The NASA Double Asteroid Redirection Test (DART) spacecraft impacted the secondary body of the binary asteroid (65803) Didymos on 2022 September 26 and altered its orbit about the primary body. Before the DART impact, we performed visible and mid-infrared observations to constrain the pre-impact thermophysical properties of the Didymos system and to model its Yarkovsky effect. Analysis of the photometric phase curve derives a Bond albedo of 0.07 ± 0.01, and a thermophysical analysis of the mid-infrared observations derives a thermal inertia of 320 ± 70 J m⁻² K⁻¹ s^−1/2 and a thermal roughness of 40° ± 3° rms slope. These properties are compatible with the ranges derived for other S-type near-Earth asteroids. Model-to-measurement comparisons of the Yarkovsky orbital drift for Didymos derives a bulk density of 2750 ± 350 kg m⁻³, which agrees with other independent measures based on the binary mutual orbit. This bulk density indicates that Didymos is spinning at or near its critical spin-limit at which self-gravity balances equatorial centrifugal forces. Furthermore, comparisons with the post-impact infrared observations presented in Rivkin et al. indicate no change in the thermal inertia of the Didymos system following the DART impact. Finally, orbital temperature simulations indicate that subsurface water ice is stable over geologic timescales in the polar regions if present. These findings will be investigated in more detail by the upcoming ESA Hera mission.

中文翻译：

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NASA DART 目标 (65803) Didymos 的撞击前热物理特性和雅可夫斯基效应

美国宇航局双小行星重定向测试 (DART) 航天器于 2022 年 9 月 26 日撞击了双小行星 (65803) Didymos 的次星体，并改变了其围绕主星体的轨道。在 DART 撞击之前，我们进行了可见光和中红外观测，以限制 Didymos 系统撞击前的热物理特性并对其雅可夫斯基效应进行建模。光度相位曲线分析得出的邦德反照率为 0.07 ± 0.01，中红外观测的热物理分析得出的热惯性为 320 ± 70 J m ^-2 K ^-1 s ^-1/2，热粗糙度为40° ± 3° 均方根斜率。这些特性与其他 S 型近地小行星的范围一致。 Didymos 的雅可夫斯基轨道漂移的模型与测量比较得出的体积密度为 2750 ± 350 kg m ^-3，这与基于双星互轨道的其他独立测量一致。这种体积密度表明，Didymos 正在以或接近其临界旋转极限旋转，在该临界旋转极限处，自重力平衡了赤道离心力。此外，与 Rivkin 等人提出的撞击后红外观测结果进行了比较。表明 Didymos 系统的热惯性在 DART 撞击后没有变化。最后，轨道温度模拟表明，如果存在的话，极地地区的地下水冰在地质时间尺度上是稳定的。即将到来的欧空局赫拉任务将更详细地研究这些发现。