Active versus standby redundancy for improved cryocooler reliability in space
Long-life space cryocooler applications, such as NASA’s Atmospheric Infrared Sounder (AIRS) instrument, require that the cryocooler system possess a very high level of reliability. This need for high reliability not only demands that high reliability coolers be used, but often requires that some form of redundancy be incorporated. One common implementation is standby redundancy; however, active redundancy is another equally viable choice. Recently, experience with both types of redundancy has been gained with the AIRS instrument. The AIRS cryocoolers were initially designed and launched as standby redundant units; they were then switched over to active redundancy after six months of in-space operation.
AIRS Pulse Tube Coolers Performance Update – Twelve Years in Space
The Atmospheric Infrared Sounder (AIRS) instrument began operation 39 days after its May 4, 2002 launch into Earth orbit. Designed with redundant cryocoolers (a primary and a backup), the instrument began operation using a single cooler to bear the load of both the detector and the non-operating, backup cooler. During the early months of the mission, contamination of the cryogenic surfaces led to increased cryocooler loads and a change in operating strategy to run both coolers simultaneously to both overcome the increased cryogenic contamination load and to allow operation at a much reduced compressor stroke level. This change led to the successful continuous operation of the coolers since November 2002, with minimal interruption of science data collection from the AIRS instrument over the past 111⁄2 years. After a brief review of the AIRS instrument cryogenic design, this paper presents detailed data on the highly successful continuous operation of the AIRS pulse tube cryocoolers and instrument thermal design over the past twelve years. The data show that early cryogenic contamination reached an equilibrium level after a year of space operation and that the cooler stroke required for constant-temperature operation has only increased a few percent since that time. This high level of operational stability not only indicates that the cryocoolers have maintained near-constant efficiency, but also that the instrument’s thermal design has presented a near-constant heat rejection and parasitic-load environment. At this time AIRS maintains continuous operation in space providing important scientific data on Earth’s atmospheric parameters. INTRODUCTION Launched in May 2002 aboard the NASA Aqua spacecraft, the AIRS instrument was designed to provide high-accuracy global air temperature data for climate studies and weather prediction. Since that time AIRS has expanded its data gathering to include creating global, three-dimensional maps of atmospheric temperature and humidity, cloud amounts and heights, greenhouse gas concentrations, and many other atmospheric phenomena. Fundamental to its operation is a precisely calibrated, high spectral resolution grating spectrometer operating between 3.7 to 15.4 ìm. The cryogenically cooled spectrometer, shown in Fig. 1, uses a pair of TRW (now Northrop Grumman Aerospace Systems—NGAS) 55¶K pulse tube cryocoolers to cool the HgCdTe focal plane to 58¶K. 1 Also shown is the ambient portion of the instrument, which contains the high power components including the instrument electronics and the P# 33 79 87 Cryocoolers 18, edited by S.D. Miller and R.G. Ross, Jr. ©¶International Cryocooler Conference, Inc., Boulder, CO, 2014
Mitochondrial aquaporin-8-mediated hydrogen peroxide transport is essential for teleost spermatozoon motility
Reactive oxygen species (ROS), particularly hydrogen peroxide (H2O2), cause oxidative cell damage and inhibit sperm function. In most oviparous fishes that spawn in seawater (SW), spermatozoa may be exposed to harmful ROS loads associated with the hyperosmotic stress of axonemal activation and ATP synthesis from mitochondrial oxidative phosphorylation. However, it is not known how marine spermatozoa can cope with the increased ROS levels to maintain flagellar motility. Here, we show that a marine teleost orthologue of human aquaporin-8, termed Aqp8b, is rapidly phosphorylated and inserted into the inner mitochondrial membrane of SW-activated spermatozoa, where it facilitates H2O2 efflux from this compartment. When Aqp8b intracellular trafficking and mitochondrial channel activity are immunologically blocked in activated spermatozoa, ROS levels accumulate in the mitochondria leading to mitochondrial membrane depolarisation, the reduction of ATP production, and the progressive arrest of sperm motility. However, the decreased sperm vitality underlying Aqp8b loss of function is fully reversed in the presence of a mitochondria-targeted antioxidant. These findings reveal a previously unknown detoxification mechanism in spermatozoa under hypertonic conditions, whereby mitochondrial Aqp8b-mediated H2O2 efflux permits fuel production and the maintenance of flagellar motility.