Steven J. Ostro

Radar Imaging of Binary Near-Earth Asteroid (66391) 1999 KW4

High-resolution radar images reveal near-Earth asteroid (66391) 1999 KW4 to be a binary system. The ∼1.5-kilometer-diameter primary (Alpha) is an unconsolidated gravitational aggregate with a spin period ∼2.8 hours, bulk density ∼2 grams per cubic centimeter, porosity ∼50%, and an oblate shape dominated by an equatorial ridge at the objects potential-energy minimum. The ∼0.5-kilometer secondary (Beta) is elongated and probably is denser than Alpha. Its average orbit about Alpha is circular with a radius ∼2.5 kilometers and period ∼17.4 hours, and its average rotation is synchronous with the long axis pointed toward Alpha, but librational departures from that orientation are evident. Exotic physical and dynamical properties may be common among near-Earth binaries.

Shape and Non-Principal Axis Spin State of Asteroid 4179 Toutatis

Radar observations of Toutatis placed hundreds to thousands of pixels per image on the asteroid and revealed it to be a non-principal axis rotator. The radar data are used to calculate Toutatiss three-dimensional shape, spin state, and the ratios of the principal moments of inertia. Toutatis is rotating in a long-axis mode characterized by periods of 5.41 days (rotation about the long axis) and 7.35 days (average for long-axis precession), and its dimensions along the principal axes are 1.92, 2.40, and 4.60 kilometers. To within the models uncertainties, Toutatiss density is homogeneous, or its inhomogeneities mimic the inertia tensor of a homogeneous body.

Titan's Rotation Reveals an Internal Ocean and Changing Zonal Winds

Cassini radar observations of Saturns moon Titan over several years show that its rotational period is changing and is different from its orbital period. The present-day rotation period difference from synchronous spin leads to a shift of ∼0.36° per year in apparent longitude and is consistent with seasonal exchange of angular momentum between the surface and Titans dense superrotating atmosphere, but only if Titans crust is decoupled from the core by an internal water ocean like that on Europa.

Shape of Asteroid 4769 Castalia (1989 PB) from Inversion of Radar Images

The inversion of previously reported, delay-Doppler images of Castalia yields a 167-parameter, three-dimensional shape model that is bifurcated into two distinct, irregular, kilometer-sized lobes. The crevice that separates the lobes has an average depth of between 100 and 150 meters and is oriented roughly perpendicular to the asteroids longest dimension. The constrained least-squares reconstruction method introduced here can be used to determine the shape, spin vector, and radar-scattering properties of any asteroid or comet for which delay-Doppler images provide sufficient signal-to-noise ratio, orientational coverage, and spatial resolution.

Radar Images of Asteroid 4179 Toutatis

Delay-Doppler images of the Earth-crossing asteroid 4179 Toutatis achieve resolutions as fine as 125 nanoseconds (19 meters in range) and 8.3 millihertz (0.15 millimeter per second in radial velocity) and place hundreds to thousands of pixels on the asteroid, which appears to be several kilometers long, topographically bifurcated, and heavily cratered. The image sequence reveals Toutatis to be in an extremely slow, non-principal axis rotation state.

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.

Galilean satellites: 1976 radar results

Abstract Radar observations of the Galilean satellites, made in late 1976 using the 12.6-cm radar system of the Arecibo Observatory, have yielded mean geometric albedos of 0.04 ± , 0.69 ± 0.17, 0.37 ± 0.09, and 0.15 ± 0.04, for Io, Europa, Ganymede, and Callisto, respectively. The albedo for Io is about 40% smaller than that obtained approximately a year earlier, while the albedos for the outer three satellites average about 70% larger than the values previously reported for late 1975, raising the possibility of temporal variation. Very little dependence on orbital phase is noted; however, some regional scattering inhomogeneities are seen on the outer three satellites. For Europa, Ganymede, and Callisto, the ratios of the echo received in one mode of circular polarization to that received in the other were: 1.61 ± 0.20 1.48 ± 0.27, and 1.24 ± 0.19, respectively, with the dominant component having the same sence of circularity as that transmitted. This behavior has not previously been encountered in radar studies of solar system objects, whereas the corresponding observations with linear polarization are “normal.” Radii determined from the 1976 radar data for Europa and Ganymede are: 1530 ± 30 and 2670 ± 50 km, in fair agreement with the results from the 1975 radar observations and the best recent optical determinations. Doppler shifts of the radar echoes, useful for the improvement of the orbits of Jupiter and some of the Galilean satellites, are given for 12 nights in 1976 and 10 nights in 1975.

Asteroid radar astrometry

Measurements of time delay and Doppler frequency are reported for asteroid-radar echoes obtained at Arecibo and Goldstone during 1980-1990. Radar astrometry is presented for 23 near-earth asteroids and three mainbelt asteroids. These measurements, which are orthogonal to optical, angular-position measurements, and typically have a fractional precision between 10 to the -5th and 10 to the -8th, permit significant improvement in estimates of orbits and hence in the accuracy of prediction ephemerides. Estimates are also reported of radar cross-section and circular polarization ratio for all asteroids observed astrometrically during 1980-1990.

Mars: Dual-polarization radar observations with extended coverage

Abstract Thirteen-centimeter-wavelength radar observations of Mars made in 1982 at Arecibo Observatory yield accurate measurements of the full backscatter spectrum in two orthogonal polarizations. The data, which were obtained for several widely separated subradar longitudes at 24°N latitude, provide the first global view of the distribution of small-scale surface roughness on Mars. The diffuse component of the echo exhibits strong spatial variations. Areas of maximum depolarization correlate well with volcanic regions (Tharsis and Elysium), while the heavily cratered upland terrain yields relatively low depolarization. Parts of Tharsis give near-complete depolarization (polaziation ratio μ c ⋟ 1 when viewed at oblique angles of incidence). Northern Martian plains regions (Tharsis, Elysium, and Amazonis) may comprise the most extensive area of severe decimeter-scale surface roughness in the inner Solar System. On the average, the northern Martian tropics yield higher diffuse radar cross sections ( σ D = 0.05–0.12) and a higher of degree disk-integrated depolarization ( μ c = 0.1–0.4) than is found for the Moon, Mercury, and Venus. Comparisons between the Moon and Mars using radar data, ground truth, and simple scattering models suggest that Mars possesses a relatively high average coverage by decimeter-scale rocks. Also discussed are several of the more interesting quasispecular scattering results, the most unsual of which were obtained over the Olympus Mons aureole region.

High-resolution model of Asteroid 4179 Toutatis

Abstract A model of the shape of Toutatis based on “high-resolution” radar images obtained in 1992 and 1996 consists of 39,996 triangular facets of roughly equal area, defined by the locations of 20,000 vertices. These define the average spatial resolution of the model as approximately 34 m, significantly finer than Hudson and Ostro’s (1995 , Science 270, 84–86) model (1600 vertices, resolution 84 m). The high-resolution model reveals complex linear features as well as circular crater-like structures down to the resolution limit. There is a dichotomy between the gravitational slope distributions on the asteroid’s two lobes. The noncraterlike features may be the manifestation of complex interior configurations involving monolithic fragments with various sizes and shapes, presumably due to collisions in various energy regimes. However, it may be impossible to decipher the interior structure and responsible collisional history of Toutatis and other small objects without extended, intensive, direct examination.