FIRST — Far Infrared and Submillimetre Space Telescope

Adv. Space Re.,. Vol. 13, No. 12, pp. (12)545—(12)548, 1993 Punted in Great Britain. All rights reserved.

0273—1177193 $6.00 + 0.00 Copyright © 1993 COSPA1~

FIRST FAR INFRARED AND SUBMILLIMETRE SPACE TELESCOPE -

G. Pilbratt Astrophysics Division/Space Science Department of the European Space Agency, ESTECJSA, P.O. Box 299, NL-2200 AG Noordwijlc, The Netherlands

ABSTRACT The present status of the ESA cornerstone mission FIRST is presented. A recent industrial study has generated a spacecraft concept employing a 4.5 m passively cooled telescope with focal plane instrument cooling provided by a superfluid helium cryostat. The model payload complement includes two direct detection instruments as well as two heterodyne instruments. After a shared launch by Ariane 5 into GTO, FIRST propels itself into the 24-hour highly eccentric operational orbit, where observations can be conducted up to 17 hours per day with an expected approximate mission duration of 3 years. An additional complementary study of a non-cryostat spacecraft option will also be performed. INTRODUCTION The Far Infra-Red and Submillimetre Space Telescope (FIRST), is one of the cornerstone missions in the “Horizon 2000” long term European Space Agency (ESA) science programme. At the end of 1991 a System Definition Study (SDS), which had been carried out for ESA by an industrial consortium led by Dornier, was completed. The aim of this study was to define in detail a mission (the goal being a multipurpose observatory serving the entire astronomical community) that attempts to maximize the scientific return from observations in the submililmetre range, covering approximately 1 mm (300 GHz) 0.1 mm = 100 pm (3000 GHz = 3 THz), while staying within the resource allocation for a cornerstone project. —

In this paper the scientific objectives of the FIRST mission, as well as the spacecraft and mission concepts generated by the SDS will be briefly described. Finally present and future activites relevant to FIRST, together with the current schedule, will be presented. SCIENTIFIC OBJECTIVES OF FIRST FIRST will open up the last major part of of the electromagnetic spectrum still mainly inaccessible for astronomers. It will, for the first time, offer near arcsecond imaging in this wavelength region and, with its high throughput and low thermal background, will result in superb sensitivity for both photometry and spectroscopy. Multiband high-resolution spectrometers will give unprecedented information on the physics, chemistry and dynamics of interstellar, circumstellar, planetary and cometary gas and dust. The observational results will benefit many fields of astrophysics. Opening up a new part of the spectrum may also lead to serendipitous discoveries, however, with the present knowledge we expect the primary impact of FIRST to occur in the following areas: The physics of the interstellar medium including its chemistry, kinematics and thermodynamics in both our own Galaxy, and in external galaxies. The Galactic Centre is the only galactic nucleus where measurements of the gas dynamics, energetics and mass distribution have sufficient spatial resolution to quantitatively test different models. It thus serves as a “test laboratory” for investigating the basic processes that may occur in the nuclei of external galaxies or quasars. (12)545

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Star formation mechanisms in our own and in external galaxies, and in particular the variation from galaxy to galaxy. Of great interest for our understanding of star formation are detailed investigations of the density and temperature structure of galactic protostellar condensations where high excitation lines of molecules from outflows and shocks occur. FIRST observations of galaxies at large look-back times to study the way star formation has evolved in time in galaxies is a daunting prospect. Studies of early evolution of galaxies and cosmology. Cosmic evolution is one of the fundamental themes in modern astrophysics; deep sky surveys of faint infrared galaxies over limited sky regions free of local emission will give important information on the effect of luminosity evolution and wifi allow the search for protogalaxies and starburst galaxies at large redshifts. Properties of primitive solar system material. The formation and evolution of our own solar system is another crucial area to be addressed with FIRST. By observing planets at high spectral resolution in many submillimetre spectral lines, new important information on the chemical abundances of elements in the gaseous atmospheres of the giant planets will be gained.

Fig. 1. Cutaway picture of the SDS baseline FIRST spacecraft. The main reflector, tripod and subreflector are visible. The telescope is protected by a sunshield which is supported by its inflatable space rigidized structure. The cryostat is attached to the service module which partially obscures the solar panel and also supports two telemetry antenna booms. FIRST SPACECRAFT AND MISSION During the SDS the industrial contractors, assisted by external scientists, made certain trade-offs of possible designs regarding fundamental areas such as antenna diameter, payload cooling concept

FIRST

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and payload complement. The resulting concept for FIRST (ci. Fig. 1) calls for a free-flyer, with a passively cooled non-deployable 4.5 m diameter main reflector Cassegrain telescope, which is protected by a thermal shield. The focal plane instruments will be cryogenically cooled by a superfluid helium cryostat. The spacecraft is split into two modules with relatively simple interfaces. The service module contains the supporting structure and spacecraft thermal control, solar array, electrical subsystems, attitude and reaction subsystems as well as some payload electronics. The payload module is made up of the submiffimetre telescope, helium cryostat with focal plane assembly, attitude measurement sensors and the thermal shield. The single most challenging item on the spacecraft is the telescope main reflector. Its rms surface accuracy requirement is 6 pm. The baseline design is similar in concept to modern ground-based submiffimetre antennas with the surface being made up of CFRP panels supported by a stiff framework, also made primarily from CFRP, in the form of tubes. An alternative design concept is an a11-CFRP sandwich monolithic “panel-only” construction, without a separate backing structure. The thermal shield around the whole payload module as8embly, offering protection from the solar radiation, is vital. It is an inflatable space rigidized structure and can therefore be folded for launch, inflated by gas in orbit, rigidized, and finally vented. Protected by this shield the antenna will cool to a mean operating temperature of approximately 150 K. The sky coverage is optimized for Galactic Centre region, however, objects anywhere on the celestial sphere wifi be accessible for observation at least once per year. The pointing accuracy is 1.8 arcseconds absolute, with a stability of 0.9 arcseconds. The operational orbit (cf. Fig. 2) is a highly eccentric (1000 x 70600 km) 24-hour (ISO-type) orbit with low (10 degrees) inclination, potentially offering 17 hours of observations per orbit. There will be no observations while in eclipse, and the operative mode will be near real-time with direct transmission of data (telemetry rate 60 kbps) to either of the two ground stations. OperattOflal Orbit

period 24 hours

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RadiatIon belts

hh~= —1000km 70600 km

ihour

2

I

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4

lime after perigee passage

Fig. 2. Following injection of the FIRST spacecraft into a geostationary transfer orbit (GTO), the perigee altitude is raised to eliminate atmospheric drag and the apogee to well outside the radiation belts. The overall system dimensions are constrained by the available envelope inside the Ariane 5 miring, FIRST being the upper passenger in shared launch to GTO. The launch mass is 3400 kg, including the 3200 litres of helium expected to offer a lifetime of about three years.

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FIRST MODEL PAYLOAD FIRST wifi have a full complement of instruments for high and medium resolution spectroscopy, imaging and photometry over the submillimetre and far-infrared range. The model payload as presently defined has been used to define requirements, interfaces, operation and performance of the spacecraft for study purposes. The actual payload to be flown wifi be proposed by individual institutes or consortia as a response to an invitation issued by ESA; it may (and most likely wifi) differ from the model payload used in the course of the SDS study. The model payload comprises four instruments, cf. Table 1. They fall into two categories: direct and heterodyne detection instruments. The Far Infrared Spectrometer (FIS) and the Far Infrared Photometer (FIP) both use direct detection, employing a combination of photoconductors and bolometers as detectors. The Sub-THz Receiver (STR) and the THz Receiver (THR) are heterodyne receivers using superconducting tunnelling junctions as non-linear elements, and solid state oscillators and multipliers for local oscillator signal generation.

Instrument (designation) Sub-THz receiver (STR) TeraHertz receiver (THR) Far-IR spectrometer (FIS) Far-IR photometer (FIP) Table 1.

Operating range (total) 820 980 GHz 305 365 pm 980 1150 GHz 260 305 pm 1667 3510 GHz 85 180 pm 425 4300 GHz 70 700 pm —

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Spectral Description resolution _______________________________ up to SIS mixer with all solid state 106 local oscillator system up to SIS mixer with all solid state 106 local oscillator system 1000 or Dual channel double FP system with imaging Ge:Ga arrays i04—i05 1—3 Photoconductor and small (continuum) bolometer arrays

Overview of the FIRST model payload.

CURRENT AND FUTURE ACTIVITIES In the course of the months after the completion of the technical part of the SDS, which was considered successful, it has become evident that the present FIRST concept does not fit within the financial limits imposed on it, within the “Horizon 2000” plan, as a mission to be done by ESA by itself. In June 1992 it was decided to address this situation by (i) performing an additional complementary industrial technical study of a descoped concept employing mechanical coolers for payload cooling instead of a cryostat, and by (ii) investigating whether, or not, there is scope for bilateral or interagency collaboration. In addition, further technical studies of critical items identified in the SDS are underway. The future schedule depends on the very important selection of the third cornerstone (FIRST or ROSETTA, a cometary mission), due to be made by ESA in late 1993. The current approximate schedule for the third cornerstone is: announcement of opportunity (AO) for experiments in 1995, invitation to tender (ITT) in 1996, start of phase B in 1997, followed by phase C/D leading to a launch in 2003. ACKNOWLEDGEMENTS The work reported on in this talk is a collective effort involving a large number of individuals in the FIRST science teams, the European Space Agency, and the industrial contractors, all of whom deserve credit for their contributions.