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arXiv 0908.0796astro-ph.EP
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斯皮策对航天器目标162173的观测（1999年JU3）

Spitzer Observations of Spacecraft Target 162173 (1999 JU3)

H. Campins, J. P. Emery, M. Kelley, Y. Fernandez, J. Licandro, M. Delbo, A. Barucci, E. Dotto

备注：
4 pages, 2 figures; to be published as a Letter in Astronomy and Astrophysics
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摘要：

近地小行星162173（1999 JU 3）是隼鸟2号采样返回飞行任务的主要目标，也是马可波罗采样返回飞行任务的潜在目标。在地球上对这一物体进行研究是这些飞行任务的基础。我们提出了一个中红外光谱（5-38微米）的1999年JU 3获得与美国宇航局的斯皮策太空望远镜在2008年5月。这些观测结果对这颗小行星的表面特性提出了新的限制。为了拟合我们的光谱，我们使用了近地小行星热模型（NEATM）和更复杂的热物理模型（TPM）。然而，自旋磁极的位置，这是不确定的，是一个关键的输入参数，用于约束与TPM的热惯性，因此，我们考虑两个磁极取向。在赤道逆行几何的极端情况下，我们推导出热惯量的下限为150 J/m^2/K/s ^[0.5]。如果我们采用阿部等人（2008 a）的极点取向，我们的最佳拟合热模型得出的热惯量值为700+/-200 J/m^2/K/s^0.5，由于绝对通量校准导致的光谱形状的不确定性，甚至允许更高的值。热惯性的下限，这是不太可能的，但可能的，将与类似于小行星433爱神星的细风化层相一致。然而，热惯性预计会更高，可能类似于或大于小行星25143 Itokawa。准确确定小行星162173的自旋极将缩小其热惯性的可能值范围。

英文摘要：

Near-Earth asteroid 162173 (1999 JU3) is the primary target of the Hayabusa-2 sample return mission, and a potential target of the Marco Polo sample return mission. Earth-based studies of this object are fundamental to these missions. We present a mid-infrared spectrum (5-38 microns) of 1999 JU3 obtained with NASA's Spitzer Space Telescope in May 2008. These observations place new constraints on the surface properties of this asteroid. To fit our spectrum we used the near-Earth asteroid thermal model (NEATM) and the more complex thermophysical model (TPM). However, the position of the spin-pole, which is uncertain, is a crucial input parameter for constraining the thermal inertia with the TPM; hence, we consider two pole orientations. In the extreme case of an equatorial retrograde geometry we derive a lower limit to the thermal inertia of 150 J/m^2/K/s^0.5. If we adopt the pole orientation of Abe et al. (2008a) our best-fit thermal model yields a value for the thermal inertia of 700+/-200 J/m^2/K/s^0.5 and even higher values are allowed by the uncertainty in the spectral shape due to the absolute flux calibration. The lower limit to the thermal inertia, which is unlikely but possible, would be consistent with a fine regolith similar to wthat is found for asteroid 433 Eros. However, the thermal inertia is expected to be higher, possibly similar to or greater than that on asteroid 25143 Itokawa. Accurately determining the spin-pole of asteroid 162173 will narrow the range of possible values for its thermal inertia.

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