Let’s say that interstellar visitors show up here at Earth. They do not have an materials or technologies that violate known physics–no FTL, no antigravity, no inertialess drives, no scrith or unknown energy sources. They want to land on Earth for a visit and then return to their orbiting mothership. They do not trust their ability to buy or build their return transport on the ground, so they must bring the vehicle and fuel down with them. What are practical ways to do this (again, using real-world physics and materials.)
Isn’t that their problem?
This is such an open-ended question I’m not sure how to even frame it. What kind of propulsion and power technology do these hypothetical aliens have, and how big of a landing vessel do they need? If they are 50 ton leviathans who live in a pressurized liquid methane atmosphere the requirements will be quite different than 10 centimeter long insectoids breathing chlorine gas at ambient pressure.
Stranger
Okay, say the aliens are roughly humanoid in size and environmental needs. Or to frame it a different way, how humans would do it in visiting a planet with an Earth-like mass and atmospheric density.
As for the technology, anything that can be reasonably extrapolated to be doable with the assumption that our knowledge of physics is essentially accurate and there are no magic “get out of jail free” technological loopholes. (No repulsorlifts, no scrith.)
Sounds like old-fashioned nuclear energy would be the best bet. A lot of energy stored in a relatively small amount of material. That keeps the weight requirement relatively low.
It is not hard at all to envision a nuclear-powered engine that essentially allows you to do whatever and lift almost as much weight in and out of orbit as you want.
Any alien civilization capable of travel over interstellar distances and is in any way like us from a physiological standpoint, e.g. are discrete, mobile life forms with a lifespan on the order of 100 Earth years, will by definition have command of such energy that getting from surface to orbit is essentially just a matter of collecting reaction mass of some kind (water, compressed air, hydrogen, whatever). They certainly won’t be dependent upon fundamentally limited chemical combustion, and they will almost certainly have some kind of energy technology capable of generating far more power per unit mass than even nuclear fusion just by virtue of being able tomcross interstellar distances in anything less than millennia.
Stranger