Periodic X-ray outbursts from A0535+26, 2S1145-619 and GX301-2

We report Ariel V(SSI) observations of three X-ray pulsars A0535+26, 2S1145-619 and GX301-2 (2S1223-624). These sources exhibit X-ray outbursts which appear, on the basis of observations extending over 5 years, to have recurrence periods of 110 days, 187.5 days and 41.4 days respectively. If these periods are orbital in origin, the observed X-ray modulation may be explained in terms of an appreciable orbital eccentricity giving rise to time variable accretion. (In the case of GX301-2 published pulse timing data already provide independent evidence for a 41.4 day orbital period and an eccentricity e 0.4). The optical counterparts are all early-type stars; A0535+26 and 2S1145-619 are identified with Be main sequence stars and GX301-2 with a B2 supergiant which also shows an emission line spectrum. The implications of the observations in relation to possible mass transfer mechanisms in such systems are discussed.     

Effects of simulated space radiation on immunoassay components for life-detection experiments in planetary exploration missions

Abstract The Life Marker Chip (LMC) instrument is part of the proposed payload on the ESA ExoMars rover that is scheduled for launch in 2018. The LMC will use antibody-based assays to detect molecular signatures of life in samples obtained from the shallow subsurface of Mars. For the LMC antibodies, the ability to resist inactivation due to space particle radiation (both in transit and on the surface of Mars) will therefore be a prerequisite. The proton and neutron components of the mission radiation environment are those that are expected to have the dominant effect on the operation of the LMC. Modeling of the radiation environment for a mission to Mars led to the calculation of nominal mission fluences for proton and neutron radiation. Various combinations and multiples of these values were used to demonstrate the effects of radiation on antibody activity, primarily at the radiation levels envisaged for the ExoMars mission as well as at much higher levels. Five antibodies were freeze-dried in a variety of protective molecular matrices and were exposed to various radiation conditions generated at a cyclotron facility. After exposure, the antibodies' ability to bind to their respective antigens was assessed and found to be unaffected by ExoMars mission level radiation doses. These experiments indicated that the expected radiation environment of a Mars mission does not pose a significant risk to antibodies packaged in the form anticipated for the LMC instrument. Key Words: Life-detection instruments-Planetary habitability and biosignatures-Radiation-Mars-Life in extreme environments. Astrobiology 12, 718-729.     

The low earth orbit environment observed using CREAM and CREDO.

The Cosmic Radiation Environment and Dosimetry experiment (CREDO) has been operational on board the Advanced Photovoltaics & Electronics Experiment Spacecraft since August 1994. Extensive measurements of cosmic ray linear energy transfer spectra (using data to January 1996) and total dose (using data to November 1994) have been made, and compared with predictions of standard models. Detailed consideration of spacecraft shielding effects have been made. Predictions are shown to overestimate the measured linear energy transfer spectra. The CREAM experiment was flown on STS-63 in the SpaceHab module. Results show penetration of high energy electrons into the SpaceHab module.     

Observations and predictions of secondary neutrons on Space Shuttle and aircraft.

The Cosmic Radiation Effects and Activation Monitor has flown on six Shuttle flights between September 1991 and February 1995 covering the full range of inclinations as well as altitudes between 220 and 570 km, while a version has flown at supersonic altitudes on Concorde between 1988 and 1992 and at subsonic altitudes on a SAS Boeing 767 between May and August 1993. The Shuttle flights have included passive packages in addition to the active cosmic ray monitor which comprises an array of pin diodes. These are positioned at a number of locations to investigate the influence of shielding and local materials. Use of both metal activation foils and scintillator crystals enables neutron fluences to be inferred from the induced radioactivity which is observed on return to Earth. Supporting radiation transport calculations are performed to predict secondary neutron spectra and the energy deposition due to nuclear reactions in silicon pin diodes and the induced radioactivity in the various scintillator crystals. The wide variety of orbital and atmospheric locations enables investigation of the influence of shielding on cosmic ray, trapped proton and solar flare proton spectra.     

Real time recovery of fault tolerant processing elements

A critical problem in the design of ultra-reliable fault tolerant systems is that of how to bring a redundant member back on-line, after a transient fault, without degrading critical real-time functions. Recovery from transients is imperative to maintain necessary system reliability in the face of transient errors which have been estimated to occur at a rate of 5 to 100 times that of permanent failures. Excessive delays associated with recovery become a problem when as much as 1 Mbytes of RAM in the faulty processor must be made congruent with the processing majority while maintaining full functionality of critical, real-time control algorithms. This paper describes a hardware assisted recovery technique which uses memory “tags” to determine which memory segments need to be restored such that recovery can be performed incrementally without affecting real-time operational tasks. Also presented is performance data associated with this technique's application to a Draper Laboratory quad-redundant processor responsible for vehicle control of a manned undersea vehicle