(urth) Cumaean???

[thalassocrat at nym.hush.com]

On Sun, 16 Dec 2007 11:35:16 +1100 thalassocrat at nym.hush.com wrote:
>On Sun, 16 Dec 2007 07:55:28 +1100 "Roy C. Lackey" 
><rclackey at stic.net> wrote:
>>
>>If the inhumi had to depend on spacecraft to travel between Blue 
>>and Green,
>>there would be no need to wait for conjunction. Sure, as a matter 

>
>>of physics
>>it's b.s., but so is astral travel.
>
>Maybe the lander only has the delta-vee for the trip when Blue & 
>Green are at or near conjunction? That's when Horn's trip to Green 

>
>on it happens.

I am cluless when it comes to orbital mechanics, but if Wikipedia 
can be trusted I think that lander voyages between Blue and Green 
will only be feasible at around the time of conjunction. 

Note that landers don't have some kind of super constant-thrust 
propulsion system - whatever it is, it works like familiar rockets 
in that thrust is applied for limited periods, shutting down in 
between.

It's surely reasonable to assume that some kind of fuel is being 
used and also that a lander has a finite fuel capacity. Even if 
they are much more efficient than current spacecraft, they'll be 
bound by the same considerations.

Presumably this means you need to plan the trip with the same kind 
of Hohmann transfer orbit used for trips in the solar system. And 
that means you need to take into account the same kind of launch 
window restrictions as you do in eg a trip from Earth to Mars. 

Launch windows are apparently tied to the synodic period, the time 
for the destination planet to return to the same position in the 
sky as observed from the launch planet. So for Green/Blue trips, 
around the time of conjunction.

Using one of the formulas here 
http://en.wikipedia.org/wiki/Synodic_period you get a synodic 
period of about 6 Blue years for Green as seen from Blue, if you 
assume that the Green year is about 85% of the Blue year.

Obviously, all this subject to correction from anybody who actually 
knows what they're talking about :)