Patrick A. Taylor

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

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.

The Extreme Kuiper Belt Binary 2001 QW322

The study of binary Kuiper Belt objects helps to probe the dynamic conditions present during planet formation in the solar system. We report on the mutual-orbit determination of 2001 QW322, a Kuiper Belt binary with a very large separation whose properties challenge binary-formation and -evolution theories. Six years of tracking indicate that the binarys mutual-orbit period is ≈25 to 30 years, that the orbit pole is retrograde and inclined 50° to 62° from the ecliptic plane, and, most surprisingly, that the mutual orbital eccentricity is <0.4. The semimajor axis of 105,000 to 135,000 kilometers is 10 times that of other near-equal-mass binaries. Because this weakly bound binary is prone to orbital disruption by interlopers, its lifetime in its present state is probably less than 1 billion years.

Asteroid Systems: Binaries, Triples, and Pairs

In the past decade, the number of known binary near-Earth asteroids has more than quadrupled and the number of known large main belt asteroids with satellites has doubled. Half a dozen triple asteroids have been discovered, and the previously unrecognized populations of asteroid pairs and small main belt binaries have been identified. The current observational evidence confirms that small ( 20 km) binaries with small satellites are most likely created during large collisions.

Radar imaging and characterization of the binary near-Earth asteroid (185851) 2000 DP107

Potentially hazardous asteroid (185851) 2000 DP107 was the first binary near-Earth asteroid to be imaged. Radar observations in 2000 provided images at 75 m resolution that revealed the shape, orbit, and spin-up formation mechanism of the binary. The asteroid made a more favorable flyby of the Earth in 2008, yielding images at 30 m resolution. We used these data to obtain shape models for the two components and to improve the estimates of the mutual orbit, component masses, and spin periods. The primary has a sidereal spin period of 2.7745 +/- 0.0007 h and is roughly spheroidal with an equivalent diameter of 863 m +/- 5 %. It has a mass of 4.656 +/- 0.43 x 10^11 kg and a density of 1381 +/- 244 kg m^{-3}. It exhibits an equatorial ridge similar to the (66391) 1999 KW4 primary, however the equatorial ridge in this case is not as regular and has a ~300 m diameter concavity on one side. The secondary has a sidereal spin period of 1.77 +/- 0.02 days commensurate with the orbital period. The secondary is slightly elongated and has overall dimensions of 377 x 314 x 268 m (6 % uncertainties). Its mass is 0.178 +/- 0.021 x 10^{11} kg and its density is 1047 +/- 230 kg m^{-3}. The mutual orbit has a semi-major axis of 2.659 +/- 0.08 km, an eccentricity of 0.019 +/- 0.01, and a period of 1.7556 +/- 0.0015 days. The normalized total angular momentum of this system exceeds the amount required for the expected spin-up formation mechanism. An increase of angular momentum from non-gravitational forces after binary formation is a possible explanation. The two components have similar radar reflectivity, suggesting a similar composition consistent with formation by spin-up. The secondary appears to exhibit a larger circular polarization ratio than the primary, suggesting a rougher surface or subsurface at radar wavelength scales.

Tidal End States of Binary Asteroid Systems with a Nonspherical Component

Abstract We derive the locations of the fully synchronous end states of tidal evolution for binary asteroid systems having one spherical component and one oblate- or prolate-spheroid component. Departures from a spherical shape, at levels observed among binary asteroids, can result in the lack of a stable tidal end state for particular combinations of the system mass fraction and angular momentum, in which case the binary must collapse to contact. We illustrate our analytical results with near-Earth Asteroids (8567) 1996 HW 1 , (66391) 1999 KW 4 , and 69230 Hermes.

Radar observations of Asteroids 64 Angelina and 69 Hesperia

Context. In the past decade, more than one hundred asteroid models wer e derived using the lightcurve inversion method. Measured by the number of derived models, lightcurve inversion has be come the leading method for asteroid shape determination. Aims. Tens of thousands of sparse-in-time lightcurves from astro metric projects are publicly available. We investigate the se data and use them in the lightcurve inversion method to derive new asteroid models. By having a greater number of models with kn own physical properties, we can gain a better insight into the na ture of individual objects and into the whole asteroid popul ation. Methods. We use sparse photometry from selected observatories from t he AstDyS database (Asteroids – Dynamic Site), either alone or in combination with dense lightcurves, to determine new a steroid models by the lightcurve inversion method. We inves tigate various correlations between several asteroid parameters and char acteristics such as the rotational state and diameter or fam ily membership. We focus on the distribution of ecliptic latitudes of pole di rections. We create a synthetic uniform distribution of lat itudes, compute the method bias, and compare the results with the distributi on of known models. We also construct a model for the long-ter m volution of spins. Results. We present 80 new asteroid models derived from combined data sets where sparse photometry is taken from the AstDyS database and dense lightcurves are from the Uppsala Asteroi d Ph tometric Catalogue (UAPC) and from several individual observers. For 18 asteroids, we present updated shape solutions based o n new photometric data. For another 30 asteroids we present t h ir partial models, i.e., an accurate period value and an estimate of the ecliptic latitude of the pole. The addition of new models inc reases the total number of models derived by the lightcurve inversion m ethod to∼200. We also present a simple statistical analysis of physic al properties of asteroids where we look for possible correlat ions between various physical parameters with an emphasis o n the spin vector. We present the observed and de-biased distribution s of ecliptic latitudes with respect to di fferent size ranges of asteroids as well as a simple theoretical model of the latitude distribut ion and then compare its predictions with the observed distr ibutions. From this analysis we find that the latitude distribution of small asteroids ( D < 30 km) is clustered towards ecliptic poles and can be explained by the YORP thermal e ff ct while the latitude distribution of larger asteroids ( D > 60 km) exhibits an evident excess of prograde rotators, probably of primordial origin.

Radar imaging and physical characterization of near-Earth Asteroid (162421) 2000 ET70

We observed near-Earth Asteroid (162421) 2000 ET70 using the Arecibo and Goldstone radar systems over a period of 12 days during its close approach to the Earth in February 2012. We obtained continuous wave spectra and range-Doppler images with range resolutions as fine as 15 m. Inversion of the radar images yields a detailed shape model with an effective spatial resolution of 100 m. The asteroid has overall dimensions of 2.6 km � 2.2 km � 2.1 km (5% uncertainties) and a surface rich with kilometer-scale ridges and concavities. This size, combined with absolute magnitude measurements, implies an extremely low albedo (� 2%). It is a principal axis rotator and spins in a retrograde manner with a sidereal spin period of 8.96 ± 0.01 h. In terms of gravitational slopes evaluated at scales of 100 m, the surface seems mostly relaxed with over 99% of the surface having slopes less than 30, but there are some outcrops at the north pole that may have steeper slopes. Our precise measurements of the range and velocity of the asteroid, combined with optical astrometry, enables reliable trajectory predictions for this potentially hazardous asteroid in the interval 460–2813.

CAPABILITIES OF EARTH-BASED RADAR FACILITIES FOR NEAR-EARTH ASTEROID OBSERVATIONS

We evaluated the planetary radar capabilities at Arecibo, the Goldstone 70-m DSS-14 and 34-m DSS-13 antennas, the 70-m DSS-43 antenna at Canberra, the Green Bank Telescope, and the Parkes Radio Telescope in terms of their relative sensitivities and the number of known near-Earth asteroids (NEAs) detectable per year in monostatic and bistatic configurations. In the 2015 calendar year, monostatic observations with Arecibo and DSS-14 were capable of detecting 253 and 131 NEAs respectively, with signal-to-noise ratios (SNRs) greater than 30/track. Combined, the two observatories were capable of detecting 276 NEAs. Of these, Arecibo detected 77 and Goldstone detected 32, or 30% and 24% the numbers that were possible. The two observatories detected an additional 18 and 7 NEAs respectively, with SNRs of less than 30/track. This indicates that a substantial number of potential targets are not being observed. The bistatic configuration with DSS-14 transmitting and the Green Bank Telescope receiving was capable of detecting about 195 NEAs, or ~50% more than with monostatic observations at DSS-14. Most of the detectable asteroids were targets of opportunity that were discovered less than 15 days before the end of their observing windows. About 50% of the detectable asteroids have absolute magnitudes > 25, which corresponds diameters < ~30 m.

Migration of interplanetary dust.

Abstract: We numerically investigate the migration of dust particles with initial orbits close to those of the numbered asteroids, observed trans‐Neptunian objects, and comet Encke. The fraction of silicate asteroidal particles that collided with the Earth during their lifetime varied from 11% for 100 micron particles to 0.008% for 1 micron particles. Almost all asteroidal particles with diameter d≥ 4 microns collided with the Sun. For migrating asteroidal dust particles, the peaks in semimajor axis distribution at the n:(n+ 1) resonances with Earth and Venus, and the gaps associated with the 1:1 resonances with these planets are more pronounced for larger particles. The probability of collisions of cometary particles with the Earth is smaller than for asteroidal particles, and this difference is greater for larger particles.