Free Falling | by Michael Ross

Yesterday, Austrian skydiver Felix Baumgartner broke the world record for the highest manned balloon flight and highest skydive. He was also likely the first human to exceed the speed of sound without the assistance of a vehicle. While the bravery and personality of Mr. Baumgartner is undeniably impressive, the science behind the system that brought him all the way up there is also fascinating.

The Numbers:
The jump took place from a massive helium balloon at an altitude of 39.05 kilometers above sea level; well into the earth’s stratosphere. Here, the temperature is typically around -35 Celsius, and the air pressure is only about 0.1% of what it is down here on dry land. In fact, Felix would be far enough from the surface of the earth that the gravity he’s experiencing is 2% less than what we’re stuck with down here. On the trip up he passed the infamous Armstrong Limit – the altitude where the atmospheric pressure is low enough that your body heat is enough to boil water. Needless to say, a pressurized suit was a critical component of the trip.

OH NO! Kitty forgot his spacesuit! Silly kitty.

The balloon used to get Baumgartner up was a 30 million cubic foot helium balloon made out of 0.0008 inch thick polyethylene (in news reporter units, that’s around 20% of a human hair). When un-inflated, the balloon alone weighs 3,700 pounds. When it’s inflated the balloon is over 330 feet tall.

The science of weather balloons themselves is actually pretty cool. The previous record for extreme altitude jumps was 102,800 feet with a balloon that had only 3 million cubic feet of gas. Baumgartner’s balloon was ten times larger, but he was only able to jump 25% higher than the previous record. The extreme change in pressure in the surrounding air at this height causes the balloons to expand as the helium gets to a higher relative pressure than the outside. These expanding forces can cause balloon failure, and this limits the height the balloon can reach. Balloons are also severely limited just based on the thinness of the atmosphere – quite simply, nothing can float unless it’s less denser than the air it’s displacing; at these altitudes there just isn’t much air to displace.

The coolest record that Baumgartner broke yesterday was breaking the speed of sound by simply falling. Under normal circumstances this is impossible. A normal skydiver can only ever reach about 15% of the speed of sound because the force pulling on you due to gravity gets balanced out due to the speed of the air pushing against you as wind. Baumgartner was only able to break this because the air was so thin where he started that he essentially had no wind resistance for the first couple of minutes of his fall (in fact, he was on record saying the lack of wind was particularly disorienting as he couldn’t control his direction of fall). Because of this, during the free-fall portion of his skydive he would have started off speeding up, but ended up slowing down as the air got thicker and thicker. Very cool indeed.

For more information on the Stratos Jump, check out the official site here.

Michael Ross is a master’s student in structural engineering. Get a hold of him on twitter @Mikerobe007or at his blog !

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