If You Dropped a Corn Kernel From Space, Would it Pop During Re-Entry?

Popcorn in Space Gary C. Knapp/AP Photo

There’s a little bit of water inside each kernel of popcorn, and if you can heat the kernel above 212°F, that water should boil, turn into high-pressure steam, and pop the kernel. But in orbit, things aren’t so simple. First off, the cold vacuum of space would suck all the water out of the kernel before it could pop the corn. So any ordinary kernels would drop, not pop. But let’s say we figured out a way to keep the kernel watertight. In that case, it all depends.

Anything falling through the atmosphere has what’s called a terminal velocity. This is the speed at which the upward force, or drag, from air resistance equals the downward force of gravity. Typically, a falling object, like a skydiver, speeds up until it reaches terminal velocity. If something like the space shuttle starts out in the airless vacuum of space, it can reach a speed higher than its terminal velocity. But as soon as it starts passing through the atmosphere, friction will slow it down. This friction generates heat—about 3,000° for the space shuttle. Just how much heat depends on how fast the object is going, as well as its size, shape and mass.

If an astronaut were to throw a watertight kernel out of that space shuttle moving at 17,000 mph, would the kernel reach hot enough temperatures to pop as it flew through the atmosphere? It’s possible, says Kenneth Libbrecht, a physics professor at the California Institute of Technology, but he can’t run the numbers to say for sure, because no one has measured how much friction a kernel generates when it moves through the air. “Of course, the other possibility is that it will heat too quickly and the outer husk will burn off before the kernel has a chance to pop,” Libbrecht says. And so, for now at least, there’s no way to know. Note to the guys on the ISS: Let loose a pan of Jiffy Pop.

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Technology, Stuart Fox, fyi, january 2008, popcorn, space, stuart fox