The problem stemmed from the simple problem that would arise when two or three aliens same as man had accidentally descended on our Earth.Would they be able to return back to their Earth? If we go land in their earth world with all the technology we have, can we ever design a launch vehicle capable of ferrying to and fro? If so how would it be feasible? A simple paper design should be attempted. I feel larger the planet greater will be the complexity. A planet without gravity close to a fly by is perhaps the most feasible one in a limiting sense.
Remi,
Quite.
If the target world is host to water, and has a equiv-tech civilization that can generate and store cryogens, then that (in principle) reduces the size to *merely* one Saturn launcher.
I f we can descend a space capsule from the present space station and bring it back to the space station solves 40 percent of the problem. A similar space station should be orbiting the sister Earth where the same exercise can be achieved. Then the remaining 20 percent of the problem is ferrying astronauts from one orbiting space station to the other. The space stations have to be filled with fuel cryogenic or otherwise. Of course the space capsule cannot have logistic support from place it descended as it would be too far fetched even to imagine.
One can imagine the problem in designing a vehicle to Mars and back. A futuristic system which beats gravity,uses renewable energy source along its journey may solve the problem.
An expendable or partially reusable vehicle is like designing a cone with the astronauts at the apex. If the astronauts have to come back it would mean designing another cone inverted on its top and its size would depend very much on the atmosphere and gravity conditions of the sister planet to be descended. Storing fuel in orbiting bodies and using it as a fuel charging center would involve large efforts and motivation factor to embark on such programs unless a feasible plan has been struck.