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.

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    $\begingroup$ Earth would not be possible if Moon was Earth-like. Or at least it would be vastly different from the Earth as we know it. $\endgroup$
    – SF.
    Feb 26, 2019 at 9:47
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    $\begingroup$ @alchimista Pluto and Charon are doing just fine, in terms of orbital stability. $\endgroup$ Feb 26, 2019 at 12:11
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    $\begingroup$ Interestingly, a tidally locked pair of Earths in a circular orbit might make the space elevator idea a lot more feasible. It could be hard to go to the other Earth and return in Apollo-like fashion, but in some time we could be travelling there by train. $\endgroup$
    – Pere
    Feb 27, 2019 at 20:12
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    $\begingroup$ @SF. two "Earths" orbiting each other in a tidally locked situation would experience smaller tides than we see on the actual Earth. The reason tides "move" is because the moon doesn't stay fixed in the sky. In the former case the only time-varying tides would be induced by the sun. $\endgroup$
    – ben
    Mar 3, 2019 at 0:59
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    $\begingroup$ @ben: True the tides would be smaller, not true "only by the Sun" - orbit being elliptic and non-equatorial induces other tidal forces, and not all that weak really - Io is volcanic thanks to these! But yes, wweaker than currently likely - OTOH the day length would make the planet completely unlivable, 2 weeks of day + 2 weeks of night. $\endgroup$
    – SF.
    Mar 3, 2019 at 13:33

6 Answers 6


I think the answer depends on what you mean by "Apollo-style". It would not be plausible to use a single rocket to do a second-earth-orbit-rendezvous mission, for the reasons other answers have indicated.

But if you only want to use Apollo-style technology to go to the second Earth and back, I think there are more possibilities. Steve Linton and ANone sketch out some ideas for that. But I thought it was worth painting a more intuitive picture of the issues.

Getting there is easy enough. The lander would have to be more robust than the LM, but if everything required for return is handled separately, the weight budget probably would work out.

When the astronauts are ready to come back, they will need a rocket powerful enough to get them to orbit (so, much smaller than a Saturn V since it doesn't have to send them back to our Earth), and waiting for them in orbit they will need a fueled-up return craft. Those could be ferried into place, in pieces if necessary, using multiple Saturn launches.

The biggest challenge would be safely landing the return-to-orbit rocket on the second Earth. I'm arguing that the mission would be possible, but this task could be the step that would thwart 1960s engineers.


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.

enter image description here

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.

  • $\begingroup$ What would be Mars? Just on braking landing and rejoining an orbiter. It would be possible for a kind of lem leave the planet and rendezvous with an orbiter? Just to see if there is impossibility already at this level or it is arising from the need to carry much more stuff for the long cruuse? Already up voted, by the way. $\endgroup$
    – Alchimista
    Feb 26, 2019 at 14:35
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    $\begingroup$ What you'd probably want to do is launch half a dozen missions to deliver the various stages of the return rocket (as well as an assembly rig) and then essentially construct and fuel the rocket in-situ for a future manned mission. It'd be exorbitantly difficult to accomplish, However, you have the perk of being on a habitable earthlike world, without the complexities of working in space suits or hard-vacuum, you'd have oceans to gather hydrogen/oxygen from, and an atmosphere to aerobrake with. Exploring that other world would be a matter for drone aircraft I think. $\endgroup$ Feb 26, 2019 at 15:15
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    $\begingroup$ @Alchimista: The mission to Mars considers emptying the tanks during Mars landing, then refueling by producing fuel from the Mars atmosphere to launch back. It's viable because Mars gravity is just 1/3 of Earth, so the escape velocity is much lower, and so a single stage to orbit from Mars surface is viable. Same spaceship that on full tanks can launch from Mars, enter trajectory to Earth and land on Earth, when launching from Earth will need a massive booster it drops halfway up, and arrives into orbit with empty tanks. Six orbital refueling missions follow before departure to Mars. $\endgroup$
    – SF.
    Feb 26, 2019 at 16:37
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    $\begingroup$ The return vehicle wouldn't need to be quite as massive as the Saturn V, since the payload would be lighter (no need for the LEM) and wouldn't need fuel for a capture burn at the destination (since it can just aerobrake, like the real Apollo capsule did when returning from the Moon). Even so, you'd still be looking at something at least the size of the Saturn IB or so, so your overall conclusion certainly still holds. $\endgroup$ Feb 26, 2019 at 17:16
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    $\begingroup$ @Mark: How do you propose to soft-land the Mercury-Atlas? Atlas-D launch mass is 118 tons vs ~30 tons of Apollo so the booster would need to be quite mighty too. Regardless, it all ceases to be an Apollo-style mission - "everything off-Earth in a single launch". $\endgroup$
    – SF.
    Feb 27, 2019 at 3:48

Yes, I think this would be possible, though I doubt plausible at the time of the Apollo missions. We don't currently have anything that would let this happen. The direct approach is sort of out: You'd need order of 500,000 Kg to the other earth, Current tech means takeoff weight would be about 100 higher than this: 50,000,000 Kg. This is a bit silly, but with reusable rockets in-orbit construction etc maybe sort of do-able. The earth-MKII rendezvous is a bit more plausible. 100,000 Kg to the surface + 20,000 Kg in orbit which leads to a 12,000,000 Kg rocket if you could aero-brake without wasting anything. Pure guesswork here as how would make a big difference but maybe 20,000,000 Kg would make that work.

I can't see anyone making an order of magnitude bigger than an Saturn-V rocket to go to the moon, but if we had a twin earth... I like to think we'd give it a shot.


While one way mission may be much easier to do and deliver the larger payload (no rocket fuel is required to land on a planet with the Earth-like atmosphere), returning back does not look doable as the landed spacecraft made only tiny part of the initial rocket, required for covering the distance.

Hence one way robotic missions and later probably colonization would be much more in the emphasis.


Suppose we had a double planet, two bodies of the mass of the Earth, both with atmospheres roughly as dense as the Earths and as far apart as the Earth and the Moon. Let us call them Earth-1 and Earth-2

Strangely, perhaps, it is easier to get from the surface of Earth-1 to the surface of Earth-2 than it was to get from Earth to the Moon for Apollo, for one simple reason, you can use the atmosphere of Earth-2 to slow down, removing the need for (almost) all the LEM descent stage fuel (about 8 tons of it).

The hard part is getting back into low Earth 2 orbit and then breaking away from that orbit to get back to Earth-1. You don't need a whole Apollo stack, but something at least as powerful as the Atlas LV-3B used for Mercury. This masses about 120 tons ready to launch, so you'd need to aim to land about 150 tons on Earth-2. About 10 times the mass of the Apollo LEM (including its descent fuel).

You'd also need more fuel to get a return rocket out of Low Earth-2 orbit.

So you'd end up needing a launcher perhaps 10 times the size of the Saturn V, and probably two of them -- one to launch the lander and one to launch the return vehicle that would then wait in LE2O for the return part of the lander.

You also mention this being "currently" impossible, which I'm going to take to include in development systems, especially the SpaceX SuperHeavy/Starship combo, which I will, optimistically, assume works as currently portrayed by SpaceX.

A Starship refuelled in Earth orbit has more than enough delta-V and reentry capability to land intact on Earth-2. So again we face the challenge of getting back.

The same Starship, fully fuelled and with zero payload, has a delta-V capability of about 9.4 km/s, very very nearly enough to reach orbit, so we need to find a way to (a) refuel it on Earth-2 and (b) add a modest booster or upper stage increasing its capabilities enough to get a small payload into LE2O ready for return. A booster would let us reuse the Starship (just refuel it in LE2O and head home). An upper stage would mean abandoning it to burn up (or maybe land on E2 again) while the crew and scientific samples rendezvous in LE2O with a separate return vehicle.

If you can find, or make, methane and oxygen on Earth-2 then refueling is not too hard. Similar technology is being developed for Mars. If not, you will need to land a number of disposable tankers on Earth-2 each carrying as much fuel as it can safely reenter and land with. You might be better instead to fall back on using a smaller, disposable rocket to get the crew and samples back (like the Atlas LV-3V) and just abandon the vehicle you used to get there.


The question is vague about what it means by "Earth-like". Let's look at some increasingly Earth-like scenarios on the second planet (which I shall call "Terra").

  • Same gravity as Earth (probably the same mass and diameter as Earth), but none of the other resources described below. You would need to build and launch everything from Earth. When you get to Terra, you need to propulsively land (no atmosphere), then launch against Earth-scale gravity. The latter requires at least a Saturn I sized rocket, all of which you had to launch from Earth and land on Terra. As @SF points out, the tyranny of the rocket equation requires such an impossibly large vehicle to be launched from Earth.

  • ...and an atmosphere as dense as Earth. This simplifies landing on Terra -- you can use a heat shield and parachutes -- but still doesn't do away with the problem of launching from Terra. If the atmospheric chemistry is favorable, perhaps you could generate some fuel from the atmosphere. However, it's still impossible.

  • ...and oceans. This gives you an option for a softer landing in the ocean, but then you are in the ocean, with all of its issues. Not really an improved situation.

  • ...and life. This might give your crew breathable air and a food source, if the biochemistry is right. It lengthens the time a mission can stay on Terra, and makes survival possible if you are stranded, but still does not solve the problem of the return trip.

  • ...and fossil fuels. This raises the possibility of landing an unfueled ascent rocket, and then extracting the fuel for it locally. This is just marginally possible, but would take a long time on Terra. You would have to send the return rocket stages in advance (probably landing them in the Terran ocean), have your crew retrieve and assemble them, and extract and process the fuel.

  • ...and intelligent, cooperative life. Making contact with another civilization would certainly give meaning to such a mission. Hopefully they are friendly! The Terrans could possibly assist with a food supply, retrieval of the return rocket stages, and fuel extraction.

  • ...with manufacturing. You could theoretically manufacture your return ship on Terra, launching from Earth only a Saturn V with a standard Apollo CSM. This is plausible, but risky as to whether the Terrans would actual be able to produce a proper return ship.

  • ...and radio. You could ask the Terrans for help in advance and provide them instructions for building a return ship.

  • ...and space technology. You only need to send a one-way ship, and the Terrans will build you one for the return trip. This is trivial.

  • $\begingroup$ Yes when Earth 2 has Baikonur or Cape Canaveral like (just to remain vague ;-) facilities, or after it has become a colony, then it is trivial. Perhaps I should have add that there is not a technological society there. $\endgroup$
    – Alchimista
    Jun 9, 2021 at 7:57

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