在2017年10月22号SpaceX飞过洛杉矶后,是什么造成了30分钟后还有发光的云?In the SpaceX rocket seen over L.A. on Dec. 22, 2017, what explains the light emitting cloud that lingered for 30 minutes afterwards?

@ Jeffrey Naujok 的回答

The simple answer is ice.

The same effect, when it happens naturally because of a massive updraft of moisture, produces what are known as “noctilucent clouds” (literally: night-light clouds).

当大量上升的湿气流时,会产生的同样的效果,即“夜光云”(字面意义就是在晚上发光的云)。

These clouds are lit by the sun, because they are miles higher than the ground, and thus the sun hasn’t “set” for them yet.

这些云能被太阳照亮,因为它们比地面高出英里,因此太阳虽然已经落入了地平线以下,但对这些云来说还不算“下落”。

The Falcon 9 burns RP-1, which is basically kerosene, and liquid oxygen. This results in an exhaust plume composed almost entirely of carbon dioxide and water in the form of super-heated steam.

猎鹰9用RP-1做燃烧剂,它基本上是煤油和液氧。这会导致排气出来的几乎完全是由二氧化碳和水构成的超热蒸汽形式。

The exhaust plume, several miles up in the atmosphere when the second stage ignites, expands rapidly into the near-vacuum at the top edge of the atmosphere.

排气羽烟,有一些在第二阶段点火时向上爬升几英里,然后迅速膨胀到在大气的顶部边缘,接近真空。

As a gas expands, it cools rapidly. In this case, it expanded into a plume several miles across. That gets it very, very cold, and the water turns to ice.

当气体膨胀时,它迅速冷却。也就是说,它膨胀到了几英里宽,但是非常非常冷,能把水变成冰。

That high in the atmosphere, there’s very little turbulence, and the air is very cold anyway, so the tiny ice crystals hang in the air, barely moving.

大气中的那么高,有很少的紊流,所以空气非常冷,这样微小的冰晶悬在空中,几乎不会动。

The SpaceX launch simply created an artificial version of these noctilucent clouds. This is actually a very well known phenomenon when rockets are launched just after sunset.

SpaceX只是创建了这些夜光云的人工版本。这实际上是在日落之后发射火箭的非常常见的现象。

For example, from the launch of a sounding rocket in 2013…

例如,在2013发射的一枚探空火箭…

Or this one from a Minuteman II that was also launched from Vandenberg in September of 2002.

或者这一个在2002年9月从范登堡空军基地发射的 Minuteman II 。

The main difference with the SpaceX launch was that the ten Iridium satellites needed to be launched into a polar orbit, so the launch was nearly straight South, rather than the typical easterly tracks for most rockets, or the westward launches of missile tests out over the ocean from Vandenberg. The southerly track created a large, broad-side view of the plume that was very high, and very apparent because it covered a huge swath of the sky.

SpaceX公司的发射与别的发射明显的区别是,十颗铱星计划的卫星发射到极地轨道,所以推出几乎笔直向南,而不像是大多数火箭弹或导弹向东发射,或者从范登堡向西发射越过海洋的测试导弹。向南发射会导致又大又宽的羽流,也非常高,透明度很高,因为它遮住了很大一片天空。

@ C Stuart Hardwick 的回答

Rocket exhaust and cloud formation seeded by rocket exhaust, both illuminated by the setting sun.

火箭排气或者由火箭排气形成的云,被日落时的太阳照亮了。

What some folk don’t seem to realise is that when the sun has set at ground level it’s still shining brightly up in the stratosphere. (Thanks Malcolm)

很多人没意识到的时,太阳下落时虽然已经落下了地平线,无法再看见,但它仍然照亮着平流层。(谢谢Malcolm)。

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