COBRAS/SAMBA is an ESA mission designed for extensive, accurate mapping of the anisotropies of the Cosmic Background Radiation, with angular sensitivity from 

sub-degree scales up to and overlapping with the COBE-DMR resolution. This will allow a full identification of the primordial density perturbations which grew to form the large-scale structures observed in the present universe. The COBRAS/SAMBA maps will provide powerful tests for the inflationary model and decisive answers on the origin of cosmic structure. A combination of bolometric and radiometric instrumentation will ensure the sensitivity and wide spectral coverage required for accurate foreground discrimination. A far-Earth orbit has been selected to minimize the unwanted emission from the Earth. The project is currently in the Phase A study within the European Space Agency M3 programme

Bersanelli, M.

Cesarsky, C.

Danese, L.

Efstathiou, G.

Griffin, M.

Lamarre, J. M.

Mandolesi, N.

Norgaard-Nielsen, H. U.

Pace, O.

Puget, J. L.

Raisanen, A.

Smoot, G. F.

Tauber, J.

Volontè, S.

Scientific Open-access Literature Archive and Repository

IL NUOVO CIMENTO VOL. 20 C, N. 5 Settembre-Ottobre 1997The COBRAS/SAMBA space mission()N. MANDOLESI(1), M. BERSANELLI(2), C. CESARSKY(3), L. DANESE(4)G. EFSTATHIOU(5), M. GRIFFIN(6), J. M. LAMARRE(7)H. U. NORGAARD-NIELSEN(8), O. PACE(9), J. L. PUGET(7)A. RAISANEN(10), G. F. SMOOT(11), J. TAUBER(12)and S. VOLONTE´(13)(1) Istituto TESRE/CNR - Bologna, Italy(2) Istituto di Fisica Cosmica, CNR - Milano, Italy(3) SAP/DAPNIA - CE Saclay, France(4) Universita` di Padova - Padova, Italy(5) University of Oxford - Oxford, UK(6) Queen Mary and Westfield College - London, UK(7) Institut d’Astrophysique Spatiale - Orsay, France(8) Danish Space Research Institute - Lyngby, Denmark(9) ESA/ESTEC, Future Project Study Office - Noordwijk, The Netherlands(10) Helsinki University of Technology - Espoo, Finland(11) Lawrence Berkeley Laboratory - Berkeley, USA(12) ESA/ESTEC, Space Science Department - Noordwijk, The Netherlands(13) ESA/HQ, Directorate of Scientific Programmes - Paris, France(ricevuto il 5 Dicembre 1996; approvato il 27 Febbraio 1997)Summary. — COBRAS/SAMBA is an ESA mission designed for extensive, accuratemapping of the anisotropies of the Cosmic Background Radiation, with angular sen-sitivity from sub-degree scales up to and overlapping with the COBE-DMR resolu-tion. This will allow a full identification of the primordial density perturbations whichgrew to form the large-scale structures observed in the present universe. The CO-BRAS/SAMBA maps will provide powerful tests for the inflationary model and decisiveanswers on the origin of cosmic structure. A combination of bolometric and radiomet-ric instrumentation will ensure the sensitivity and wide spectral coverage required foraccurate foreground discrimination. A far-Earth orbit has been selected to minimizethe unwanted emission from the Earth. The project is currently in the Phase A studywithin the European Space Agency M3 programme.PACS 96.40 – Cosmic rays.PACS 01.30.Cc – Conference proceedings.() Paper presented at the VII Cosmic Physics National Conference, Rimini, October 26-28, 1994.G Societa` Italiana di Fisica 685686 N. MANDOLESI, M. BERSANELLI, C. CESARSKY, ETC.1. – IntroductionIn April 1992 the COBE team announced the detection of intrinsic temperature fluc-tuations in the CBR at angular scales larger than  7, with brightness amplitudeT=T  10 5 (Smoot et al., 1992). These fluctuations have been interpreted as due tothe Sachs-Wolfe effect (Sachs and Wolfe, 1967) and yield information on the spectrum ofprimordial density fluctuations. This result represents a major milestone for cosmologyand establishes the inflationary Big Bang model as the theoretical paradigm. The an-gular resolution of COBE-DMR, however, provides no direct information on scales thatcorrespond to any of the structures observed in the present universe. The COBE-DMRangular resolution corresponds to present day physical size >1000 h 1 Mpc, or about100 times larger than the scales typical of clusters of galaxies. The COBRAS/SAMBAmission is designed to image nearly the whole sky with resolution  100–300 and with sen-sitivity approaching T=T  10 6. These observations will provide decisive answers toseveral major open questions relevant to the structure formation epoch and will providepowerful tests for the inflationary model.The COBRAS/SAMBA mission is the result of the merging of two proposals pre-sented in 1993 to the European Space Agency M3 Call for Mission Ideas: COBRAS(Cosmic Background Radiation Anisotropy Satellite; Mandolesi et al., 1993) and SAMBA(Satellite for Measurements of Background Anisotropies; Puget et al., 1993). The CO-BRAS/SAMBA team completed the ESA assessment study in May 1994, and the projecthas been selected for the Phase A study.2. – Scientific objectivesThe COBRAS/SAMBA mission will produce near all-sky maps of the backgroundanisotropies in 8 frequency bands in the range 30–800 GHz, with peak sensitivity T=T 10 6. Individual hot and cold regions should be identified above the statistical noise level,at all angular scales from <100 up to very large scales, thus providing a high-resolutionimaging of the last scattering surface. It is the information contained in this wide rangeof angular scales (or large number of multipoles in a spherical harmonics decomposition)that can probe the various proposed scenarios of structure formation and the shape of theprimordial fluctuation spectrum.The statistics of the T=T distribution as observed by COBRAS/SAMBA will providea key test for structure formation mechanisms. The inflationary model predicts Gaus-sian statistics for the fluctuations of the CBR anisotropies, while alternative models basedon the presence of topological defects predict non-Gaussian statistics (e.g., Coulson et al.1994). The angular resolution and sensitivity of COBRAS/SAMBA will allow discrimina-tion between these alternatives.Extensive measurements with a wide range of angular scales will allow an accuratemeasure of the spectral index n of the primordial fluctuation spectrum:()2/ (1 n);where  is the potential fluctuation responsible for the CBR anisotropies, and  is thescale of the density perturbation. The proposed observations will be able to verify ac-curately the scale-invariant “Harrison-Zel’dovich” spectrum (n = 1) predicted by in-flation. The COBE-DMR limit on the spectral index after two years of observationsTHE COBRAS/SAMBA SPACE MISSION 687Fig. 1. – Estimated fluctuation levels due to galactic polar emission (dashed lines) and to extra-galactic sources (solid lines), from Toffolatti et al. (1994). The vertical dotted lines show the CO-BRAS/SAMBA frequencies.(n = 1:1+0:3 0:4, 68% CL; Gorski et al., 1994) can be constrained  10 times better by theCOBRAS/SAMBA results.Temperature anisotropies on large angular scales can be generated by gravita-tional waves (tensor modes), in addition to the energy-density perturbation component(scalar modes). Inflationary models predict that the ratio of the tensor and scalarmode anisotropies is related to the spectral index n (e.g., Davis et al., 1992). TheCOBRAS/SAMBA maps will allow to verify this relationship, since the temperatureanisotropies from scalar and tensor modes vary with multipoles in different ways.Small angular scale anisotropies are sensitive to the ionization history of the universe(e.g., Sugiyama et al., 1993). They can be erased if the intergalactic medium underwentreionization at high redshifts. Temperature anisotropies at sub-degree scales depend onkey cosmological parameters, such as the initial spectrum of irregularities, the baryondensity of the universe, the nature of dark matter, and the geometry of the universe(see, e.g., Crittenden et al., 1993; Bond et al., 1994; Kamionkowski et al., 1994). The CO-BRAS/SAMBA maps will provide constraints on these parameters within the context of688 N. MANDOLESI, M. BERSANELLI, C. CESARSKY, ETC.specific theoretical models.Finally, the high-resolution, high-frequency channels will measure the Sunyaev-Zel’dovich effect for more than 1000 rich clusters. Combined with X-ray observationsthese measurements can be used to estimate the Hubble constant H0.3. – Foreground radiationThe temperature fluctuations measured by a CBR anisotropy experiment need to bewell understood in terms of the various components that add to the CBR signal. Fore-ground structures are caused by unresolved extragalactic sources, galactic radiation (in-terstellar dust, free-free and synchrotron radiation), and interplanetary dust (fig. 1). TheCOBRAS/SAMBA observations will be performed near the minimum foreground emis-sion, and will reach the required control on the various components in two ways. First,the large sky coverage will allow accurate modeling of these foregrounds where they aredominant. Second, the observations will be performed in a spectral range as broad aspossible. The frequency channels will span the spectral region of minimum foregroundintensity (50–300 GHz), but with enough margin at high and low frequency to monitor“in real time” the effect of the various foreground components (see, e.g., Brandt et al.,1994). By using the COBRAS/SAMBA spectral information and modeling the spectraldependence of galactic and extragalactic emissions it will be possible to remove the fore-ground contributions with high accuracy. In the high-frequency channels ( >140 GHz)the main foreground components will be dust emission, while towards the low-frequencyrange ( <70 GHz) galactic synchrotron and free-free emission become increasingly im-portant (Toffolatti et al., 1995).In most COBRAS/SAMBA channels the final limitation to the cosmological informa-tion is expected to be due to the residual uncertainties in the separation of the foregroundcomponents rather than statistical noise. Therefore the overall design of the instrumentand payload is highly driven by the need of achieving a spectral coverage as wide as possi-ble. Performing measurements where the dominant foreground components are differentwill permit a powerful cross check on residual systematic errors in the CBR temperaturefluctuation maps.4. – The payloadThe COBRAS/SAMBA model payload (see fig. 2) consists mainly of a shielded, off-axisGregorian telescope, with a parabolic primary reflector and a secondary mirror, leadingto an integrated instrument (Focal Plane Assembly, FPA). The payload is part of a spin-ning spacecraft, with a spin rate of 1 rpm. The focal plane assembly is divided into low-frequency (LFI) and high-frequency (HFI) instrumentation according to the technologyof the detectors, each covering four observing bands (see table I). The highest-frequencyLFI channel and the lowest HFI channel overlap near the minimum foreground region.A clear field of view is necessary for the optics of a high-sensitivity CBR anisotropyexperiment to avoid spurious signals from the mirrors or from supports and mechanicalmounting. A Gregorian configuration has been chosen, with a primary parabolic mirrorof 1.5 m, and an elliptic secondary mirror (0.57 m diameter). Stray satellite radiationand other off-axis emissions are minimized by underilluminating the low-emissivity optics.Blockage is a particularly important factor to avoid the effect of local radiation (e.g. fromthe Earth, the Sun and the Moon). A large, flared shield surrounds the entire telescopeTHE COBRAS/SAMBA SPACE MISSION 689Fig. 2. – System diagram of the COBRAS/SAMBA payload, showing the concept of the shieldingand optics design. The inset shows a top view of the Focal Plane Assembly, with the bolometricsystem (HFI) cooled at 0.1 K, sharing the focal plane with the arrays of passively cooled feed horns(LFI).and FPA, to screen the detectors from contaminating sources of radiation. The shield alsoplays an important role as an element of the passive thermal control of the spacecraft.The necessary wide spectral range requires the use of two different technologies,bolometers and radiometers, incorporated in a single instrument. Both technologies haveshown impressive progress in the last ten years or so, and more is expected in the nearfuture.4.1. The low-frequency instrument. – The LFI consists of an array of 26 corrugated,conical horns, each exploited in the two orthogonal polarization modes, feeding a set of690 N. MANDOLESI, M. BERSANELLI, C. CESARSKY, ETC.TABLE I. – Payload characteristics.Telescope 1.5 m diameter Gregorian; system emissivity 1%Viewing direction offset 30 from spin axisInstrument LFI HFICenter frequency (GHz) 31.5 53 90 125 140 222 400 714Wavelength (mm) 9.5 5.7 3.3 2.4 2.1 1.4 0.75 0.42Bandwidth () 0.15 0.15 0.15 0.15 0.4 0.5 0.7 0.6Detector technology HEMT receiver arrays Bolometers arraysDetector temperature  100 K 0.1–0.15 KCooling requirements Passive Cryocooler + dilution systemNumber of detectors 13 13 13 13 8 11 16 16Angular resolution (arcmin) 30 30 30 30 10.5 7.5 4.5 3Optical trasmission 1 1 1 1 0.3 0.3 0.3 0.3TTSensitivity (1; 10 6 units, 1.7 2.7 4.1 7.2 0.9 1.0 8.2 10490% sky coverage, 2 years)TTSensitivity (1; 10 6 units, 0.6 0.9 1.4 2.4 0.3 0.3 2.7 50002 % sky coverage, 2 years)state-of-the-art, high-sensitivity radio receivers. The receivers will be based on MMIC(Monolithic Microwave Integrated Circuits) technology with HEMT (High Electron Mo-bility Transistor) ultra-low noise amplifiers (see, e.g. Pospieszalski, 1993). Since the wholeLFI system will be passively cooled, it can be operated for a duration limited only byspacecraft consumables (up to 5 years). The three lowest center frequencies match theCOBE-DMR channels.4.2. The high-frequency instrument. – About 50 bolometers will be used in the HFIinstrument, which require cooling at  0:1 K. The cooling system combines active coolersreaching 4 K with a dilution refrigeration system working in zero gravity. The refrigera-tion system will include two pressurized tanks of 3He and 4He for an operational lifetimeof 2 years.5. – Orbit and observation strategyCOBRAS/SAMBA will operate from a far-Earth orbit to avoid unwanted radiationfrom the Earth, a serious potential contaminant at the high goal sensitivity and angularresolution. Adopting a low-Earth orbit, such as that used by the COBE satellite, the re-quirement on straylight and sidelobe rejection (for both LFI and HFI) would be a factorof 1013, which is beyond the capabilities of present microwave and sub-mm systems andtesting equipment. COBRAS/SAMBA will operate from a small orbit around the L5 La-grangian point of the Earth-Moon system, at a distance of about 400 000 km from both theTHE COBRAS/SAMBA SPACE MISSION 691Earth and the Moon. From this location the required rejection is relaxed by four ordersof magnitude, which is achievable with careful, standard optical designs. The selected L5orbit is also very favorable from the point of view of passive cooling and thermal stabil-ity (Farquhar and Dunham, 1990). The telescope optical axis is offset by 30 from thespin axis. Thus at each spacecraft spin rotation the telescope pointing direction sweeps a60 circle in the sky. The spacecraft will be normally operated in the anti-solar direction,with part of the sky observations performed within 40 from anti-solar.The main goal of the mission is to observe nearly the whole sky (>90%) with a sensi-tivity of 10–15 K within the two year mission lifetime. Deeper observation of a limited( 2%) sky region with low foregrounds would significantly contribute to the cosmologicalinformation. Preliminary simulations have shown that these observational objectives canbe achieved simultaneously in a natural way, using the spinning and orbit motion of thespacecraft, with a relatively simple scheme.6. – ConclusionsAn accurate, extensive imaging of CBR anisotropies with sub-degree angular reso-lution would provide decisive answers to several major open questions on structure for-mation and cosmological scenarios. While ground-based and balloon-borne experimentsprovide very important results, the observational requirements for a full characterizationof the primordial CBR anisotropy can be met only by a carefully planned space missionwith a far-Earth orbit (e.g., Danese et al., 1996)The assessment study has shown that a mission combining bolometric and radiometrictechnologies is feasible within the European Space Agency Medium Mission scope, thusensuring a powerful control over foreground structures and other potential systematiceffects. The wide spectral range (30–800 GHz), limited only by the unavoidable galacticand extragalactic emissions, the sensitivity to a very broad spectrum of angular scales( 100 to several degrees), and the choice of the L5 Lagrangian orbit are the main featuresthat characterize the COBRAS/SAMBA mission.  Several people contributed to the work so far accomplished. In particular we acknowl-edge all the researchers and groups who supported the original COBRAS and SAMBAproposals, and the ESA staff members at Estec and Esoc who have contributed to theAssessment Study.REFERENCESBOND J. R., CRITTENDEN R., DAVIS R. L., EFSTATHIOU G. and STEINHARDT P. J., Phys. Rev.Lett., 72 (1994) 13.BRANDT W. N., LAWRENCE C. R., READHEAD A. C. S., PAKIANATHAN J. N. and FIOLA T. M.,Astrophys. J., 424 (1994) 1.COULSON D., FERREIRA P., GRAHAM P. and TUROK N., Nature, 368 (1994) 27.CRITTENDEN R., BOND J. R., DAVIS R. L., EFSTATHIOU G. and STEINHARDT P., Phys. Rev.Lett., 71 (1993) 324.DANESE L., TOFFOLATTI L., FRANCESCHINI A., BERSANELLI M. and MANDOLESI N.,Astrophys. Lett. & Commun., 33 (1996) 257.DAVIS R. L., HODGES H. M., SMOOT G. F., STEINHARDT P. J. and TURNER M. S., Phys. Rev.Lett., 69 (1992) 1856.692 N. MANDOLESI, M. BERSANELLI, C. CESARSKY, ETC.FARQUHAR R. W. and DUNHAM D. W., Observatories in Earth Orbit and Bejond, edited by Y.KONDO (Kluwer) 1990, p. 391.GORSKI K., HINSHAW G., BANDAY A. J., BENNETT C. L., WRIGHT E. L., KOGUT A., SMOOTG. F., and LUBIN P. M., Astrophys. J., 430 (1994) L89.KAMIONKOWSKI M., SPERGEL D. N. and SUGIYAMA N., Astrophys. J., 426 (1994) L57.MANDOLESI N., SMOOT G. F., BERSANELLI M., CESARSKY C., LACHIEZE-REY M., DANESEL., VITTORIO N., DE BERNARDIS P., DALL’OGLIO G., SIRONI G., CRANE P., JANSSENM., PARTRIDGE B., BECKMAN J., REBOLO R., PUGET P. L., BUSSOLETTI E., RAFFELTG., DAVIES R., ENCRENAZ P., NATALE V., TOFANI G., MERLUZZI P., TOFFOLATTI L.,SCARAMELLA R., MARTINEZ-GONZALES E., SAEZ D., LASENBY A. and EFSTATHIOU G.,COBRAS, proposal submitted to ESA M3 (1993).POSPIESZALSKI M. W., Astrophys. Lett. & Commun, 32 (1995) 125.PUGET J. L., ADE P., BENOIT A., DE BERNARDIS P., BOUCHET F., CESARSKY C., DESERTF. X., GISPERT R., GRIFFIN M., LACHIEZE-REY M., LAMARRE J. M., DE MARCILLACP., MASI S., MELCHIORRI F., PAJOT F., ROWAN-ROBINSON M., SERRA G., TORRE J. P.,VIGROUX L. and WHITE S., SAMBA, proposal submitted to ESA M3 (1993).SACHS R. K. and WOLFE A. M., Astrophys. J., 147 (1967) 73.SMOOT G. F. et al., Astrophys. J., 396 (1992) L1.SUGIYAMA N., SILK J. and VITTORIO N., Astrophys. J., (1993) L1.TOFFOLATTI L. et al., Astrophys. Lett. & Commun., 32 (1995) 125.

The COBRAS/SAMBA space mission

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The COBRAS/SAMBA space mission

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