The AIRS thermal system is responsible for keeping the various parts of the instrument at specific temperatures throughout the mission. These regions are
- the detectors, inside the dewar, at 58 K (-215 °C)
- the spectrometer optics at 155 K (-118 °C)
- the thermal shield surrounding the optics at 190 K (-83 °C)
- the Scan Mirror Assembly at 273 K (0 °C)
- the instrument base at 293 K (20 °C)
- the radiometric calibrator at 308.3 K (35 °C).
Detectors & Dewar
The detectors are kept at 58 K by using an active cryocooler.
The first stage passive radiator keeps the thermal shield cold by radiating into space. This radiator is connected to a thermal shield around the spectrometer, and keeps the temperature of the thermal shield to be 190 K.
The second stage passive radiator further cools the optics to 155 K. The radiators are called first and second because the second stage leverages the already cool environment inside the thermal shield created by the first stage. The second stage has a 3 W heater to actively control the temperature.
The Earth Shield keeps infrared radiation from the warm Earth from striking the radiators. The Earth shield was stowed on launch and deployed with a motor once in orbit.
Instrument Base & Heat Rejection System
The instrument base temperature is controlled by mounting to the Aqua spacecraft Heat Rejection System (HRS).
The job of the AIRS cryocooler system is to lower the temperature of focal plane assembly (FPA) inside the dewar to 58 K, relative to the outside of the dewar at 155 K. The challenging cryocooler requirements which were all met are
- long lifetime (over 17 years in space)
- excellent temperature control (±0.01 K to maintain detector stability)
- good power efficiency (wasted power produces excessive heat)
- low vibration output (shaking reduces image quality).
The cryocoolers use split Stirling cycle refrigerator, with a pulse tube, where a passive orifice and gas reservoir replaces the moving displacer in the traditional cold head. The result is an accessible cold spot for FPA cooling without the penalties of cold head vibration, electromagnetic interference, and displacer reliability concerns inherent with two displacers.
Both cryocoolers achieved a thermodynamic efficiency (system input power/cooling capacity) of approximately 62 W/W at 55 K operation with a net cooling capacity of 1.5 Watts. Heat from electronics in the dewar as well as and conduction and heat radiation losses on the way to the FPA bring that up to 58 K.
The AIRS focal plane cryocooler is fully redundant. Each assembly consists of an actively balanced compressor, separate pulse tube cold head, and independent control electronics.
The overall mass of the AIRS cryocooler system is 37 kg including both redundant assemblies along with the support structure.
The AIRS space qualified pulse tube cryocooler was a major advance in cryogenic technology. TRW, in Redondo Beach, California, developed the cryocoolers. TRW has since become Northrop-Grumman Space Technologies (NGST). The AIRS coolers were the first pulse tube coolers used in space. A variety of missions have since used pulse tube coolers.
Ross, R.G., Jr. and Green K., “AIRS Cryocooler System Design and Development,” Cryocoolers 9, Plenum Publishing Corp., New York, 1997, pp. 885–894. Google Scholar
Ross, R.G., Jr., Johnson, D.L., Collins, S.A., Green K. and Wickman, H. “AIRS PFM Pulse Tube Cooler System-level Performance,” Cryocoolers 10, Plenum Publishing Corp., New York, 1999, pp. 119–128. Google Scholar
Johnson, D.L., Collins, S.A., Heun, M.K. and Ross, R.G., Jr., “Performance Characterization of the TRW 3503 and 6020 Pulse Tube Coolers,” Cryocoolers 9, Plenum Press, New York (1997), pp. 183–193. Google Scholar
AIRS has a very complex focal plane assembly of detectors, and so testing the detectors on the ground before launch was essential. A type of vacuum thermos called a dewar keeps the detectors cold enough (58 K) to operate during testing. The outside of the dewar mates directly to the 155 K spectrometer optics, while the detectors inside are kept cold by a cryocooler.
The dewar outer shell is primarily stainless steel and uses a glass inner bore to help thermally isolate the 58-K focal plane assembly inside from the 155-K dewar body.
The dewar window is made from Germanium to enable light from the optics to strike the focal plane. The window is anti-reflection coated to reduce stray light and maintain good transmission through the optical system.
A single-crystal aluminum oxide (sapphire) rod provides low loss heat transfer from the focal plane to the cryocooler. Attachment of the rod to the cryocooler uses a flexible coupling to mechanically isolate the vibrating cryocooler.