Nucleic acid analogues and the origins of replication.

Recent interest in the properties of "nucleic acid-like structures" has been stimulated by difficulties encountered in the synthesis and nonenzymatic oligomerization of nucleotides. However, none of the newly proposed monomers has yet been synthesized in a plausibly prebiotic manner. Arguments are presented that analogues based on 8-hydroxymethyladenine and 5-hydroxymethyluracil are promising candidates for primitive nucleotide precursors.     

Relative efficacy of the proposed Space Shuttle antimotion sickness medications

Space motion sickness has been estimated as affecting between 1/3 and 1/2 of all space flight participants. NASA has at the moment proposed a combination of promethazine and ephedrine (P/E) and one of scopolamine and dextroamphetamine (S/D), both given orally, as well as a transdermally applied scopolamine (TAS), as preventive and ameliorative measures. The reported double-blind study, tests the early phase actions and efficacy of the transdermal scopolamine (Transderm (TM)-V of ALZA Corporation) and compares these in detail to the oral medications. Motion sickness resistance was tested by standardized head movements while accelerating at 0.2 degree/sec2 to a maximum rotation of 240 degrees/sec, with an intermediate plateau of 10 min at 180 degrees/sec. To permit weighting motion sickness protection against other system influences, cardiovascular, psychological (subjective and objective), and visual parameter changes were documented for the three therapeutic modes. The relative impact of the various modalities on operational and experimental components of space missions is discussed. A comparison to intramuscularly administered promethazine (a backup therapeutic mode suggested for Space Shuttle use) is also included.     

Microphysics in Astrophysical Plasmas

Although macroscale features dominate astrophysical images and energetics, the physics is controlled through microscale transport processes (conduction, diffusion) that mediate the flow of mass, momentum, energy, and charge. These microphysical processes manifest themselves in key (all) boundary layers and also operate within the body of the plasma. Crucially, most plasmas of interest are rarefied to the extent that classical particle collision length- and time-scales are long. Collective plasma kinetic phenomena then serve to scatter or otherwise modify the particle distribution functions and in so-doing govern the transport at the microscale level. Thus collisionless plasmas are capable of supporting thin shocks, current sheets which may be prone to magnetic reconnection, and the dissipation of turbulence cascades at kinetic scales. This paper lays the foundation for the accompanying collection that explores the current state of knowledge in this subject. The richness of plasma kinetic phenomena brings with it a rich diversity of microphysics that does not always, if ever, simply mimic classical collision-dominated transport. This can couple the macro- and microscale physics in profound ways, and in ways which thus depend on the astrophysical context.     

Quickest detection for sequential decentralized decision systems

Quickest detection procedures are techniques used to detect sudden or abrupt changes (also called disorders) in the statistics of a random process. The goal is to determine as soon as possible that the change occurred, while at the same time minimizing the chance of falsely signaling the occurrence of a disorder before the change. In this work the distributed quickest detection problem when the disorder occurs at an unknown time is considered. The distributed local detectors utilize a simple summing device and threshold comparator, with a binary decision at the output. At the fusion center, the optimal maximum likelihood (ML) procedure is analyzed and compared with the more practical Page procedure for quickest detection. It is shown that the two procedures have practically equivalent performance. For the important case of unknown disorder magnitudes, a version of the Hinkley procedure is also examined. Next, a simple method for choosing the thresholds of the local detectors based on an asymptotic performance measure is presented. The problem of selecting the local thresholds usually requires optimizing a constrained set of nonlinear equations; our method admits a separable problem, leading to straightforward calculations. A sensitivity analysis reveals that the resulting threshold settings are optimal for practical purposes. The issue of which sample size to use for the local detectors is investigated, and the tradeoff between decision delay and communication cost is evaluated. For strong signals, it is shown that the relative performance deteriorates as the sample size increases, that is, as the system cost decreases. Surprisingly, for the weak signal case, lowering the system cost (increasing the sample size) does not necessarily result in a degradation of performance     

Bit-Plane Encoding: A Technique for Source Encoding

This paper describes bit-plane encoding, a technique for reducing redundancy in data gathered in space probes. The results of testing bit-plane encoding on particular data gathered by Explorer XII are summarized, and the technique is compared with alternative schemes for encoding the same data. Bit-plane encoding is intended for use aboard spacecraft. The technique presumes input data in binary form. The encoder implementation consists of a memory to store data samples, a monitor, and a code box. Both the monitor and the code box perform simple operations on binary sequences. Bit-plane encoding is especially useful when the data have an amplitude spectrum which is concentrated in different ranges in different time intervals. With a stored group of 128 samples, bit-plane encoding could be used to describe energetic particle counts gathered by Explorer XII with less than 50 percent as many bits as were actually used, and with no loss of information. The technique also conveniently allows certain useful information destroying operations.     

On Optimal Minimax Jamming and Detection of Radar Signals

A game between an intelligent jammer J and decision maker DM is considered. DM seeks to detect a coherent slowly fading narrowband signal under a Neyman-Pearson criterion. His observations are corrupted with additive narrowband noise, the source of which is J's jamming with a power constraint, but otherwise almost arbitrary statistics. DM knows J's action but the converse is not true. When the number of samples increases asymptotically, a minimax solution for the game exists where the jamming is Gaussian, independent of the desired signal amplitude level and probability distribution. The same result also holds for detection of a nonrandom baseband signal.     

Space station gas-grain simulation facility: Application to exobiology

The Space Station provides an environment in which the forces required to suspend particles during an experiment can be reduced by as much as six orders of magnitude. This reduction in levitation force enables us to perform many new experiments in a variety of disciplines. We have grouped these experiments into two catatgories: 1) those involving an individual particle or the interaction between a few particles and 2) those involving clouds of particles. We consider only particle experiments at this stage because cloud experiments suffer from electrostatic interactions and levitation-forced coalescence therefore requiring considerably more space, mass and crew interaction. The displacement of a particle resulting from g-jitter for ballistic, Knudsen and Stokes flow regimes is considered in detail and the radiation, acoustic, electrostatic and electromagnetic levitation mechanisms to control this motion are reviewed. We have selected the simulation of organic haze production on Titan as an example experiment for detailed study. The objective of this experiment is to simulate the photolysis of methane and the subsequent formation of the organic haze particles in the upper atmosphere of Titan.