Pergamon Acra Astronauricu Vol. 40, No. 2-8, pp. 467-416, 1991

01997 International Astronautical Federation. Published by Elsevier Science Ltd Printed in Great B&run

PII: SOO94-5765(97)00113-6 0094-5765/97 $17.00 + 0.00

IAF-96-Q.1.02

ESA’s INFRARED SPACE OBSERVATORY (ISO): THE DEVELOPMENT PROGRAMME

Johan A. STEINZ

EUROPEAN SPACE AGENCY / ESTEC P.O. Box 299.2200 AG Noordwijk, THE NETHERLANDS

Abstract

EM’s Infrared Space Observatory, ISO, was put into orbit by Ariane on 17 November 1995 and it has since entered the routine operations phase, providing excellent results for the scientific community. IS0 provides an unprecedented opportunity to make scientific observations of very weak infrared radiation sources.

To make such measurements, the telescope and scientific instruments are kept at near absolute zero temperature by enclosing them in a helium-cooled cryostat, effectively a large thermos-flask with superfluid helium at 1.7 K. The development of the observatory proved to be a very challenging task as there was little experience available with the advanced technologies required for the mission, e.g. cryogenics, optics and attitude control.

This paper describes the mission, the satellite, the development programme and the organisation of the project. Both the development problems encountered and the technology achievements of the programme are emphasized. The performance of the satellite in orbit is also reported. IS0 is an excellent observatory for the scientific community. It is a unique facility and will be the only one in the world with such capabilities for at least the next decade. Q 1997 International Astronautical Federation. Publishedby Elsevier Science Ltd.

Introduction

EM’s Infrared Space Observatory (ISO) satellite is the world’s first true astronomical observatory in space operating at infrared wavelengths. Astronomers are able to choose specific targets in the sky and point IS0 towards them for up to ten hours at a time to

take observations with versatile instruments of unprecedented sensitivity. During its expected lifetime of about 2 years IS0 will be used to study a very large variety of infrared radiation sources in every field of astronomy from solar system objects, through the birth and death of stars, to the most distant extragalactic sources. Figure 1 shows an image taken by IS0 of the Whirlpool Galaxy.

Figure 1. The Whirlpool Galaxy (MSl) imaged by the ISOCAM instrument at a wavelength of 15 microns.

The first major step in this direction was taken with the Infrared Astronomical Satellite (IRAS), which surveyed almost the entire sky in the 8 - 120 micron wavelength range in 1983. Compared with IRAS, IS0 has longer operational time, wider wavelength coverage (2.5 - 240 microns), better angular resolution, more sophisticated instruments.

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Figure 2. ISO’s Orbit.

and a sensitivity gain of several orders of magnitude. Most importantly, IS0 is operated as an observatory, able to point at specific objects in space and observe them continuously. Such opportunities are rare for infrared astronomers. The next possibility using a European satellite after IS0 (in 1995) will be with ESA’s FIRST (Far Infrared and Submillimetre Telescope), which is currently proposed for launch in 2007.

IS0 was conceived in the early 1970’s. The main milestones in the project’s development were:

1982 - Feasibility study completed 1985 - Scientific instruments selected 1987 - Satellite development started 1995 - Satellite launched 1996 - Routine operations underway.

ISO’s specified lifetime is 18 months. Its performance suggests that its actual life may be 2 years.

This paper is but a summary of the IS0 development programme More extensive information is given in the references listed.

Scientific Needs

As observational astronomy has expanded beyond the narrow visible range of the electromagnetic spectrum accessible from the ground, it has become clear that a full understanding of the properties and physics of astronomical sources can only be obtained by studying them across the widest possible frequency range.

The infrared wavelength range (l- 5OOFm), much wider than the visible range of (0.4-0.7pm), is of great scientific interest because it is here that cold objects (lo-100K) radiate most of their energy, but also because of the large variety of spectral features which can be analysed. Dust which is present in large amounts obscures observations in the visible range. Infrared radiation, due to its much longer wavelength, has the great benefit that it can pass through the dusty fog allowing astronomers to observe far-away objects. Observations of cold objects and gases in space down to the lowest possible temperatures, preferably several degrees Kelvin, are of primary interest.

The satellite has to be put in space to avoid the Earth’s atmosphere, which masks almost all the weak infrared radiation from space (through absorption and, being warm, its own radiation emission). The telescope and scientific instruments also have to be kept extremely cold, near absolute zero, to ensure that their own emitted radiation is much less than that of the cold objects being observed.

The Mission

ISO’s orbit has a period of 24 h, a perigee height of 1000 km, an apogee height of 70 500 km, and an inclination to the equator of 5”. The scientifically useful time in this orbit is that spent outside the Earth’s radiation belts, nearly 17 h per day. Instrument performance is generally degraded when in the radiation belts, where operations are therefore generally restricted to satellite and instrument setup, etc.

IS0 is operated from ESA’s Villafranca ground station, near Madrid, Spain. A second ground station, at Goldstone in California (USA), is dedicated to IS0 under a cooperative agreement with NASA and the Japanese Institute of Space and Astronautical Sciences (ISAS). The Goldstone station will be used to relay telecommands and telemetry for some time each day. In return for the support given, both NASA and ISAS may each use IS0 for 0.5 h per day. Over one third of ISO’s scientifically useful time is guaranteed time for the parties involved in the development of the scientific instruments, and the remaining time is available to the general astronomical community.

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IS0 is a true astronomical observatory, the operating concept of which is shown in Figure 3. Astronomers submit their proposals which are screened by peer review groups. Following selection the full proposals are entered electronically, scheduled taking into account all relevant constraints, and then executed by the satellite which sends the data that has been collected to the ground.

Figure 3. An overview of the activities involved in planning, executing and analysing an IS0 observation.

The guaranteed observing time, for those scientists involved in the construction and operation of the facility covers some 13,000 observations. About 35,000 observations in total from some 1,000 proposals have been programmed into the IS0 mission database. The duration of most observations is between 10 and 60 minutes; only a few hundred observations have a duration of l-10 hours.

The Scientific Instruments

The IS0 instrument complement consists of a camera, an imaging photo-polarimeter and two spectrometers (Table 1). Observers are provided with a range of photometric, polarimetric, spectroscopic and imaging capabilities across the entire IS0 wavelength range. Their capabilities in terms of spectral resolution are shown in Fig. 4. These unique instruments will reach out to new frontiers, probing fainter sources with higher spectral and spatial resolution than ever before at these wavelengths. The instruments are so sensitive that they could easily detect the heat radiated from an ice cube from a distance of 1,000 km or that from a human being at the distance of the Moon from the Earth.

Table 1. IS0 Scientific Instruments

Instrument Wavelength (Microns)

Camera and polarimetry Isocam, Saclay (F)

Two detector arrays 32x32 elements

Imaging photopolarimetry Isophot, Heidelberg (D)

Photo-polarimeter (3-110pm) Camera (30-240pm) Spectrophotometer (2.512pm)

Short-wavelength spectrometer SWS, Groningen (NL)

2 Gratings + 2 Fabry-Perot interferometers

Long-wavelength spectrometer LWS, London (UK)

1 Grating + 2 Fabrv-Perot interferometers

2.5-17

2.5240

2.545

45-200

Each instrument was developed by an international consortium of institutes and industries using national funding. and then delivered to ESA for integration into the satellite. These instruments are all very sophisticated and have been optimised to form a complete complementary and versatile common-user facility.

The Spacecraft

The spacecraft has to provide an extremely cold environment for the telescoee and its instruments in order to detect and

Figure 4. Spectroscopic and photometric capabilities of the IS0 scientific instruments.

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Figure 5. Configuration of the IS0 satellite.

measure extremely weak heat sources. In the case of ISO, this is done by placing both the instruments and the telescope in a large, thermally insulated vacuum vessel filled with superfluid helium at a temperature of about 1.7 K (about -271°C). The helium slowly evaporates to provide the necessary cooling and is vented to space. The initial volume of helium in the tank therefore determines the satellite’s lifetime.

The spacecraft consists of a payload module (the upper cylindrical part in Fig. 5) and the service module (below) with its

Figure 6. The IS0 flight-model satellite after mechanical/environmental testing at ESTEC in Noordwijk (NL).

interface to the Ariane launch vehicle. The main characteristics of the IS0 spacecraft are summarised in Table 2. Figures 6 and 7 show the flight-model satellite.

Table 2. IS0 Spacecraft Characteristics

Dimensions: height width

Total mass Superfluid helium (2 K) Telescope aperture Pointing accuracy (jitter) Electrical power Telemetry rate (S-band)

5.3 m 3.5 m 2500 kg 21001 600 mm 2.7 arcsec 580 W 32 kbitls

The payload module is essentially a large cryostat, with a long toroidal tank filled with superfluid helium. The telescope is mounted in the centre of the cryostat. The scientific instruments are mounted behind the telescope’s primary mirror. Some instrument detectors are strapped directly to liquid- helium tank to keep them at very cold temperatures (2-4 K). Infrared radiation from

Figure 7. IS0 flight-model satellite immediately prior to launch on the Ariane -44P vehicle.

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an object in space passes through the telescope and into the instruments as indicated in Figure 5. The payload- module cryostat is well-insulated with thermal blankets and the Sunshield mounted on the side protects it from direct solar heating.

The telescope, shown in Figure 8, has a 60 cm diameter aperture. Its mirrors are polished to optical quality to permit accurate measurements on the ground and at cryogenic temperatures with already developed “standard” optical instruments (the quality of the primary mirrqr is such if scaled up to the radius of curvature of the Earth then the hills and valleys would be within +lm of the ideal shape). The design and development of the optical subsystem including baffles is presented in ref. 3.

The service module at the bottom of the spacecraft provides the traditional spacecraft functions such as power conditioning, data handling, communications and attitude control. The attitude-control subsystem can point the satellite to an object with an accuracy of a few arcseconds, for periods of up to 10 hours independent of their relative motion (equivalent to finding and following a

moving human 100 km away). The onboard computer system also provides for safe autonomous satellite operations in the event of loss of control from the ground for at least three days. This system ensures that the telescope will never be pointed towards the Sun or the Earth, to avoid damaging the scientific instruments.

The Development Programme

The IS0 satellite and its scientific

instruments employ very advanced technologies and therefore called for an extensive development plan which had to be sufficiently flexible to cope with unexpected problems. The approach taken was to try to identify and solve development problems early, and at the lowest level in the integration sequence, thereby reducing the number of problems to be solved at system level to the strict minimum.

The main technical challenges encountered were with the scientific instruments, the telescope, the cryogenic subsystem and cryostat, the attitude control subsystem and the star tracker. The most important technological difficulties that had to be overcome are discussed below in order of the integration sequence and level. The key problem areas, all successfully resolved, are briefly mentioned, full explanations not being within the scope of this paper.

Unit and suhsvstem level develoument

The main technological problem areas resolved were:

The scientific instruments are highly sophisticated employing the ultimate in state- of-the-art technology. Their capabilities are unique and their development took more than 8 years. The main new technologies developed were a large variety of infrared detectors and the cryo-readout electronics which have to work at near zero temperature in the liquid-helium cryostat. Furthermore. the development of reliable mechanisms operating at liquid-helium temperatures was a major achievement.

Figure 8. The IS0 telescope. The telescope’s development is

described in a parallel paper (ref. 3). The main

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challenges were in achieving the required high quality mirror surfaces, cleanliness control and alignment and image quality testing at liquid-helium temperatures.

The cryogenic subsystem required extensive development at all levels, starting with individual components. The overall development can only be completed at full- scale cryostat level, viz. at module and system level (see below).

The attitude control subsystem is completely new, has stringent pointing requirements and is extremely complex. primarily because of the autonomy requirement (which ensures that the satellite can maintain safe attitudes for at least 3 days without ground contact). This requirement put heavy demands on the design, testing and simulations to be performed. The attitude sensors were newly developed. The gyroscope electronics units needed special attention, not least to resolve problems encountered on other projects.

The star tracker development was arduous. The software development was intricate, a new CCD image sensor had to be developed and calibration was difficult. The result is a high-performance, state-of-the-art star tracker.

Satellite module level develoDment

The satellite service module and payload modules were developed separately.

The service module was qualified thermally and mechanically using a structural/thermal model. In parallel, the electrical subsystems were developed and qualified at unit/subsystem level.

The payload module development was a very challenging task, involving a considerable amount of laboratory-type investigations requiring inventive solutions to unforeseen problems. The operation of the cryostat was demonstrated first in ambient laboratory conditions and later in a vacuum chamber that simulated the in-orbit environment. These tests showed that several components of the cryogenic system had to be improved, e.g. vent-line elements, a filling

port. Mechanical tests showed that the liquid- helium tank was not rigid enough (and later, during flight-model tests an important interaction between the compressible liquid- helium and the flexible structure was found). Subsequently, it was found that the liquid- helium valves had to be redeveloped to improve their leakage characteristics and to ensure high reliability. All the necessary improvements were implemented in the final flight-model build, the proto-flight model, as qualification had still to be achieved.

Satellite system development

The overall development at system level was ultimately accomplished using two models:

A Development Model (DM): The Service Module was essentially a structural/thermal model with dummy mass units. The Payload Module (cryostat) was built in full flight configuration. Nearly all development problems were resolved using this model.

A Proto flight Model (PFM): All of the DM’s shortcomings were corrected on the PFM and the latter was then subjected to all qualification tests. A straylight test was also performed for the first time to demonstrate that the cryostat interior would be sufficiently dark in space for the instruments to make meaningful measurements of faint objects. The test and analysis of results had to be done with great care to ensure proper interpretation of the results (one had to show that IS0 would be one of the coldest and darkest places in the Universe). The overall approach was extremely successful: the final PFM test sequence did not reveal any major new problems, confirming that all such problems had been identified and resolved on the DM.

All units were required to be delivered in two models, a flight model and a flight spare (which is generally a refurbished qualification model). The availability of flight spare units contributed greatly to the success of the programme: small problems could be easily resolved by simply exchanging units and thus avoiding any major delays. The scientific instruments also benefitted because the flight model and flight

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spare could be alternately improved in parallel

with satellite development.

Science and Spacecraft Flight Operations

The science operations software development was a very difficult undertaking, both because of the complexity of the scientific instruments and because of the high degree of automation required to perform the scientific operations. The spacecraft operations software development, although complex, proceeded smoothly.

Special efforts were made to establish an integrated test, simulation and training plan involving the total ground segment, all flight operations software, the satellite and its instruments. Two all-up tests each of one week’s duration demonstrated the adequacy of the total system. Subsequently, many simulations and training sessions were undertaken in preparation for the launch.

Launch and Flight Operations

ISO’s launch and subsequent flight has been a true success story, the major milestones being:

- Launch by the Ariane 44P vehicle on 17 November 1996

- Launch and early orbit phase (4 days) controlled from ESOC, Darmstadt, Germany; IS0 put into final orbit.

- Satellite commissioning (21 days) controlled from IS0 Flight Control Centre, Villafranca, Spain: cryocover

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that closed cryostat was ejected and all

spacecraft subsystems and scientific instruments were shown to perform well. Performance Verification Phase (56 days): scientific instruments’ perform- ance demonstrated, their core calibrations established and planned operating mode\ validated. Routine operations started on 4 February 1996. All above operations were completed as planned and on schedule.

In-Orbit Performance

The performance of the satellite and scientific instruments is excellent in every respect with all satellite subsystems working perfectly.

The cryostat is working as expected and the latest estimate is that ISO’s in-orbit lifetime will be about 2 years compared to the 18 months specified (measurements are still to be made of the helium consumption which will allow more accurate predtctions of the lifetime). The cryostat thermal performance is very close to prediction as is shown by Figures 9 and 10. The final equilibrium temperature of the cryostat outer wall is about 115 K and that of the superfluid helium in the tank is 1.73 K.

ISO’s pointing performance is excellent. far better than specified, as can be seen in Table 3.

Figure 9. Cryostat outer wall temperature. Figure 10. Superfluid helium temperature.

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Table 3. ISO’s Pointing Performance

2 sigma, half cone Units Spec. Now

Relative Error (jitter -30 set)

Absolute Drift

arcsec

arcsec/h

< 2.7 < 0.5

< 2.8 <O.l

Absolute Error arcsec c11.7 - 3.5

The scientific instruments are all working extremely well. Figures I and 1 l-l 3 show some typical images and spectra taken. It has been found that the sensitivity is affected to varying degrees by ‘glitches’ caused by high-energy cosmic-ray particles impacting on the infrared detectors. These glitches result primarily in increased noise, but in some cases have necessitated modifications to instrument settings and changes in observing strategy, to optimise instrument performance. All instruments are giving scientific data of excellent quality.

Project Organisation

The overall project organisation is shown in Figure 14. Central to this organisation is the ESA Project Team located at EM’s European Research and Technology Centre, ESTEC, in Noordwijk, The Netherlands. This project team, part of EM’s Scientific Programme Directorate, was responsible for the management of the development, launch and in-orbit commissioning of the satellite. The ESA Space Science Department assumes responsibility during the subsequent routine operations phase of the mission, with spacecraft operations being delegated to ESOC.

The main groups involved in the project’s development have been:

Principal Investigator groups: IS0 has four Principal Investigators (PIs), one for each of the scientific instruments. The scientific instruments were developed with national funding, with each Principal Investigator being responsible for his or her own scientific instrument, each of which was developed by The in-flight performance of the

instruments has shown that they can be operated somewhat deeper into the Van Allen radiation belts than foreseen prior to launch. They are therefore operated for 16.7 h each day rather than the 16 h originally foreseen. The overall science operations system is very efficient: about 95% of the 16.7h/day science window provides meaningful scientific data.

Figure 12. Tbe C II cooling line as seen by the LWS instrument.

Figure 11. ISOPHOT image at a wavelength of 60 microns of the supernova remnant MSH 11-54.

Top left: S 106, an H II reglon, consisting of ionlsed gas and dust

Bottom left: NGC 7027, a planetary nebula, a cloud of dust and gas ejected from a dying star

Top right: Infrared Cirrus, cold wispy dust and gas

Bottom right: the Antennae, two colliding galaxies

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a group involving many institutes and industries (over 45 organisations in total).

The PIs were responsible to the ESA Project for the timely delivery of their scientific instruments for integration and testing with the satellite. In return for their efforts, they are guaranteed about one-third of ISO’s total operations time in orbit. The PIs plan this guaranteed time in great detail and share it with their many Co-Investigators and

Scientific Associates (about 100 astronomers in total).

Science Team: The IS0 Science Team (IST) is an advisory group appointed by the Director of ES-A’s_ Scientific Program-me to advise the Agency on all scientific aspects of the mission. The IST consists of the ESA Project Scientist. the four PIs, five Mission Scientists providing independent advice, and one representative each from the ESA Project Team, from ISAS and from NASA.

Satellite Prime Contractor: The Satellite Prime Contractor, Aerospatiale (F). was responsible for the design and development of the satellite and for the integration and testing of the scientific instruments. Aerospatiale’s main subcontractors were:

DSS (D) - Linde(D) - Aerospatiale - Fokker (NL) - Alenia (I) -

MMS (UK) -

Payload module Cryogenic subsystem Optical subsystem Attitude control Data handling and telecommunications Reaction control

The total industrial organisation required to design and develop the hardware required to build and test the satellite consisted of 32

companies.

Launcher authority: Arianespace provided the launch vehicle and all associated launch services.

Science and snacecraft onerations: ESA’s Space Operations Centre (ESOC) in Darmstadt. Germany, is responsible for the ground segment and for operating the spacecraft. ESOC also coordinates its operations with NASA-JPL which provides the second ground station for IS0 at Goldstone. and with ISAS in Japan which provides support for ISO’s flight operations.

The ESA Space Science Department is responsible for the science operations, i.e. the in-flight operations of the scientific instruments. It assumed full responsibility for the IS0 mission once the satellite commissioning was completed in December 1995.

Conclusion

IS0 is an excellent space observatory. It produces a vast quantity of high-quality data, carrying out an average of 50 observations per day. Scientists the world over are highly enthusiastic about this remarkable new facility, the only one with such capabilities foreseen for at least the next decade.

Figure 13. ISOCAM image of the colliding galaxies known as the Antennae at the wavelength of 15 microns.

Figure 14. IS0 project organisation.

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ISO’s development was extremely challenging. The mission’s success represents a major step forward in Europe’s capabilities and is a tribute to the many people whose efforts have made IS0 the success it is.

References

1. ESA Bulletin No. 81, November

1995. - The IS0 Mission - The IS0 Spacecraft - The IS0 Scientific Instruments - The IS0 Programme - Using IS0

2. ESA Bulletin No. 86, May 1996 - The First Results from IS0 - The IS0 Ground Segment at

Villafranca: Its Integration and End-to-End Validation

3. IS0 Optical Subsystem Design and Performances, C. Singer and M. Anderegg, IAF paper IAF-96-Q. 101, October 1996.