European Space Agency

Planck: the Cosmic Background Anisotropy mission

J.A. Tauber

Planck homepage

ESA issued a call in late 1992 for proposals for candidates to become the third medium-sized mission (M3) of the Agency's Horizon 2000 long-term science programme. In response to this call, two proposals (COBRAS and SAMBA) devoted to the study of the anisotropies of the cosmic microwave background (CMB) were submitted. The Space Science Advisory Committee (SSAC) recommended that a study be carried out to assess the feasibility of a mission dedicated to map the primordial fluctuations of the CMB. Early in the assessment study, it was recognised that merging the two originally proposed instruments into one payload would be the only way to obtain a definitive and credible answer to the scientific questions to be addressed.

Following the assessment study of the combined Planck mission, the Phase A study was carried out with Matra Marconi Space (MMS Toulouse) as industrial prime contractor. Between December 1994 and February 1996, MMS developed a detailed design concept for the Planck spacecraft. In parallel and over the same period, the Planck Science Team (ST) considerably reinforced the scientific case for the mission, and developed detailed designs for the payload instruments and telescope.

The industrial/scientific Phase A study confirmed and enhanced the scientific case for Planck and proved that the mission could be carried out within the technical and financial constraints of the M3 call for proposals. The selection of Planck was confirmed by ESA's Science Programme Committee in November 1996. The present schedule calls for a launch in late 2004.

The Planck model payload (see Table 3.1.7/1) consists of a 1.5 m off-axis telescope with an optical design that minimises off- axis distortions and two arrays of detectors sharing the focal plane. The low frequencies (30-125 GHz) are covered by 56 tuned radio receivers grouped into four channels, and the high frequencies are covered by 56 bolometers divided among five channels.

Characteristics Planck payload
Table 3.1.7/1: Characteristics Planck Payload

The former are based on amplifiers using High Electron Mobility Transistors (HEMT) and operated at ~100 K (achieved by passive cooling). The bolometers must be cooled to 100 mK by a dilution cooler coupled to a mechanical cryocooler. This complement of detectors will make it possible to achieve instrumental sensitivities of mK magnitude in each resolution element. The detector arrays will be incorporated into two separate instruments, each developed and funded by ESA member states, via PIs. An AO for the instruments will be issued in mid- 1997. It is expected that the telescope will be procured by a Danish consortium led by the Danish Space Research Institute.

The straylight level as well as the thermal stability of the telescope are controlled by a system of optical baffles. To minimise the contributions of the strong sources of radiation in the sky (Earth, Sun and Moon), and to reach more easily the required temperature and thermal stability, the satellite will orbit around the outer (L2) Lagrange point of the Sun-Earth system. The technical requirements on the satellite are very moderate by today's standards, and can be implemented in a spin- stabilised spacecraft, which minimises cost.

The sky scanning strategy is simple. The optical axis of the telescope is offset by 70° from the rotation axis, and scans one full circle on the sky every minute. Near-full sky coverage is achieved by periodically displacing the rotation axis to remain within 15° of the anti-solar direction. The nominal mission duration calls for two coverages of the sky, to be achieved within 14-15 months of routine operations. Very simple spacecraft operations will consist of a daily set of 12 pre- planned manoeuvres automatically carried out by the onboard computer. Instrument operations will be under the responsibility of the instrument PIs via a Mission Operations Centre managed by ESOC.

The primary output of the mission will be nine calibrated all- sky maps ranging in frequency from 30 GHz (lambda=1 cm) to 900 GHz (lambda=350 µm) and in angular resolution from 30 arcmin to 4.5 arcmin. The information contained in these maps will give Planck the ability to separate the observed microwave signal into the various astrophysical components that contribute to it. The galactic foregrounds around 100 GHz are much weaker than the CMB in the cleanest 50% of the sky. In this region, Planck will be able to determine the amplitude of CMB fluctuations to an uncertainty better than Delta T/T~2x10-6 at all angular scales larger than ~10 arcmin.

The unprecedented accuracy of the Delta T/T map will allow the determination of the angular power spectrum and of the statistical properties of the fluctuations originated in the early Universe. Since particle physics at the extremely large energies involved (1016 GeV) cannot be tested in any accelerator, this determination constitutes the most direct way of testing not only the physics of the early Universe, but also fundamental theories of high energy physics. In particular, it will establish the nature of the primordial fluctuations, e.g. whether they are due to topological defects or quantum fluctuations. In either case, fluctuations at angular scales larger than 1° depend primarily on the primordial spectrum but, as smaller angular scales are probed, the power spectrum depends more on physical processes sensitive to most of the fundamental cosmological parameters. Very accurate measurements of the CMB anisotropies at high angular resolutions can therefore be used to constrain strongly (to a few percent) basic parameters such as the Hubble constant, the geometry of the Universe as characterised by the total density parameter omega0, the cosmological constant Lambda, the baryon content and the nature of the dark matter.

Planck will yield not only CMB anisotropies, but also near- all-sky maps of all the major sources of microwave emission, opening a broad expanse of astrophysical topics to scrutiny. In particular, the physics of dust at long wavelengths and the relative distribution of interstellar matter (neutral and ionised) and magnetic fields will be investigated using dust, free-free and synchrotron maps. One specific and local distortion of the CMB that will be mapped by Planck is the Sunyaev-Zeldovich (SZ) effect arising from the Compton interaction of CMB photons with the hot gas of clusters of galaxies. Measurement of the SZ effect in galaxy clusters will be used, together with X-ray data, to study the physics of structure formation and ultimately of galaxy formation and evolution. A signal can also be extracted from the SZ data that is sensitive to deviations of cluster velocities from the Hubble flow: the sensitivity of Planck will allow the determination of the large scale peculiar velocity fields as traced by ensembles of clusters.

Conceived as a PI mission, all of the scientific data processing tasks in Planck will be carried out under the responsibility of the instrument PIs. This includes analysis of instrument health and performance during operations, creation of the scientific data products, and their archival and distribution to the community. As is usual for ESA scientific missions, the PIs will have exclusive use of the data for scientific analysis for a period of 1 year before it is delivered to the community.

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Published August 1997.