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arXiv 1402.5573astro-ph.EP
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OSIRIS-REx目标小行星（101955）Bennu的轨道和体积密度

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

Steven R. Chesley, Davide Farnocchia, Michael C. Nolan, David Vokrouhlicky, Paul W. Chodas, Andrea Milani, Federica Spoto, Benjamin Rozitis, Lance A. M. Benner,… 展开作者

Steven R. Chesley, Davide Farnocchia, Michael C. Nolan, David Vokrouhlicky, Paul W. Chodas, Andrea Milani, Federica Spoto, Benjamin Rozitis, Lance A. M. Benner, William F. Bottke, Michael W. Busch, Joshua P. Emery, Ellen S. Howell, Dante S. Lauretta, Jean-Luc Margot, Patrick A. Taylor

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摘要：

OSIRIS-REx小行星取样返回任务的目标小行星是（101955）Bennu（原1999 RQ $_{36}$），它是一颗半公里长的近地小行星，轨道受到非常严格的限制。从1999年到2013年的大量光学天体测量数据和1999年、2005年和2011年对Bennu的高质量雷达延迟测量揭示了Yarkovsky效应的作用，其平均半长轴漂移率为$da/dt = (-19.0 \pm 0.1)\times 10^{-4}$ au/Myr或$284\pm 1.5\;\rm{m/yr}$。这一结果的准确性关键取决于观测和动力学模型的保真度。作为一个例子，忽略地球在近距离接近过程中的相对论摄动会影响$da/dt$中具有$3σ$意义的轨道。轨道偏离纯粹的引力动力学使我们能够推导出雅科夫斯基效应的加速度，而Bennu的已知物理特性使我们能够独立地模拟热辐射引起的力。这两种分析的结合产生了体积密度为$ρ= 1260\pm70\,\rm{kg/m^3}$，这表明大孔隙率在$40\pm10$ %范围内的体积密度可能的模拟陨石，这表明一个碎石堆的内部结构。相关质量估计值为$(7.8\pm0.9)\times 10^{10}\, \rm{kg}$和$GM = 5.2\pm0.6\,\rm{m^3/s^2}$。Bennu的地球接近方法在1654- 2135年的时间间隔内是确定性的，超过这个时间间隔的预测是统计性质的。特别是，2135年的近距离接近可能在月球距离内，导致强烈的散射，因此在随后的几年里，从2175年到2196年，可能会有许多潜在的影响。最高的单个撞击概率是2196年的$9.5\times 10^{-5}$，累积撞击概率是$3.7\times 10^{-4}$，导致累积巴勒莫尺度为-1.70。

英文摘要：

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--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 $da/dt = (-19.0 \pm 0.1)\times 10^{-4}$ au/Myr or $284\pm 1.5\;\rm{m/yr}$. The accuracy of this result depends critically on the fidelity of the observational and dynamical model. As an example, neglecting the relativistic perturbations of the Earth during close approaches affects the orbit with $3σ$ significance in $da/dt$. The orbital deviations from purely gravitational dynamics allow us to deduce the acceleration of the Yarkovsky effect, while the known physical characterization of Bennu allows us to independently model the force due to thermal emissions. The combination of these two analyses yields a bulk density of $ρ= 1260\pm70\,\rm{kg/m^3}$, which indicates a macroporosity in the range $40\pm10$% for the bulk densities of likely analog meteorites, suggesting a rubble-pile internal structure. The associated mass estimate is $(7.8\pm0.9)\times 10^{10}\, \rm{kg}$ and $GM = 5.2\pm0.6\,\rm{m^3/s^2}$. Bennu's Earth close approaches are deterministic over the interval 1654--2135, beyond which the predictions are statistical in nature. In particular, the 2135 close approach is likely within the lunar distance and leads to strong scattering and therefore numerous potential impacts in subsequent years, from 2175--2196. The highest individual impact probability is $9.5\times 10^{-5}$ in 2196, and the cumulative impact probability is $3.7\times 10^{-4}$, leading to a cumulative Palermo Scale of -1.70.

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