LINDER 8/29/01 9:36 AM Page 2 r bulletin 107 — august 2001 Cryogenics in Space - A review of the missions and technologies M. Linder Mechanical Engineering Department, ESA Directorate of Technical and Operational Support, ESTEC, Noordwijk, The Netherlands N. Rando, A. Peacock Space Science Department, ESA Directorate of Scientific Programmes, ESTEC, Noordwijk, The Netherlands B. Collaudin Alcatel Space Industries, Cannes, France Applications of cryogenics in space against their development and operating costs, The development, in-orbit commissioning, and and compared with alternative technologies. operation of cryogenic instrumentation in space raises the level of mission complexity, risk and Cryogenic detectors for space applications associated cost. Any application must then be Cryogenic photon detectors offer two main justified on the basis of its specific return. In the advantages over conventional sensors: case of scientific missions, the cryogenic – their much higher sensitivity (expressed by detectors and related payloads are the only the Noise Equivalent Power or NEP, i.e. the candidates for the accomplishment of the amount of incident power required to achieve mission objectives, offering unmatched a signal-to-noise ratio equal to unity) performance and unique advantages. In the – the better energy resolution (expressed in case of other applications, such as terms of resolving power, i.e. the ratio E/∆E telecommunications, the advantages offered by = λ/∆λ, with ∆E representing the full width at superconducting devices need to be evaluated half maximum of the detector response to a monochromatic excitation of energy E). Since the first liquefaction of 4He and the discovery of super- conductivity by H. Kamerlingh-Onnes (1908 and 1911), cryogenics and Cryogenic detectors have driven the utilisation its applications have come a long way. The continuous improvement of cryogenics in space, determining the of cryogenic equipment has made it easier and easier to achieve requirements in terms of operating temperature, temperatures well below the liquefaction point of nitrogen temperature stability and architecture of the payload system. This trend is now well- (77 K), either by means of cryogens (liquid gases such as Xe, H2, O2, 4 3 established across the electromagnetic N2, He and He) or by means of mechanical coolers. Cryogenic devices, such as sensors and cold electronics, have taken advantage spectrum. Figure 1 provides a summary of the of the progress made in materials science, thereby offering a reliable different detectors, including operating photon and effective solution to otherwise unsolvable problems. energy and temperature range. Table 1 provides an overview of other characteristics of the In the last 15 years, several spacecraft have employed cryogenic detectors, including typical power dissipations, equipment, mostly in the context of astrophysics missions, targeting array sizes and operating temperatures. the electromagnetic radiation emitted by celestial objects over a wavelength range that it is difficult to cover from the ground. Such Applications involving the lower energy end of missions include IRAS (Infrared Astronomical Satellite, launched in the electromagnetic spectrum (i.e. sub- 1983), COBE (Cosmic Background Explorer, launched in 1989) and ISO millimetre wave and infrared) are the ones that (Infrared Space Observatory, launched in 1995). Several new missions benefit most from the utilisation of cryogenic are currently in preparation, including Herschel/Planck, SIRTF and the detectors. Dramatic developments have recently Next-Generation Space Telescope (NGST). In the higher temperature taken place in infrared detector technology, range, between 100 and 10 K, many missions are already operational driven mainly by the vast investments made by or under development. They include military reconnaissance satellites the US Department of Defense during the (such as Helios), Earth-observation satellites (Spot) and meteoro- 1980s. Such developments embrace a very logical spacecraft (MSG, Meteosat Second Generation), with infrared large spectral range, from the Near-IR (NIR, λ = detectors operating at about 85 K. 1 µm) to the Far-IR (FIR, λ = 200 µm), and have focused on low-background, high-sensitivity 92 LINDER 8/29/01 9:36 AM Page 3 cryogenics in space Figure 1. Overview of photon detectors and related operating temperatures. Note the extended sensitivity range of the cryogenic detectors, such as STJs and bolometers Table 1. Main characteristics of photon detectors and SQUIDs Detector type Temperature range (K) Dissipation range (W) Detector size (pixel and array) Min. Max. Min. Max. Pixel (µm) Array (n x n) Wavelength Ge crystal 50 100 0 0 10 000 <10 Gamma CCD 150-200 300 0.1 20 10-30 106 X-ray / Vis STJs 0.01 1 10-9 10-6 20-50 <103 X-ray–UV-Vis-NIR µ-Calorimeters 0.05 0.3 10-12 10-11 100 <100 X-ray TESs 0.05 0.3 10-11 10-9 100 <100 X-ray-UV-Vis-NIR Photo-conductors-NIR 30 100 0.01 0.02 30-50 106 NIR Photo-conductors-MIR 2 20 0.01 0.02 50-100 <104 MIR Photo-conductors-FIR 1 2 0.001 0.003 50-100 <103 FIR Sub-mm bolometers 0.1 0.3 10-9 10-8 100-500 <102 Sub-mm SQUIDs (LTS) 1 4 10-12 10-11 na na Read-out/accelerometer and large-format arrays. IRAS used a total of Bolometers have also been used to detect sub- 62 detector elements, while since 1995 large- millimetre photons. Neutron-Transmutation- format arrays for IR astronomy have been Doped (NTD) Ge detectors are well-established available with total pixel counts in excess of 106. and operate at temperatures between 300 and 100 mK, with NEPs of order 10-17 W/Hz1/2. Such Astronomical observations in the far-infrared devices will be used onboard ESA’s Planck investigate objects that are colder than those spacecraft. observed in the visible or in the near-infrared, as blackbody radiation in the 30 – 300 µm In the sub-millimetre-wavelength range, heterodyne wavelength range (emitted by bodies at receivers provide very high sensitivities up to temperatures ranging from 100 to 10 K). An frequencies as high as 500 GHz. Several example of such cold objects is the interstellar laboratories have shown that receivers based dust in our galaxy (at 20 – 30 K), as detected on Superconductor-Isolator-Superconductor (SIS) by IRAS in 1983, which both confirmed the devices (such as Nb-based Superconducting existence of interstellar dust and detected its Tunnel Junctions) offer better performance than thermal emission. Photo-conductors represent the conventional Schottky-diode-based systems. the main detection technique use throughout Operating temperatures are of order 2 K. At the IR range. At low temperatures and low frequencies η > 500 GHz, the so-called Hot- photon fluxes, the conductivity of these semi- Electron Bolometers (HEBs) compete with SIS conducting materials is influenced by the and Schottky diodes for the next generation of absorbed IR photons, which can ionise heterodyne receivers (e.g. on ESA’s Planck and impurities and free charge carriers. Such Herschel missions). In such devices, the photoconductors are typically operated at incoming radiation excites the electron T < 3 K. In the case of Isophot, a broad-band population, thus determining changes in the photometer flown onboard ISO, Ge:Ga resistance of the device, according to a non- detectors were combined with low-noise linear behaviour, used for mixing the signal CMOS integrating preamplifiers and voltage with the local oscillator voltage. multiplexers operating at 2 K to achieve a NEP Operating temperatures range from 70 K of order 10-18 W/Hz1/2. (2 deg, InSb HEBs) to 0.3 K (NIS HEBs). 93 LINDER 8/29/01 9:36 AM Page 4 r bulletin 107 — august 2001 In the NIR (at wavelengths between 1 and 5 µm), and 35 K and have already been used onboard other photo-conductors are used, mainly PtSi, the Hubble Space Telescope. HgCdTe and InSb. Over the last decade, the introduction of two-dimensional InSb arrays A new generation of photon detectors is has drastically changed the field of IR represented by Superconducting Tunnel astronomy, with a 1 k x 1 k pixel array based on Junctions (STJs) and Transition Edge Detectors hybrid technology. These detectors have (TESs), both photon-counting in the visible and operating temperatures ranging between 77 NIR, with intrinsic spectroscopic capability. Table 2. Summary of cryogenic space programmes (Space Science) Mission Application Type/Class Launch year Cryogenic system IRAS (NASA,NIVR,SERC) Science / IR Satellite (surveyor) 1983 4He cryostat COBE (NASA) Science / IR Satellite (surveyor) 1989 4He cryostat ISO (ESA) Science / IR Satellite (observat.) 1995 4He cryostat SFU (ISAS/NASDA/MITI) Science / IR Instrument (IRST) 1995 4He cryostat + 3He SC MSX (BMDO,US) MP/UV to FIR Satellite (observat.) 1996 sH2 cryostat HST (NASA) Science / NIR Instrument (Nicmos) 1997 sN2 cryostat WIRE (NASA) Science / IR Satellite (surveyor) 1999 Dual, sH2 cryostat STEP (ESA) Science / FP Satellite 4He cryostat Astro-E (ISAS, NASA) Science / X Satellite (observat.) 2000 sNe + 4He cryost.+ADR INTEGRAL (ESA) Sci. / Gamma Instrument (observ.) 2001 Stirling cooler SIRTF (NASA) Science / IR Satellite (observat.) 2002 4He cryostat Submillimetron (ASC) Sci. / Sub mm ISS telescope > 2004 4He cryost. + 3He SC XEUS (ESA) Science / X Instrument (observ.) 2005 Stirling cool. + ADR Herschel (ESA) Science / IR Satellite (observat.) 2007 4He cryost. + 3He SC 4 Planck (ESA) Science / FIR Satellite (surveyor) 2007 H2 & He JT + DR NGST (NASA) Science / NIR Satellite (observat.) 2008 Passive rad. + cooler Constellation-X (NASA) Science / X Satellite (observat.) 2008-10 Astro-E like / coolers ARISE (NASA) Sci. / Radio Satellite (VLBI) 2008 Cryo-cooler + H2 JT DARWIN (ESA) Science / IR Satellite (VLBI) >2009 Cryo-cooler + H2 JT TPF (NASA) Science / IR Satellite (VLBI) 2010 Passive rad. + cooler. Rosetta (ESA) Sci. / Comet Instrument (probe) 2003 Stirling cooler FP = Fundamental Physics MP = Multipurpose mission (defence + science) Table 3.