Imagine you are an astronaut aboard the International Space Station. Your “day” is taken up mostly by conducting experiments, making repairs, and communicating with Earth. But you’re in space! You can’t help but take some time to explore the physics that can only be seen while perpetually falling around our planet. And one of the cooler things you’ll see happens as a result of just spinning something around:
What is going on here? Why is this small handle flitting around like an extraterrestrial hummingbird? In a recently Q&A with astronaut Scott Kelly, we actually got to ask him a question about this odd rotation while he was orbiting above us:
The motion does look like something that could only happen in space, but what is going on here is actually a famous principle from 19th century classical physics that applies everywhere — the intermediate axis theorem.
Imagine flipping a tennis racket down here on Earth. It can spin without tumbling erratically along two of three principal axes, y and z (passing through the COM or Center of Mass):
In this example, the most stable rotations happen around the y and z axes. However, try to spin the racket around the x axis, like you might do towards yourself after rocketing a ball down a baseline, and the racket tumbles and spins before returning to your hand. This is because the x axis is the intermediate axis of rotation, in between the minor and major axes. Because the racket is compelled by physics to return to spinning around the y or z axis, any extra movement flips the racket and causes it to tumble.
So the Leatherman tool and the ISS handle above are tumbling and flipping between axes because the astronauts are giving each spin a bit of a wobble that the objects are exquisitely sensitive to along their intermediate axes of rotation!
On Earth, we can’t really see this weirdness when we’re flipping tennis rackets because there isn’t enough time to see it (and there’s that pesky thing called gravity). That’s why space is so awesome — it’s a playground for science.
