The COBE (Cosmic Background Explorer) project.

As described in the paper by John Mather and Tom Kelsall, the COBE (Cosmic Background Explorer) project is designed to make a substantial improvement in our knowledge of the condition in the universe at large red shifts. The focus of the mission is to study attributes of the primeval cosmic background radiation and to set limits on the universal radiation energy density at shorter wavelengths - radiation emanating from distant sources but at later times than the primeval fireball. In introducing the accompanying paper, I thought it might be worthwhile to describe the scientific strategy driving the COBE mission.

The COBE mission has been under study since 1974 by a team of scientists consisting of S. Gulkis (Jet Propulsion Laboratory), M. Hauser (NASA Goddard Space Flight Center), J. Mather (NASA), G. Smoot (University of California, Berkeley), R. Weiss (MIT), and D. Wilkinson (Princeton). The team members have been involved in ground based, balloon borne and airborne experiments to measure the background radiation and are keenly aware of limitations of these platforms and the arguments in favor of a space borne experiment. It is worth repeating these arguments:

(1) Freedom from atmospheric emission and fluctuations in the emission.

(2) Full sky coverage with a single instrument.

(3) A benign and controlled thermal environment to reduce systematic errors.

(4) The ability to perform absolute primary calibration in flight without the necessity of windows to avoid condensation of the atmosphere on calibrators and instruments.

(5) Sufficient time both to perform tests for systematic errors and to gain the increase in sensitivity permitted by extended observation time.

In planning the COBE mission, the team came to know the peculiar difficulties of carrying out a space mission. Aside from the ever present problem of maintaining the project within an assigned budget, the mission had to be designed so that it would still be the "right thing to do" in the field after the 5 to 10 years it takes between initial planning and execution. One had to anticipate the progress that could still be made by using other platforms and blend this with the fact that in the space mission one would be dealing with technology that was 3 to 5 years behind the state of the art (to allow time for space quahfication).

In an interim report to NASA in 1977, the conclusion of the team was that the technology was sufficiently advanced and the instrument systematic noise sources were well enough understood or controllable so that the major limitation in a space mission to perform precision measurements of the background would be the "noise" produced by the local astrophysical environment. The complement of instruments chosen for the mission, as well as the need for full sky coverage and extended observation time, are based primarily on the hope that the local astrophysical "noise" can be discriminated from the cosmic background by its peculiar set of spectra and anisotropic angular distribution. In the COBE mission, the data from one instrument truly serves to enhance the value of that from another. Examples of this are discussed in the accompanying paper.