Mid-mission update of Spitzer Space Telescope cryogenic performance

The payload of the Spitzer Space Telescope, known as the Cryogenic Telescope Assembly (CTA), houses the science instruments and provides them a 1.2 K thermal sink. The CTA also includes a 0.85-meter telescope, which is selectively controlled between 5 K and 13 K to achieve the appropriate low photon background for the instruments while conserving helium. The CTA cryogenic system supplies cooling through a combination of passive radiation and controlled vapor flow from a superfluid helium cryostat. The CTA was designed to meet a required cryogen lifetime of 2.5 years after on-orbit commissioning, with a design goal of 5 years. Launched warm in August 2003 with 49 kg of helium, the CTA has been performing superbly with a current helium loss rate of only 9 kg per year after the initial cool-down period. Remaining helium is monitored so that mission lifetime can be accurately determined. Due in large part to the success of CTA, the warm launch design has become the standard for future cryogenic space telescopes. This report describes the current flight performance of the CTA cryogenic system including porous plug operation, cryogenic radiator performance, telescope temperature control, and helium mass gauging.