Though classic sci-fi movies often depict extraterrestrial beings visiting Earth in spinning disks, astrophysicist Neil deGrasse Tyson explained Thursday that only stupid aliens who didn't understand the laws of physics would build such an interstellar craft.

"There are some fundamental problems with an entire rotating ship," he told co-host Eugene Mirman on StarTalk Radio. "If you set it into rotation, something else has to be set rotating the opposite direction. It's a little thing, you learn it in the second week of physics 101... it's the conservation of angular momentum."

The physical law holds that if one part of a system is given angular momentum, then some other part of that system must simultaneously be given the same angular momentum in the opposite direction.

"The laws of physics are quite commanding -- or rather quite descriptive -- because the universe doesn't obey our laws," Tyson added. "The laws describe what we see and one of the deepest laws of physics we know is the conservation of angular momentum. Once a system has a certain amount of momentum, it never changes within itself unless something from outside of it touches it."

"So, I don't buy it," he concluded. "Why spin at all? I think it's because -- when did the Frisbee come out?"

Watch video, uploaded to YouTube, below:

-- --

[Flying saucer via Shutterstock]