ON-ORBIT THERMAL-BEHAVIOR OF THE IRTS CRYOGENIC SYSTEM

The IRTS (Infrared Telescope in Space) was launched at 08:01 on 18 Mar ch 1995 UT by the H-ll rocket as a part of the Space Flyer Unit (SFU), which is a re-usable free flyer designed as a multi-purpose com mon f acility for scientific and engineering experiments. The He II tank was estimated to be approximately 85% full at 1.84 K at the moment of lau nch, 11 h after pumping was disconnected and the tank was sealed. The vapour evacuation through a porous plug was resumed 28 min after the l aunch. The thermal state reached the anticipated steady state at 1.91 K on the third day without any anomalous thermal behaviour of the poro us plug in the initial zero-g state. Over the course of 10 days the SF U transferred to a low earth orbit (LEG) at 486 km from the initial ph ase orbit at 340 km with the telescope aperture lid ejecting on day 11 after launch. A He-3 refrigerator was then activated to decrease the temperature of some bolometer detectors down to 302 mK for observation s in the far IR region. Every detector worked quite satisfactorily. Th e stable thermal state of He II at 1.91 K was maintained until the dep letion of He II at 09:30 on 24 April UT. The cold life is found to be fairly close to the prediction on the basis of the ground test data. O n April 27 the sun shield was jettisoned for the safe retrieval by a U S shuttle in January 1996, and the IRTS mission was completed because of the regular shift of the SFU mission time-line to another experimen t; In this report the flight data of the thermal performance of the cr yogenic system are discussed. (C) 1996 Elsevier Science Limited