Fusion Message Board

In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.

Subject: Vacuum/Earth Trivia
Date: Jul 27, 1:18 pm
Poster: Richard Hull

On Jul 27, 1:18 pm, Richard Hull wrote:

Many of us never make a very firm mental link with vacuum, gas molecule density and the Earth's atmosphere and space's lack of one. It turns out that even interplanetary vacuums contain a lot of molecules/cc

The verbal bombast which follows is a form of digestable data reduction from a singularly unique and very handy chart which graced the back cover of Electric Spacecraft Journal a couple of years ago.

1. At sea level there are about 2X10e19 gas molecules in each cubic cm of space. That is
over 20 quintillion molecules banging against one another at hypersonic velocities that would put to shame the fastest rifle bullet. All in a 1/3 inch cube!

2. At sea level there are very few free electons, being less than 10 per cc.

3. Thirty miles up, in the stratopause, we have a near equivalent of the Martian atmosphere at just a few mm or torr or pressure. There are still about 100 quadrillion air molecules per cc at this level.

4. At 50 miles up the atmosphere is just thick enough so that things moving 5 miles/second heading down start to heat up due to friction with the air molecules. (meteors start to burn - Spacecraft re-entering heat up). There are now only a mere 100 Trillion molecules per cc and the pressure is at 10e-2 mm (torr) or 10 microns. This is about the pressure that most mechanical vacuum pumps start balking as they are out of transitional flow and deep into molecular flow.

5. At 60 miles (beginning of the E Region) the pressure is on the order of 1 micron with still about 10 Trillion or so molecules in every cc of space. This is where most simple fusors are fusing and represents the absolute realizable, sustainable, limit of a well maintained, standard, two stage mechanical pump.

6. The E Region is the region where aurora occur and extends from 60 miles (10e-3 torr) to ~90 miles (10-5 torr). At the very extreme high end of this region, we still have about 100 billion gas molecules jammed into that same little 1/3 inch cube. This is about the level of operation of a vacuum system sporting a poorly operated oil diffusion pump. This is also the area (10e-5 torr) where the practical limit of the earth's atmosphere is reached and a near frictionless orbit is possible. (frictionless!!!? with 100 billion molecules/cc?.........You Betcha).

7. The space shuttle normally orbits in the upper F1 region at around 130 to 140 miles. The outside pressure is about 10e-6 torr or in the middle of the high vacuum range about where a decent diffusion pump works without operational heroics or cryotraps. This is about where most ion pumps are turned on to go to higher vacuums.
there ae now only one Billion gas atoms in a cc or space. (still sounds like an awful lot..don't it?)

8. Our best diff pumps play out some where are about 180 miles and 10e-7 torr. This is the middle of the F2 region. Still over 100 million molecules/cc. Interestingly, the number of free electrons in the atmosphere peaks in this region after steadily rising with altitude. There are about 10 million free electrons/cc at the transition of the F1 and F2 regions 155 miles up. This makes this the most conductive portion of our atmosphere. Free electrons will now slowly decline with altitude as the number of gas atoms decrease.

9. At about 250 miles where the space station MIR orbits, we have a vacuum of about 10e-8 torr.

10. The Hubble Space Telescope orbits at about 370 miles and the pressure is near 10e-9 torr or about where UHV (ultra-high vacuum)starts for the professional vacuumist. We are now in the range of millions of molecules/cc.

11. At 450 miles we reach the pressure of about 10e-10 torr. Between 450 and 600 miles the pressure can rise or fall depending on solar wind and other variables.

12. Above 600 miles the pressure falls off to 10e-11 which is about the lowest I have personally ever witnessed in a pulled down vacuum system (not mine). This is near the upper limit of worldly vacuum systems with all the bells and whistles such as ion pumps, Ti sub pumps and other getter and cryo traps. Also most gauges of even the most sophisticated types get "flakey" beyond here.

13. The finest and most meticulously maintained vacuum systems can hit the deep space vacuum of 10e-12 torr which represents only a few tens of thousands of gas atoms/cc.

14. I am given to understand that, with ungodly expensive, super cooled, liquid He at below 4 degrees kelvin, the Jefferson National Labs here in Virginia, which has the most potent continuous electron beam accelerator in the world, the vacuum they achieve is just simply not measurable and is certainly greater than 10e-14 torr. The head engineering expert at CBAF who attended our meetings when he was here in VA, said that if their best gauging efforts read anything at all they knew they had to keep pumping. He estimated fewer than several hundreds of remenant atoms existed per CC. This level is most likely a nearly interstellar level of vacuum, but no one has ever really measured that.

Richad Hull