Saturday, October 20, 2012

Extreme Skydiving

A couple days ago, I got the following email (fmail? it came via Facebook) from a non-space friend:

Ok, I have a space question for you....! So when the space shuttle re enters the earth.. There is a point when it is very hot and the bottom of the shuttle is the shield for the jumping over to this guy that just jumped from 23 miles up, .. It looked as though he was just about in could see the curve of the earth. Did his balloon not reach the barrier that the shuttle encounters? How does that work when a rocket or balloon leaves the earth? Do they still encounter the same restrictions as the re-entry?

Just in case I get similar questions from other non-space friends, I thought I'd take a shot at answering this publicly (my friend agreed). So what follows is a combination of what I know with what I had to research to "get it right." If you're a space-minded friend and you find something grievously wrong, by all means, let  me know and I'll be certain to correct it.

Shuttle Tiles
The Space Shuttle reaches a speed of 17,500 miles per hour or so to reach a 250-mile-up orbit. It has a thermal protection system--broken up into thousands of tiles--to protect it from the friction of flying that quickly through the atmosphere. An example people can relate to in their daily lives is if they stick their hand out the window of an accelerating car--the faster you go, the more heat builds up on your hand due to collision with more air molecules. Your hand can get pretty warm just going from 0 to 55 miles per hour. Now imagine going from 0 to 17,500 miles per hour. There is little to no air at the altitudes the Shuttle flew--the air only gets thicker the closer you get to Earth--so the combination of very high speed and increasing friction generates temperatures over 3,500 degrees Fahrenheit.

The shuttle uses that atmospheric friction to slow down to a safe landing speed. Going back to the car analogy, your hand builds up more heat and air resistance if it's held perpendicular to your forward motion, less, if your palm is facing down toward the ground.

Felix Baumgartner and High-Altitude Skydiving
The official boundary of "space" is around 62 miles (100 kilometers) above sea level. Above that level the thickness of Earth's atmosphere is negligible. It's not much thicker at 23 miles (~121,000 feet), but the atmosphere is more or less starting at that level.

There are a few things affecting the "re-entry" heat on the Shuttle vs. a high-altitude skydiver:
  • Size/Mass: The larger the object, the more air resistance. Sort of like the difference between running against a strong wind alone and running against a strong wind while carrying a large board or poster. The Space Shuttle is much larger and heavier than a human being and reenters the atmosphere at a high angle of attack to expose as much of its underside as possible to the air to slow down.
  • Speed: Baumgartner was coming down with an initial speed of zero, as opposed to the Shuttle's 17,500 miles per hour, so he had a much lower initial velocity. His maximum velocity was 824 miles per hour, so he wasn't generating nearly the same amount of heat.
While it appears Baumgartner's suit was pressurized and insulated to some extent, it didn't need to withstand nearly the heat or pressure that the Shuttle did. However, there are folks who are interested in doing skydiving from orbit, which would require a much more durable suit. They almost included a stunt like that in Star Trek: Generations.

Regardless of all this, the Baumgartner dive was a remarkable feat, one that hasn't even been attempted since the 1960s. The idea is that his dive will provide the basis for safe recovery of an astronaut from a launching or reentering spacecraft if there is an emergency and time to bail out.

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