ALBERT

Description: Stabilogram-diffusion analysis was used to examine how prolonged periods in microgravity affect the open-loop and closed-loop postural control mechanisms. It was hypothesized that following spaceflight: (1) the effective stochastic activity of the open-loop postural control schemes in astronauts is increased; (2) the effective stochastic activity and uncorrelated behavior, respectively, of the closed-loop postural control mechanisms in astronauts are increased; and (3) astronauts utilized open-loop postural controls schemes for shorter time intervals and smaller displacements. Four crew members and two alternates from the 14-day Spacelab Life Sciences 2 Mission were included in the study. Each subject was tested under eyes-open, quiet-standing conditions on multiple preflight and postflight days. The subjects' center-of-pressure trajectories were measured with a force platform and analyzed according to stabilogram-diffusion analysis. It was found that the effective stochastic activity of the open-loop postural control schemes in three of the four crew members was increased following spaceflight. This result is interpreted as an indication that there may be in-flight adaptations to higher-level descending postural control pathways, e.g., a postflight increase in the tonic activation of postural muscles. This change may also be the consequence of a compensatory (e.g., "stiffening") postural control strategy that is adopted by astronauts to account for general feeling of postflight unsteadiness. The crew members, as a group, did not exhibit any consistent preflight/postflight differences in the steady-state behavior of their closed-loop postural control mechanisms or in the functional interaction of their open-loop and closed-loop postural control mechanisms. These results are interpreted as indications that although there may be in-flight adaptations to the vestibular system and/or proprioceptive system, input from the visual system can compensate for such changes during undisturbed stance.

Description: The authors describe results in applying robust control techniques of an active precision truss structure at the Jet Propulsion Laboratory. The active structure incorporates piezoelectric members which serve as both structural and actuator/sensor elements. Several characterizations of uncertainty are studied and the controllers are compared experimentally.

Description: It is pointed out that real parametric modal frequency and damping variation for lightly damped systems does not resemble disks on the complex plane. Using complex uncertainty (and therefore disklike) models can introduce conservativeness in the design method. An alternative means of developing suitable complex uncertainty models is presented. It involves treating the uncertainty as perturbations to the system eigenvalues and using a particular linear fractional transformation to cover this uncertainty. This approach is applicable to modes within the bandwidth of control. It can be used in conjunction with the standard perturbation modeling approaches. A simple SISO (single-input single-output) example, motivated by a flexible truss experiment at the Jet Propulsion Laboratory, is discussed in order to illustrate the proposed approach.

Description: Consideration is given to the problem of estimating, from experimental data, real parameters for a model with uncertainty in the form of both additive noise and norm-bounded perturbations. Such models frequently arise in robust control theory, and a framework is introduced for the consideration of experimental data in robust control analysis problems. If the analysis tools applied include robust stability tests for real parameter variations (real mu), the framework can be used to address the problem of robust parameter identification. While the techniques discussed can quickly become computationally overwhelming when applied to physical systems and real data, the approach introduces a novel way of looking at the identification problem and may be helpful in arriving at a more tractable methodology.

Description: All five gases in interstellar silicon carbide grains have grossly nonsolar isotopic and elemental abundances that vary with grain size but are strikingly similar to calculated values for the helium-burning shell of low-mass carbon stars. Apparently these grains formed in carbon-star envelopes, and were impregnated with noble gas ions from a stellar wind. Meteoritic SiC provides a detailed record of nuclear and chemical processes in carbon stars.

Description: Primitive meteorites contain up to 400 ppm of a very fine-grained type of carbon tentatively called C-delta. Evidence is presented here that part of all of the C-delta is primary, not shock-produced, diamond formed by stellar condensation as a metastable phase. It appears that interstellar dust contains diamond.

Description: This paper describes recent advances in structural quieting technology as applied to active truss structures intended for high precision space based optics applications. The active structure incorporates piezoelectric active members which exert control forces internal to the structure and thereby improve the structure's dimensional stability. The control architecture involves two layers of feedback control. The first utilizes collocated measurements of force and velocity at the active member to achieve active damping, the second utilizes noncollocated measurements of acceleration at the location of a simulated optical component to achieve structural stabilization. The local control loops are based on the concept of impedance matching, the global control loops are designed using robust control methods. These two levels of control are intended to operate simultaneously; however, in this paper each approach is applied individually. The combined implementation is left for future work.

Description: Early results from an ongoing search for isotopically distinctive noble gases as evidence for presolar oxide grains are presented. With some qualifications, we do not see such evidence in spinel rich acid residue fractions from the Allende meteorite. We remain hopeful that less abundant mineral separates may yet be fruitful. Presolar grains, micro-diamonds, silicon carbide, and graphite, were found in primitive meteorites. While the abundances of these three refractory C rich grains are low, a few hundred ppm, a few ppm, & less than 1 ppm, respectively in primitive meteorites, they are tagged with high concentrations of isotopically anomalous noble gas components, Xe-HL, KR & Xe-s and Ne-E(H), and Ne-E(L). These tags have served as tracers and allowed the development of techniques for their purification and eventual identification. One might expect similar amounts of refractory presolar oxides to have survived, but so far only three cases exist for their identification. The first two cases are individual corundum oxide grains. Huss et al. found one such grain from an Orgueil residue with an Al-26/Al-27 ratio of 8.9 x 10(exp -5), about 18 times higher than the canonical initial solar system value. The second corundum grain, from Murchison, was found by Nittler et al. to have unusual oxygen in addition to a similar Al-26/Al-27 ratio of 8.7 x 10(exp -4). The oxygen was depleted in O-18 by 22 percent and enriched in O-17 by a factor of 2. The third case is a measurement by Zinner et al. on an aggregate of fine grained spinels from a Murray residue with an O-17 enrichment and a possible O-18 depletion similar to the second grain, but much subdued. This is consistent with a few such presolar grains diluted by a much larger population of isotopically normal corundum grains and an even larger number of normal spinel grains.

Description: Reported here is the discovery of crystals of titanium carbide in a grain of silicon carbide which formed as a circumstellar dust particle in the atmosphere of a carbon-rich star. Just as in the case of terrestrial rocks, whose assemblage of minerals gives us clues to the composition and conditions of the environment in which they formed, the titanium carbide crystals and their textural relationship to the silicon carbide give us important clues to the nature of the stellar atmosphere in which they formed. From microscopic studies of the relationships between the atomic planes of the silicon carbide and the titanium carbide, we can show that the titanium carbide cannot have existed as already-formed crystals in a gas around which silicon carbide subsequently condensed. An alternative possibility is that both minerals grew quickly and simultaneously from condensing gas in the rapidly cooling and expanding stellar atmosphere. Other microscopic features of the silicon carbide, such as abundant atomic layer disorder and crystal twinning, similarly suggest rapid grain growth. However, another possibility is that the titanium carbide grew inside of the silicon carbide by diffusion of titanium atoms. Our calculations suggest that this scenario is less likely, given the relatively short times (a year or less) for which stellar condensates can be expected to be exposed to temperatures high enough to make diffusion sufficiently rapid.

Description: The discovery of crystals of titanium carbide in an interstellar graphite spherule is reported. The new species is particularly interesting in that it came in a protective wrapping (the graphite spherule) which eliminated the possibility of chemical alteration during its residence in the interstellar medium and in the meteorite in which it was discovered.