哈勃Space Telescope对3200法厄同的最近距离观测
Hubble Space Telescope Observations of 3200 Phaethon At Closest Approach
摘要：
我们目前的哈勃太空望远镜观测活动小行星（和双子座流父）3200法厄同时，在其最接近地球（分离0.07 AU）在2017年12月。在轨道平面的$\sim$ 1 \degr ~范围内记录图像，对大颗粒尾迹散射引起的低表面亮度提供额外的灵敏度。我们把这样一个尾迹的表面亮度上限定为27.2角秒$^{-2}$，对应于平面内光学深度$\le 3\times10^{-9}$。没有检测到比绝对星等26.3更亮的同向移动源，对应于圆形等效半径$\sim$ 12米（假设为0.12）。法厄同太热了，近地表的冰无法生存。我们简要地考虑了深埋冰的热力学稳定性，发现它的生存将需要一个非常小的（风化）的热扩散率（$< 10^{-8}$ m $^2$ s $^{-1}$），或意外最近注入法厄同（时间尺度$\lesssim$ 10 $^6$年）到其目前的轨道，或两者兼而有之。
英文摘要：
We present Hubble Space Telescope observations of the active asteroid (and Geminid stream parent) 3200 Phaethon when at its closest approach to Earth (separation 0.07 AU) in 2017 December. Images were recorded within $\sim$1\degr~of the orbital plane, providing extra sensitivity to low surface brightness caused by scattering from a large-particle trail. We placed an upper limit to the apparent surface brightness of such a trail at 27.2 magnitudes arcsecond$^{-2}$, corresponding to an in-plane optical depth $\le 3\times10^{-9}$. No co-moving sources brighter than absolute magnitude 26.3, corresponding to circular equivalent radius $\sim$12 m (albedo 0.12 assumed), were detected. Phaethon is too hot for near-surface ice to survive. We briefly consider the thermodynamic stability of deeply-buried ice, finding that its survival would require either a very small (regolith-like) thermal diffusivity ($< 10^{-8}$ m$^2$ s$^{-1}$), or the unexpectedly recent injection of Phaethon (timescale $\lesssim$ 10$^6$ yr) into its present orbit, or both.
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