COsmic Background Radiation Anisotropy Satellite/
SAtellite to Measure Background Anisotropies


The COBE discovery of anisotropy in the cosmic microwave background radiation combined with the confirmation and new 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 would 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, and
  3. the geometry, dynamics, and topology of the universe.
COBRAS/SAMBA is a mission proposed to the European Space Agency (ESA). The COBRAS/SAMBA mission has completed and 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 will be an ESA meeting in December to review the fate of the CLUSTER mission which was destroyed in the the Ariane V launch and the review the Middle Class mission program including COBRAS/SAMBA which is designated M3 (Middle Class mission #3).

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 dicovered 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. Now 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:

Summary of COBRAS/SAMBA Scientific Goals

The primary goal of COBRAS/SAMBA mission is the production of high-sensitivity (one part per million), high-angular resolution (10 arcminutes) 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 arcminutes to large angular scales. This provides the information necessary for large scale structure formation theory.

  2. Test Inflation/Primodial 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} arcminutes) by an ionized intergalactic medium. A careful measurement can determine or limit the possbility 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. COBRAS/SAMBA 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 COBRAS/SAMBA 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

COBRAS/SAMBA Mission Description

CMB and Foreground Emissions

The COBRAS/SAMBA 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 and the Lagrange Point 5 (L5) of the Earth-Moon system. The baseline orbit is the Earth-Sun L2 point (see second figure below).

Schematic of one considered orbit

Schematic of Baseline Orbit

Space craft and Instrument

Artist concept of the Spacecraft

More on COBRAS/SAMBA payload for the Phase A Study (Red Book) version.
More on the COBRAS/SAMBA Project

European Space Agency (ESA) Page for COBRAS/SAMBA is maintained by Project Scientist Jan Tauber.

Return to the Smoot Group page for a complete description of Dr. Smoot's group's research activities.

Revised 13 August 1996;