Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] homemade spacecraft!

Regarding only one part of the thread below, the escape of helium (and hydrogen) into space:

The explanation I have previously encountered is this: these lighter gases float to the top of Earth's atmosphere, where solar radiation provides the energy and momentum to allow these molecules to attain escape velocity. They do not remain in the Earth's atmosphere, but gradually spread through interplanetary (or perhaps interstellar?) space. This occurs for all terrestrial planets, in contrast to the situation for the jovian planets, whose escape velocities are sufficiently high to guarantee the retention of the hydrogen and helium in their atmospheres.

I don't have numbers to back up this explanation, but it seems like it would be a reasonably easy thing to check. What is not clear to me is whether escape events are more likely to occur from collisions with photons (treatable based on the blackbody emission curve and setting up the collision with the photon in a way similar to the treatment of Compton scattering -- except that here we are interested in the recoil velocity of the H2 molecule or He atom) or from collisions with ions making up the "solar wind" (mostly protons and electrons). The Wikipedia article on "solar wind", http://en.wikipedia.org/wiki/Solar_wind, refers to the atmospheric stripping of Mercury and Luna, and the partial atmosphere depletion of Venus and Mars due to the solar wind, giving the impression that this is the main factor involved. It also gives typical velocities and components of the solar wind. Hmmm, I see a reference to a more complicated mechanism involving atmosphere caught in magnetic field being ripped away by the solar wind.

I'd be interested if anyone here can refer us to more information. Has anyone crunched the numbers for either the photon or ion collision mechanism for atmosphere loss? I googled "solar wind atmosphere" and found some interesting hits, including this article from Astronomy, http://www.astronomy.com/asy/default.aspx?c=a&id=9661, that claims that 1/3 of Mars' atmospheric loss occurs during high pressure solar wind pulses.

Highly interesting!

Ken Caviness
Physics
Southern Adventist University

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of chuck britton
Sent: Wednesday, 20 October 2010 10:03 AM
To: Forum for Physics Educators; tap-l@lists.ncsu.edu
Subject: Re: [Phys-l] homemade spacecraft!

I confess to having left the 'lifting power' comparison as a bit of a troll.
Yes H2 is only 8% more effective than He but the fact remains that He is a limited resource that has better uses. And Helium released into the atmosphere does irrecoverably escape into space.

The price of H2 must be quite a bit cheaper as well.

I'm still looking for youtube or such info on a straight unconfined
H2 explosion.

I see fireballs from H2 bubbles.

At 5:37 PM -0700 10/19/10, M. Horton wrote:
"I'm surprised at how many groups are still using He (a VERY limited
and nonrenewable resource) instead of the more buoyant H2 alternative."

Chuck, it sounds like you creeped up on, but didn't quite directly
state a very common misconception about the lifting power of hydrogen and helium.
Many people think that because hydrogen has half the mass (and
therefore half the density) of helium, that it will have twice the
lifting power. But that's not how lifting power (or buoyant force) works.

The lifting power of a gas confined inside of a balloon is the
difference between the mass of the gas and the mass of an equal volume
of air (minus the mass of the balloon). So, if we assume exactly a
22.4L balloon at standard temperature and pressure (i.e. one mole of
gas), hydrogen has a mass of 2.0 grams, helium has a mass of 4.0 grams,
and air (approximately)
28.8 grams. So the lifting power of a hydrogen-filled balloon of this
size would be 26.8 grams and the lifting power of a helium-filled
balloon would be 24.8 grams. That's only an 8% increase. It's
probably not worth the additional cost and danger. A 10% larger helium
balloon would be far cheaper and safer.

I can't comment on the renewability. I doubt that it really went out
of the atmosphere, it would have exploded long before that. So, the
helium is probably still in the atmosphere somewhere.

I have an episode of my Science Misconception Podcast on this topic at
http://scienceinquirer.wikispaces.com.

Mike


----- Original Message -----
From: "chuck britton" <cvbritton@mac.com>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>;
<tap-l@lists.ncsu.edu>
Sent: Tuesday, October 19, 2010 12:30 PM
Subject: Re: [Phys-l] homemade spacecraft!


As someone else has already noted - there's a lot of this going on
these days.
FAA regs allow for lightweight packages with no rigamarole (sp?)

I'm surprised at how many groups are still using He (a VERY limited
and nonrenewable resource) instead of the more buoyant H2 alternative.
> And the H2 adds even MORE excitement to an already exciting
adventure ;-)
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l