Source: https://youtu.be/l8W19r5m_GU?t=2605
At 43:24 in the above live stream of the SpaceX CRS-19 Launch, something slides around the engine. Someone remarked that this looked a bit like a mouse, and some of the more imaginative conspiracy theorists took this to be evidence that the footage was faked.

The video is shot from two sides, alternating between viewpoints, which makes it difficult to follow exactly what is going on. But it's much clearer if we take just the shots from the "mouse" side, zoom in, and speed them up 10x.
Source: https://www.youtube.com/watch?v=tnbSMjFebPk
A real time version of this is here.
Here we see the "mouse" build up at the top, then break free and slide around. It's followed by a bunch more of what looks like slushy ice (possibly actual ice - or some other byproduct of the jet engine that built up around that point, maybe from what is venting near the top of the image). This slides around and looks like it maybe melts a bit.
So, a fascinating display of space physics. But not a mouse.
[Update Jan 18 2020]
It's not ice, it's actually solid oxygen. The white jet at the top is venting liquid oxygen, which is what causes the build-up of solid oxygen.
People why the oxygen does not just boil off into space. The answer is that some of it does, and what's left freezes. The principle is called "Vacuum freezing". Below the triple point of oxygen (or water, etc), it can only exist only as a solid and gas. A vacuum is obviously below the triple point in terms of pressure. So in the vented liquid oxygen the high energy molecules rapid boil away, cooling down the remaining liquid, which will solidify. Putting liquid oxygen into a vacuum is actually HOW YOU MAKE solid oxygen. Here's a demonstration with oxygen:
Source: https://www.youtube.com/watch?v=6RRLBD1bB3E
And similar with water:
Source: https://www.youtube.com/watch?v=Ti9C_cLSR0A
At 43:24 in the above live stream of the SpaceX CRS-19 Launch, something slides around the engine. Someone remarked that this looked a bit like a mouse, and some of the more imaginative conspiracy theorists took this to be evidence that the footage was faked.
The video is shot from two sides, alternating between viewpoints, which makes it difficult to follow exactly what is going on. But it's much clearer if we take just the shots from the "mouse" side, zoom in, and speed them up 10x.
Source: https://www.youtube.com/watch?v=tnbSMjFebPk
A real time version of this is here.
Here we see the "mouse" build up at the top, then break free and slide around. It's followed by a bunch more of what looks like slushy ice (possibly actual ice - or some other byproduct of the jet engine that built up around that point, maybe from what is venting near the top of the image). This slides around and looks like it maybe melts a bit.
So, a fascinating display of space physics. But not a mouse.
[Update Jan 18 2020]
It's not ice, it's actually solid oxygen. The white jet at the top is venting liquid oxygen, which is what causes the build-up of solid oxygen.
People why the oxygen does not just boil off into space. The answer is that some of it does, and what's left freezes. The principle is called "Vacuum freezing". Below the triple point of oxygen (or water, etc), it can only exist only as a solid and gas. A vacuum is obviously below the triple point in terms of pressure. So in the vented liquid oxygen the high energy molecules rapid boil away, cooling down the remaining liquid, which will solidify. Putting liquid oxygen into a vacuum is actually HOW YOU MAKE solid oxygen. Here's a demonstration with oxygen:
Source: https://www.youtube.com/watch?v=6RRLBD1bB3E
And similar with water:
Source: https://www.youtube.com/watch?v=Ti9C_cLSR0A
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