Radar Studies of Comet Nuclei and Grain Comae
Radar Studies of Comet Nuclei and Grain Comae
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彗核和颗粒彗发的雷达研究
DOI：
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发表时间：
2005
期刊：
影响因子：
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通讯作者：
D. Campbell
中科院分区：
文献类型：
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作者：
J. Harmon;M. Nolan;S. Ostro;D. Campbell
近距离接近的彗星可以显示出可检测到的来自其核的回声，或者来自大的彗星颗粒，或者两者兼而有之。自1980年以来，阿雷西博和戈德斯通雷达已经探测到9颗彗星；其中包括6次探测到核和5次探测到彗星颗粒。原子核雷达截面跨越了与原子核大小的10倍范围一致的很大范围的值。与这些彗星的独立大小估计进行比较，支持这个大小范围，并给出雷达反照率为0.04-0.1，这大约是典型小行星雷达反照率的一半。反照率与原子核表面密度~0.5-1.5g/cm~3相对应。基于简单的颗粒抛射理论的彗发回波模型可以使用合理的颗粒尺寸分布来解释雷达横截面，其中包括大量厘米大小的颗粒；在一种情况下，有证据表明尺寸分布的截止点与重力限制的最大可提升颗粒尺寸一致。模型表明，一些彗星以与其气体和尘埃产生速率相当的速率(~106g/S)排放大颗粒。彗星雷达的主要目标是获得原子核的延迟多普勒图像。在接下来的20年里，可能会发现11颗短周期彗星，其中几颗可能适合成像。这些可以通过偶然出现的新彗星的近距离显现来补充。
A close-approaching comet can show detectable echoes from its nucleus, or from large coma grains, or both. Nine comets have been detected since 1980 with the Arecibo and Goldstone radars; this includes six nucleus detections and five grain-coma detections. The nucleus radar cross sections span a large range of values consistent with a factor-of-10 range of nucleus sizes. Comparisons with independent size estimates for these comets support this size range and give radar albedos of 0.04–0.1, which is about half the typical asteroid radar albedo. The albedos correspond to nucleus surface densities ~0.5–1.5 g/cm3. Coma echo models based on simple grain ejection theories can explain the radar cross sections using reasonable grain size distributions that include a substantial population of centimeter-sized grains; in one case there is evidence for a cutoff in the size distribution consistent with a gravity-limited maximum liftable grain size. The models indicate that some comets emit large grains at rates (~106 g/s) that are comparable with their gas and dust production rates. The primary goal of cometary radar is to obtain delay-Doppler images of a nucleus. Eleven short-period comets are potentially detectable over the next two decades, a few of which may be suitable for imaging. These could be supplemented by chance close apparitions of new comets.