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We present a physical model and spin-state analysis of the potentially hazardous asteroid (23187) 2000 PN9. As part of a long-term campaign to make direct detections of the Yarkovsky–O'Keefe–Radzievskii–Paddack (YORP) effect, we collected optical light curves of the asteroid between 2006 and 2020. These observations were combined with planetary radar data to develop a detailed shape model, which was used to search for YORP acceleration. We report that 2000 PN9 is a relatively large top-shaped body with a sidereal rotation period of 2.53216 ± 0.00015 h. Although we find no evidence for rotational acceleration, YORP torques smaller than |$\sim 10^{-8}\, \rm rad\,{d}^{-2}$| cannot be ruled out. It is likely that 2000 PN9 is a YORP-evolved object, and may be an example of YORP equilibrium or self-limitation. [ABSTRACT FROM AUTHOR]

Phase curves of asteroids are typically considered to depend solely on the scattering properties of airless particulate surfaces and the size of the object being studied. In this study, we demonstrate the additional dependence of phase curves on object shape, rotation pole orientation, and viewing geometry over an apparition. Variations in the phase curve of near-Earth asteroid (159402) 1999 AP10 over its apparition from 2020 July to 2021 January are verified to be due to aspect changes over the apparition. This is achieved through shape modelling of the asteroid and simulation of the phase curve over the apparition. We present simulations of asteroid phase curves over a range of geometries to understand the potential magnitude of this aspect effect, and under which circumstances it can begin to dominate in the phase curves. This dependence on aspect may introduce significant additional uncertainty in the properties derived from phase-curve data. We provide and demonstrate software code to estimate the aspect-related uncertainty in near-Earth asteroid phase curves through simulation and model fitting of a randomly generated sample of ellipsoidal asteroid models over the observed viewing geometry. We demonstrate how ignoring this effect may lead to misleading interpretations of the data and underestimation of uncertainties in further studies, such as those in the infrared that use phase curve derived parameters when fitting physical properties of an asteroid. [ABSTRACT FROM AUTHOR]

Context. The potentially hazardous asteroid (85990) 1999 JV6 has been a target of previously published thermal-infrared observations and optical photometry. It has been identified as a promising candidate for possible Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect detection. Aims. The YORP effect is a small thermal-radiation torque considered to be a key factor in spin-state evolution of small Solar System bodies. In order to detect YORP on 1999 JV6 we developed a detailed shape model and analysed the spin-state using both optical and radar observations. Methods. For 1999 JV6, we collected optical photometry between 2007 and 2016. Additionally, we obtained radar echo-power spectra and imaging observations with Arecibo and Goldstone planetary radar facilities in 2015, 2016, and 2017. We combined our data with published optical photometry to develop a robust physical model. Results. We determine that the rotation pole resides at negative latitudes in an area with a 5° radius close to the south ecliptic pole. The refined sidereal rotation period is 6.536787 ± 0.000007 h. The radar images are best reproduced with a bilobed shape model. Both lobes of 1999 JV6 can be represented as oblate ellipsoids with a smaller, more spherical component resting at the end of a larger, more elongated component. While contact binaries appear to be abundant in the near-Earth population, there are only a few published shape models for asteroids in this particular configuration. By combining the radar-derived shape model with optical light curves we determine a constant-period solution that fits all available data well. Using light-curve data alone we determine an upper limit for YORP of 8.5 × 10−8 rad day−2. Conclusions. The bifurcated shape of 1999 JV6 might be a result of two ellipsoidal components gently merging with each other, or a deformation of a rubble pile with a weak-tensile-strength core due to spin-up. The physical model of 1999 JV6 presented here will enable future studies of contact binary asteroid formation and evolution. [ABSTRACT FROM AUTHOR]

Context. The near-Earth asteroid (1917) Cuyo was subject to radar and light curve observations during a close approach in 1989, and observed up until 2008. It was selected as one of our ESO Large Programme targets, aimed at observational detections of the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect through long-term light curve monitoring and physical modelling of near-Earth asteroids. Aims. We aim to constrain the physical properties of Cuyo: shape, spin-state, and spectroscopic and thermo-physical properties of the surface. Methods. We acquired photometric light curves of Cuyo spanning the period between 2010 and 2013, which we combined with published light curves from 1989 to 2008. Our thermal-infrared observations were obtained in 2011. Rotationally resolved optical spectroscopy data were acquired in 2011 and combined with all available published spectra to investigate any surface material variegation. Results. We developed a convex light curve-inversion shape of Cuyo that suggests the presence of an equatorial ridge, typical for an evolved system close to shedding mass due to fast rotation. We determine limits of YORP strength through light curve-based spin-state modelling, including both negative and positive acceleration values, between − 0.7 × 10−8 and 1.7 × 10−8 rad day−2. Thermo-physical modelling with the ATPM provides constraints on the geometric albedo, pV = 0.24 ± 0.07, the effective diameter, Deff = 3.15 ± 0.08 km, the thermal inertia, Γ = 44 ± 9 J m−2 s−1∕2 K−1, and a roughness fraction of 0.52  ±  0.26. This enabled a YORP strength prediction of ν = (−6.39 ± 0.96) × 10−10 rad day−2. We also see evidence of surface compositional variation. Conclusions. The low value of YORP predicted by means of thermo-physical analysis, consistent with the results of the light curve study, might be due to the self-limiting properties of rotational YORP, possibly involving movement of sub-surface and surface material. This may also be consistent with the surface compositional variation that we see. The physical model of Cuyo can be used to investigate cohesive forces as a way to explain why some targets survive rotation rates faster than the fission limit. [ABSTRACT FROM AUTHOR]

Context. Comets are believed to hold a relatively pristine record of the physical and chemical processes that occurred during the formation and evolution of the solar system. Thorough investigations of these small bodies, such as the one that will be performed by the ESA/Rosetta cornerstone mission, are thus supposed to bring strong and unique constraints on the origins of the solar system. Aims. Because comet 67P/Churyumov-Gerasimenko was only recently selected as the target for the ESA/Rosetta mission, there has been little opportunity to study its pre-perihelion activity. This phase is, however, very important for the mission, since the global mapping of the nucleus and the choice of landing site for Philae will be performed during this pre-perihelion phase. Here, we report previously unpublished data of the last pre-perihelion passage of this comet, observed between May and September 2008. Methods. The gas and dust activity of comet 67P/Churyumov-Gerasimenko are studied through visible spectroscopy and broadband imaging, respectively, covering a range of pre-perihelion heliocentric distances between 2.99 and 2.22 AU. Results. The data we have gathered on the dust activity are consistent with trends observed by other authors and show a strong asymmetry between the pre- and post-perihelion phases of the orbit. The spectra do not show any lines due to the emission of volatiles, and upper limits on their production rates are typically one order of magnitude lower than at the equivalent post-perihelion heliocentric distances. The asymmetry in the pre- and post-perihelion phases of the activity may be due to a dusty crust quenching the activity at the surface of 67P. We estimate that this crust could be about 12 cm thick, although not uniform across the surface. Even if no gas is individually detected, the coma surface brightness profiles might indicate a possible contamination from gaseous species emitted before the comet actually reaches perihelion. [ABSTRACT FROM AUTHOR]

Context. Near-Earth asteroid (25143) Itokawa was visited by the Hayabusa spacecraft in 2005, resulting in a highly detailed shape and surface topography model. This model has led to several predictions for the expected radiative torques on this asteroid, suggesting that its spin rate should be decelerating. Aims. To detect changes in rotation rate that may be due to YORP-induced radiative torques, which in turn may be used to investigate the interior structure of the asteroid. Methods. Through an observational survey spanning 2001 to 2013 we obtained rotational lightcurve data at various times over the last five close Earth-approaches of the asteroid. We applied a polyhedron-shape-modelling technique to assess the spin-state of the asteroid and its long term evolution. We also applied a detailed thermophysical analysis to the shape model determined from the Hayabusa spacecraft. Results. We have successfully measured an acceleration in Itokawa's spin rate of dω=dι = (3:54 ± 0.38) × 10-8 rad day-2, equivalent to a decrease of its rotation period of ∼45 ms year-1. From the thermophysical analysis we find that the centre-of-mass for Itokawa must be shifted by ∼21 m along the long-axis of the asteroid to reconcile the observed YORP strength with theory. Conclusions. This can be explained if Itokawa is composed of two separate bodies with very different bulk densities of 1750 ± 110 kg m-3 and 2850 ± 500 kg m-3, and was formed from the merger of two separate bodies, either in the aftermath of a catastrophic disruption of a larger differentiated body, or from the collapse of a binary system. We therefore demonstrate that an observational measurement of radiative torques, when combined with a detailed shape model, can provide insight into the interior structure of an asteroid. Futhermore, this is the first measurement of density inhomogeneity within an asteroidal body, that reveals significant internal structure variation. A specialised spacecraft is normally required for this. [ABSTRACT FROM AUTHOR]