Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA.
The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA.
Nature. 2019 Apr;568(7750):55-60. doi: 10.1038/s41586-019-1033-6. Epub 2019 Mar 19.
NASA'S Origins, Spectral Interpretation, Resource Identification and Security-Regolith Explorer (OSIRIS-REx) spacecraft recently arrived at the near-Earth asteroid (101955) Bennu, a primitive body that represents the objects that may have brought prebiotic molecules and volatiles such as water to Earth. Bennu is a low-albedo B-type asteroid that has been linked to organic-rich hydrated carbonaceous chondrites. Such meteorites are altered by ejection from their parent body and contaminated by atmospheric entry and terrestrial microbes. Therefore, the primary mission objective is to return a sample of Bennu to Earth that is pristine-that is, not affected by these processes. The OSIRIS-REx spacecraft carries a sophisticated suite of instruments to characterize Bennu's global properties, support the selection of a sampling site and document that site at a sub-centimetre scale. Here we consider early OSIRIS-REx observations of Bennu to understand how the asteroid's properties compare to pre-encounter expectations and to assess the prospects for sample return. The bulk composition of Bennu appears to be hydrated and volatile-rich, as expected. However, in contrast to pre-encounter modelling of Bennu's thermal inertia and radar polarization ratios-which indicated a generally smooth surface covered by centimetre-scale particles-resolved imaging reveals an unexpected surficial diversity. The albedo, texture, particle size and roughness are beyond the spacecraft design specifications. On the basis of our pre-encounter knowledge, we developed a sampling strategy to target 50-metre-diameter patches of loose regolith with grain sizes smaller than two centimetres. We observe only a small number of apparently hazard-free regions, of the order of 5 to 20 metres in extent, the sampling of which poses a substantial challenge to mission success.
美国宇航局的起源、光谱解释、资源识别和安全风化层探测器（OSIRIS-REx）飞船最近抵达近地小行星（101955）贝努，这是一个原始天体，代表着可能将前生物分子和挥发物如水带到地球的物体。贝努是一颗低反照率 B 型小行星，与富含有机物质的水合碳质球粒陨石有关。这些陨石是由于从母体中喷出并受到大气进入和地球微生物的污染而发生变化的。因此，主要任务目标是将贝努的原始样本送回地球，即不受这些过程影响的样本。OSIRIS-REx 飞船携带了一套复杂的仪器，用于描述贝努的全球特性，支持采样地点的选择，并以亚厘米级的分辨率记录该地点。在这里，我们考虑 OSIRIS-REx 对贝努的早期观测结果，以了解小行星的特性如何与预先设想的结果相比较，并评估样本返回的前景。贝努的整体成分似乎是水合的和富含挥发物的，这是预期的。然而，与预先设想的贝努热惯性和雷达偏振比的建模形成对比的是，这表明了一个通常是由厘米级颗粒覆盖的光滑表面，解析成像揭示了意想不到的表面多样性。反照率、纹理、颗粒大小和粗糙度都超出了航天器设计规格。根据我们预先设想的知识，我们制定了一个采样策略，目标是瞄准直径为 50 米的松散风化层斑块，颗粒尺寸小于两厘米。我们只观察到少数显然没有危险的区域，其范围约为 5 到 20 米，对这些区域的采样对任务成功构成了重大挑战。
