PRIMORDIAL STRUCTURE INVESTIGATION (PSI) - A LOW-COST SPACE MISSION TO IMAGE THE INTERMEDIATE-SCALE COSMIC BACKGROUND ANISOTROPY

Preliminary results are presented for the Primordial Structure Investi gation mission study. PSI is a concept for a high-sensitivity, low-cos t mission to image the Cosmic Microwave Background on intermediate ang ular-scales (0.5 degrees-10 degrees). The mission objective is to meas ure the anisotropy of the CMB with sufficient sensitivity, and over a large enough solid angle, to unlock the wealth of evidence that the an isotropy holds for the origin of structure and the nature of the unive rse. PSI consists of an 80-cm aperture fed by 32 radiometers (eight at each of four frequencies), carried into heliocentric orbit on a spinn ing spacecraft. Total power radiometers, based on current HEMT (High E lectron Mobility Transistor) and MMIC (Monolothic Microwave Integrated Circuit) technologies, provide low noise, stable gain, and low power dissipation to allow passive cooling. The spacecraft is constrained to fit the projected launch capabilities of a Taurus vehicle, has a mass of 213 kg, consumes 230 W, and is designed for a 5-year lifetime. The mission will image similar to 10(3) pixels to a sensitivity of 1 mu K , or similar to 10(4) pixels to a sensitivity of 3 mu K, at each frequ ency. This study supports the feasibility of a low-cost space approach to the problem of determining the intermediate angular-scale anisotro pies. Costing of our baseline design has not been completed, but we an ticipate a mission within the guidelines of the anticipated NASA Mid-s ized Explorer Program. However, further work is needed in the areas of HEMT development and characterization, optics design, and the multi-f requency strategy for the removal of galactic foreground emission to f ully justify the present concept.