无尾彗星可能对地球构成威胁(上)

Tailless comets could threaten Earth, but they also offer an explanation of the solar system's earliest days.

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's dart mission was an impressive success.

NASA的“飞镖”任务取得了重要成功。

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.

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。

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.

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。

Though the results are not yet in, it seems likely that they could.

虽然任务结果还没有公布，这似乎是可行的。

But that depends on seeing them early enough.

但这取决于人类观测到小行星的时间是否足够早。

Asteroids have predictable orbits, so cataloguing them is feasible, and is currently being done.

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。

This does not generally apply to comets.

但彗星不同。

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.

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。

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.

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。

The first Manx comet, so called after the tailless cats of the Isle of Man, was spotted in 2014 by a telescope in Hawaii.

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”。

Since then, several dozen more have been detected.

从那时起，人类又发现了几十颗无尾彗星。

And, as a meeting of the American Astronomical Society in London, Ontario, heard on October 5th, they are objects of great interest.

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。

The chance—albeit remote—that such a comet might hit Earth is one reason.

这种彗星撞击地球的可能性——尽管很小——是其中一个原因。

Another is that they are a surprising source of evidence for what happened in the solar system's earliest days.

另一个原因出人意料——它们是说明太阳系早期形成发展情况的来源之一。

Manx comets, like their tail-wagging brethren, come from the Oort cloud.

马恩岛猫彗星，和它们带有慧尾的兄弟一样，来自奥尔特云。

This is a spherical shell of debris left over from the solar system's formation that is way farther from the Sun than the farthest planet, Neptune, and farther even than the Kuiper belt, a disc-like repository of asteroids and minor planets, the most famous inhabitant of which is Pluto.

这是一个由太阳系形成时遗留下来的碎片组成的球体，它和太阳的距离比离太阳最远的行星海王星还要远得多，甚至比柯伊伯带还远，柯伊伯带是一个圆盘状的小行星聚集区域，其中最著名的是冥王星。

The Oort cloud is thought to contain trillions of icy chunks, some of which, because of a collision or the pull of a passing star, from time to time fall inward.

人们认为奥尔特云包含数以万亿计的冰块，其中一些由于碰撞或擦肩而过的恒星的引力，不时向太阳系内坠落。

As these inbound bolides approach the Sun, their ice sublimes, gas and dust escape, and a comet spreads its tail.

当这些冰块接近太阳时，冰块开始升华、形成气体和尘埃，一颗彗星就这样展开了它的尾巴。

A Manx comet, by contrast, is presumably a solid (and thus ice-free) lump of rock.

相比之下，马恩岛猫彗星很可能是一块固体的(不含冰的)岩石。

A tailed comet on a collision course with Earth might possibly be detected early enough for a successor to dart, stationed in outer space, to rush in and push it out of harm's way.

如果存在一颗会与地球相撞的彗星，会拖尾的彗星可能很快就会被发现，驻扎在外层空间的“飞镖”的继任者就能及时赶到，把它推到不会危及地球的地方。

For a Manx comet, the warning time would be hopelessly short.

而如果是马恩岛猫彗星，预警时间将短得令人绝望。