If instead of earth/moon we had 2 earths, one with people the one without people, but with all other life.
We want to design a mission like Apollo, man to the 2nd earth, have him walk around a bit, then safely return him to the first one. Are we capable of this in a single shot - is it within our current technology? Issues I see is where to land the thing (water, land), if water how to have the man actually walk on land (small boat?), and how to relaunch from earth 2 back to earth 1
The relaunch from Earth 2 is the killer. You have six times higher gravity to contend with and atmospheric resistance too. Also your mission vehicle to Earth 2 has to be re-entry capable because it is going to be hitting an Earth-like atmosphere at about 17,000mph.
I don’t see how you can do it even with present-day technology let alone Apollo-type rocketry. You need something like a space shuttle to touch down with and something like what’s needed to launch one in the present day in order to take off again. Hope like hell you can find something like Bonneville Salt Flats to use as your landing strip, which needs to be many miles long and mighty flat.
That means lifting a massive payload from Earth 1, although you could do it in stages, and every part of what you’re using to relaunch has to be dropped in something like another space shuttle, as there are limits to how small you can make things (small payloads could just be dropped in something like an Apollo command module).
Divide the payload into several parts each massing about the same as a CSM and a LM put together, and for each payload you need a Saturn V to launch it… and that’s assuming it’s even possible to put that many tons of rocket fuel into re-entry.
Best bet is to wait for controlled fusion. Then you will need nothing much except reaction mass in order to take off again, and if Earth 2 has water then you’re laughing. What’s more you can maybe use braking thrust to land, which means you don’t have to hit the upper atmosphere at low-orbital speed.
Of course, an Earthlike world with no life as yet will have a nasty atmosphere - life was well-established on Earth before oxygen became abundant and we kissed the ammonia and methane goodbye…
There’s always the possibility of a one way mission. A team of astronauts could land on Earth 2, keep in radio contact with Earth and live the rest of their lives (or at least until someone invents a better spaceship) on Earth 2. Of course if Earth was a twined with another Earthlike world instead of the Moon human cultural development/history would be profoundly different.
I think we could do it, if we ignored treaties and built an Orion spacecraft, the one powered by nuclear bombs ( just don’t stand downwind 
). It would take a major effort to build the thing and work out the kinks, but as far as I know it’s perfectly possible for the technique to work.
How close would the two Earths need to be to get tidally locked?
Conversely, how far away can they get before the orbits of other planets eventually toss one
or the other out of their little arrangement?
Assuming neither of the above is a consideration, we’d need a SSTO vehicle, designs
for some are just now coming onto the drawing boards. I’m sure a novel on this very
subject has been written by now, just can’t think of any offhand…
The closest that comes to mind is the alternate world novel A World of Difference by Harry Turtledove, in which Mars is replaced by a life bearing world, Minerva. Funding for space travel goes way up when the Viking lander gets taken out by an alien with a spear . . .
How about the return solution consisting of some kind of X-plane vehicle that takes off horizontally and flies to very high altitude to rendevous with a command module that dips down in orbit to scoop it up. I think the meetup would be a pretty hairy affair, but I think there would probably be some way to achieve it.
Instead of having them relaunch and return, have them start building a connective space elevator… 
If the two Earth-worlds were tidally locked and motionless relative to each other, yes.
Basically, you’d have to send a large enough expedition to build industry capable of building the launch facilities and vehicles you need. Several thousand people at least, who would be stuck on the New World until they got the new systems working. When the expedition is that large, you have to build farms, industries, etc, and you’re basically colonising the place.
I agree that relaunch is the joker in the deck, but it’s not quite the impossible hurdle you are assuming. Simply put, given the OP’s hypothesis, there’s no need to bring the fuel for the return in your return vehicle.
Take a page from Zubrin’s Mars Direct plan, and send ahead several robitc payloads before sending the manned probe. The first one sets up a turbine to compress oxygen from the atmosphere, and the second electolyizes hydrogen from water. Per the OP, we know, given that this second Earth has life, that there is free oxygen in the atmosphere, and that it has to have some kind of hydrosphere.
You don’t launch the manned probe until you already have the fuel made, and stored, in the prepared modules. So, now you’re not looking at carrying enough fuel to return to Earth with, but only enough fuel to get to Earth 2. (With some small margin for error.) That’s going to reduce mass requirements by 50% with a much, much greater reduction in the engineering needed to launch the probe initially.
Also, while gantries are used for launching our space probes, now, I’m not sure they’re absolutely necessary - underwater launch of ICBMs is a proven technology, and the difference between a space exploration launch vehicle and an ICBM is rather small. Make use of the assumed hydrology on this second Earth, and assume a launch from there.
It’s still going to be a difficult undertaking. But it’s not quite as impossible as some are saying.
Yeah, I don’t think it would be impossible either. If we had to, we could put a space station in orbit around Earth 2 (we’d probably have one there anyway). Then we could just send a bunch of unmanned rockets to rendevous with the space station carrying fuel and supplies. Then you’d eventually ship over something like a DC-X, a VTOL SSTO type aircraft. Find something like a salt flats on the other planet, and go land. Then parachute down a whole bunch of fuel and have the thing refueled on the ground for the return trip.
The thing is, if it was really an Earth 2 and inhabitable, we’d be shipping people over like crazy. Huge numbers of explorer-types would be lined up for the trip. Could you imagine? An entire alien world, and you can live in your shirtsleeves and stay alive indefinitely. If such a world existed, my guess is that we’d have found a way to drop people on it no more than a few years later than Apollo, and by now there would be hundreds of people there, probably including the first generation of children. Once it became safe and there was some sort of community foothold, my guess is that we’d be firing capsules of people over on a regular basis. A one-way trip for now, but if the place was truly inhabitable, who cares? We might even have built emigration ships carrying 50-100 people at a time, if we found a compelling enough reason to get them there.
I agree relaunching from a body much heavier than the present Moon is physically difficult.
On the other hand, having another Earth visible in the sky, several times larger in angular size and with visible weather patterns, might have inspired the sciences to take off millenia earlier. How hard would space flight be today if we’d had the World Wide Web 30,000 years earlier?
The hypothetical reduction would be far more than 50%–hauling the requisite amount of fuel would require more fuel, by a factor of about 100,000:1 or greater–but even assuming you have some automated way of manufacturing fuel and oxidizer on Earth 2 (which is vastly more complicated than you allow for) you still have an issue with reusing your launch vehicle. You’re going to have to perform an aggressive aerobraking/reentry maneuver to land on Earth 2, then refurbish it for the flight home. A truely reusable spacecraft–one that can launch, land, and relaunch without significant refurbishment–is well and beyond current thermal protection technology, and performing such without a significant infrastructure is beyond any reasonable extension of current capability.
Not hardly. A Trident D5 missile has a payload of 2800kg to a ballistic suborbital trajectory. A Saturn V booster has a payload capability of 118,000kg to a circular Low Earth Orbit, a difference of almost two and a half orders of magnitude. They don’t even begin to compare.
Another issue is that your Two Earth system is going to have a greater average orbital speed (1.44km/s) as opposed to the Moon’s average speed (1.022 km/s), requiring longer curves or more impulse to achieve orbital insertion.
Tidal locking isn’t just an issue of distance, but also of rotational speed, mass distribution, et cetera. All bodies (that demonstrate hysteresis) will eventually lose rotational energy to tides and become tidally locked or enter into a tidal resonance. Assuming similar rotational speeds, though, you’re certainly going to have far more aggressive tides in your Two Earth system than the Earth-Moon system.
Stranger
Actually, I was thinking specifically of the Atlas launch vehicle, which AIUI is both an orbital launch platform, and an ICBM. I agree the current generation of submarine launched ICBMs cannot match the same performance that even the Atlas, let alone the Saturn V, could make - but I do believe that it’s reasonable to talk about matching Atlas level launch capabilities with a water launch.
Which doesn’t negate your points on the size and complexity of the Saturn V.
I was deliberately skimming over the difficulties of manufacturing fuel on Earth 2. One problem that comes to my mind right off the bat, that your accurate and on-target analysis neglected to mention, is that the OP said that there’s life on Earth 2. Can you imagine the difficulties that would happen if the processing facility landed on Earth 2’s version of Kudzu? Automatic machines are wonderful, but they don’t always seem to work well with biological complications.
My intent was less to say that we could do it, now, than to suggest there are strategies available to allow some ‘cheating’ on the engineering challenges involved in a manned ‘there and back’ mission.
I’ve considered writing a story about this basic setup because it opens another possibility.
Even without space travel, you could easily have communication between two such planets. In the 19th century people made proposals like making a giant geometrical diagram in the Sahara (say of the Pythagorean Theorem), then filling it with something flammable and lighting it. That would attract the attentions of the Martians 9who were building those canals), and alert them to the presence of intelligent life on Earth. The same idea would work even better on a pair of earth-sized worlds orbiting around each other as they went around the sun. Even before radio developed, you’d have some sort of mechanical telegraphy for communicating across the gulf. That would make for some interesting developments.
of course, it seems to me that two such large planets, tidally locked and performing such gyrations in orbit would end up with some extreme environmental differences from ours. Weird tides, maybe extreme temperature swings (or maybe very little ones, deending on how things are arranged). I suspect you’d know pretty quick that you’re not on the Earth.
I don’t know about that; the Trident D5 is roughly comperable in end performance to an Atlas D-F (the ICBM version of the Atlas) but not even close to the commercial Atlas V EELV. And the Trident is about as jam-packed with performance as you can safely make a booster, including Class 1.1 propellent in all boost stages, and some other special performance enhancements like telescoping nozzles and forward “aerospike” (not to be confused with the aerospike engine design, which is an entirely different thing). Admittedly, this is in part due to the requirement to fit into the vertical package of a ballistic missile submarine (which is itself dictated by available draft into ports) but scaling it up to something the size of an Ares I rocket is beyond reason, methinks.
However, I agree with you that it is not conceptually infeasable, merely that it is significantly more difficult than the Moon landing. I think, as Sam Stone argues, that you would need a single stage to orbit vehicle (to minimize integration and launch preparation), the ability to manufacture fuel on Earth 2, and an ability to do some kind of booster integration or refueling in orbit. A vehicle like the Chrysler Shuttle SERV (assuming that you could make all the technology work, and make it suitable for cislunar transit) would be ideal for this, especially if you could make the engines truly restartable without refurbishment.
As for the difficulty of life on Earth 2, there are pluses to that as well; specifically, the likelyhood of hydrocarbon deposits that could be refined into a stable, storable fuel. While given the presence of water and sufficient energy you could crack out diatomic hydrogen and oxygen, it would be a lot easier to use kerosene or gasoline as your fuel and hydrogen peroxide as the oxidizer, which would eliminate the need for cryogenic storage and would be only moderately corrosive (compared to, say, hydrazine or RFNA.
Stranger
Without safe biological decontamination, the whole point is moot.
Can you say “Andromida Strain” kiddies?
Can you say bad science fiction? Extraterrestrial viruses and bacteria (or the equivilent thereof) aren’t likely to be compatible with terrestrial life. They might compete for resources (which, if they are particularly virulent, could be a problem in and of itself) but are unlikely to infest and hijack a human body the way agents that are evolved to do so can.
Stranger
Not necessarily. In a closely-coupled situation like we’re talking about, I suspect that it would be likely that both ecologies came from a common source, or at least influenced each other heavily.
