Radar studies in the solar system

We are actively engaged in observations of asteroids and comets, both as planned targets and as targets of opportunity. The most recent example of the latter is asteroid 1991 AQ, which was discovered optically on January 14, 1991 and found to be rapidly nearing Earth. Due in part to our previous success in obtaining useful radar data on short notice, we were able to schedule time on the Arecibo radar on January 28-31, when the asteroid was within the Arecibo declination window. Each day during the period, a fresh ephemeris was made including all available data, both optical and radar. With further processing, the results of the observations are expected to include detailed two-dimensional images of the asteroid and information on its surface properties, size, shape and spin. The observing program also included two other asteroids, two of the four Galilean satellites of Jupiter, the satellites of Mars, and the planet Mercury. Analysis of newly available radar observations of Venus has led to a refinement of the spin vector of that planet and has thereby provided a coordinate basis for the Magellan spacecraft mapping mission

Studies of regional and global tectonics and the rotation of the earth using very-long baseline interferometry

Progress in the areas of data analysis, atmospheric delay calibration and software conversion is reported. Over 800 very long baseline interferometry (VLBI) experiments were analyzed in the last 6 months. Reprocessing of the Mark III VLBI data set is almost completed. Results of analysis of the water-vapor radiometer (WVR) data were submitted and a preprint of a related paper is attached. Work on conversion of the VLBI analysis software from HP1000 to Unix based workstations is continuing

Radar studies in the solar system

The ephemerides needed to acquire radar data at Arecibo from observations of various solar-system objects was developed. The resultant data to test fundamental laws of gravitation and to determine the size, shape, topography, and spin vectors of the targets were analyzed. The surface properties of these objects through their radar scattering and polarization characteristics were studied. The observing program also included Arecibo observations of Mercury, Venus, and the Galilean satellites of Jupiter. The Mercury observations at Arecibo were all within a week of the epoch of a closure point (same surface position as a previous observation), and one was matched by a near-simultaneous observation at Goldstone

Solar-System Tests of Gravitational Theories

This research is aimed at testing gravitational theory, primarily on an interplanetary scale and using mainly observations of objects in the solar system. Our goal is either to detect departures from the standard model (general relativity) - if any exist within the level of sensitivity of our data - or to support this model by placing tighter bounds on any departure from it. For this project, we have analyzed a combination of observational data with our model of the solar system, including planetary radar ranging, lunar laser ranging, and spacecraft tracking, as well as pulsar timing and pulsar VLBI measurements

HST Observations and Models of The Gravitational Lens System MG 0414+0534

Quadruple gravitational lens systems offer the possibility of measuring time delays for image pairs, microlensing effects, and extinction in distant galaxies. Observations of these systems may be used to obtain estimates of H_o and to study the various mass components of lens galaxies at high redshifts. With HST, we have observed the reddest known gravitational lens system, MG 0414+0534. We used WFPC2/PC1 to obtain deep, high-resolution images with two filters, F675W and F814W. We present a detailed analysis of all of the components, as well as macrolens models. Our main results are: (1) confirmation that MG 0414+0534 is inescapably a gravitational lens system; (2) discovery of a blue arc connecting the 3 brightest images of the QSO central core; (3) accurate positions and apparent brightnesses for all 4 known images of the QSO central core and for the lens galaxy G; (4) a good representation of the brightness distribution of G by elliptical isophotes with a De Vaucouleurs profile, characteristic of an elliptical galaxy; (5) models that consist of simple elliptical potentials and account qualitatively, not quantitatively, for the HST image positions, arc morphology and radio flux ratios for the images of the QSO central core; (6) a possible new test to distinguish between reddening in the host galaxy of the QSO and in the lens galaxy, based on future accurate measurements of spatial variations in the color of the arc; and (7) the suggestion that microlensing is a plausible cause for the differences between the radio and optical flux ratios for the brightest images, A1 and A2. Further observations and measurements such as of the redshift of the lens galaxy, can be used fruitfully to study microlensing for this system.Comment: 27 pages, 8 .ps figs, AAS Latex, AJ, in press, Feb 199

Io's radar properties

Arecibo 13 cm wavelength radar observations during 1987-90 have yielded echoes from Io on each of 11 dates. Whereas Voyager imaged parts of the satellite at resolutions of several km and various visible/infrared measurements have probed the surfaces's microscale properties, the radar data yield new information about the nature of the surface at cm to km scales. Our observations provide fairly thorough coverage and reveal significant heterogeneity in Io's radar properties. A figure is given showing sums of echo spectra from 11 dates

Studies of regional and global tectonics and the rotation of the earth using very-long-baseline interferometry

Work is continuing on the study of atmospheric gradients. We include a preprint entitled 'The effect of turbulence on atmospheric gradient parameters determined from ground-based radiometric and space geodetic measurements'. Work has begun on a study of solid Earth tidal deformations using the VLBI data set. We have examined deformations at the semi-diurnal tidal period using the IRIS data set

Asteroid 1986 DA: Radar evidence for a metallic composition

Radar observations of the near-Earth asteroid 1986 DA were carried out at the Arecibo Observatory in April 1986, two months after its discovery. Radar results are consistent with the hypothesis that 1986 HA is a piece of NiFe metal derived from the interior of a much larger object that melted, differentiated, cooled and subsequently was disrupted in a catastrophic collision. This 2-km asteroid might be (or have been part of) the parent body of some iron meteorites. Or 1986 DA might share the parentage and/or part of the dynamical history of some meteorites without ever having contributed any of its own ejecta to our meteorite sample. Analysis of the samples returned from 1986 DA might ultimately involve economic considerations. Meteoritic metal is mostly iron with about 8 percent nickel, but also contains substantial concentrations of precious and strategic metals, including approx. 1 ppm of gold and approx. 10 ppm of platinum group elements. If these abundances apply to 1986 DA, it contains some 10(exp 16) g of iron, 10 (exp 15) g of nickel, 10(exp 11) g of platinum group metals, and 10(exp 10) g of gold

VLBI for Gravity Probe B. VII. The Evolution of the Radio Structure of IM Pegasi

We present measurements of the total radio flux density as well as very-long-baseline interferometry (VLBI) images of the star, IM Pegasi, which was used as the guide star for the NASA/Stanford relativity mission Gravity Probe B. We obtained flux densities and images from 35 sessions of observations at 8.4 GHz (wavelength = 3.6 cm) between 1997 January and 2005 July. The observations were accurately phase-referenced to several extragalactic reference sources, and we present the images in a star-centered frame, aligned by the position of the star as derived from our fits to its orbital motion, parallax, and proper motion. Both the flux density and the morphology of IM Peg are variable. For most sessions, the emission region has a single-peaked structure, but 25% of the time, we observed a two-peaked (and on one occasion perhaps a three-peaked) structure. On average, the emission region is elongated by 1.4 +- 0.4 mas (FWHM), with the average direction of elongation being close to that of the sky projection of the orbit normal. The average length of the emission region is approximately equal to the diameter of the primary star. No significant correlation with the orbital phase is found for either the flux density or the direction of elongation, and no preference for any particular longitude on the star is shown by the emission region.Astronom