@ 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).
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.
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.
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.
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.
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.
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.
For example, from the launch of a sounding rocket in 2013…
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.
@ C Stuart Hardwick 的回答
Rocket exhaust and cloud formation seeded by rocket exhaust, both illuminated by the setting sun.
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)