Skydiver Leaps From 18 Miles Up In 'Space Jump' Practice

3. I understand objects falling from that altitude tend to encounter very little air resistance, which means they pick up a lot of speed. The kind of speed that causes brilliant fireballs to appear in place of anything falling from that height, like asteroids, satellites, and space shuttles.

Not really. Most things that cause brilliant fireballs have a very high initial velocity (and kinetic energy) which must be dissipated when they first reach the atmosphere. This skydiver started with a vertical velocity of zero.

And since the density gradient of the atmosphere is low, a skydiver's air resistance will build up slowly bleeding off this energy gradually.

All that must be done is to bleed off the skydiver's potential energy. For a 115kg (person + gear. I'm pulling figures out of my *ss here) at 29,500m altitude, this is aprox. 32,700 Joules. Dissipated in 630 seconds, this is an average rate of 51 Watts. Warm, but not out of line with being wrapped in an electric blanket.

That same individual hitting the atmosphere at 7750 m/sec (Shuttle re-entry velocity) would have kinetic energy of 3.45E9 Joules. Over 630 seconds this would be 5.5 megawatts, although the 630 second figure does not represent the re-entry time anymore. That time would be less, giving a higher average dissipation rate. And nothing but a few ashes reaching the ground.

Source: http://rss.slashdot.org/~r/Slashdot/slashdotScience/~3/Y5xSWhwN3K4/skydiver-leaps-from-18-miles-up-in-space-jump-practice

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