Flight Performance of the AKARI Cryogenic System

TL;DR

The paper reports on the successful flight performance of AKARI's hybrid cryogenic system, combining liquid helium and mechanical coolers, maintaining cryogenic temperatures for over 500 days and demonstrating stable on-orbit operation.

Contribution

It introduces a novel hybrid cryogenic system for space telescopes that effectively extends cryogenic lifetime and maintains performance in orbit.

Findings

Cryogenic system maintained for over 500 days

Boil-off gas flow rate was minimal at 0.32 mg/s

Orbit increase explained by helium venting thrust

Abstract

We describe the flight performance of the cryogenic system of the infrared astronomical satellite AKARI, which was successfully launched on 2006 February 21 (UT). AKARI carries a 68.5 cm telescope together with two focal plane instruments, Infrared Cameras (IRC) and Far Infrared Surveyor (FIS), all of which are cooled down to cryogenic temperature to achieve superior sensitivity. The AKARI cryogenic system is a unique hybrid system, which consists of cryogen (liquid helium) and mechanical coolers (2-stage Stirling coolers). With the help of the mechanical coolers, 179 L (26.0 kg) of super-fluid liquid helium can keep the instruments cryogenically cooled for more than 500 days. The on-orbit performance of the AKARI cryogenics is consistent with the design and pre-flight test, and the boil-off gas flow rate is as small as 0.32 mg/s. We observed the increase of the major axis of the AKARI orbit, which can be explained by the thrust due to thermal pressure of vented helium gas.