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I'm a big fan of space movies, especially when they're based on actual science and/or near-future. So of course I went to see Ad Astra (IMDB). In a nutshell, Brad Pitt's character flies into an orbit around Neptune on a rescue mission. This involved flying to the Moon to get on a rocket to Mars then blast off in a heavy rocket to make the actual trip.

My main issue with the movie was that, according to my limited understanding of space exploration (primarily thanks to KSP), the Moon would offer a better launch as the thrust requirements to get back into space should be significantly lower than Mars. My girlfriend's argument, somewhat as devil's advocate, is that Mars is better because it's closer to Neptune. The only reason I can think of to use Mars instead of the Moon is if the transfer window for Earth-Neptune is closed but Earth-Mars is good and Mars-Neptune is also good.

Which location would serve better for deep space vehicles, the Moon or Mars?

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  • $\begingroup$ In a sense, we typically launch from Jupiter, using the giant planet as a "slingshot" to get on the trajectory we want. $\endgroup$ – Oscar Lanzi Oct 15 at 15:49
  • $\begingroup$ @OscarLanzi True. However, I'm more interested in the start of trip to Neptune/deep space than the whole trip overall. $\endgroup$ – Lux Claridge Oct 15 at 15:50
  • $\begingroup$ The only reason I can think of to land on Mars would be that you have a propellant manufacturing operation there. If the propellant is being shipped from Earth, then it's always better just to start from Earth and skip the Mars landing. $\endgroup$ – Steve Linton Oct 15 at 16:00
  • $\begingroup$ Energetically, the moon is closer to Neptune than the (comparatively) deep gravity well of Mars. $\endgroup$ – AtmosphericPrisonEscape Oct 15 at 21:16
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    $\begingroup$ Great question! I made some small changes and removed some of the plot-spoiling information. $\endgroup$ – uhoh Oct 16 at 2:08
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If you look at this delta-v map of the solar system, you can see that to get from the surface of the Moon to Neptune transfer requires about 7.67 km/s of delta-v.

enter image description here To get to Neptune from the surface of Mars requires 10.56 km/s of delta-v. So even if you were magically transported to Mars first, it would still take more energy to get to Neptune than it would if you were starting from the surface of the Moon. And of course it takes energy to get to Mars in the first place.

So no, if all we care about is getting to Neptune as efficiently as possible, it would be better to launch from the Moon. Not having seen the movie, I don't know if there were other considerations.

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  • $\begingroup$ I think the movie had other considerations (Brad's mission was classified and Moon pirates) so there maybe administrative reasons for a Mars launch. However, I feel that including that in the question would make it off-topic here and on topic SFF or Movies and TV stacks. $\endgroup$ – Lux Claridge Oct 15 at 17:34
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    $\begingroup$ @LuxClaridge I like your question an it is certainly possible to ask related but different questions about the film in all of these sites at the same time. Best way to to do that is to reference your other question(s) and in a short sentence make a note of how it is different than the other. $\endgroup$ – uhoh Oct 16 at 2:10
  • $\begingroup$ You can't use a delta-V map like this to go from an arbitrary planet to another. The core assumption of a delta-V map like this is that your origin or destination is Earth, because it's primarily specific orbital energy that determines where you go, and delta-V required does not scale linearly with that. As a result, if you try to use this map to get the delta-V to go from Mars to Neptune, your values will be incorrect. $\endgroup$ – notovny Oct 16 at 16:06

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