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The interaction of the disks of spiral galaxies and small companions is discussed. The gravitational drag effects of the disk on small satellites are of particular interest. Studies of the Andromeda Galaxy and its satellites, M32 and NGC 205, reveal the usefulness of few-body test-particle simulations in explaining many features of spiral galaxies and their satellites.
The objective of this theoretical research effort was to improve the understanding of the growth of Pb(x)Sn(1-x)Te and especially how crystal quality could be improved utilizing the microgravity environment of space. All theoretical growths are done using the vertical Bridgman method. It is believed that improved single crystal yields can be achieved by systematically identifying and studying system parameters both theoretically and experimentally. A computational model was developed to study and eventually optimize the growth process. The model is primarily concerned with the prediction of the thermal field, although mass transfer in the melt and the state of stress in the crystal were of considerable interest. The evolution is presented of the computer simulation and some of the important results obtained. Diffusion controlled growth was first studied since it represented a relatively simple, but nontheless realistic situation. In fact, results from this analysis prompted a study of the triple junction region where the melt, crystal, and ampoule wall meet. Since microgravity applications were sought because of the low level of fluid movement, the effect of gravitational field strength on the thermal and concentration field was also of interest. A study of the strength of coriolis acceleration on the growth process during space flight was deemed necessary since it would surely produce asymmetries in the flow field if strong enough. Finally, thermosolutal convection in a steady microgravity field for thermally stable conditions and both stable and unstable solutal conditions was simulated.
An overview is presented of the microgravity fluid physics program at Lewis Research Center. One of the main reasons for conducting low gravity research in fluid physics is to study phenomena such as surface tension, interfacial contact angles, and diffusion independent of such gravitationally induced effects as buoyant convection. Fluid physics is at the heart of many space-based technologies including power systems, thermal control systems, and life support systems. Fundamental understanding of fluid physics is a key ingredient to successful space systems design. In addition to describing ground-based and space-based low-gravity facilities, selected experiments are presented which highlight Lewis work in fluid physics. These experiments can be categorized into five theme areas which summarize the work being conducted at Lewis for OSSA: (1) isothermal/iso-solutal capillary phenomena; (2) capillary phenomena with thermal/solutal gradients; (3) thermal-solutal convection; (4) first- and second-order phase transitions in a static fluid; and (5) multiphase flow.
If the Galaxy possesses a dark halo of massive objects, they will produce small but potentially detectable distortions in the images of distant, extended objects due to gravitational lensing effects. An illustrative analytic and a more realistic numerical model are presented for calculating the number and angular sizes of these distortions, and a specific example of a digital filter is described which could be used for the identification of such distortions. The sensitivity of possible searches for the effect in the image of M31 and the Galactic Center are discussed in some detail. Simulated images of distortions of various angular sizes in these fields as they would be expected to appear in ground-based and Space Telescope CCD frames are presented. A Galactic halo of black holes such as that proposed by Lacey and Ostriker might well be detectable ina practically realizable experiment.
If neutron stars have radii as small as has been argued by some, observations of accretion-powered X-rays could verify the existence of innermost stable circular orbits (predicted by general relativity) around weakly magnetized neutron stars. This may be done by detecting X-ray emission from clumps of matter before and after they cross the gap (where matter cannot be supported by rotation) between the inner accretion disk and the stellar surface. Assuming the validity of general relativity, it would then be possible to determine the masses of such neutron stars independently of any knowledge of binary orbital parameters. If an accurate mass determination were already available through any of the methods conventionally used, the new mass determination method proposed here could then be used to quantitatively test strong field effects of gravitational theory.
Proposed segmented robotic arm for positioning and assembly of components of large structures contains motor-and-flywheel angular-momentum devices in each arm segment. Transfers angular momentum to and from segments of arm to produce desired motion of arm. Conceived for use in outer space, concept adaptable to terrestrial uses in which buoyancy cancels effect of gravitation - (floating on or under water).
A detailed study of the effects of gravitational radiation on the relative positions of objects was carried out with the hope that astrometric detection of gravitational radiation might be possible. The results are discouraging. It would appear that narrow-field instruments in the Astrometric Telescope Facility (ATF) class are still several orders of magnitude less accurate than would be required for this very delicate kind of measurement. However, the situation changes considerably when wide-field instruments are considered. An instrument such as POINTS ought to be able to detect gravitational radiation at this level with ease. The list of bright quasars has been augmented. Of particular interest is the finding of quasars near two open clusters and one planetary nebula. These would be useful in determining absolute parallaxes of these objects.
The possibility of placing constraints on the mass distribution of a cluster of galaxies by analyzing the cluster's gravitational lensing effect on the images of more distant galaxies is investigated theoretically in the limit of weak distortion. The steps in the proposed analysis are examined in detail, and it is concluded that detectable distortion can be produced by clusters with line-of-sight velocity dispersions of over 500 km/sec. Hence it should be possible to determine (1) the cluster center position (with accuracy equal to the mean separation of the background galaxies), (2) the cluster-potential quadrupole moment (to within about 20 percent of the total potential if velocity dispersion is 1000 km/sec), and (3) the power law for the outer-cluster density profile (if enough background galaxies in the surrounding region are observed).
An extensive body of data, from meteors, zodiacal light, spacecraft-borne impact detectors (helios, Pioneer, Galileo, and Ulysses), and other sources, forms the basis of a new numerical model for the distributions of interplanetary meteoroids. For each of the five populations in this model it is possible to evaluate meteoroid concentration and flux for oriented surfaces or detectors having arbitrary position and velocity in interplanetary space (Divine, 1992, in preparation). For a spacecraft in geocentric orbit, the effects of gravitational focusing and shielding by the Earth were derived with full attention to the directionality of the particles, both on approach (i.e., relative to a massless Earth) and at the target. This modeling approach was exercised to provide an estimate of meteoroid fluence for each of several oriented surfaces on the Long Duration Exposure Facility (LDEF).
A dynamical qualitative study of the rotational motion for cometary-type bodies submitted to gravitational perturbations has been performed by numerical simulations, including the Sun and Jupiter's disturbing torques in the model. Results show small gravitational disturbing effects from the Sun on Halley-type orbits, as well as from Jupiter on most close-approach configurations. Only a very close-approach induces notable effects, presenting then some interesting sensitivity to initial conditions.
It has been said the decades of the 1990's represents a second golden age of science. We are in the midst of one of the most exciting and interesting periods in the history of exploration -- a period which holds the potential for expanding our understanding of scientific phenomena ranging from intricacies of the universe to the subtleties of our own planet. The opportunities to study the effects of microgravity in space seem limitless during the next decade. Using the European Space Agency (ESA) built Spacelab, we will maximize opportunities for development of new microgravity facilities in order to obtain new information and increase our understanding of the effects of gravitational forces on phenomena in biological, chemical and physical systems. NASA intends to move aggressively, but sensibly, to develop additional space research facilities as we built toward the unique Space Station Freedom resources becoming available towards to end of this decade.
Microgravity Science in the U.S.A. involves research in fluids science, combustion science, materials science, biotechnology, and fundamental physics. The purpose is to achieve a thorough understanding of the effects of gravitational body forces on physical phenomena relevant to those disciplines. This includes the study of phenomena which are usually overwhelmed by the presence of gravitational body forces and, therefore, chiefly manifested when gravitational forces are weak. In the pragmatic sense, the research involves gravity level as an experimental parameter. Calendar year 1992 is a landmark year for research opportunities in low earth orbit for Microgravity Science. For the first time ever, three Spacelab flights will fly in a single year: IML-1 was launched on January 22; USML-1 was launched on June 25; and, in September, SL-J will be launched. A separate flight involving two cargo bay carriers, USMP-1, will be launched in October. From the beginning of 1993 up to and including the Space Station era (1997), nine flights involving either Spacelab or USMP carriers will be flown. This will be augmented by a number of middeck payloads and get away specials flying on various flights. All of this activity sets the stage for experimentation on Space Station Freedom. Beginning in 1997, experiments in Microgravity Science will be conducted on the Space Station. Facilities for doing experiments in protein crystal growth, solidification, and biotechnology will all be available. These will be joined by middeck-class payloads and the microgravity glove box for conducting additional experiments. In 1998, a new generation protein crystal growth facility and a facility for conducting combustion research will arrive. A fluids science facility and additional capability for conducting research in solidification, as well as an ability to handle small payloads on a quick response basis, will be added in 1999. The year 2000 will see upgrades in the protein crystal growth and fluids science facilities. From the beginning of 1997 to the fall of 1999 (the 'man-tended capability' era), there will be two or three utilization flights per year. Plans call for operations in Microgravity Science during utilization flights and between utilization flights. Experiments conducted during utilization flights will characteristically require crew interaction, short duration, and less sensitivity to perturbations in the acceleration environment. Operations between utilization flights will involve experiments that can be controlled remotely and/or can be automated. Typically, the experiments will require long times and a pristine environment. Beyond the fall of 1999 (the 'permanently-manned capability' era), some payloads will require crew interaction; others will be automated and will make use of telescience.
Lunar spherical harmonic gravity coefficients are estimated from simulated observations of a near-circular low altitude polar orbiter disturbed by lunar mascons. Lunar gravity sensing missions using earth-based nearside observations with and without satellite-based far-side observations are simulated and least squares maximum likelihood estimates are developed for spherical harmonic expansion fit models. Simulations and parameter estimations are performed by a modified version of the Smithsonian Astrophysical Observatory's Planetary Ephemeris Program. Two different lunar spacecraft mission phases are simulated to evaluate the estimated fit models. Results for predicting state covariances one orbit ahead are presented along with the state errors resulting from the mismodeled gravity field. The position errors from planning a lunar landing maneuver with a mismodeled gravity field are also presented. These simulations clearly demonstrate the need to include observations of satellite motion over the far side in estimating the lunar gravity field. The simulations also illustrate that the eighth degree and order expansions used in the simulated fits were unable to adequately model lunar mascons.
An extensive body of data, from meteors, zodiacal light, spacecraft-borne impact detectors (Helios, Pioneer, Galileo, Ulysses), and other sources, forms the basis of a new numerical model for the distributions of interplanetary meteoroids. For each of the five populations in this model it is possible to evaluate meteoroid concentration and flux for oriented surfaces or detectors having arbitrary position and velocity in interplanetary space. For a spacecraft in geocentric orbit the effects of gravitational focussing and shielding by the Earth have been newly derived with full attention to the directionality of the particles, both on approach (i.e., relative to a massless Earth) and at the target. This modeling approach was exercised to provide an estimate of meteoroid fluence for each of several oriented surfaces on LDEF.
The Mars Pathfinder mission (formerly Mars Environmental Survey or MESUR) is scheduled for launch in December 1996 and is designed to place a small lander on the surface of Mars. After impact, the lander unfolds to expose its solar panels and release a miniature rover. Also on board is the Imager for Mars Pathfinder (IMP) binocular camera which is elevated by a deployable mast to obtain a panoramic view of the landing area. The design of this deployable mast is based on similar designs which have a long and successful flight history. In the past when this type of self-deployable mast has been used, a rate limiter has been incorporated to control the speed of deployment. In this application, to reduce weight and complexity, it was proposed to eliminate the rate limiter so that the mast would deploy without restraint. Preliminary tests showed that this type of deployment was possible especially if the deployed length was relatively short, as in this application. Compounding the problem, however, was the requirement to deploy the mast at an angle of up to 30 degrees from vertical. The deployment process was difficult to completely analyze due to the effects of gravitational and inertial loads on the mast and camera during rapid extension. Testing in a realistic manner was imperative to verify the system performance. A deployment test was therefore performed to determine the maximum tilt angle at which the mast could reliably extend and support the camera on Mars. The testing of the deployable mast requires partial gravity compensation to simulate the smaller force of Martian gravity. During the test, mass properties were maintained while weight properties were reduced. This paper describes the testing of a deployable mast in a simulated Martian environment as well as the results of the tests.
We propose a generalized model for stellar spin-down, disk accretion, and truncation, and the origin of winds, jets, and bipolar outflows from young stellar objects. We consider the steady state dynamics of accretion of matter from a viscous and imperfectly conducting disk onto a young star with a strong magnetic field. For an aligned stellar magnetosphere, shielding currents in the surface layers of the disk prevent stellar field lines from penetrating the disk everywhere except for a range of radii about pi = R(sub x), where the Keplerian angular speed of rotation Omega(sub x) equals the angular speed of the star Omega(sub *). For the low disk accretion rates and high magnetic fields associated with typical T Tauri stars, R(sub x) exceeds the radius of the star R(sub *) by a factor of a few, and the inner disk is effectively truncated at a radius R(sub t) somewhat smaller than R(sub x). Where the closed field lines between R(sub t) and R(sub x) bow sufficiently inward, the accreting gas attaches itself to the field and is funneled dynamically down the effective potential (gravitational plus centrifugal) onto the star. Contrary to common belief, the accompanying magnetic torques associated with this accreting gas may transfer angular momentum mostly to the disk rather than to the star. Thus, the star can spin slowly as long as R(sub x) remains significantly greater than R(sub *). Exterior to R(sub x) field lines threading the disk bow outward, which makes the gas off the mid-plane rotate at super-Keplerian velocities. This combination drives a magnetocentrifugal wind with a mass-loss rate M(sub w) equal to a definite fraction f of the disk accretion rate M(sub D). For high disk accretion rates, R(sub x) is forced down to the stellar surface, the star is spun to breakup, and the wind is generated in a manner identical to that proposed by Shu, Lizano, Ruden, & Najita in a previous communication to this journal. In two companion papers (II and III), we develop a detailed but idealized theory of the magnetocentrifugal acceleration process.
The Local Supercluster kinematic properties (the Local Group infall toward the Virgo Cluster and galaxy density distribution about the Virgo Cluster) in various cosmological models are examined utilizing large-scale N-body (PM) simulations 500(exp 3) cells, 250(exp 3) particles, and box size of 400 h(exp -1) Mpc) and are compared to observations. Five models are investigated: (1) the standard, Cosmic Background Explorer Satellite (COBE)-normalized cold dark matter (CDM) model with omega = 1, h = 0.5, and sigma(sub 8) = 1.05; (2) the standard Hot Dark Matter (HDM) model with omega = 1, h = 0.75, and sigma(sub 8) = 1; (3) the tilted CDM model with omega = 1, h = 0.5, n = 0.7, and sigma(sub 8) = 0.5; (4) a CDM + lambda model with omega = 0.3, lambda = 0.7, h = 2/3, and sigma(sub 8) = 2/3; (5) the PBI model with omega = 0.2, h = 0.8, x = 0.1, m = -0.5, and sigma(sub 8) = 0.9. Comparison of the five models with the presently available observational measurements v(sub LG) = 85 - 305 km/s (with mean of 250 km/s), delta(n(sub g))/(n(sub g)-bar) = 1.40 + or - 0.35) suggests that an open universe with omega approximately 0.5 (with or without lambda) and sigma(sub 8) approximately 0.8 is preferred, with omega = 0.3-1.0 (with or without lambda) and sigma(sub 8) = 0.7-1.0 being the acceptable range. At variance with some previous claims based on either direct N-body or spherical nonlinear approaches, we find that a flat model with sigma(sub 8) approximately 0.7-1.0 seems to be reasonably consistent with observations. However, if one favors the low limit of v(sub LG) = 85 km/s, then an omega approximately 0.2-0.3 universe seems to provide a better fit, and flat (omega = 1) models are ruled out at approximately 95% confidence level. On the other hand, if the high limit of v(sub LG) = 350 km/s is closer to the truth, then it appears that omega approximately 0.7-0.8 is more consistent. This test is insensitive to the shape of the power spectrum, but rather sensitive to the normalization of the perturbation amplitude on the relevant scale (e.g., sigma(sub 8)) and omega. We find that neither linear nor nonlinear relations (with spherical symmetry) are good approximations for the relation between radial peculiar velocity and density perturbation, i.e., nonspherical effects and gravitational tidal field are important. The derived omega using either of the two relations is underestimated. In some cases, this error is as large as a factor of 2-4.
Femtolensoing is a gravitational lensing effect in which the magnification is a function not only of the position and sizes of the source and lens, but also of the wavelength of light. Femtolensing is the only known effect of 10(exp -13) - 10(exp -16) solar mass) dark-matter objects and may possibly be detectable in cosmological gamma-ray burst spectra. We present a new and efficient algorithm for femtolensing calculation in general potentials. The physical optics results presented here differ at low frequencies from the semiclassical approximation, in which the flux is attributed to a finite number of mutually coherent images. At higher frequencies, our results agree well with the semicalssical predictions. Applying our method to a point-mass lens with external shear, we find complex events that have structure at both large and small spectral resolution. In this way, we show that femtolensing may be observable for lenses up to 10(exp -11) solar mass, much larger than previously believed. Additionally, we discuss the possibility of a search femtolensing of white dwarfs in the Large Magellanic Cloud at optical wavelengths.