(urth) Dome, Dome on the Range

[Jerry Friedman]

> From: Gerry Quinn <gerryq at indigo.ie>

> From: "Jerry Friedman" <jerry_friedman at yahoo.com>
> >>  From: Gerry Quinn <gerryq at indigo.ie>
...

> >> If  an atom  leaves the top of Lune's atmosphere at lunar escape velocity, 
>the
> >>  question of whether it is likely to end up in Urth's atmosphere on Urth  
>depends
> >> on whether it exceeds Urth escape velocity at this point  (unless it's 
>headed
> >> directly for Urth, which is unlikely.
> > 
> > [snip calculation]
> > 
> > I like it, but you need to add in  the orbital velocity of Lune, which makes
> > molecules substantially more  likely to escape.  (This could actually be a 
>bigger
> > problem than  air ending up back on Urth, where as you note it's not lost.)
> 
> It doesn't  affect their likelihood to escape.  All motion is relative.

Sorry, what I wrote was unclear.  I meant their likelihood of escaping from the 
Lune-Urth
system.  That depends on their velocity relative to the center of mass of the 
system,
which is their velocity relative to Lune plus Lune's orbital velocity.  
(Velocity vectors, of
course.)

> >  Also, molecules that escape from Lune but not the Lune-Urth system will be  
>in
> > orbits that intersect Lune's orbit where they left it. Except for the  
>relatively
> > small portion aimed at Urth, they're not going to come to  rest on Urth 
until
> > their orbits are modified by collisions with other  molecules or brushes 
with
> > Lune.  I suspect they're a good deal more  likely to come to rest back on 
>Lune
> > than on Urth.
> 
> Those are  molecules that don't escape.  The ones we care about are ones which 
>achieve  escape velocity near the top of the lunar atmosphere, where the 
>atmosphere is  very thin (mostly these will be hydrogen molecules or atoms)..  
>In this  case they are unlikely to collide with anything and will escape Lune.  
>If  the top of the atmosphere is less than 3000 miles above the lunar surface, 
>they  will be fast enough to escape Urth as well.
> 
> I'd expect them to become  widely dispersed in a sphere around the region of 
>Urth's orbit, and eventually  to be blown away by the solar wind.

I expect you're right, but Jeff was assuming molecules that escaped from Lune 
and
didn't escape from the system would end up on Urth.  In fact, I think a good 
fraction of
them would make some orbits around the Lune-Urth center of mass and then collide 
with
Lune again, since their orbits all intersect Lune's orbit but don't all go near 
Urth.

Jerry Friedman