Resonance rotation of venus.

Combination of two types of radar data shows the orbital plane and equator of Venus to be included by less than 2 degrees, and the sidereal rotation period to be 243.09 +/- 0.18 days (retrograde)-remarkably close to the 243.16-day period for which the spin would be in resonance with the relative orbital motions of Earth and Venus. In this resonance, Venus would make, on average, four axial rotations as seen by an Earth observer between successive close approaches of the two planets. Estimates of the instantaneous spin period, accurate within about 0.01 day, would provide important information on the difference of Venus's equatorial moments of inertia, on their orientation, and on the magnitude of the tidal torque exerted on Venus by the sun.

New method for the detection of light deflection by solar gravity.

The prediction of Einstein's theory of general relativity that light will be deflected by the sun may be tested by sending radio waves from the earth to Venus or Mercury when either passes behind the sun and detecting the echoes with a radar interferometer.

Last of the west ford dipoles.

Radar measurements confirm that the several hundred million individually orbiting West Ford dipoles reentered the lower atmosphere in precise accord with predictions. Calculations indicate that these tiny copper wires survived reentry and floated gently back to Earth; unfortunately,the probability of finding one is minuscule. Some dipole clusters remain in orbit but almost all should return to Earth within the next 2 years.

Is the sun shrinking?

Observations of 23 transits of Mercury in front of the sun between 1736 and 1973 show no indication of any significant change in the diameter of the sun. Regression analysis yields a decrease of the angular diameter, as viewed from the earth, of under 0.3 arc second per century (> 90 percent confidence limit). This limit is incompatible with the 2 arc seconds per century decrease obtained by Eddy for the equatorial diameter from direct observations made at the Greenwich Observatory and at the U.S. Naval Observatory.

Scientific uses of pulsars.

The recently discovered celestial sources of pulsed radio energy can be used to test general relativity, to study the solar corona, and to determine the earth's orbit and ephemeris time. The vector positions and transverse velocities of pulsars can be measured with radio interferometers; in combination with pulse-arrival-time data, the distance determination will yield the average interstellar electron density.

Mainbelt asteroids: dual-polarization radar observations.

Observations of 20 asteroids in the main belt between Mars and Jupiter provide information about the nature of these objects' surfaces at centimeter-to-kilometer scales. At least one asteroid (Pallas) is extremely smooth at centimeter-to-meter scales. Each asteroid appears much rougher than the Moon at some scale between several meters and many kilometers. The range of asteroid radar albedos is very broad and implies substantial variations in porosity or metal concentration (or both). The highest albedo estimate, for the asteroid Psyche, is consistent with a surface having porosities typical of lunar soil and a composition nearly entirely metallic.

Astronomical applications of differential interferometry.

Intercomparison of radio signals received simultaneously at several sites from several sources with small mutual angular separation provides a powerful astrometric tool. Applications include tracking the Lunar Rover relative to the Lunar Module, determining the moon's libration, measuring winds in Venus's lower atmosphere, mapping Mars radiometrically, and locating the planetary system in an inertial frame.

Orbit-orbit resonance capture in the solar system.

A realistic model involving mutual gravitation and tidal dissipation for the first time provides a detailed explanation for satellite orbit-orbit resonance capture. Although applying directly only to Saturn's satellites Titan and Hyperion, the model reveals general principles of resonance capture, evolution, and stability which seem applicable to other orbit-orbit resonances in the solar system.

Martian craters and a scarp as seen by radar.

Radar observations of Mars with a surface resolution of 1.3 degrees in latitude and 0.8 degrees in longitude have been carried out during the opposition of 1971. With a precision in surface height measurement approaching 75 meters in regions of high reflectivity, it has been possible to measure the detailed characteristics of a number of craters. Many of these can be identified with craters shown in Mariner photographs of Mars. In addition, a scarp has been seen at 41 degrees west, 14 degrees south with an average slope of about 6 degrees extending over about 40 kilometers.

Galilean satellites of jupiter: 12.6-centimeter radar observations.

Observations of the Galilean satellites with the radar system at the Arecibo Observatory, Puerto Rico, show that their surfaces are highly diffuse scatterers of radio waves of length 12.6 centimeters; spectra of the radar echoes are asymmetric and broad. The geometric radar albedos for the outer three satellites-0.42 +/- 0.10, 0.20 +/- 0.05, and 0.09 +/- 0.02 for Europa, Ganymede, and Callisto, respectively-show about the same relative decreases as do the optical albedos, although the latter presumably bear only on material much nearer the surface. Radii of 1420 +/- 30, 2640 +/- 80, and 2360 +/- 70 kilometers for Europa, Ganymede, and Callisto were determined from the radar data and are in good agreement with the corresponding optically derived values. Io, observed successfully only once, appears to have an albedo comparable to Ganymede's, but no radius was estimated for it.

Discrepancies between Radar Data and the Lunar Ephemeris.

Precise measurements of the Doppler shift of radar waves reflected from Moon disclose unexpectedly large discrepancies-averaging about 0.6 centimeter per second-between the radial velocities and the predictions based on the Eckert-Brown lunar ephemeris. These residuals have a rapidly changing component corresponding to a relatively large, variable, and unexplained discrepancy in radial acceleration of about 10(-4) centimeter per second, per second, in magnitude and about 1 day in period.

Venus: density of upper atmosphere from measurements of drag on pioneer orbiter.

Measurements of the changes in orbital period of the Pioneer Venus orbiter have yielded estimates of the density of the upper atmosphere of Venus at altitudes in the range from 150 to 200 kilometers. At the lower limit of this range, the density on the dayside of the terminator exhibits a temporal variation of amplitude near 4 x 10(-14) gram per cubic centimeter aboult a mean of approximately 1.4 x 10(-13) gram per cubic centimeter. The variation appears oscillatory, with a 4- to 5-day period, but barely one cycle was observed. The density on the nightside of the terminator, sampled inthe same 150-kilometer altitude range, fluctuates about a smaller mean of approximately 4 x 10(-14) gram per cubic centimeter. The density between the altitudes of 150 and 200 kilometers, sampled only on the dayside of the terminator, imply a scale height of between 15 and 20 kilometers. The interpretation of this estimate is uncertain, however, in view of the measurements at the different altitudes having been made at different times and, hence, at different values of solar phase.

Radar Image of Asteroid 1989 PB.

Radar observations of the near-Earth asteroid 1989 PB, made shortly after its optical discovery, yield a sequence of delay-Doppler images that reveal it to consist of two distinct lobes that appear to be in contact. It seems likely that the two lobes once were separate and that they collided to produce the current "contact-binary" configuration.

Venus: topography revealed by radar data.

Surface height variations over the entire equatorial region on Venus have been estimated from extended series of measurements of interplanetary radar echo delays. Most notable is a mountainous section of about 3-kilometer peak height located at a longitude of 100 degrees (International Astronomical Union coordinate system). The eastern edge has an average inclination of about 0.5 degrees, which is unusually steep for a large-scale slope on Venus. The resolution of the radar measurements along the surface of Venus varied between about 200 and 400 kilometers with a repeatability in altitude determination generally between 200 and 500 meters. The mean equatorial radius was found to be 6050.0+/-0.5 kilometers.

Venus: radar determination of gravity potential.

We describe a method for the determination of the gravity potential of Venus from multiple-frequency radar measurements. The method is based on the strong frequency dependence of the absorption of radio waves in Venus' atmosphere. Comparison of the differing radar reflection intensities at several frequencies yields the height of the surface relative to a reference pressure contour; combination with measurements of round-trip echo delays allows the pressure, and hence the gravity potential contour, to be mapped relative to the mean planet radius. Since calibration data from other frequencies are unavailable, the absorption-sensitive Haystack Observatory data have been analyzed under the assumption of uniform surface reflectivity to yield a gravity equipotential contour for the equatorial region and a tentative upper bound of 6 x 10(-4) on the fractional difference of Venus' principal equatorial moments of inertia. The minima in the equipotential contours appear to be associated with topographic minima.

Comet encke: radar detection of nucleus.

The nucleus of the periodic comet Encke was detected in November 1980 with the Arecibo Observatory's radar system (wavelength, 12.6 centimeters). The echoes in the one sense of circular polarization received imply a radar cross section of 1.1 +/- 0.7 square kilometers. The estimated bandwidth of these echoes combined with an estimate of the rotation vector of Encke yields a radius for the nucleus of l.5(+2.3)(-1.0) kilometers. The uncertainties given are dependent primarily on the range of models considered for the comet and for the manner in which its nucleus backscatters radio waves. Should this range prove inadequate, the true value of the radius of the nucleus might lie outside the limits given.

Viking lander location and spin axis of Mars: determination from radio tracking data.

Radio tracking data from the Viking lander have been used to determine the lander position and the orientation of the spin axis of Mars. The areocentric coordinates of the lander are 22.27 degrees N, 48.00 degrees W, and 3389.5 kilometers from the center of mass; the spin axis orientation, referred to Earth's mean equator and equinox of 1950.0, is 317.35 degrees right ascension and 52.71 degrees declination.

Transcontinental baselines and the rotation of the Earth measured by radio interferometry.

Nine separate very-long-baseline interferometry (VLBI) experiments, carried out in 1972 and 1973 with radio telescopes 3900 kilometers apart, yielded values for the baseline length with a root-mean-square deviation about the mean of less than 20 centitneters. The corresponding fractional spread is about five parts in 10(8). Changes in universal time and in polar motion were also detertnined accurately from these data; the root-mean-square scatter of these results with respect to those based on optical methods were 2.9 milliseconds and 1.3 meters, respectively. Solid-earth tides were apparently detected, but no useful estimate of their amplituide was extracted.

Wind velocities on venus: vector determination by radio interferometry.

To determine the wind directions and speeds on Venus, as each Pioneer probe fell to the surface we tracked its motion in three dimensions using a combination of Doppler and long-baseline radio interferometric methods. Preliminary results from this tracking, coupled with results from test observations of other spacecraft, enable us to estimate the uncertainties of our eventual determinations of the velocity vectors of the probes with respect to Venus. For altitudes below about 65 kilometers and with time-averaging over 100-second intervals, all three components of the velocity should have errors of the order of 0.3 meter per second or less.

Asteroid 1986 DA: Radar Evidence for a Metallic Composition.

Echoes from the near-Earth object 1986 DA show it to be significantly more reflective than other radar-detected asteroids. This result supports the hypothesis that 1986 DA 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-kilometer asteroid, which appears smooth at centimeter to meter scales but extremely irregular at 10- to 100-meter scales, might be (or have been a part of the parent body of some iron meteorites.