Faster than the speed of sound..

[jonjel]

Like most of you I watched the man jumping from a great height over Mew Mexico. Brave of foolhardy or both depending on your viewpoint.

However one thing was mentioned, that it was a Helium balloon. If so, why?

In other words why not use Hydrogen, which is a better lift gas and abundant as opposed to Helium which is a depleting resource.

Hardly a fire risk as the gas is in an enclosed Oxygen free envelope, and even if there was a remote possibility of it catching fire from what I saw the chap had a parachute.

[principled]

Jonjel,
I suspect the reason for using helium was to prevent any possible risk of fire at lower altitudes (lightning strike, static) however remote the possibility.
Talking about the balloon, I was was discussing with my son as to what would be the maximum height the balloon would reach, assuming it never burst of course. It only rises if the density inside is lower than that outside, but the higher one goes, the nearer to a vacuum one is getting. When he jumped, the outside pressure was just over 0.1PSI. As I see it, their must be an increasing pressure differential between inside the balloon and out as the outside pressure reaches zero. If not, the balloon envelope would have to extend infinitely to reduce its internal pressure to zero.

One other point. I noticed that as the balloon climbed, the external temperature dropped (minus 45 C) as one would expect and then rose again (minus 16 ish) just before the jump. Bearing in mind space temp. is near absolute zero, I found this temperature inversion strange. I believe this has been discussed previously, but I can't remember the outcome. Anyone care to comment?
P