Adaptation of neuromuscular activation patterns during treadmill walking after long-duration space flight

The precise neuromuscular control needed for optimal locomotion, particularly around heel strike and toe off, is known to he compromised after short duration (8- to 15-day) space flight. We hypothesized here that longer exposure to weightlessness would result in maladaptive neuromuscular activation during postflight treadmill walking. We also hypothesized that space flight would affect the ability of the sensory-motor control system to generate adaptive neuromuscular activation patterns in response to changes in visual target distance during postflight treadmill walking. Seven crewmembers, who completed 3- to 6-month missions, walked on a motorized treadmill while visually fixating on a target placed 30 cm (NEAR) or 2 m (FAR) from the subject's eyes. Electronic foot switch data and surface electromyography were collected from selected muscles of the right lower limb. Results indicate that the phasic features of neuromuscular activation were moderately affected and the relative amplitude of activity in the tibialis anterior and rectus femoris around toe off changed after space flight. Changes also were evident after space flight in how these muscles adapted to the shift in visual target distance.     

Solar particle event dose distributions: parameterization of dose-time profiles

Calculations of total dose and dose equivalent as a function of time since the start of the event are presented for four of the major solar particle events that occurred during the period from August to December 1989. Results are presented for exposures to the skin, ocular lens and bone marrow shielded by a nominal thickness of aluminum shielding, comparable to that provided by a spacesuit. The calculated curves of organ dose and dose equivalent versus time are parameterized using a Weibull functional form for the fitting equation. The fitting parameters are determined using least squares regression techniques. These results provide a useful starting point for the development of methods to predict the cumulative doses and times to reach various dose limits from a limited number of dose measurements early in a solar particle event.     

Using electrical impedance spectroscopy to detect water in planetary regoliths

We present data in examination of the utility of electrical impedance spectroscopy measurements for in situ surveys to determine the water content, distribution, and phase in unconsolidated planetary regolith. We conducted calibration experiments under conditions relevant to Mars: the concentration of electrolytes in solution was varied up to 1 M to simulate the effects of unsaturated dissolved minerals and brines. We also varied the water content of heterogeneous water/sand mixtures, made with these electrolytic solutions from 0.01 wt% to 10 wt%. Tests were performed at temperatures from +25 degrees C to -65 degrees C. Conductivity and dielectric permittivity calculated from the impedance measurements indicate an expected dependence on electrolyte concentration and relative independence from electrolyte type for both liquid water and water ice. Conductivity and calculated dielectric relaxation times for these aqueous solutions agree with existing data in the literature. The relative permittivity for heterogeneous water/sand mixtures is dominated by polarization effects for the electrode configuration used. However, the characteristic orientational relaxation of ice is still visible. The conductivity retains the strong dependence on electrolyte concentration, and the permittivity is still not affected by electrolyte type. A "universal" curve between conductivity and water content establishes detectability limits of <0.01 wt% and approximately 0.3 wt% for water/sand mixtures containing liquid water and ice, respectively.     

Artificial gravity: head movements during short-radius centrifugation.

Short-radius centrifugation is a potential countermeasure to long-term weightlessness. Unfortunately, head movements in a rotating environment induce serious discomfort, non-compensatory vestibulo-ocular reflexes, and subjective illusions of body tilt. In two experiments we investigated the effects of pitch and yaw head movements in participants placed supine on a rotating bed with their head at the center of rotation, feet at the rim. The vast majority of participants experienced motion sickness, inappropriate vertical nystagmus and illusory tilt and roll as predicted by a semicircular canal model. However, a small but significant number of the 28 participants experienced tilt in the predicted plane but in the opposite direction. Heart rate was elevated following one-second duration head turns. Significant adaptation occurred following a series of head turns in the light. Vertical nystagmus, motion sickness and illusory tilt all decreased with adaptation. Consequences for artificial gravity produced by short-radius centrifuges as a countermeasure are discussed. Grant numbers: NCC 9-58.     

Human interactions in space: results from Shuttle/Mir.

Background: Anecdotal reports from space and results from simulation studies on Earth have suggested that space crewmembers may experience decrements in their interpersonal environment over time and may displace tension and dysphoria to mission control personnel. Methods: To evaluate these issues, we studied 5 American astronauts, 8 Russian cosmonauts, and 42 American and 16 Russian mission control personnel who participated in the Shuttle/Mir space program. Subjects completed questions from subscales of the Profile of Mood States, the Group Environment Scale, and the Work Environment Scale on a weekly basis before, during, and after the missions. Results: Among the crewmembers, there was little evidence for significant time effects based on triphasic (U-shaped) or linear models for the 21 subscales tested, although the presence of an initial novelty effect that declined over time was found in three subscales for the astronauts. Compared with work groups on Earth, the crewmembers reported less dysphoria and perceived their crew environment as more constraining, cohesive, and guided by leadership. There was no change in ratings of mood and interpersonal environment before, during, and after the missions. Conclusions: There was little support for the presence of a moderate to strong time effect that influenced the space crews. Crewmembers perceived their work environment differently from people on Earth, and they demonstrated equanimity in mood and group perceptions, both in space and on the ground. Grant numbers: NAS9-19411.     

Precision bone and muscle loss measurements by advanced, multiple projection DEXA (AMPDXA) techniques for spaceflight applications.

An advanced, multiple projection, dual energy x-ray absorptiometry (AMPDXA) scanner system is under development. The AMPDXA is designed to make precision bone and muscle loss measurements necessary to determine the deleterious effects of microgravity on astronauts as well as develop countermeasures to stem their bone and muscle loss. To date, a full size test system has been developed to verify principles and the results of computer simulations. Results indicate that accurate predictions of bone mechanical properties can be determined from as few as three projections, while more projections are needed for a complete, three-dimensional reconstruction.     

Future launchers strategy : the ariane 2010 initiative

With the new cryogenic upper stage ESC, the European heavy launcher Ariane 5+ is perfectly suited to the space market envisioned for the coming decade: flexible to cope with any payload and commercially attractive despite a fierce competition.Current Arianespace projections for the following years 2010–2020 indicate two major trends:

• satellites may still become larger and may require very different final orbits; today's market largely dominated by GEO may well evolve, influenced by LEO operations such as those linked to ISS or by constellations,

• to remain competitive, the launch cost has to be reduced.

The future generation of the European heavy launcher has therefore to focus on an ever increased flexibility with a drastic cost reduction.Two strategies are possible to achieve this double goal:

• reusable launchers, either partially or totally, may ease the access to space, limiting costly expendable stages; the assessment of their technical feasibility and financial viability is undergoing in Europe under the Future Launchers Technology Program (FLTP),

• expendable launchers, derived from the future Ariane 5+.

This second way started by CNES at the end of year 1999 is called the “Ariane 2010 initiative”.The main objectives are simultaneously an increase of 25% in performance and a reduction of 30% in launch cost wrt Ariane 5+.To achieve these very ambitious goals, numerous major modifications are studied:

• technical improvements :

◦ modifications of the Solid Rocket Boosters may consist in filament winding casing, increased loading, simplified casting, improved grain, simplified Thrust Vector Control, …

◦ evolution of the Vulcain engine leading to higher efficiency despite a simplified design, flow separation controlled nozzle extension, propellant management of the two cryogenic stages,

◦ simplified electrical system,

◦ increased standardization, for instance on flanged interfaces and manufacturing processes,

• operational improvements such as launch cycle simplification and standardization of the coupled analyses,

• organizational improvements such as a redistribution of responsibilities for the developments.

All these modifications will of course not be implemented together; the aim is to have a coherent catalogue of improvements in order to enable future choices depending on effective requirements. These basic elements will also be considered for the development of other launchers, in the small or medium size range.     

ENVISAT RA-2 advanced radar altimeter: Instrument design and pre-launch performance assessment review

RA-2 is a second generation spaceborne radar altimeter designed and developed by ALENIA AEROSPAZIO (ALS) in the frame of the European Space Agency (ESA) ENVISAT programme. The project, started at beginning of 1990, reached its completion in October 1998 with the delivery of the Instrument Flight Model to Daimler-Benz Aerospace, Dornier (the mission prime contractor of ENVISAT) for its integration on the platform.

RA-2 only partly resembles in its characteristics those of ERS-1 / ERS-2 first generation Radar Altimeters, designed by ALS. Principal innovative features of the RA-2 are: dual frequency operation (Ku and S band) for ionospheric bias correction of the range measurements, enhanced precision range measurements over oceans, capability of including in the instrument telemetry data down linked to ground In-phase and Quadrature samples of individual Ku band radar echoes, resolution adaptivity and robust tracking algorithms to allow range measurements and radar echoes collection not only over open oceans but also at land-ocean boundaries, ice and land regions with the real objective of continuous global Earth topography mapping.

Aim of this paper is to provide an overview of the instrument key design characteristics and to present instrument pre-launch performance results derived from the evaluation of the test results gathered during the instrument integration and characterisation campaign of the Flight Model (FM).     

Optimization of aircraft container loading

We address the problem of loading as much freight as possible in an aircraft while balancing the load in order to minimize fuel consumption and to satisfy stability/safety requirements. Our formulation methodology permits to solve the problem on a PC, within ten min, by off-the-shelf integer linear programming software. This method decides which containers to load (and in which compartment) and which to leave on the ground.     

Dual-frequency distortion predictions for the Cutler VLF array

The VLF transmitting system in Cutler, ME currently broadcasts at one frequency through a two-element antenna system. This study investigates simultaneous operation at 24.0 and 17.8 kHz. Detailed nonlinear PSPICE models were generated for the system including the amplifier triodes, saturable dynamic-tuning reactors, and the closely-coupled antenna elements. The predicted behavior is in close agreement with available observations. Upper limits are placed on harmonic content, <-59.8 dBc, and intermodulation distortion, <-78 dBc.