[ExI] A Geophysics problem

Dan TheBookMan danust2012 at gmail.com
Wed Sep 30 02:55:14 UTC 2015

On Tue, Sep 29, 2015 at 7:30 PM, Keith Henson <hkeithhenson at gmail.com>

> Sorry to interrupt the high culture fest.
> I was explaining the electric propulsion from LEO up to GEO to someone
> recently and he asked what happens to the 20-25 km/s exhaust.  I
> mentioned that it was well above escape velocity and he pointed out
> that it's being expended in the Earth's magnetic field.  That field
> traps particles with a lot more energy than this exhaust.  I realized
> that I had no idea what would happen to the exhaust.  I have asked
> several people, including a PhD from Ad Astra Rockets.  So far
> nobody has an answer.

That's a good question. I wonder how significant it would be considering
the local environment up there. The plasma density is high at 300 km then
tappers off.

> At peak production about 2 million tons of ionized hydrogen gets
> expended between LEO and GEO every year.

How quickly will that be neutralized before it has a chance to do any
serious interaction? And 2 million tons over the whole area seems rather
small. Plasma density at 300 km is roughly 10^13 gm / m^3. Your daily
output over the whole space is roughly 10^6 gm, so divide by much much than
a cubic meter. I think it'll be trivial.

But it might matter more at certain regions where the exhaust might be
comparable to the amount of plasma or even neutral gas there. My guess,
though, is most of this will quickly dissipate. Even so, I'm no expert.

> It's a geophysics problem.  Can you think of anyone I should ask?
> Also I might note that it is hard to sell people on climate change, it
> is at least that hard to sell a solution that gets humanity off fossil
> fuels without crushing the economy.

If there are any problems too they might be mitigated by changing paths and
schedules. Maybe daytime is best to have solar radiation quickly sweep up
anything. Maybe not.

By the way, I'm relying on Tribble's _The Space Environment: Implications
for Spacecraft Design_ for my numbers. I wonder if there's an empirical
data given that ion propulsion and such have been used before and are used
now for station keeping. They might give some clues, no?


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