Five millinewton field emission thruster testing

A compact cluster of two 80-mm-wide emitters has been designed, built and tested. The field emission thruster using this cluster is able to produce thrust in excess of 5 mN at high mass efficiency.     

The first cell membranes

Organic compounds are synthesized in the interstellar medium and can be delivered to planetary surfaces such as the early Earth, where they mix with endogenous species. Some of these compounds are amphiphilic, having polar and nonpolar groups on the same molecule. Amphiphilic compounds spontaneously self-assemble into more complex structures such as bimolecular layers, which in turn form closed membranous vesicles. The first forms of cellular life required self-assembled membranes that were likely to have been produced from amphiphilic compounds on the prebiotic Earth. Laboratory simulations show that such vesicles readily encapsulate functional macromolecules, including nucleic acids and polymerases. The goal of future investigations will be to fabricate artificial cells as models of the origin of life.     

Vertical test integration

Today, most every weapon system is electronics intensive. Digital computers are at the core of military aircraft, ship, and vehicle weapons systems. Indeed, each weapon system's performance is largely determined by its digital computers and other electronics. This electronics dependency is necessary in order to provide the speed, functional compliance, and accuracy to achieve the required weapon system performance. Historically, test systems required for the various maintenance levels and also at the manufacturing site have each been uniquely developed, employed, and sustained for only one area of weapon system electronics support. There are a number of reasons for this situation including different organizations responsible for the levels of support, different funding sources, timing limitations, and technical feasibility. In recent years, however, commercial-off-the-shelf (COTS) advances in measurement and stimulation hardware, personal computers, Windows operating systems, flexible test programming languages, and more, have made it technically feasible to develop a family of test systems that provide common weapon system electronics support at all levels. Some use the phrase Vertical Test Integration to describe this concept of factory/field/depot integration     

Uniform versus nonuniform sampling when tracking in clutter

Many target tracking subsystems have the ability to schedule their own data rates; essentially they can "order" new information whenever they need it, and the cost is in terms of the sensor resource. But among the unmanaged schemes, uniform sampling, in which a new measurement is requested periodically and regularly, is the most commonly-used sampling scheme; deliberately nonuniform schemes are seldom given serious consideration. In this paper, however, we show that such schemes may have been discarded prematurely: a nonuniform sampling can have its benefits. Specifically, the nonuniform and uniform sampling schemes are compared for two kind of trackers: the probabilistic data association filter (PDAF), which updates its track based on a single scan of information at a time; and N-D assignment (an optimization-based implementation of the multi-hypothesis tracker (MHT)), in which the sliding window involves many scans of observations. We find that given the ground rule of maintenance of the same overall scan rate (i.e., the same sensor effort) uniform sampling is always optimal for the single-scan tracker in the sense of track life. However, nonuniform sampling can outperform uniform sampling if a more sophisticated multi-scan tracker is used, particularly when 1) the target has a high process noise, and/or 2) the false alarm density is high, and/or 3) the probability of detection is high.     

Potential experiments with superconducting magnets in Earth orbit

Several experiments that can be performed in Earth orbit with a superconducting magnet are discussed. They are divided into 2 classes, pure plasma physics experiments that can be performed in near Earth orbit and planetary magnetosphere simulation experiments that are best conducted in weak background fields distant from the Earth. The later are all based on the Minimag concept where plasma is directed toward a large dipolar magnet in Earth orbit to form a model miniature magnetosphere. Several experiments that cannot be performed in ground based laboratories and tests that cannot be made in the real magnetosphere can be carried out in Earth orbit. The creation of a miniature model of the magnetosphere (Minimag) forms the basis for several of these experiments.     

The THEMIS Constellation

The Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission is the fifth NASA Medium-class Explorer (MIDEX), launched on February 17, 2007 to determine the trigger and large-scale evolution of substorms. The mission employs five identical micro-probes (termed “probes”), which have orbit periods of one, two and four days. Each of the Probes carries five instruments to measure electric and magnetic fields as well as ions and electrons. Each probe weighs 134 kg including 49 kg of hydrazine fuel and measures approximately 0.8×0.8×1.0 meters (L×W×H) and operates on an average power budget of 40 watts. For launch, the Probes were integrated to a Probe Carrier and separated via a launch vehicle provided pyrotechnic signal. Attitude data are obtained from a sun sensor, inertial reference unit and the instrument Fluxgate Magnetometer. Orbit and attitude control use a RCS system having two radial and two axial thrusters for roll and thrust maneuvers. Its two fuel tanks and pressurant system yield 960 meters/sec of delta-V, sufficient to allow Probe replacement strategies. Command and telemetry communications use an S-band 5 watt transponder through a cylindrical omni antenna with a toroidal gain pattern. This paper provides the key requirements of the probe, an overview of the probe design and how they were integrated and tested. It includes considerations and lessons learned from the experience of building NASA’s largest constellation.     

Heliopause stability revisited: dispersion analysis and numerical simulations

The heliopause, a surface separating the tenuous hot heliosheath flow and the dense, strongly magnetized interstellar flow, is subject to instabilities of the Rayleigh–Taylor and Kelvin–Helmholtz types. The dynamic evolution of this discontinuity is of considerable importance for understanding the neutral atom and cosmic-ray filtration at the interface. Here, we investigate the stability of the upwind heliopause in the presence of charge exchange collisions using both an analytic (dispersion relation) approach and a numerical model that includes the interstellar magnetic field. The linear analysis yields a cubic dispersion relation that admits imaginary solutions for the full range of wavenumbers, implying that the stagnation point on the heliopause is unconditionally Rayleigh–Taylor unstable to small perturbations propagating parallel to the discontinuity surface. We confirm this result by following the nonlinear development of the instability with a time-dependent simulation using a four fluid MHD-neutral numerical code. For the typical solar wind and LISM conditions, we obtain cyclical evolution of the upwind heliopause with a period of the order of 100 years. We also identify two areas of space physics where the instability may have important implications.     

A detailed FLUKA-2005 Monte-Carlo simulation for the ATIC detector

We have performed a detailed Monte-Carlo (MC) simulation for the Advanced Thin Ionization Calorimeter (ATIC) detector using the MC code FLUKA-2005 which is capable of simulating particles up to 10 PeV. The ATIC detector has completed two successful balloon flights from McMurdo, Antarctica lasting a total of more than 35 days. ATIC is designed as a multiple, long duration balloon flight, investigation of the cosmic ray spectra from below 50 GeV to near 100 TeV total energy; using a fully active Bismuth Germanate (BGO) calorimeter. It is equipped with a large mosaic of silicon detector pixels capable of charge identification, and, for particle tracking, three projective layers of x–y scintillator hodoscopes, located above, in the middle and below a 0.75 nuclear interaction length graphite target. Our simulations are part of an analysis package of both nuclear (A) and energy dependences for different nuclei interacting in the ATIC detector. The MC simulates the response of different components of the detector such as the Si-matrix, the scintillator hodoscopes and the BGO calorimeter to various nuclei. We present comparisons of the FLUKA-2005 MC calculations with GEANT calculations and with the ATIC CERN data.