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.     

Huygens' Surface Science Package

The design and performance of the Surface Science Package (SSP) on the Huygens probe are discussed. This instrument consists of nine separate sensors that are designed to measure a wide range of physical properties of Titan's lower atmosphere, surface, and sub-surface. By measuring a number of physical properties of the surface it is expected that the SSP will be able to constrain the inferred composition and structure of the moon's near-surface environment. Although the SSP is primarily designed to sense properties of the surface, some of its sensors will also make measurements of the atmosphere along the probe's entry path and will complement the data gathered by other experiments on the Huygens probe. This revised version was published online in August 2006 with corrections to the Cover Date.     

Power factor correction circuits with robust current control technique

A dynamically robust current control method to synthesize a sinusoidal input current for AC-to-DC converters with boost type topology is presented. Under this control strategy, the inductor current and the diode current of the boost converter are fed back and combined in a special way which makes the input current of the AC-to-DC converter stable and robust. The input current is solely determined by the reference current. When the reference current signal is derived from the sinusoidal input voltage, the input current is sinusoidal and in phase with the input voltage. Theoretical analysis is first provided. Small signal analysis shows that the current loop is inherently stable and has a fast dynamic response. Large signal analysis reveals that the control system is not affected by large disturbances in supply voltage or output load. Computer simulations have been carried out and experimental prototype models have been built to verify the analysis and demonstrate the feasibility of the control strategy. A power factor of 0.998 and a total harmonic distortion (THD) of 3.18% are measured.     

The Coma of Comet 9P/Tempel 1

As comet 9P/Tempel 1 approaches the Sun in 2004–2005, a temporary atmosphere, or “coma,” will form, composed of molecules and dust expelled from the nucleus as its component icy volatiles sublimate. Driven mainly by water ice sublimation at surface temperatures T > 200 K, this coma is a gravitationally unbound atmosphere in free adiabatic expansion. Near the nucleus (≤ 10² km), it is in collisional equilibrium, at larger distances (≥10⁴ km) it is in free molecular flow. Ultimately the coma components are swept into the comet’s plasma and dust tails or simply dissipate into interplanetary space. Clues to the nature of the cometary nucleus are contained in the chemistry and physics of the coma, as well as with its variability with time, orbital position, and heliocentric distance. The DI instrument payload includes CCD cameras with broadband filters covering the optical spectrum, allowing for sensitive measurement of dust in the comet’s coma, and a number of narrowband filters for studying the spatial distribution of several gas species. DI also carries the first near-infrared spectrometer to a comet flyby since the VEGA mission to Halley in 1986. This spectrograph will allow detection of gas emission lines from the coma in unprecedented detail. Here we discuss the current state of understanding of the 9P/Tempel 1 coma, our expectations for the measurements DI will obtain, and the predicted hazards that the coma presents for the spacecraft. An erratum to this article is available at .