Tailless comets could threaten Earth, but they also offer an explanation of the solar system's earliest days.NASA's dart mission was an impressive success.On September 26th, using a probe weighing 600kg, it hit Dimorphos, an asteroid 160 metres across that was, at the time, 11m kilometres away from Earth.The purpose was to determine whether asteroids which might otherwise strike Earth could have their paths altered by what is, in effect, a game of cosmic billiards.Though the results are not yet in, it seems likely that they could.But that depends on seeing them early enough.Asteroids have predictable orbits, so cataloguing them is feasible, and is currently being done.This does not generally apply to comets.Though some (like Halley's) have known orbits, most sweep in from the farthest reaches of the solar system, with little notice of their arrival.And that lack of notice applies, a fortiori, to a newly discovered class of them that do not flag up their appearance in the conventional way, by developing a tail.The first Manx comet, so called after the tailless cats of the Isle of Man, was spotted in 2014 by a telescope in Hawaii.Since then, several dozen more have been detected.And, as a meeting of the American Astronomical Society in London, Ontario, heard on October 5th, they are objects of great interest.
无尾彗星可能对地球构成威胁，但也为太阳系的早期形成提供了解释。NASA的“飞镖”任务取得了重要成功。9月26日，重达600公斤的“飞镖”探测器击中了一颗直径160米的小行星迪莫弗斯，当时该小行星距离地球1100万公里。其目的是确定可能撞击地球的小行星是否会因为一场本质上属于“宇宙台球”的游戏而改变其运行轨迹。虽然任务结果还没有公布，这似乎是可行的。但这取决于人类观测到小行星的时间是否足够早。小行星的轨道是可预测的，因此可以对它们进行编目，目前这项任务正在进行之中。但彗星不同。虽然有些彗星(如哈雷彗星)的轨道已经确定，但大多数彗星是从太阳系最远的地方接近的，人类几乎无法观测到它们的来临。最重要的是，这种观测不到的情况在新发现的一类彗星上更加突出，此类彗星和传统彗星外表不同，不会产生彗尾。2014年，人类利用夏威夷的一台望远镜发现了第一颗无尾彗星，并将其以马恩岛上的无尾猫命名，称为“马恩岛猫彗星”。从那时起，人类又发现了几十颗无尾彗星。正如10月5日在安大略省伦敦市举行的美国天文学会会议上所称，人们对无尾彗星非常感兴趣。
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