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The Yarkovsky effect is a thermal process acting upon the orbits of small celestial bodies, which can cause these orbits to slowly expand or contract with time. The effect is subtle (da/dt ~ 10^-4 au/My for a 1 km diameter object) and is thus generally difficult to measure. We analyzed both optical and radar astrometry for 600 near-Earth asteroids (NEAs) for the purpose of detecting and quantifying the Yarkovsky effect. We present 247 NEAs with measured drift rates, which is the largest published set of Yarkovsky detections. This large sample size provides an opportunity to examine the Yarkovsky effect in a statistical manner. In particular, we describe two independent population-based tests that verify the measurement of Yarkovsky orbital drift. First, we provide observational confirmation for the Yarkovsky effect's theoretical size dependence of 1/D, where D is diameter. Second, we find that the observed ratio of negative to positive drift rates in our sample is 2.34, which, accounting for bias and sampling uncertainty, implies an actual ratio of $2.7^{+0.3}_{-0.7}$. This ratio has a vanishingly small probability of occurring due to chance or statistical noise. The observed ratio of retrograde to prograde rotators is two times lower than the ratio expected from numerical predictions from NEA population studies and traditional assumptions about the sense of rotation of NEAs originating from various main belt escape routes. We also examine the efficiency with which solar energy is converted into orbital energy and find a median efficiency in our sample of 12%. We interpret this efficiency in terms of NEA spin and thermal properties.

The population of large asteroids is thought to be primordial and they arethe most direct witnesses of the early history of our Solar System. Thosesatellites allow study of the mass, and hence density and internal structure.We study here the properties of the triple asteroid (107) Camilla fromlightcurves, stellar occultations, optical spectroscopy, and high-contrast andhigh-angular-resolution images and spectro-images. Using 80 positions over 15years, we determine the orbit of its larger satellite to be circular,equatorial, and prograde, with RMS residuals of 7.8 mas. From 11 positions inthree epochs only, in 2015 and 2016, we determine a preliminary orbit for thesecond satellite. We find the orbit to be somewhat eccentric and slightlyinclined to the primary's equatorial plane, reminiscent of the inner satellitesof other asteroid triple systems. Comparison of the near-infrared spectrum ofthe larger satellite reveals no significant difference with Camilla. Hence,these properties argue for a formation of the satellites by excavation fromimpact and re-accumulation of ejecta. We determine the spin and 3-D shape ofCamilla. The model fits well each data set. We determine Camilla to be largerthan reported from modeling of mid-infrared photometry, with aspherical-volume-equivalent diameter of 254 $\pm$ 36 km (3 $\sigma$uncertainty), in agreement with recent results from shape modeling(Hanus2017+). Combining the mass of (1.12 $\pm$ 0.01) $\times$ 10$^{19}$ kgdetermined from the dynamics of the satellites and the volume from the 3-Dshape model, we determine a density of 1,280 $\pm$ 130 SI. From this density,and considering Camilla's spectral similarities with (24) Themis and (65)Cybele (for which water ice coating on surface grains was reported), we infer asilicate-to-ice mass ratio of 1-6, with a 10-30% macroporosity.

Near-Earth asteroid (NEA) 1566 Icarus ($a=1.08\,\mathrm{au}$, e = 0.83, $i=22\buildrel{\circ}\over{.} 8$) made a close approach to Earth in 2015 June at 22 lunar distances (LD). Its detection during the 1968 approach (16 LD) was the first in the history of asteroid radar astronomy. A subsequent approach in 1996 (40 LD) did not yield radar images. We describe analyses of our 2015 radar observations of Icarus obtained at the Arecibo Observatory and the DSS-14 antenna at Goldstone. These data show that the asteroid is a moderately flattened spheroid with an equivalent diameter of 1.44 km with 18% uncertainties, resolving long-standing questions about the asteroid size. We also solve for Icarus's spin-axis orientation ($\lambda =270^\circ \pm 10^\circ ,\beta =-81^\circ \pm 10^\circ $), which is not consistent with the estimates based on the 1968 light-curve observations. Icarus has a strongly specular scattering behavior, among the highest ever measured in asteroid radar observations, and a radar albedo of ~2%, among the lowest ever measured in asteroid radar observations. The low cross section suggests a high-porosity surface, presumably related to Icarus's cratering, spin, and thermal histories. Finally, we present the first use of our orbit-determination software for the generation of observational ephemerides, and we demonstrate its ability to determine subtle perturbations on NEA orbits by measuring Icarus's orbit-averaged drift in semimajor axis ($(-4.62\pm 0.48)\times {10}^{-4}$ au My−1, or ~60 m per revolution). Our Yarkovsky rate measurement resolves a discrepancy between two published rates that did not include the 2015 radar astrometry.

We analyze radio tracking data obtained during 1311 orbits of the MESSENGER space- craft in the period March 2011 to April 2014. A least-squares minimization of the residuals between observed and computed values of two-way range and Doppler allows us to solve for a model describing Mercury’s gravity, tidal response, and spin state. We use a spherical harmonic representation of the gravity field to degree and order 40 and report error bars corresponding to 10 times the formal uncertainties of the fit. Our estimate of the product of Mercury’s mass and the gravitational constant, GM = (22031.87404 ± 9 × 10−4) km3s−2, is in excellent agreement with published results. Our solution for the geophysically important second-degree coefficients (C2,0 = −2.25100 × 10−5 ± 1.3 × 10−9, C2,2 = 1.24973 × 10−5 ± 1.2 × 10−9) confirms previous estimates to better than 0.4% and, there- fore, inferences about Mercury’s moment of inertia and interior structure. Our estimate of the tidal Love number k2 = 0.464 ± 0.023 indicates that Mercury’s mantle may be hotter and weaker than previously thought. Our spin state solution suggests that gravity-based estimates of Mercury’s spin axis orientation are marginally consistent with previous measurements of the orientation of the crust.

It has been suggested that the timing of pollination in Ephedra foeminea coincides with the full moon in July. The implication is that the plant can detect the full moon through light or gravity and that this trait is an evolutionary adaptation that aids the navigation by pollinating insects. Here we show that there is insufficient data to make such a claim and we predict that pollinations of E. foeminea do not in general coincide with the full moon.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.10.028 Byline: S.J. Peale (a), M. Yseboodt (b), J.-L. Margot (b) Keywords: Mercury; Rotational dynamics Abstract: Planetary perturbations of Mercury's orbit lead to forced librations in longitude in addition to the 88-day forced libration induced by Mercury's orbital motion. The forced librations are a combination of many periods, but 5.93 and 5.66 years dominate. These two periods result from the perturbations by Jupiter and Venus respectively, and they lead to a 125-year modulation of the libration amplitude corresponding to the beat frequency. Other periods are also identified with Jupiter and Venus perturbations as well as with those of the Earth, and these and other periods in the perturbations cause several arc second fluctuations in the libration extremes. The maxima of these extremes are about 30a[sup.3] above and the minima about 7a[sup.3] above the superposed [approximately equal to]60.sup.a[sup.3], 88-day libration during the 125-year modulation. Knowledge of the nature of the long-period forced librations is important for the interpretation of the details of Mercury's rotation state to be obtained from radar and spacecraft observations. We show that the measurement of the 88-day libration amplitude for the purposes of determining Mercury's core properties is not compromised by the additional librations, because of the latter's small amplitude and long period. If the free libration in longitude has an amplitude that is large compared with that of the forced libration, its [approximately equal to]10-year period will dominate the libration spectrum with the 88-day forced libration and the long-period librations from the orbital perturbations superposed. If the free libration has an amplitude that is comparable to those of the long-period forced libration, it will be revealed by erratic amplitude, period and phase on the likely time span of a series of observations. However, a significant free libration component is not expected because of relatively rapid damping. Author Affiliation: (a) Department of Physics, University of California, Santa Barbara, CA 93106, USA (b) Department of Astronomy, Cornell University, Ithaca, NY 14850, USA Article History: Received 24 April 2006; Revised 26 October 2006

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.05.014 Byline: J.K. Harmon (a), M.C. Nolan (a), J.-L. Margot (b), D.B. Campbell (b), L.A.M. Benner (c), J.D. Giorgini (c) Keywords: Comets; Radar observations; Comets; nucleus; Comets; dust Abstract: Arecibo radar observations of Comet P/2005 JQ5 (Catalina) have produced the first delay-Doppler images of a comet nucleus and the first radar detection of large-grain ejection from a Jupiter-family comet. The nucleus is small (1.4 km diameter), rough, and rapidly rotating. The large (>cm) grains have low velocities ([approximately equal to]1 m/s) and a low production rate. Author Affiliation: (a) National Astronomy and Ionosphere Center, Arecibo Observatory, Arecibo, PR 00612, USA (b) Department of Astronomy, Cornell University, Ithaca, NY 14853, USA (c) Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA Article History: Received 22 March 2006; Revised 26 May 2006

Photometric data on 17 binary near-Earth asteroids (15 of them are certain detections, two are probables) were analysed and characteristic properties of the near-Earth asteroid (NEA) binary population were inferred. We have found that binary systems with a secondary-to-primary mean diameter ratio [D.sub.s]/[D.sub.p] [greater than or equal to] 0.18 concentrate among NEAs smaller than 2 km in diameter; the abundance of such binaries decreases significantly among larger NEAs. Secondaries show an upper size limit of [D.sub.s] = 0.5-1 km. Systems with [D.sub.s]/[D.sub.p] [less than or equal to] 0.5 are abundant but larger satellites are significantly less common. Primaries have spheroidal shapes and they rotate rapidly, with periods concentrating between 2.2 to 2.8 h and with a tail of the distribution up to ~4 h. The fast rotators are close to the critical spin for rubble piles with bulk densities about 2 g/[cm.sup.3]. Orbital periods show an apparent cut-off at [P.sub.orb] ~ 11 h; closer systems with shorter orbital periods have not been discovered, which is consistent with the Roche limit for strengthless bodies. Secondaries are more elongated on average than primaries. Most, but not all, of their rotations appear to be synchronized with the orbital motion; nonsynchronous secondary rotations may occur especially among wider systems with [P.sub.orb] > 20 h. The specific total angular momentum of most of the binary systems is similar to within [+ or -] 20% and close to the angular momentum of a sphere with the same total mass and density, rotating at the disruption limit; this suggests that the binaries were created by mechanism(s) related to rotation near the critical limit and that they neither gained nor lost significant amounts of angular momentum during or since formation. A comparison with six small asynchronous binaries detected in the main belt of asteroids suggests that the population extends beyond the region of terrestrial planets, but with characteristics shifted to larger sizes and longer periods. The estimated mean proportion of binaries with [D.sub.s]/[D.sub.p] [greater than or equal to] 0.18 among NEAs larger than 0.3 km is 15 [+ or -] 4%. Among fastest rotating NEAs larger than 0.3 km with periods between 2.2 and 2.8 h, the mean proportion of such binaries is (66 [+ or -] 110)%. Keywords: Asteroids; Photometry

Integration of the orbit of asteroid (29075) 1950 DA, which is based on radar and optical measurements spanning 51 years, reveals a 20-minute interval in March 2880 when there could be a nonnegligible probability of the 1-kilometer object colliding with Earth. Trajectory knowledge remains accurate until then because of extensive astrometric data, an inclined orbit geometry that reduces in-plane perturbations, and an orbit uncertainty space modulated by gravitational resonance. The approach distance uncertainty in 2880 is determined primarily by uncertainty in the accelerations arising from thermal re-radiation of solar energy absorbed by the asteroid. Those accelerations depend on the spin axis, composition, and surface properties of the asteroid, so that refining the collision probability may require direct inspection by a spacecraft., Useful predictions of asteroid encounters with planets, in which the statistical uncertainty in the time of closest approach is ± 10 days or less, are generally limited to an interval [...]