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Some lunar probes such as GRAIL travelled on a different trajectory to the Moon that went even much farther than the Moon itself is from Earth, to a Langrangian point, so why isn't this option ever brought up for human Moon missions? It takes weeks to reach the Moon by that, but on the ISS astronauts are spending much longer times. And such cruise wouldn't require a carrier rocket as strong as the Saturn V / SLS, or does it for human flights? Wouldn't a Falcon 9 or Falcon Heavy suffice for such low-energy cruise?

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    $\begingroup$ "so why isn't this option ever brought up for human Moon missions?" Life support and other consumables. $\endgroup$
    – Organic Marble
    Oct 20, 2022 at 14:11
  • $\begingroup$ @OrganicMarble So how did astronauts survive for more than two months on SkyLab? $\endgroup$
    – Nullnummer
    Oct 20, 2022 at 15:57
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    $\begingroup$ They weren't launched to the moon carrying all their consumables along. $\endgroup$
    – Organic Marble
    Oct 20, 2022 at 16:14
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    $\begingroup$ "such cruise wouldn't require a carrier rocket as strong as the Saturn V" - most of the mass of the Saturn V was needed to get the Apollo stack to orbit. Only about 1/30th of its propellant and engine power was used for the trans-lunar injection burn. $\endgroup$
    – Cadence
    Oct 20, 2022 at 18:01
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    $\begingroup$ @ColonelCornieliusCornwall It's spelled "Cornelius" BTW $\endgroup$
    – Organic Marble
    Oct 20, 2022 at 21:19

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You would only save a bit more fuel by doing that if you sent a probe. If you send humans on a longer mission then you will have to have more food, water and oxygen. Overall you will need more fuel to ship everything the humans would need to survive the longer trip. Also we could send humans around the world on zeppelins and save a lot of fuel doing that, but airplanes are preferred because they are much faster. Time is an important factor when humans are onboard.

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  • $\begingroup$ People are for six months on the ISS. Wouldn't a Space Shuttle-like space glider be able to carry more supply? $\endgroup$
    – Nullnummer
    Oct 20, 2022 at 15:56
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    $\begingroup$ @Nullnummer Shipping food to low Earth orbit is a lot cheaper than shipping it to the moon. $\endgroup$
    – WaterMolecule
    Oct 20, 2022 at 16:23
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    $\begingroup$ The ISS receives a lot of resupply shipments. In the next 12 months there are at least 10 planned cargo shipments, plus whatever supplies are on the crew rotation vehicles. Sending supply shipments to a vessel in transit to the moon would be... nontrivial. $\endgroup$
    – Cadence
    Oct 20, 2022 at 17:34
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    $\begingroup$ @Cadence Cygnus freighter + SLS = space food! $\endgroup$
    – Deko Revinio
    Oct 20, 2022 at 19:19
  • $\begingroup$ @The Rocket Fan you are correct. time is important, because unlike probes, us humans have families and lives we gotta get back to. Would you rather get more times in space or have more time with those you care about. Yeah, I know I'm a good speaker. $\endgroup$
    – Deko Revinio
    Oct 20, 2022 at 22:14
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Comparison to Salyut, Skylab, Mir, ISS, and Tiangong is a bit inapt as all of those space stations operate within the thermosphere. The Earth's upper atmosphere and the Earth's magnetic field protect the inhabitants of those space stations from some space radiation.

Moreover, those space stations had escape mechanisms for a quick return to Earth in case something goes very wrong. The Apollo missions had a semi-quick return option in case something went wrong. This was used on Apollo 13, where something went drastically wrong. What if something goes seriously wrong on your proposed slow transfer to the Moon?

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A "low-energy cruise" is probably better known as a ballistic lunar transfer (BLT). These trajectories generally require more $\Delta V$ from the launch vehicle, but less of the payload (spacecraft), though the use of an outbound lunar flyby makes the launch C3 comparable to a direct lunar transfer (at cost of extended transfer times):

c3 vs spacecraft dV, advanced space

(Credit: Advanced Space)

The $\Delta V$ savings are significant on the payload side, but overall not as much. A crewed lunar mission still necessitates a super heavy lift launch vehicle.

Also note that a BLT does not travel to a Lagrangian point but rather uses the relative strength of the Sun's gravitational pull (being so far from the Earth and the Moon) to raise the spacecraft's "Earthian" perigee to a similar height as the Moon.

See Also: My answer to "Is there a fairly detailed outline of CAPSTONE's "highly efficient ballistic lunar transfer trajectory" from LEO to lunar near-rectilinear halo orbit?"

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