Comments on: Space Leap of Faith

By: Ben H.

Ben H. — Mon, 05 Nov 2012 12:43:42 +0000

Phil,
You basically channeled this post from my blog back in March:

http://www.rocketsfromcassiopeia.com/2012/03/how-i-learned-to-stop-worrying-and-love-the-gap/

Well said!

- Ben H.
Mission Control, TX

By: Sili

Sili — Mon, 29 Oct 2012 23:42:22 +0000

Sorry, but the leap of faith is 128 km.

By: Mathew

Mathew — Fri, 26 Oct 2012 12:28:36 +0000

You know, I think this meme actually started in Australia not the US. It’s completely true in that case.

By: “That awkward moment when you realize a an energy drink has a better space program than your nation.” | A Singularity

Mon, 22 Oct 2012 12:34:16 +0000

[...] Why? Because of several reasons, as enumerated by Phil Plait of Bad Astronomy in one of his articles. [...]

By: Gunnar

Gunnar — Mon, 22 Oct 2012 07:51:49 +0000

@James #85

I’m sure they meant the “local speed of sound” at high altitude, which is certainly less than the speed of sound at sea level, but since his top speed during the fall was more than 800 mph, he clearly exceeded the speed of sound, even if referring to the sea level speed of sound.

By: Congratulations … | Glory, Gold and Honor

Congratulations … | Glory, Gold and Honor — Mon, 22 Oct 2012 00:50:22 +0000

[...] here – http://blogs.discovermagazine.com/badastronomy/2012/10/16/space-leap-of-faith/ [...]

By: James

James — Sun, 21 Oct 2012 18:14:23 +0000

I’ve been a little curious about that “speed of sound” thing – do they mean he exceeded the sea level speed of sound, or that he exceeded the local speed of sound?

Don’t Mach numbers cease to make sense somewhere around the altitute He jumped from?

By: Gunnar

Gunnar — Sun, 21 Oct 2012 11:41:59 +0000

@Rift
Check out this nifty NASA website. http://www.grc.nasa.gov/WWW/K-12/airplane/termv.html. It features a nifty calculator for determining terminal velocities at various altitudes. You will quickly see that while terminal velocity for a 150 lb (man sized) object with a cross sectional area of 5 sq feet and a coefficient of drag of 0.7 is only about 189 ft/sec (130 mph) at sea level, Baumgartner fell through altitudes where the terminal velocity greatly exceeds the speed of sound. Remember, terminal velocity is merely the velocity at which the drag force (which varies as the square of the velocity and with the density of the air through which one is falling) equals the weight of the falling object.

By: Gunnar

Gunnar — Sun, 21 Oct 2012 10:42:04 +0000

@Rift

Terminal velocity is determined by air resistance and varies according to the density of the air one is falling through, which in turn varies with altitude. Obviously, terminal velocity would be very much higher when falling through the near vacuum at 120,000 feet, and could very easily exceed the speed of sound at or near that altitude. By my calculations, the initial acceleration of a falling body would be between 31.7 and 31.8 feet per second squared, which is not much less that the 32.2 feet per second squared (1 g) we experience on the earth’s surface. The speed of sound is in the neighborhood of 760 mph at sea level, and is somewhat less than that at high altitudes (due to the lower temperature there).

Neglecting air resistance (which would be very low at that altitude, anyway) it would only take aproscimately 35 seconds or so to reach 760 mph after jumping, and he would have fallen only 20,000 feet by the time he reached that velocity, which is still 100,000 feet up, where the air is still very thin, compared to sea level. According to his blog, he passed the speed of sound 33 seconds after beginning his fall, which makes sense when you realize that the speed of sound at that altitude is less than at sea level (somewhere in the neighborhood of 670 to 680 mph IIRC correctly, or maybe even a bit less). 33 seconds of acceleration at 31.7 feet per second squared would give him a velocity of 713 mph. That his velocity was actually somewhat less than that after 33 seconds also makes sense, since at that high velocity, the atmospheric drag would begin to become noticeable even at that high altitude, and would have slowed his acceleration rate somewhat.

So, in summary, given the acceleration due to the earth’s gravity and the thinness of the air at his jumping altitude, it is entirely believable that he could have reached the maximum velocity he claimed before hitting the denser altitude near sea level and deploying his parachute.

By: Rift

Rift — Fri, 19 Oct 2012 17:48:29 +0000

This was a mere stunt, didn’t whistle or shout for the guy, didn’t watch, don’t care. Same category as Evil Keneval jumping the Snake River Canyon. Yawn.

Are you SERIOUSLY comparing this to the moon landings???

I doubt seriously he broke the sound barrier, terminal velocity would have stopped him from traveling faster than sound.