-wavelength detectors in the Short Wavelength Spectrometer were used to measure the 2.36 - 4.08 micron spectrum of nearly 300 stars to extend their spectral classification to the infrared.

The `last light' of ISO occurred just before midnight on 10 May and was an SWS spectrum of Brackett alpha emission in a hot supergiant star. Preliminary results show that this star may well have a disk!

Two `de-orbiting' manoeuvres were carried out on 11 and 14   May to lower the perigee from 1380km to 715km. In this new orbit, it is expected that ISO\ will re-enter the Earth's atmosphere in some 20-30 years time. At 14.00 CET on 16 May, the final ground command to turn off the transmitter was executed, thus ending the operational life of ISO.

Just prior to exhaustion of liquid helium, some of ISO's latest results were announced to the Press at a briefing in London. Among these discoveries were the finding of water vapour on Saturn's moon Titan, complementing ISO's many detections of water throughout the cosmos, detections of young stars in the Orion region of the sky and measurements of infrared galaxies at immense distances.

The detailed analysis and interpretation of the ISO results is only just beginning. All ISO data will be re-processed with the latest software over the summer and an archive will be opened to the astronomical community in the autumn. Thereafter, there will be an international effort, co-ordinated by the ESA ISO Data Centre at Villafranca in Spain, to maximise the exploitation of the ISO data and to prepare the best possible final archive to leave as a legacy from ISO to future astronomers. Many more discoveries still await us!

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A small pool of ISO observing time, called ``Discretionary Time'', was kept available for observations which could not have been foreseen at the time of the proposal process. A list of discretionary time proposals for which observing time was recommended since the last newsletter follows. Note however, that observations were not made for all of these programmes due to the date at which ISO's liquid Helium became depleted.