Michael C. Nolan

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.

Orbit and Bulk Density of the OSIRIS-REx Target Asteroid (101955) Bennu

The target asteroid of the OSIRIS-REx asteroid sample return mission, (101955) Bennu (formerly 1999 RQ 36), is a half-kilometer near-Earth asteroid with an extraordinarily well constrained orbit. An extensive data set of optical astrometry from 1999 to 2013 and high-quality radar delay measurements to Bennu in 1999, 2005, and 2011 reveal the action of the Yarkovsky effect, with a mean semimajor axis drift rate da=dt ¼ð � 19:0 � 0:1 Þ� 10

Dynamical Configuration of Binary Near-Earth Asteroid (66391) 1999 KW4

Dynamical simulations of the coupled rotational and orbital dynamics of binary near-Earth asteroid 66391 (1999 KW4) suggest that it is excited as a result of perturbations from the Sun during perihelion passages. Excitation of the mutual orbit will stimulate complex fluctuations in the orbit and rotation of both components, inducing the attitude of the smaller component to have large variation within some orbits and to hardly vary within others. The primarys proximity to its rotational stability limit suggests an origin from spin-up and disruption of a loosely bound precursor within the past million years.

Focused 70-cm Wavelength Radar Mapping of the Moon

We describe new 70-cm wavelength radar images of the lunar near-side and limb regions obtained via a synthetic-aperture-radar patch-focusing reduction technique. The data are obtained by transmitting a circularly polarized pulsed waveform from the Arecibo telescope in Puerto Rico and receiving the echo in both senses of circular polarization with the Robert C. Byrd Green Bank Telescope in West Virginia. The resultant images in both polarizations have a spatial resolution as fine as 320 m 450 m near the lunar limb. The patch-focusing technique is a computationally efficient method for compensating for range migration and Doppler (azimuth) smearing over long coherence times, i.e., 983 s, which is needed to achieve the required Doppler resolution. Three to nine looks are averaged for speckle reduction and to improve the signal-to-noise ratio. At this long wavelength, the radar signal penetrates up to several tens of meters into the dry lunar surface materials, thus revealing details of the bulk loss properties and decimeter-scale rock abundance not evident in multispectral and other remote-sensing data. Application of the new radar images to the analysis of basalt flow complexes in Mare Serenitatis shows that the long-wavelength radar data are sensitive to differences in both flow age and composition, and may be particularly useful for studies of smaller deposits that do not have robust crater statistics. The new 70-cm lunar radar data are archived at the National Aeronautics and Space Administration Planetary Data System.

Radar Observations of Comet 103P/Hartley 2

Comets rarely come close enough to be studied intensively with Earth-based radar. The most recent such occurrence was when Comet 103P/Hartley 2 passed within 0.12 AU in late 2010 October, less than two weeks before the EPOXI flyby. This offered a unique opportunity to improve pre-encounter trajectory knowledge and obtain complementary physical data for a spacecraft-targeted comet. 103P/Hartley 2 is only the fourth comet nucleus to be imaged with radar and already the second to be identified as an elongated, bilobate object based on its delay-Doppler signature. The images show the dominant spin mode to be a rotation about the short axis with a period of 18.2 hr. The nucleus has a low radar albedo consistent with a surface density of 0.5-1.0 g cm{sup -3}. A separate echo component was detected from large (>cm) grains ejected anisotropically with velocities of several to tens of meters per second. Radar shows that, in terms of large-grain production, 103P/Hartley 2 is an unusually active comet for its size.

Radar imaging of the lunar poles.

We have used a radio telescope at Arecibo Observatory, Puerto Rico, to map features of the lunar poles — some as small as 300 metres across — by collecting long-wavelength radar images that can penetrate several metres of lunar dust. We find that areas of the crater floors at the poles that are in permanent shadow from the Sun, which are potential cold traps for water or other volatiles, do not give rise to strong radar echoes like those associated with thick ice deposits in the polar craters on Mercury. Any lunar ice present within regions visible to the Arecibo radar must therefore be in the form of distributed grains or thin layers.

Radar observations of comets

Seven comets have been detected by Earth-based radars during the period 1980‐1998. All but one of these gave a detectable echo from the nucleus, while three of the comets also showed a broad-band echo from large (0cm-size) grains in the inner coma. Although all observations have been of the CW (continuous-wave) type, which precludes direct size measurement, the radar cross sections are consistent with nucleus diameters averaging a few kilometers and varying over a range of ten. Comparisons with independent size estimates indicate relatively low radar albedos, implying nucleus surface densities of 0.5 to 1 g/cm 3 . The surfaces of comet nuclei appear to be as rough as typical asteroid surfaces, but are considerably less dense. Analysis of coma echoes indicates that some comets emit large grains at rates (0ton/s) which are comparable with their gas and dust production rates. There is also some indirect evidence for grain evaporation or fragmentation within a few hundred to a few thousand kilometers of the nucleus. The highest priority of future radar observations will be to obtain delay-Doppler images of a nucleus, which would give direct size and shape estimates as well as a more reliable albedo. Delay-Doppler or interferometric imaging of the coma echo would also help to better characterize the grain halo. Ten short-period comets are potentially detectable during the next two decades, although the best radar opportunities may well come from comets yet to be discovered. # 1999 Elsevier Science Ltd. All rights reserved.

Mueller Matrix Parameters for Radio Telescopes and Their Observational Determination

Modern digital cross-correlators permit the simultaneous measurement of all four Stokes parameters. However, the results must be calibrated to correct for the polarization transfer function of the receiving system. The transfer function for any device can be expressed by its Mueller matrix. We express the matrix elements in terms of fundamental system parameters that describe the voltage transfer functions (known as the Jones matrix) of the various system devices in physical terms and thus provide a means for comparing with engineering calculations and investigating the effects of design changes. We describe how to determine these parameters with astronomical observations. We illustrate the method by applying it to some of the receivers at the Arecibo Observatory.

OSIRIS-REx: Sample Return from Asteroid (101955) Bennu

In May of 2011, NASA selected the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) asteroid sample return mission as the third mission in the New Frontiers program. The other two New Frontiers missions are New Horizons, which explored Pluto during a flyby in July 2015 and is on its way for a flyby of Kuiper Belt object 2014 MU69 on January 1, 2019, and Juno, an orbiting mission that is studying the origin, evolution, and internal structure of Jupiter. The spacecraft departed for near-Earth asteroid (101955) Bennu aboard an United Launch Alliance Atlas V 411 evolved expendable launch vehicle at 7:05 p.m. EDT on September 8, 2016, on a seven-year journey to return samples from Bennu. The spacecraft is on an outbound-cruise trajectory that will result in a rendezvous with Bennu in November 2018. The science instruments on the spacecraft will survey Bennu to measure its physical, geological, and chemical properties, and the team will use these data to select a site on the surface to collect at least 60 g of asteroid regolith. The team will also analyze the remote-sensing data to perform a detailed study of the sample site for context, assess Bennu’s resource potential, refine estimates of its impact probability with Earth, and provide ground-truth data for the extensive astronomical data set collected on this asteroid. The spacecraft will leave Bennu in 2021 and return the sample to the Utah Test and Training Range (UTTR) on September 24, 2023.

All‐Stokes Parameterization of the Main Beam and First Sidelobe for the Arecibo Radio Telescope

Radio astronomical measurements of extended emission require knowledge of the beam shape and response because the measurements need correction for quantities such as beam efficiency and beamwidth. We describe a scheme that characterizes the main beam and sidelobe in all Stokes parameters employing parameters that allow reconstruction of the complete beam patterns and, also, afford an easy way to see how the beam changes with azimuth, zenith angle, and time. For the main beam in Stokes I, the parameters include the beamwidth, ellipticity and its orientation, coma and its orientation, the point-source gain, and the integrated gain (or, equivalently, the main-beam efficiency); for the other Stokes parameters, the beam parameters include beam squint and beam squash. For the first sidelobe ring in Stokes I, the parameters include an eight-term Fourier series describing the height, radius, and radial width; for the other Stokes parameters they include only the sidelobes fractional polarization.We illustrate the technique by applying it to the Arecibo telescope. The main-beam width is smaller and the sidelobe levels higher than for a uniformly illuminated aperture of the same effective area. These effects are modeled modestly well by a blocked aperture, with the blocked area equal to about 10% of the effective area (this corresponds to 5% physical blockage). In polarized emission, the effects of beam squint (difference in pointing direction between orthogonal polarizations) and squash (difference in beamwidth between orthogonal polarizations) do not correspond to theoretical expectation and are higher than expected; these effects are almost certainly caused by the blockage. The first sidelobe is highly polarized because of blockage. These polarization effects lead to severe contamination of maps of polarized emission by spatial derivatives in brightness temperature.