Gsyfuy

I suspect that actual technology results in the apparently paradoxical situation that is easier to visit the moon rather than a comfortable planet able to sustain life. I am obviously supposing the crew must return to Earth.

Am I correct thinking that if there would be a copy of Earth at the place of the Moon not manned missions would have been yet possible? This seems obvious but is rarely realised by people in general.

Similar (just similar) considerations apply to manned mission to Mars, which are getting more and more discussed in divulgative TV channels but seems to me almost totally fictional, at the current stage.

(I consider the various energy requirements for getting there, braking, descending, and back, and life sustaining stuff such as food, water and breathable gas. While the design of Apollo did it for the Moon, a two ways trip to a Earth like moon or Mars differ by many points and seems to me currently impossible.

Please note that I am aware that replacing moon with an Earth would change the solar system and perhaps a binary planet system orbiting a star is even physically impossible. I am concerned with distance and escaping speed only).

Edit: I would like to stress that I could have place two earth masses of concrete in my question instead of a couple of earths. Atmosphere is important in the sense that there are different braking mechanisms / dragging etc. There is not much emphasis in my Q about habitability, except that I have mentioned that counter-intuitively is easier to go to the moon than to some kind of a big planet supposed to have a breathable atmosphere. Just to avoid debate not inherent here.

Under (very stretched) assumptions that the "Other Earth" would not affect Earth's natural conditions, then an Apollo-like mission would be impossible.

Tyranny of the rocket equation makes the launch mass to scale exponentially with delta-V budget. The enormous Saturn V rocket was capable of lifting and launching towards the Moon the command/service module and the lunar module, and little more than that. Compare scale/size of these - the Apollo spacecraft in upper right corner to the rest of the rocket.

Now instead of the lunar module, you'd need an entire stack of size comparable to Saturn V - additionally, capable of soft landing in the rough terrain of the uninhabited "other Earth". And now you need to add a rocket to bring it to the "Other Earth" - one, that by size, comparing to Saturn V, is as Saturn V is to the Apollo spacecraft. And obviously, by cost too.

The same style mission would be plain impossible. The delta-V to take off from Earth is high enough by itself. Doubling it, scaling the mass ratios exponentially yields an impossibly huge construction.

That is not to say a successful mission including return couldn't be launched. If the other planet was inhabitable, a permanent colony could be established with multiple one-way missions delivering supplies and technology. Utilizing in-situ resources and technology from Earth, the colony could construct rockets capable of entering the "Other Earth's" orbit, where they could rendez-vous with vehicles resembling Apollo CSM and the crew could use it to return to Earth. Thing is the timeline between first landing and first return would be decades, not two weeks.