I enjoyed reading this this morning.
From the XKCD: What If? series : http://what-if.xkcd.com/
Question:
I bike to class sometimes. It's annoying biking in the wintertime, because it's so cold. How fast would I have to bike for my skin to warm up the way a spacecraft heats up during reentry?
—David Nai
Answer:
Reentering spacecraft heat up because they’re compressing the air in front of them (not, as is commonly believed, because of air friction).
According to this calculator, to increase the temperature of the air layer in front of your body by twenty degrees Celsius (enough to go from freezing to room temperature), you would need to be biking at 200 m/s.
The fastest human-powered vehicles at sea levels are recumbent bicycles enclosed in streamlined aerodynamic shells. These vehicles have an upper speed limit near 40 m/s. This is the speed at which the human can just barely produce enough thrust to balance the drag force from the air.
Since drag increases with the square of the speed, this limit would be pretty hard to push any further. Biking at 200 m/s would require at least 25 times the power output needed to go 40 m/s.
At those speeds, you don’t really have to worry about the heating from the air—a quick back-of-the-envelope calculation suggests that if your body were doing that much work, your core temperature would reach fatal levels in a matter of seconds.
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