system
Given the following post, respond as if you're commenting on Reddit:
human
How would Superman's cape actually behave if he was flying in space?
gpt
If Superman is "flying" in space, we can remove two of the effects that usually act on a cape: gravity and aerodynamic effects. We're left with the forces on the part of the cape attached to Superman's neck, which will generally follow his flight, and internal friction forces in the material which will eventually damp out any oscillations in the cape, dissipating that energy as heat.

So, let's assume he's going to accelerate in one direction and then just coast. As he accelerates, the same magical force also causes the top part of the cape to accelerate in the same fashion, and the rest of the material will follow suit as the fabric is pulled taut. This will introduce some crazy, unpredictable motions in the cape, but due to the aforementioned internal friction, this motion will be damped out somewhat quickly. Now, we have a cape that is extending straight behind its attachment at the neck with no flapping or movement, much in the same way that a short piece of string held up in the air will swing and oscillate but eventually come to rest along a vertical line.

But this isn't the end of the story, if we assume he's going to stop accelerating at some point. When he does, the force on the neck of the cape stops, and, assuming the cape has some stretch to it, the energy stored in those little "springs" in the material will be released, pulling the material back in the direction in which it was previously being tensioned. This motion will be damped out again, but not before the cape has become more or less rumpled up along Superman's back depending on how springy its material is. Then, it holds that shape until he decides to accelerate again (in any direction).

In the end, Superman just kinda looks like crap, with a rumpled cape that isn't streaming behind him as expected.