system
Given the following post, respond as if you're commenting on Reddit:
human
What is the mechanism behind the theory that Uranus' tilt was caused by an impact?

The current theory as to how Uranus tilted is that it was hit by a large object that caused a drastic change in its orientation.
This has always puzzled me. I would think that being a large ball of gas, an "impact" would be different from that of a solid planet, that there's nothing there to hit and it would simply plunge into a cloud of gas.
Does gas at that mass behave differently? Is the impacting object so large that it doesn't matter what it's interacting with? Or am I just fundamentally misunderstanding some basic aspect of physics/fluid dynamics?
gpt
When two things collide, conservation of momentum (both linear and angular) will hold. It doesn't really matter what they're made of; rock, gas, water, a person, whatever. 

If Uranus was spinning normally, west-to-east (let's call that angular momentum +1) , and something large came in fast retrograde (so, going east to west, so its angular momentum around the system's center of mass is negative), and they collided in *any way t*hat resulted in a single object after the dust settles, then the angular momentum of that final object will be the original +1, added to the incoming object's X, where if X is <-1, you now get a backwards rotating planet. Uranus might not have been exactly this scenario, and of course the incoming object would probably have angular momentum around a different axis too, but, same idea.