Introduction
The COBE discovery of anisotropy in the cosmic microwave background radiation combined with the confirmation and data of other experiments has opened a new direct probe of conditions in the early universe. A high-resolution, high-sensitivity nearly full sky map of these anisotropies would provide a wealth of new and highly important cosmological data. These data relate to three major areas:
1. The initial conditions for large scale (galactic and larger) structure form.
2. The high energy physics of the early universe, e.g. inflation or topological defects.
3. The geometry, dynamics, and topology of the universe.
Planck, formerly known as COBRAS/SAMBA, is an European Space Agency (ESA) mission . The COBRAS/SAMBA mission completed the assessment study and was judged a viable mission under ESA guidelines. The COBRAS/SAMBA mission completed its 18 month Phase A Study in May 1996. At the end of the Phase A Study COBRAS/SAMBA was selected to continue onto space flight. There was an ESA meeting to review the fate of the CLUSTER mission which was destroyed in the Ariane V launch. At that meeting COBRAS/SAMBA was reconfirmed. In January the following year, COBRAS/SAMBA was renamed the Max Planck Surveyor mission or "Planck" for short.
Scientific Rationale
Cosmology is one of the oldest subjects of human inquiry and at the same time one of the newest sciences. Questioning the origin and evolution of the Universe has been a characteristic human endeavor since before recorded history. The science of cosmology emerged in this century when it first became possible to probe great distances through the Universe. At first, the observational breakthroughs occurred infrequently. Hubble discovered the external galaxies and expansion of the Universe in the 1920's. Penzias and Wilson discovered the cosmic microwave background in the 1960's. The compelling evidence for primordial nucleosynthesis of the elements amassed during the 1970's and 1980's. The discovery of anisotropy in the cosmic microwave background allows decisive cosmological tests because the microwave background directly probes the oldest and farthest features in the Universe. Anisotropy measurements that provide a spectrum of precise, quantitative information would relate to three major areas of cosmology:
(1) The initial conditions for large scale (galactic and larger) structure formation,
(2) The high energy physics of the early universe, e.g. inflation or topological defects, and
(3) The geometry, dynamics, and topology of the universe.
In addition to testing cosmological models one will be able to determine their critical parameters to unprecedented accuracy.
Summary of Planck Scientific Goals
The primary goal of Planck mission is the production of high-sensitivity (one part per million), high-angular resolution (10 arc minutes) maps of the microwave sky and thus of the cosmic microwave background. In addition to having an actual high signal-to-noise map on which one can do morphological and topological work, the goals include:
1. Determine the Precise Primordial Fluctuation Spectrum
The first goal is to find to high precision (few per cent or cosmic sample limit) the fluctuation spectrum over the range from 10 arc minutes to large angular scales. This provides the information necessary for large scale structure formation theory.
2. Test Inflation/Primordial Gravity Waves
Detection of primordial gravitons/gravity waves and test of the relationships expected for inflation.
3. Statistics of the CMB Anisotropies
The anisotropy statistics reveal information on their source. Inflation generally predicts a gaussian parent distribution while topological defects predict deviation from gaussian distributions and rare peak fluctuations. The statistics of the fluctuations are also important in setting the time scale for first structure formation.
4. Small-scale Anisotropies and Reionization
Anisotropy is partially erased on small to medium angular scales (30 Omega{-1/2} arc minutes) by an ionized intergalactic medium. A careful measurement can determine or limit the possibility of an early ionized intergalactic medium and place constraints on or determine the thermal history of the Universe.
5. Small-Scale Anisotropies and Galaxy Clustering
10' to 30' corresponds to galactic cluster scales (10h{-1} Mpc).
6. Sunyaev-Zel'dovich Effect in Clusters of Galaxies
Low energy CMB photons are scattered by energetic electrons of the hot intracluster (seen in X-rays) medium. Planck should collect data on over 1,000 clusters. The ROSAT satellite is compiling an all-sky X-ray catalog of clusters of galaxies (among other things). Of particular interest now is the question of the temperature profiles of clusters of galaxies, as this impinges on the use of the Sunyaev--Zel'dovich method for determining Ho. The combination of Planck and ROSAT data will be particularly powerful in studying the Sunyaev-Zel'dovich effect.
Information on the potential Non-cosmological Scientific Results and alternate discussion of the Cosmological Scientific Rationale
Planck Mission Description
CMB and Foreground Emissions
The Planck mission is designed to map the sky at multiple frequencies to both measure the CMB anisotropies and the various Galactic and extragalactic foreground emissions. Multiple frequencies combined with the known spectral dependence and measurements at very different wavelengths allow the separation of the various components.
Foreground emissions gif
More on the Separation of the Foregrounds from the Primordial Fluctuations
Solar System Effects
Emission from the Earth, Sun, and other solar system bodies is the next major concern. The effects of solar system emissions is controlled and minimized by using special far from Earth orbits. Examples are Lagrange Point 2 (L2) of the Earth-Sun system.
Schematic of one considered orbit
Schematic of Baseline Orbit
Spacecraft and Instrument
Artist concepts of the Planck payload, a combined and a joint Planck/First mission.
Foreground emissions gif 
Schematic of one considered orbit
Schematic of Baseline Orbit