Anomalous on-orbit behaviour of the NASA Cosmic Background Explorer (COBE) Dewar

Abstract

The cryogenic operation of NASA's Cosmic Background Explorer (COBE) ended on September 21, 1990, with the depletion of the liquid helium cryogen, after COBE had completed more than 10 months of successful Dewar and instrument operation. In this report we provide a brief summary of the nominal cryogenic performance of the Dewar (for more detail consult Reference 1). We present in detail several aspects of the helium and spacecraft dynamical behaviour. We discuss the occurrence of temperature and pressure oscillations in the Dewar porous plug. We review the impact of internal instrument malfunctions and of external radiation sources on the performance of the Dewar and of the instruments. From measurements of the COBE spacecraft spin rate we are able to monitor the spin coupling of the liquid helium to the walls of the Dewar. We analyse the spin measurements and present a model for the coupling. Finally, we review a number of helium Dewar ‘lessons learned’ from the COBE mission, and remark on the applicability of these lessons to future missions involving cryogenic payloads.