Gravity and perceptual stability during translational head movement on earth and in microgravity

We measured the amount of visual movement judged consistent with translational head movement under normal and microgravity conditions. Subjects wore a virtual reality helmet in which the ratio of the movement of the world to the movement of the head (visual gain) was variable. Using the method of adjustment under normal gravity 10 subjects adjusted the visual gain until the visual world appeared stable during head movements that were either parallel or orthogonal to gravity. Using the method of constant stimuli under normal gravity, seven subjects moved their heads and judged whether the virtual world appeared to move “with” or “against” their movement for several visual gains. One subject repeated the constant stimuli judgements in microgravity during parabolic flight. The accuracy of judgements appeared unaffected by the direction or absence of gravity. Only the variability appeared affected by the absence of gravity. These results are discussed in relation to discomfort during head movements in microgravity.     

On the effect of considering more realistic particle shape and mass parameters in MMOD risk assessments

One of the primary mission risks tracked in the development of all spacecraft is that due to micro-meteoroids and orbital debris (MMOD). Both types of particles, especially those larger than 0.1 mm in diameter, contain sufficient kinetic energy due to their combined mass and velocities to cause serious damage to crew members and spacecraft. The process used to assess MMOD risk consists of three elements: environment, damage prediction, and damage tolerance. Orbital debris risk assessments for the Orion vehicle, as well as the Shuttle, Space Station and other satellites use ballistic limit equations (BLEs) that have been developed using high speed impact test data and results from numerical simulations that have used spherical projectiles. However, spheres are not expected to be a common shape for orbital debris; rather, orbital debris fragments might be better represented by other regular or irregular solids. In this paper we examine the general construction of NASA’s current orbital debris (OD) model, explore the potential variations in orbital debris mass and shape that are possible when using particle characteristic length to define particle size (instead of assuming spherical particles), and, considering specifically the Orion vehicle, perform an orbital debris risk sensitivity study taking into account variations in particle mass and shape as noted above. While the results of the work performed for this study are preliminary, they do show that continuing to use aluminum spheres in spacecraft risk assessments could result in an over-design of its MMOD protection systems. In such a case, the spacecraft could be heavier than needed, could cost more than needed, and could cost more to put into orbit than needed. The results obtained in this study also show the need to incorporate effects of mass and shape in mission risk assessment prior to first flight of any spacecraft as well as the need to continue to develop/refine BLEs so that they more accurately reflect the shape and material density variations inherent to the actual debris environment.     

Analysis of the Micrometeoroid and Debris Hazard Posed to an Orbiting Parabolic Mirror

Many concepts of future space systems involve the use of parabolic mirrors for optical applications. The need for a highly reflective finish means that performance of such systems will be particularly vulnerable to space debris and micrometeoroids. A case study was performed to examine the micrometeoroid and debris hazard posed to an orbiting parabolic mirror. The mirror considered was nominally Earth-pointed in a circular orbit with two candidate altitudes in low Earth orbit (LEO), well within the region inhabited by man-made debris. The timeframes of interest for the two missions were 2002–2004 and 2005–2015. Microgram and larger particles were considered.To perform this study, it was necessary to determine the debris and meteoroid flux across the parabolic surface. To assess sensitivity of results to uncertainity in available data, two approaches were taken. The first approach was an analytical procedure based on use of long duration exposure facility (LDEF) data and published theoretical results. The second approach used two readily available computer models: the ESA MASTER model and NASA's ORDEM96. In addition, an in-house implementation of the Grün meteoroid model was used. While multiple results were available for the total flux and flux distributed over azimuth, only the MASTER model was available for generating the desired elevation data to obtain the flux distribution over the parabolic mirror. In an attempt to bound the uncertainty in the knowledge of the elevation distribution, the results from both the MASTER and ORDEM96 models were processed together to form a separate, hybrid prediction. In addition, results were used in the preliminary design of a protective skirt.This case study elucidated the practical obstacles and considerations in performing a sufficiently accurate debris and meteoroid analysis using data and tools that are readily available to the broad space sector. The resulting procedures are useful in the assessment of the risk posed to optics by the meteoroid and debris environment and in the design of protection.     

The relative role of visual and non-visual cues in determining the perceived direction of "up": experiments in parabolic flight

In order to measure the perceived direction of "up", subjects judged the three-dimensional shape of disks shaded to be compatible with illumination from particular directions. By finding which shaded disk appeared most convex, we were able to infer the perceived direction of illumination. This provides an indirect measure of the subject's perception of the direction of "up". The different cues contributing to this percept were separated by varying the orientation of the subject and the orientation of the visual background relative to gravity. We also measured the effect of decreasing or increasing gravity by making these shape judgements throughout all the phases of parabolic flight (0 g, 2 g and 1 g during level flight). The perceived up direction was modeled by a simple vector sum of "up" defined by vision, the body and gravity. In this model, the weighting of the visual cue became negligible under microgravity and hypergravity conditions.     

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Contents to Volume