<title>Science objectives lead to contamination requirements for the Cosmic Background Explorer</title>

Abstract

The Cosmic Background Explorer (COBE) was recently launched and is now acquiring spectacular science data on the cosmic structure of the universe. The instruments developed to obtain this science data are extremely sensitive to contamination. The task of the project scientists and the contamination engineers was to develop specific contamination requirements which were linked to the mission science objectives. A description of the mission the instruments the science goals the derivation of the subsequent contamination requirements an overview of the contamination control program and a brief summary of contamination program results will be presented. The COBE Project was initiated to answer basic questions concerning the formation of the universe: What was the primeval explosion that started the expanding universe? What made it uniform on such a gigantic scale? What started the formation of galaxies? What caused them to be arranged in giant clusters of galaxies? Is there a center or an edge of the universe?'' The answers to many of these ambitious questions are now being obtained by the scientific instruments on board COBE: the Diffuse Infrared Background Experiment (DIRBE) the Far Infrared Absolute Spectrophotometer (FIRAS) and the Differential Microwave Radiometers (DMRs). The DIRBE is a cryogenically cooled instrument with a primary mirror highly sensitive to particulate scattering. The FIRAS is a cryogenically cooled spectrophotometer with a sky horn also intolerant of particulate scattering. The DMRs are radiometers that have antennae with narrow throats which can become obscured due to the presence of particles. The cryogenic cooling of the DIRBE and FIRAS led to the need for relatively strict molecular requirements and the scattering and obscuration issues led to the need for extremely rigid particulate requirements. Analytical studies and testing results were used to quantify cleanliness levels that would allow for the achievement of the mission objectives. 1. THE MISSION The COBE shown in Figure 1 is a 19 foot long 13 foot diameter spacecraft weighing 5000 pounds. The spacecraft was launched in November 1989 into a near polar circular orbit 559miles above the Earth''s surface. To achieve this orbit COBE was launched on a Delta rocket from the Vandenburg Air Force Base. The COBE''s spin axis was oriented 94 away from the sun and was directed outward from Earth. To shield the instruments from the earth sun and moon''s light COBE carried a sunshade. Each instrument in the three instrument complement works in unison as will be briefly described in the following sections. 16 / SPIE Vol. 1329 Optical System Contamination: Effects Measurement Control 11(1990)