Lunar habitat concept employing the space shuttle external tank

The space shuttle external tank, which consists of a liquid oxygen tank, an intertank structure, and a liquid hydrogen tank, is an expendable structure used for approximately 8.5 min during each launch. A concept for outfitting the liquid oxygen tank-intertank unit for a 12-person lunar habitat is described. The concept utilizes existing structures and openings for both man and equipment access without compromising the structural integrity of the tank. Living quarters, instrumentation, environmental control and life support, thermal control, and propulsion systems are installed at Space Station Freedom. The unmanned habitat is then transported to low lunar orbit and autonomously soft landed on the lunar surface. Design studies indicate that this concept is feasible by the year 2000 with concurrent development of a space transfer vehicle and manned cargo lander for crew changeover and resupply.     

Synthesis Techniques for a Class of SSB-AM-PM Signals

This paper develops synthesis techniques for a particular type of single-sideband sinusoidal carrier which is phase modulated by a subcarrier. Mathematical expressions for signal efficiency, sensitivity of design to parameter variation, and ratio of peak to average power are derived and incorporated in a computer program. Given the desired power ratios for modulated signal components, the program solves for the corresponding modulation parameters and evaluates signal efficiency, design sensitivity, and peak to average power ratio. A sample signal design is presented for clarity.     

Optical communication systems for avionics

Avionic information processing and transmission requirements are increasing geometrically, with no end in sight, The only feasible way to meet them is to incorporate fiber-optic communication systems into avionic platforms, Such systems can be employed to replace standard electronic components, to augment existing systems, or to enable new technologies. In this paper, we survey the field of low-power communication systems from the avionics engineering perspective. We review the fundamental merits of optical fiber for information transmission and discuss various information modulation schemes. These modulation techniques determine the performance requirements for laser transmitters, We describe the horizontal- and vertical-cavity semiconductor lasers currently available for these systems, and measure their performance against the additional environmental constraints of avionics platforms. Finally, we outline expected near- and far-term trends in system and device development     

Observations of interplanetary shocks: Recent progress

Interplanetary shock observations since the prior Solar Terrestrial Physics Symposium in 1978 are reviewed. Since the interval coincides with the recent solar maximum, emphasis is placed on shocks associated with transient solar phenomena, including coronal transients and eruptive prominences as well as flares. A good correlation between shocks and Storm Sudden Commencements has persisted into the recent maximum. Shocks have been identified that are associated with disappearing filaments and coronal transients rather than with flares. Significant progress has been made in the indirect observation of shocks near the Sun as a result of radio wave measurements in interplanetary space and measurement of the scintillation and spectral broadening of spacecraft radio transmissions. Preliminary results regarding the thickness of interplanetary shocks have appeared. Several quasi-parallel shocks propagating more nearly along, rather than across, the magnetic field have been identified. The plasma drivers accompanying interplanetary shocks have received increased attention and distinctive features have been found in electron, ion and magnetic field data.     

The Plasma and Suprathermal Ion Composition (PLASTIC) Investigation on the STEREO Observatories

The Plasma and Suprathermal Ion Composition (PLASTIC) investigation provides the in situ solar wind and low energy heliospheric ion measurements for the NASA Solar Terrestrial Relations Observatory Mission, which consists of two spacecraft (STEREO-A, STEREO-B). PLASTIC-A and PLASTIC-B are identical. Each PLASTIC is a time-of-flight/energy mass spectrometer designed to determine the elemental composition, ionic charge states, and bulk flow parameters of major solar wind ions in the mass range from hydrogen to iron. PLASTIC has nearly complete angular coverage in the ecliptic plane and an energy range from ～0.3 to 80 keV/e, from which the distribution functions of suprathermal ions, including those ions created in pick-up and local shock acceleration processes, are also provided.     

The Delivery of Water During Terrestrial Planet Formation

The planetary building blocks that formed in the terrestrial planet region were likely very dry, yet water is comparatively abundant on Earth. Here we review the various mechanisms proposed for the origin of water on the terrestrial planets. Various in-situ mechanisms have been suggested, which allow for the incorporation of water into the local planetesimals in the terrestrial planet region or into the planets themselves from local sources, although all of those mechanisms have difficulties. Comets have also been proposed as a source, although there may be problems fitting isotopic constraints, and the delivery efficiency is very low, such that it may be difficult to deliver even a single Earth ocean of water this way. The most promising route for water delivery is the accretion of material from beyond the snow line, similar to carbonaceous chondrites, that is scattered into the terrestrial planet region as the planets are growing. Two main scenarios are discussed in detail. First is the classical scenario in which the giant planets begin roughly in their final locations and the disk of planetesimals and embryos in the terrestrial planet region extends all the way into the outer asteroid belt region. Second is the Grand Tack scenario, where early inward and outward migration of the giant planets implants material from beyond the snow line into the asteroid belt and terrestrial planet region, where it can be accreted by the growing planets. Sufficient water is delivered to the terrestrial planets in both scenarios. While the Grand Tack scenario provides a better fit to most constraints, namely the small mass of Mars, planets may form too fast in the nominal case discussed here. This discrepancy may be reduced as a wider range of initial conditions is explored. Finally, we discuss several more recent models that may have important implications for water delivery to the terrestrial planets.     

Regolith X-Ray Imaging Spectrometer (REXIS) Aboard the OSIRIS-REx Asteroid Sample Return Mission

The Regolith X-ray Imaging Spectrometer (REXIS) is the student collaboration experiment proposed and built by an MIT-Harvard team, launched aboard NASA’s OSIRIS-REx asteroid sample return mission. REXIS complements the scientific investigations of other OSIRIS-REx instruments by determining the relative abundances of key elements present on the asteroid’s surface by measuring the X-ray fluorescence spectrum (stimulated by the natural solar X-ray flux) over the range of energies 0.5 to 7 keV. REXIS consists of two components: a main imaging spectrometer with a coded aperture mask and a separate solar X-ray monitor to account for the Sun’s variability. In addition to element abundance ratios (relative to Si) pinpointing the asteroid’s most likely meteorite association, REXIS also maps elemental abundance variability across the asteroid’s surface using the asteroid’s rotation as well as the spacecraft’s orbital motion. Image reconstruction at the highest resolution is facilitated by the coded aperture mask. Through this operation, REXIS will be the first application of X-ray coded aperture imaging to planetary surface mapping, making this student-built instrument a pathfinder toward future planetary exploration. To date, 60 students at the undergraduate and graduate levels have been involved with the REXIS project, with the hands-on experience translating to a dozen Master’s and Ph.D. theses and other student publications.     

High Dynamic GPS Receiver Using Maximum Likelihood Estimationand Frequency Tracking

A new high dynamic global positioning system (GPS) receiver ispresented and its performance characterized by analysis,simulation, and demonstration. The demonstration receiver is abreadboard model capable of tracking a single simulated satellitesignal in pseudorange and range rate. Pseudorange and range rateestimates are made once every 20 ms, using a maximum likelihoodestimator, and are tracked by means of a third-order fadingmemory filter in a feedback configuration. The receiver trackspseudorange with rms errors of under 1 m when subjected tosimulated 50 g, 40 g/s circular trajectories. The tracking thresholdis approximately 28 dB·Hz, which provides 12 dB margin relativethe the minimum specified signal strength, assuming 3.5 dB systemnoise figure and 0 dB antenna gain.     

A General Model for Switching Converters

A low frequency, small signal model, valid for all types of converters, both in the heavy and the light mode of conduction, is developed in block diagram form. The model provides a clear insight into the behavior of such converters and enhances the respective contributions of input and output filters.