- Email: [email protected]
The INFRARED SPACE OBSERVATORY (ISO)
0273—1177/93 $6.00 + 0.00 Copyright ~ 1993 COSPAR
Adv. Space Res. Vol. 13, No. 12, pp. (12)485—(12)494, 1993
Printed in Great Britain. All rights reserved.
THE INFRARED SPACE OBSERVATORY
(ISO) M. F. Kessler* and H. J. Habing** * Space Science Department ofESA, Postbus 299, 2200 AG Noordw~jlcThe Netherlands ** Sterrewacht Leiden, Postbus 9513, 2300 RA Leiden, The Netherlands
ABSTRACI’
The INFRARED SPACE OBSERVATORY (ISO), a project of the European Space Agency (ESA), will make various astronomical observations in the wavelength range of 2 to 200 p.m. Two-thirds of its observing time will be available to guest observers via the traditional route of proposal submission. ISO will be launched in 1995 by an Ariane 4 from Kourou (Guyana) and be brought into a veiy elongated orbit with a 24hr period of which 16 hr can be used for astronomical observations. The payload module is essentially a large cryostat with a tank that will be filled with over 2000 litres of superfluid helium to keep the instruments cold (2 to 4 K) during an expected lifetime of 18 months. Inside the dewar, there is a Ritchey-Chretien telescope with a primaiy mirror of diameter 60cm. Four instruments, each provided by a different P1 consortium, share the ISO focal plane. These instruments are : a photometer, a camera and two spectrometers one for the wavelength range 2-45 p.m and the other for 45-180 p.m. -
INTRODUCTION The scientific potential of infrared astronomy was first demonstrated by observations with ground-based telescopes operating in the few atmospheric windows, as well as by observations with aircraft- and balloon-borne telescopes. However, all of these systems are limited by atmospheric absorption of the incoming celestial radiation and by the addition of photon shot-noise associated with the thermal emission from the warm telescope optics and from the atmosphere. Operating outside the Earth’s atmosphere with a cooled telescope and instruments removes these limitations and allows about a thousand-fold increase in sensitivity. The first major step in this direction was taken with the highly successful Infrared Astronomical Satellite (IRAS), which surveyed nearly all the sky in four broad infrared bands between 8 and 120 p.m. ISO, being an observatory satellite, will build upon the IRAS results by making detailed observations of selected sources. Compared with IRAS, ISO will have a longer operational lifetime, wider wavelength coverage, better angular resolution, more sophisticated instruments and, through a combination of detector improvements and longer integration times, a sensitivity gain of up to several orders of magnitude. ISO results from a proposal submitted to ESA in 1979. It was selected in 1983 as the next new stait in the ESA Scientific Programme and the complement of four scientific instruments was approved in 1985. Currently, ISO is in its main development phase with the launch scheduledfor 1995. ISO will provide astronomers with a facility of unprecedented sensitivity for detailed exploration of the Universe, ranging from objects in the solar system right out to the most distant extragalactic sources. Its cryogenically-cooled telescope will be equipped with four scientific instruments, which together will permit imaging and photometric, spectroscopic and polarimetnc observations at wavelengths from 2.5 to beyond 200 jim. (12)485
(12)486
M. F. Kessler and H. J. Habing
ISO, a development model of which is shown under test in Figure 1, is designed to be a true observatory, with scientific instrumentation capable of tackling a wide range of astrophysical problems; two-thirds of its observing time will be available to the general astronomical community.
_
~*~_~b _
Figure 1: Development model of the ISO satellite undertest at Aerospatiale, Cannes (courtesy
Aerospatiale) SCIENTIFIC OBJECTIVES The ISO part of the spectrum is of great scientific interest, not only because it is here that cool objects (15-300 K) radiate the bulk of their energy, but also because it has a rich variety of atomic, ionic, molecular and solid-state spectral features. Measurements at these wavelengths permit the determination of many physical parameters, including energy balance, temperatures, abundances, densities and velocities of the interstellar gas in our galaxy and in many others. Owing to the much-reduced extinction, infrared observations are particularly well-suited to probing the properties of objects obscured at visible wavelengths. ISO will be offering high-sensitivity and sophisticated observing facilities for a relatively unexplored part of the spectrum. It is therefore expected that its scientific programme will have an impact on virtually every field of astronomy. Some possible highlights are summarised below. Another potential interest of the IR wavelength concerns the fact that highly redshifted UV and visual radiation will be detected there. This potential will probably not yet be explored by ISO, but leftfor future missions.
The Infrared Space Observatory
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In the solar system, the giant planets are among the best targets and they can be imaged at various wavelengths. Spectroscopic observations offer many possibilities, such as the discovery of new molecules, tracing of minor atmospheric constituents, or determination of isotopic ratios. The helium and deuterium abundance ratios can be measured in Uranus and Neptune, where they are expected to have their primordial values. Titan is the only satellite known to have an atmosphere, and a spectroscopic study of it may lead to a better understanding of the Earth’s original atmosphere. Spectroscopy of cometary heads may provide insight into the gas-to-dust ratio and into molecule formation. There is every hope of detecting comets in the outskirts of the solar system, thereby permitting study of the material of the presolarnebula. Stellar evolution will be studied via observations of circumstellar shells, especially those around cool giant stars. Such question can be addressed as How large is the mass-loss rate ? What is the nuclear enrichment of the outfiowing material compared with the material from which the star once formed? Absorption lines of several molecules and their isotopes are predicted to be easily detectable. When studied at high resolution, these lines wifi permit conditions in the stellar wind to be determined. Planetary nebulae will be mapped in various forbidden lines. Such lines, together with hydrogen and helium re-combination lines, can also be studied in Wolf-Rayet stars. IRAS has discovered a large number of stars with thick circumstellar shells in the bulge of our galaxy. We need to know Are these stars truly Asymptotic Giant Branch stars similar to those in the galactic disk? Are the planetary nebulae in the galactic bulge similar to the planetary nebulae in the disk of the Galaxy? Rich rewards are expected from ISO-based studies of protostars and regions with star formation. WAS has discovered point sources and small-scale structure in a large number of dark clouds. The nature of these sources will probably be revealed in the next few years through ground-based and airborne observations. However, for the many weak sources in particular, such follow-up observations will be inadequate. ISO offers colour/colour diagrams for large numbers of weak point sources, and it may also discover multiple objects or small clusters of stars. At present, no generally accepted examples of stars in their Helmholtz contraction phase are known, but they may be discovered through detailed spectroscopy in the far-infrared. Determination of the excitation levels and the line shapes of several molecular transitions may show that some point sources are indeed contracting spheres. The nature of dust particles will be an important topic. By studying the outlow from cool giants (see above) one may hope to obtain insight into the formation of the particles. A study of the spectral features between 3 and 16 p.m could explain the nature of the “hot cirrus” discovered by IRAS. Do small particles such as polycyclic aromatic hydrocarbons (PAHs), transiently heated by single ultraviolet photons, account for the emission from these dust clouds? In cold clouds, one can study the 158 p.m [CIIIline, which is the major cooling line of this gas. This line is often expected to be so strong that the hyperfine satellite lines of l3C+ may also be visible. Shock waves, an important phenomenon in the interstellar medium, produce rich line spectra. Easy and accurate measurement of fine-structure lines in the infrared will contribute significantly to the determination of dnergy budgets and accurate abundances in HII regions. This is especially true for the HII regions in the inner galaxy. They are optically invisible because ofthe interstellar extinction, but their existence is known from radio observations. Galactic gradients in metal abundances and in isotope ratios may be derived more reliably than has been possible so far and will help us greatly in understanding the evolution of the Galaxy. It may also prove possible to determine the abundance differences between our Galaxy and its neighbours, such as M3l and M33. Much information is expected to be gathered about the interstellar medium in nearby galaxies, complementing present and future millimetre-line observations. Especially promising is the 158 pm [CIII line. Maps of galaxies at near- and medium-infrared wavelengths will be made through narrow filters with angular resolutions of 6 arcsec (at 6 pm) and 180 arcsec (at 180 p.m). In combination with radio maps, one will see the position and condition of the interstellar material and will get a bird’s-eye view of the star-formation process. Short-wavelength maps will also be obtained which will be very useful for the study of the population of evolved stars.
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M. F. Kessler and H. J. Habing
IRAS has detected thousands of galaxies. But is the radiation from the centre of the disk, from starburst or merging galaxies ? ISO’s improved angular resolution and its high sensitivity between 50 and 200 p.m
will help to answer this question in a large number of cases. Galactic nuclei can be studied spectroscopically without being troubled by interstellar extinction. Infrared fine structure and recombination lines offer very good opportunities. Broadband photometry will be obtained between 3 and 200 p.m. Whether a nucleus is “normal” or “active” may be a matter of gradation, although active galaxies are also more “active” in the infrared. Finally, one comes to cosmology. A difficult but exciting task is the search for the large amount of invisible matter thought to be present in the Universe. This mass could exist in the form of brown dwarfs (stars that have never entered a nuclear-burning phase) or cool low-mass stars in the halos of galaxies. ISO may detect them. If such matter does exist, the cosmic expansion could come to a stop in the distant future. The era of galaxy formation is usually placed at z>3. How galaxy formation proceeded is largely a matter of speculation, but all scenarios agree that much infrared radiation is produced in the process. ISO may pick up galaxies out to z=6. It may even fmd out whether the brighter ones are surrounded by a cluster of other galaxies. SATELLITE DESIGN The ISO spacecraft is 5.3 m high, 2.3 m wide and will weigh approximately 2400 kg at launch. A cutaway schematic is shown in Fig. 2. It consists of a payload module (the upper cylindrical part in the figure), which carries the conical sun shade, the two star trackers and a service module, which provides the basic spacecraft functions. These include the structure and the load path to the launcher, the solar array mounted on the sun shield, and sub-systems for thermal control, data handling, power conditioning, telemetry and telecommand (using two antennas) and attitude and orbit controL The last item providesthe three-axis stabiisation to an accuracy of a few arcseconds, and also the raster pointing facilities needed for the mission. It consists of sun and earth sensors, star trackers, a quadrant star sensor on the telescope axis, gyros and reaction wheels, and uses a hydrazine reaction-control system. The downlink bit rate is 33 kbitjs, of which about 24 kbit/s are dedicated to the scientific instruments.
WNSIEWWUH
iA SERV)CE MODULE
~ SIWERFUJID HELIUM TANK
4
‘V
I 1~ INTERFACE WITH —‘\
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/
TElESCOPE WITH SCIENI1FIC INSTRUMENTS
Figure 2 : A cut-away schematic of the ISO satellite showing the majorcomponents The payload module is essentially a large cryostat. Inside the vacuum vessel is a toroidal tank containing over 2100 litres at launch of superfluid helium, which will provide an in-orbit lifetime of at least 18 months. Some of the infrared detectors are directly coupled to the helium tank and are at a temperature of
The Infrared Space Observatory
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around 2 K. All other units are cooled by means of the cold boil-off gas from the liquid helium. This is first routed through the optical support structure, where it cools the telescope and the scientific instruments to temperatures of around 3 K. It is then passed along the baffles and radiation shields, before being vented to space. A small auxiliary tank containing about 60 litres of normal liquid helium meets all of ISO’s cooling needs for the last 100 hours before launch. Mounted on the outside of the vacuum vessel is a sunshade, which prevents direct sunlight from entering the cryostat. Suspended in the middle of the tank is the telescope, which is a Ritchey-Chretien configuration with an effective aperture of 60 cm. The optical quality of its mirrors is adequate for diffraction-limited performance at a wavelength of 5 p.m; however, the pointing stability of the spacecraft means that diffraction-limited operation of the system is restricted to wavelengths longer than around 10 p.m. Stringent control over straylight, particularly that from bright infrared sources outside the telescope’s field of view, is necessary to ensure that the system’s sensitivity is not degraded. This is accomplished by imposing viewing constraints and by means of the sunshade, the Cassegrain and main baffles, and an additional light-tight shield around the instruments. The scientific instruments are mounted on the opposite side of the optical support structure to the primary mirror, each one occupying an 80 degree segment of the cylindrical volume available. The 20 arcmin
total unvignetted field of view of the telescope is distributed radially to the four instruments by a pyramid mirror. Each experiment receives a 3 arcmin unvignetted field, centred on an axis at an angle of 8.5 arcmin to the main optical axis. THE SCIENTIFIC INSTRUMENTS The ISO instrument complement consists of a camera (ISOCAM), an imaging photopolarimeter (ISOPHOT), and two spectrometers, the short wavelength spectrometer (SWS) and the long wavelength
spectrometer (LWS). Each instrument is being built by an international consortium of institutes using national funding and will be delivered to ESA for in-orbit operations. In keeping with the observatory nature of ISO, the individual instruments are being optimised to form a complete, complementary and versatile common-user package. The total payload provides photometric and imaging capabilities at various spatial and spectral resolutions from 2.5 to beyond 200 p.m and spectroscopic capabilities at medium and high resolution from 2.5 to 180 p.m. Instrument! Principal Investigator
Participating countries
Main function
Wavelength (pm)
Spectral resolution
Spatial resolution
Outline description
ISOCAM (C. Cesarsky, CEN-Saclay, F)
F. GB, I, S. USA
Camera and polarimetry
2.5—17
Broad-band, narrowband and circular variable filters
Pixel FOVs of 1.5, 3, 6 and 12 arcsec
Two channels, each with a 32 x32 element detector array
ISOPHOT (D. Lemke, MPI für Astronomic, Heidelberg, FRG)
D, DK, E, GB, IRL, SF. USA
Imaging photopolarimeter
2.5 —200
Broad-band and narrow-band filters
Variable from diffraction-limited to wide-beam
Three subsystems: (i) Multi-band, multiaperture photopolarimeter (3—120 pm) (ii) Far-infrared camera (50—240 pm) (iii) Spectrophotometer (2.5—12 pm)
Near-IR grating spectrometer with R—90
SWS (Th. de Graauw, D, NL, USA Lab, for Space Research. Groningen, ML), LWS (P. Ctegg. F, GB, I, USA Queen Mary College, London. GB)
Short-wavelength spectrometer
2.5—45
1000 across wavelength range and 2 x l0~from 15 to 35 pm
14x20 and 20 )< 30 arcsec
Two gratings and two Fabry-Ptrot interferometers
Long-wavelength spectrometer
45—180
200 and l0~across wavelength range
1.65 arcmin
Grating and two FabryPerot interferometers
Table 1: Main characteristics of the ISO instruments
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M. F. Kessler and H. J. Halting
A: ISOCAM
B : ISOPHOT
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The Infrared Space Observatory
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A summary of the characteristics of the 4 instruments is given in Table I, while Figure 3 shows a schematic diagram of each instrument. ISO’s spectroscopic and photometric capabilities are presented in
Figure 4. At all wavelengths, it will be possible to build up images by operating the satellite in its raster pointing mode. However, across much of its wavelength range, ISO will also be capable of direct imaging in broad and narrow spectral bands. The number ofpixels and the pixel field of view vary as a function of wavelength (Table 2). Wavelength Range (3m) 2.5—5 5—17 60—120 120—200
No. of Pixels
Pixel f.o.v. (arc secs) 1.5, 3, 6, 12 1.5, 3, 6, 12 44 90
___________________
32x32 32x32 3x3 2x2
Table 2 : Imaging capability ofthe ISO instruments. Further information on the individual instruments may be found in Cesarsky /1/ for ISOCAM, Lemke /2/ for ISOPHOT, de Graauw /3/ for SWS and Clegg /4/for LWS SWS F~J~y.pg~c~
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Figure 4 : Spectroscopic and Photometric capabilities of the ISO instruments.
The four instruments view adjacent areas on the sky and switching between them will involve repointing the satellite. In principle, only one instrument will be operated ata time; however, when the camera is not the main instrument, it will be used in a so-called “parallel mode” to acquire extra astronomical data. Whenever possible, the long-wavelength channel of the photometer will be used during satellite slews.
This “serendipitous” mode will lead to a partial sky survey at wavelengths out to 200 p.m, thus complementing the IRAS survey, which only extended to wavelengths of 120 p.m. THE ISO SCIENCE TEAM The ISO Science Team is advising ESA on all scientific aspects of the mission throughout the project’s lifetime. The Team’s main aims are to maximise the mission’s scientific return and to ensure that ISO maintains its principal characteristic as an observatory satisfying the needs of the scientific community at large. The Team consists of the instrument Principal Investigators (Table 1), the Mission Scientists (Th. Encrenaz, Observatoire de Paris; H. Habing, Sterrewacht, Leiden; M. Harwit, National Air and Space Museum, Washington; A. Moorwood, ESO; J.L. Puget, Institut d’Astrophysique Spatiale, Orsay) and the ESA Project Scientist and Payload Manager. The role of the Mission Scientists is to provide scientific input to the project and to represent the interest of the general astronomical community. JASS 13:t2-GG
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M. F. Kesslerand H. J. Halting
OBSERVING TIME Nearly two-thirds of ISO’s observing time will be available to the scientific community via the traditional route of proposal submission and review. In addition to this Open Time, there will also be Guaranteed Time for the groups who provide the instruments, for the Mission Scientists and for the Observatory Team, who will be responsible for all scientific operations. A coordinated programme of observations, to be carried out in the guaranteed time, is under preparation by the holders of the guaranteed time. These observations will be published with the Call for Observing Proposals and will be reserved.
The first Call for Observing Proposals will be issued about 18 months before launch. It will solicit proposals for observations to be carried out in the period from 3 to 10 months after launch. Due to the large number of observations expected to be proposed for ISO, the proposal-handling system will be automated as much as is possible. Thus, proposals will have to be submitted electronically. ESA intends to issue a software package -to run on the proposers’ own computer- to help the community prepare these electronic proposals and to check them before transmission to the Observatory, either via network file transfer or on floppy disks. This package, called the Proposal Generation Aids (cf. figure 5), will contain full details of the predicted performance of the satellite and its instruments as well as electronic “forms” or “templates” to be filled in by the proposer to specify the exact details of the observations requested. After receipt by ESA, the proposals will be subjected to peer review in the usual way and an independent Observing Time Allocation Committee (OTAC), chaired by L. Woltjer, will recommend, via allocation of priorities, which observations should be included in the ISO observing programme. Recommendations will be made on a “per object” basis, as was the case for EXOSAT, rather than on a “per shift” or “per night” basis as is done with IIJE and most ground-based telescopes. In order that the best use can be made of the mission’s limited lifetime, there will be a review of the implementation of the observing programme about 5 months after launch; if actual spacecraft, instrument or ground segment performances differ from those predicted, the Observing Time Allocation Committee will recommend suitable adjustments to the programme. During the in-orbit phase of the mission, it is planned to issue two further Calls for Proposals so as to give the community the opportunity to react to initial results. Further details of observing with ISO can be found in Kessler /5/. OPERATIONAL ASPECFS
ISO will be launched by an Ariane-44P vehicle into a transfer orbit and its hydrazine reaction-control system will then be used to attain the operational orbit. This will have a 24 h period, a perigee height of 1000 km, an apogee height of 70 000 km. and an inclination to the equator of around 5 degree. In this orbit, ISO will spend mostof its time outside the Earth’s radiation belts; this reduces the number of events in the infrared detectors induced by the trapped protons and electrons, which in turn leads to higher sensitivity for the scientific instruments.
ESA will provide only one ground station, enabling ISO to make astronomical observations for about 1213 h per day. The mission’s scientific return could be greatly increased by the addition of a second ground station, which would permit ISO to be operated for the 16 h per day that it will spend outside the trappedparticle belts. An agreement, for the provision of a second station and associated resources, is under discussion between ESA, ISAS and NASA. The in-orbit operations of the ISO spacecraft and instruments will be carried out by a team of scientists and engineers located at the ISO Control Centre at Villafranca in Spain. An functional overview of the Science Operations Centre is given in Figure 5. During its scientific use, ISO will always be in contact with the ground segment however, it is planned to minimise real-time modifications to the observing programme in order to maximise the overall efficiency of the satellite. Thus, the observing timeline will be frozen by the Mission Planning software at least 3 days in advance and the detailed instrument and
spacecraft commands stored in a “Central Command Schedule” for release to the satellite atpre-specified times. Some limited capability exists for real-time intervention in specific cases. The downcoming telemetry has ground segment information added to it to make the “Telemetry Distribution Format” TDF. A “quick look” output adequate for an initial judgement of the scientific quality of the data will be
available in real-time. Off-line data reduction will produce 3 standard levels of data products with more
The Infrared Space Observatory
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advanced processing and more detailed calibration for distribution to observers and inclusion in the archive. These products will be the ones from which guest observers make their astronomical analyses.
CENTRAL
PROGRAMME INSTRUMENT
________________
PROPOSAL GENERATION
INPUTS
AIDS
CALL FOR PROPOSALS
4 ~UNIT1
PROPOSAL HANDLING
~
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I MISSION PLANNING PHASES I a 2
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____________
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TELEMETRY NON REAL TIME
TDFCONSTRUCTION
TDFARCHIVE
REALTIME
_________________
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SCIENTIFIC PRODUCT GENERATION Edfted Raw Data
I:
Standard Processed Data
SCIENTIFIC PRODUCT DISTRIBUTION
F”1
—
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ANALYSIS
~
________
OBSERVER
Figure 5 : Functional block diagram of the Science Operations Centre CURRENT STATUS ISO is currently in its main development phase (phase C/D), which started in March 1988. Aerospatiale (F) leads an industrial team of about 35 subcontractors. A full-scale ISO satellite development model was built and successfully tested both thermally and mechanically during 1991. It contained a flight representative and fully functional cryostat but mass/thermal dummies for the electrical units. Flight model hardware manufacture is well under way and much of this hardware has been delivered to the Prime Contractor. Some technical problems have been encountered in the development of the liquid helium and optical sub-systems and are in the course of being solved. The four instrument groups are in the final stages of testing and calibrating their flight models before delivery to the Agency. In general, the measured instrument performances are in line with, or in some cases better than, those expected at the time of proposal.
M. F. Kessler and H. J. Halting
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REFERENCES
1
C.J. Cesarsky, ISOCAM, in : Infrared Astronomy with ISO, Les Houches Series,
2
Th. Encrenaz and M.F. Kessler, Eds., Nova Science Publishers Inc, pp. 31-50, 1992. D. Lemke, ISOPHOT, in: The ISO Photometer, InfraredAstronomy with ISO, Les Houches Series, Th. Encrenaz and M.F. Kessler, Eds., Nova Science Publishers mc, pp. 53-56, 1992.
3
Th. de Graauw, in: The ISO Short Wavelength Spectrometer, Infrared Astronomy with ISO, les Houches Series, Th. Encrenaz and M.F. Kessler, Eds., Nova Science Publishers Inc. pp. 105-118, 1992
4
P.E. Clegg, in : The Long-Wavelength Spectrometer in/SO Infrared Astronomy with ISO, Les
Houches Series, Th. Encrenaz and M.F. Kessler, Eds., Nova Science Publishers Inc, pp. 87-102, 1992. 5
M.F. Kessler, in: Observing with ISO, Infrared Astronomy with ISO, les Houches Series, Th. Encrenaz and M.F. Kessler, Eds., Nova Science Publishers Inc. pp. 13-29, 1992
Adv. Space Res. Vol. 13, No. 12, pp. (12)485—(12)494, 1993
Printed in Great Britain. All rights reserved.
THE INFRARED SPACE OBSERVATORY
(ISO) M. F. Kessler* and H. J. Habing** * Space Science Department ofESA, Postbus 299, 2200 AG Noordw~jlcThe Netherlands ** Sterrewacht Leiden, Postbus 9513, 2300 RA Leiden, The Netherlands
ABSTRACI’
The INFRARED SPACE OBSERVATORY (ISO), a project of the European Space Agency (ESA), will make various astronomical observations in the wavelength range of 2 to 200 p.m. Two-thirds of its observing time will be available to guest observers via the traditional route of proposal submission. ISO will be launched in 1995 by an Ariane 4 from Kourou (Guyana) and be brought into a veiy elongated orbit with a 24hr period of which 16 hr can be used for astronomical observations. The payload module is essentially a large cryostat with a tank that will be filled with over 2000 litres of superfluid helium to keep the instruments cold (2 to 4 K) during an expected lifetime of 18 months. Inside the dewar, there is a Ritchey-Chretien telescope with a primaiy mirror of diameter 60cm. Four instruments, each provided by a different P1 consortium, share the ISO focal plane. These instruments are : a photometer, a camera and two spectrometers one for the wavelength range 2-45 p.m and the other for 45-180 p.m. -
INTRODUCTION The scientific potential of infrared astronomy was first demonstrated by observations with ground-based telescopes operating in the few atmospheric windows, as well as by observations with aircraft- and balloon-borne telescopes. However, all of these systems are limited by atmospheric absorption of the incoming celestial radiation and by the addition of photon shot-noise associated with the thermal emission from the warm telescope optics and from the atmosphere. Operating outside the Earth’s atmosphere with a cooled telescope and instruments removes these limitations and allows about a thousand-fold increase in sensitivity. The first major step in this direction was taken with the highly successful Infrared Astronomical Satellite (IRAS), which surveyed nearly all the sky in four broad infrared bands between 8 and 120 p.m. ISO, being an observatory satellite, will build upon the IRAS results by making detailed observations of selected sources. Compared with IRAS, ISO will have a longer operational lifetime, wider wavelength coverage, better angular resolution, more sophisticated instruments and, through a combination of detector improvements and longer integration times, a sensitivity gain of up to several orders of magnitude. ISO results from a proposal submitted to ESA in 1979. It was selected in 1983 as the next new stait in the ESA Scientific Programme and the complement of four scientific instruments was approved in 1985. Currently, ISO is in its main development phase with the launch scheduledfor 1995. ISO will provide astronomers with a facility of unprecedented sensitivity for detailed exploration of the Universe, ranging from objects in the solar system right out to the most distant extragalactic sources. Its cryogenically-cooled telescope will be equipped with four scientific instruments, which together will permit imaging and photometric, spectroscopic and polarimetnc observations at wavelengths from 2.5 to beyond 200 jim. (12)485
(12)486
M. F. Kessler and H. J. Habing
ISO, a development model of which is shown under test in Figure 1, is designed to be a true observatory, with scientific instrumentation capable of tackling a wide range of astrophysical problems; two-thirds of its observing time will be available to the general astronomical community.
_
~*~_~b _
Figure 1: Development model of the ISO satellite undertest at Aerospatiale, Cannes (courtesy
Aerospatiale) SCIENTIFIC OBJECTIVES The ISO part of the spectrum is of great scientific interest, not only because it is here that cool objects (15-300 K) radiate the bulk of their energy, but also because it has a rich variety of atomic, ionic, molecular and solid-state spectral features. Measurements at these wavelengths permit the determination of many physical parameters, including energy balance, temperatures, abundances, densities and velocities of the interstellar gas in our galaxy and in many others. Owing to the much-reduced extinction, infrared observations are particularly well-suited to probing the properties of objects obscured at visible wavelengths. ISO will be offering high-sensitivity and sophisticated observing facilities for a relatively unexplored part of the spectrum. It is therefore expected that its scientific programme will have an impact on virtually every field of astronomy. Some possible highlights are summarised below. Another potential interest of the IR wavelength concerns the fact that highly redshifted UV and visual radiation will be detected there. This potential will probably not yet be explored by ISO, but leftfor future missions.
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In the solar system, the giant planets are among the best targets and they can be imaged at various wavelengths. Spectroscopic observations offer many possibilities, such as the discovery of new molecules, tracing of minor atmospheric constituents, or determination of isotopic ratios. The helium and deuterium abundance ratios can be measured in Uranus and Neptune, where they are expected to have their primordial values. Titan is the only satellite known to have an atmosphere, and a spectroscopic study of it may lead to a better understanding of the Earth’s original atmosphere. Spectroscopy of cometary heads may provide insight into the gas-to-dust ratio and into molecule formation. There is every hope of detecting comets in the outskirts of the solar system, thereby permitting study of the material of the presolarnebula. Stellar evolution will be studied via observations of circumstellar shells, especially those around cool giant stars. Such question can be addressed as How large is the mass-loss rate ? What is the nuclear enrichment of the outfiowing material compared with the material from which the star once formed? Absorption lines of several molecules and their isotopes are predicted to be easily detectable. When studied at high resolution, these lines wifi permit conditions in the stellar wind to be determined. Planetary nebulae will be mapped in various forbidden lines. Such lines, together with hydrogen and helium re-combination lines, can also be studied in Wolf-Rayet stars. IRAS has discovered a large number of stars with thick circumstellar shells in the bulge of our galaxy. We need to know Are these stars truly Asymptotic Giant Branch stars similar to those in the galactic disk? Are the planetary nebulae in the galactic bulge similar to the planetary nebulae in the disk of the Galaxy? Rich rewards are expected from ISO-based studies of protostars and regions with star formation. WAS has discovered point sources and small-scale structure in a large number of dark clouds. The nature of these sources will probably be revealed in the next few years through ground-based and airborne observations. However, for the many weak sources in particular, such follow-up observations will be inadequate. ISO offers colour/colour diagrams for large numbers of weak point sources, and it may also discover multiple objects or small clusters of stars. At present, no generally accepted examples of stars in their Helmholtz contraction phase are known, but they may be discovered through detailed spectroscopy in the far-infrared. Determination of the excitation levels and the line shapes of several molecular transitions may show that some point sources are indeed contracting spheres. The nature of dust particles will be an important topic. By studying the outlow from cool giants (see above) one may hope to obtain insight into the formation of the particles. A study of the spectral features between 3 and 16 p.m could explain the nature of the “hot cirrus” discovered by IRAS. Do small particles such as polycyclic aromatic hydrocarbons (PAHs), transiently heated by single ultraviolet photons, account for the emission from these dust clouds? In cold clouds, one can study the 158 p.m [CIIIline, which is the major cooling line of this gas. This line is often expected to be so strong that the hyperfine satellite lines of l3C+ may also be visible. Shock waves, an important phenomenon in the interstellar medium, produce rich line spectra. Easy and accurate measurement of fine-structure lines in the infrared will contribute significantly to the determination of dnergy budgets and accurate abundances in HII regions. This is especially true for the HII regions in the inner galaxy. They are optically invisible because ofthe interstellar extinction, but their existence is known from radio observations. Galactic gradients in metal abundances and in isotope ratios may be derived more reliably than has been possible so far and will help us greatly in understanding the evolution of the Galaxy. It may also prove possible to determine the abundance differences between our Galaxy and its neighbours, such as M3l and M33. Much information is expected to be gathered about the interstellar medium in nearby galaxies, complementing present and future millimetre-line observations. Especially promising is the 158 pm [CIII line. Maps of galaxies at near- and medium-infrared wavelengths will be made through narrow filters with angular resolutions of 6 arcsec (at 6 pm) and 180 arcsec (at 180 p.m). In combination with radio maps, one will see the position and condition of the interstellar material and will get a bird’s-eye view of the star-formation process. Short-wavelength maps will also be obtained which will be very useful for the study of the population of evolved stars.
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IRAS has detected thousands of galaxies. But is the radiation from the centre of the disk, from starburst or merging galaxies ? ISO’s improved angular resolution and its high sensitivity between 50 and 200 p.m
will help to answer this question in a large number of cases. Galactic nuclei can be studied spectroscopically without being troubled by interstellar extinction. Infrared fine structure and recombination lines offer very good opportunities. Broadband photometry will be obtained between 3 and 200 p.m. Whether a nucleus is “normal” or “active” may be a matter of gradation, although active galaxies are also more “active” in the infrared. Finally, one comes to cosmology. A difficult but exciting task is the search for the large amount of invisible matter thought to be present in the Universe. This mass could exist in the form of brown dwarfs (stars that have never entered a nuclear-burning phase) or cool low-mass stars in the halos of galaxies. ISO may detect them. If such matter does exist, the cosmic expansion could come to a stop in the distant future. The era of galaxy formation is usually placed at z>3. How galaxy formation proceeded is largely a matter of speculation, but all scenarios agree that much infrared radiation is produced in the process. ISO may pick up galaxies out to z=6. It may even fmd out whether the brighter ones are surrounded by a cluster of other galaxies. SATELLITE DESIGN The ISO spacecraft is 5.3 m high, 2.3 m wide and will weigh approximately 2400 kg at launch. A cutaway schematic is shown in Fig. 2. It consists of a payload module (the upper cylindrical part in the figure), which carries the conical sun shade, the two star trackers and a service module, which provides the basic spacecraft functions. These include the structure and the load path to the launcher, the solar array mounted on the sun shield, and sub-systems for thermal control, data handling, power conditioning, telemetry and telecommand (using two antennas) and attitude and orbit controL The last item providesthe three-axis stabiisation to an accuracy of a few arcseconds, and also the raster pointing facilities needed for the mission. It consists of sun and earth sensors, star trackers, a quadrant star sensor on the telescope axis, gyros and reaction wheels, and uses a hydrazine reaction-control system. The downlink bit rate is 33 kbitjs, of which about 24 kbit/s are dedicated to the scientific instruments.
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Figure 2 : A cut-away schematic of the ISO satellite showing the majorcomponents The payload module is essentially a large cryostat. Inside the vacuum vessel is a toroidal tank containing over 2100 litres at launch of superfluid helium, which will provide an in-orbit lifetime of at least 18 months. Some of the infrared detectors are directly coupled to the helium tank and are at a temperature of
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around 2 K. All other units are cooled by means of the cold boil-off gas from the liquid helium. This is first routed through the optical support structure, where it cools the telescope and the scientific instruments to temperatures of around 3 K. It is then passed along the baffles and radiation shields, before being vented to space. A small auxiliary tank containing about 60 litres of normal liquid helium meets all of ISO’s cooling needs for the last 100 hours before launch. Mounted on the outside of the vacuum vessel is a sunshade, which prevents direct sunlight from entering the cryostat. Suspended in the middle of the tank is the telescope, which is a Ritchey-Chretien configuration with an effective aperture of 60 cm. The optical quality of its mirrors is adequate for diffraction-limited performance at a wavelength of 5 p.m; however, the pointing stability of the spacecraft means that diffraction-limited operation of the system is restricted to wavelengths longer than around 10 p.m. Stringent control over straylight, particularly that from bright infrared sources outside the telescope’s field of view, is necessary to ensure that the system’s sensitivity is not degraded. This is accomplished by imposing viewing constraints and by means of the sunshade, the Cassegrain and main baffles, and an additional light-tight shield around the instruments. The scientific instruments are mounted on the opposite side of the optical support structure to the primary mirror, each one occupying an 80 degree segment of the cylindrical volume available. The 20 arcmin
total unvignetted field of view of the telescope is distributed radially to the four instruments by a pyramid mirror. Each experiment receives a 3 arcmin unvignetted field, centred on an axis at an angle of 8.5 arcmin to the main optical axis. THE SCIENTIFIC INSTRUMENTS The ISO instrument complement consists of a camera (ISOCAM), an imaging photopolarimeter (ISOPHOT), and two spectrometers, the short wavelength spectrometer (SWS) and the long wavelength
spectrometer (LWS). Each instrument is being built by an international consortium of institutes using national funding and will be delivered to ESA for in-orbit operations. In keeping with the observatory nature of ISO, the individual instruments are being optimised to form a complete, complementary and versatile common-user package. The total payload provides photometric and imaging capabilities at various spatial and spectral resolutions from 2.5 to beyond 200 p.m and spectroscopic capabilities at medium and high resolution from 2.5 to 180 p.m. Instrument! Principal Investigator
Participating countries
Main function
Wavelength (pm)
Spectral resolution
Spatial resolution
Outline description
ISOCAM (C. Cesarsky, CEN-Saclay, F)
F. GB, I, S. USA
Camera and polarimetry
2.5—17
Broad-band, narrowband and circular variable filters
Pixel FOVs of 1.5, 3, 6 and 12 arcsec
Two channels, each with a 32 x32 element detector array
ISOPHOT (D. Lemke, MPI für Astronomic, Heidelberg, FRG)
D, DK, E, GB, IRL, SF. USA
Imaging photopolarimeter
2.5 —200
Broad-band and narrow-band filters
Variable from diffraction-limited to wide-beam
Three subsystems: (i) Multi-band, multiaperture photopolarimeter (3—120 pm) (ii) Far-infrared camera (50—240 pm) (iii) Spectrophotometer (2.5—12 pm)
Near-IR grating spectrometer with R—90
SWS (Th. de Graauw, D, NL, USA Lab, for Space Research. Groningen, ML), LWS (P. Ctegg. F, GB, I, USA Queen Mary College, London. GB)
Short-wavelength spectrometer
2.5—45
1000 across wavelength range and 2 x l0~from 15 to 35 pm
14x20 and 20 )< 30 arcsec
Two gratings and two Fabry-Ptrot interferometers
Long-wavelength spectrometer
45—180
200 and l0~across wavelength range
1.65 arcmin
Grating and two FabryPerot interferometers
Table 1: Main characteristics of the ISO instruments
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A: ISOCAM
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A summary of the characteristics of the 4 instruments is given in Table I, while Figure 3 shows a schematic diagram of each instrument. ISO’s spectroscopic and photometric capabilities are presented in
Figure 4. At all wavelengths, it will be possible to build up images by operating the satellite in its raster pointing mode. However, across much of its wavelength range, ISO will also be capable of direct imaging in broad and narrow spectral bands. The number ofpixels and the pixel field of view vary as a function of wavelength (Table 2). Wavelength Range (3m) 2.5—5 5—17 60—120 120—200
No. of Pixels
Pixel f.o.v. (arc secs) 1.5, 3, 6, 12 1.5, 3, 6, 12 44 90
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The four instruments view adjacent areas on the sky and switching between them will involve repointing the satellite. In principle, only one instrument will be operated ata time; however, when the camera is not the main instrument, it will be used in a so-called “parallel mode” to acquire extra astronomical data. Whenever possible, the long-wavelength channel of the photometer will be used during satellite slews.
This “serendipitous” mode will lead to a partial sky survey at wavelengths out to 200 p.m, thus complementing the IRAS survey, which only extended to wavelengths of 120 p.m. THE ISO SCIENCE TEAM The ISO Science Team is advising ESA on all scientific aspects of the mission throughout the project’s lifetime. The Team’s main aims are to maximise the mission’s scientific return and to ensure that ISO maintains its principal characteristic as an observatory satisfying the needs of the scientific community at large. The Team consists of the instrument Principal Investigators (Table 1), the Mission Scientists (Th. Encrenaz, Observatoire de Paris; H. Habing, Sterrewacht, Leiden; M. Harwit, National Air and Space Museum, Washington; A. Moorwood, ESO; J.L. Puget, Institut d’Astrophysique Spatiale, Orsay) and the ESA Project Scientist and Payload Manager. The role of the Mission Scientists is to provide scientific input to the project and to represent the interest of the general astronomical community. JASS 13:t2-GG
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OBSERVING TIME Nearly two-thirds of ISO’s observing time will be available to the scientific community via the traditional route of proposal submission and review. In addition to this Open Time, there will also be Guaranteed Time for the groups who provide the instruments, for the Mission Scientists and for the Observatory Team, who will be responsible for all scientific operations. A coordinated programme of observations, to be carried out in the guaranteed time, is under preparation by the holders of the guaranteed time. These observations will be published with the Call for Observing Proposals and will be reserved.
The first Call for Observing Proposals will be issued about 18 months before launch. It will solicit proposals for observations to be carried out in the period from 3 to 10 months after launch. Due to the large number of observations expected to be proposed for ISO, the proposal-handling system will be automated as much as is possible. Thus, proposals will have to be submitted electronically. ESA intends to issue a software package -to run on the proposers’ own computer- to help the community prepare these electronic proposals and to check them before transmission to the Observatory, either via network file transfer or on floppy disks. This package, called the Proposal Generation Aids (cf. figure 5), will contain full details of the predicted performance of the satellite and its instruments as well as electronic “forms” or “templates” to be filled in by the proposer to specify the exact details of the observations requested. After receipt by ESA, the proposals will be subjected to peer review in the usual way and an independent Observing Time Allocation Committee (OTAC), chaired by L. Woltjer, will recommend, via allocation of priorities, which observations should be included in the ISO observing programme. Recommendations will be made on a “per object” basis, as was the case for EXOSAT, rather than on a “per shift” or “per night” basis as is done with IIJE and most ground-based telescopes. In order that the best use can be made of the mission’s limited lifetime, there will be a review of the implementation of the observing programme about 5 months after launch; if actual spacecraft, instrument or ground segment performances differ from those predicted, the Observing Time Allocation Committee will recommend suitable adjustments to the programme. During the in-orbit phase of the mission, it is planned to issue two further Calls for Proposals so as to give the community the opportunity to react to initial results. Further details of observing with ISO can be found in Kessler /5/. OPERATIONAL ASPECFS
ISO will be launched by an Ariane-44P vehicle into a transfer orbit and its hydrazine reaction-control system will then be used to attain the operational orbit. This will have a 24 h period, a perigee height of 1000 km, an apogee height of 70 000 km. and an inclination to the equator of around 5 degree. In this orbit, ISO will spend mostof its time outside the Earth’s radiation belts; this reduces the number of events in the infrared detectors induced by the trapped protons and electrons, which in turn leads to higher sensitivity for the scientific instruments.
ESA will provide only one ground station, enabling ISO to make astronomical observations for about 1213 h per day. The mission’s scientific return could be greatly increased by the addition of a second ground station, which would permit ISO to be operated for the 16 h per day that it will spend outside the trappedparticle belts. An agreement, for the provision of a second station and associated resources, is under discussion between ESA, ISAS and NASA. The in-orbit operations of the ISO spacecraft and instruments will be carried out by a team of scientists and engineers located at the ISO Control Centre at Villafranca in Spain. An functional overview of the Science Operations Centre is given in Figure 5. During its scientific use, ISO will always be in contact with the ground segment however, it is planned to minimise real-time modifications to the observing programme in order to maximise the overall efficiency of the satellite. Thus, the observing timeline will be frozen by the Mission Planning software at least 3 days in advance and the detailed instrument and
spacecraft commands stored in a “Central Command Schedule” for release to the satellite atpre-specified times. Some limited capability exists for real-time intervention in specific cases. The downcoming telemetry has ground segment information added to it to make the “Telemetry Distribution Format” TDF. A “quick look” output adequate for an initial judgement of the scientific quality of the data will be
available in real-time. Off-line data reduction will produce 3 standard levels of data products with more
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advanced processing and more detailed calibration for distribution to observers and inclusion in the archive. These products will be the ones from which guest observers make their astronomical analyses.
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Figure 5 : Functional block diagram of the Science Operations Centre CURRENT STATUS ISO is currently in its main development phase (phase C/D), which started in March 1988. Aerospatiale (F) leads an industrial team of about 35 subcontractors. A full-scale ISO satellite development model was built and successfully tested both thermally and mechanically during 1991. It contained a flight representative and fully functional cryostat but mass/thermal dummies for the electrical units. Flight model hardware manufacture is well under way and much of this hardware has been delivered to the Prime Contractor. Some technical problems have been encountered in the development of the liquid helium and optical sub-systems and are in the course of being solved. The four instrument groups are in the final stages of testing and calibrating their flight models before delivery to the Agency. In general, the measured instrument performances are in line with, or in some cases better than, those expected at the time of proposal.
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REFERENCES
1
C.J. Cesarsky, ISOCAM, in : Infrared Astronomy with ISO, Les Houches Series,
2
Th. Encrenaz and M.F. Kessler, Eds., Nova Science Publishers Inc, pp. 31-50, 1992. D. Lemke, ISOPHOT, in: The ISO Photometer, InfraredAstronomy with ISO, Les Houches Series, Th. Encrenaz and M.F. Kessler, Eds., Nova Science Publishers mc, pp. 53-56, 1992.
3
Th. de Graauw, in: The ISO Short Wavelength Spectrometer, Infrared Astronomy with ISO, les Houches Series, Th. Encrenaz and M.F. Kessler, Eds., Nova Science Publishers Inc. pp. 105-118, 1992
4
P.E. Clegg, in : The Long-Wavelength Spectrometer in/SO Infrared Astronomy with ISO, Les
Houches Series, Th. Encrenaz and M.F. Kessler, Eds., Nova Science Publishers Inc, pp. 87-102, 1992. 5
M.F. Kessler, in: Observing with ISO, Infrared Astronomy with ISO, les Houches Series, Th. Encrenaz and M.F. Kessler, Eds., Nova Science Publishers Inc. pp. 13-29, 1992