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The film "The Martian" opens with the expedition ending early due to a dust storm - the crew evacuates to orbit and their mothership, the Hermes. The Hermes then departs for Earth, and after quite some time, Earth discovers what has happened to Watney (the Hermes crew doesn't know).

This all lays the impression that after the early evacuation, the Hermes departed for Earth immediately, moving up their departure by some weeks.

However, missions to Mars require some fairly critical ∆v calculations and timing (so the planets are in the right places) to minimize fuel burn. Wouldn't you expect Hermes to remain in Mars orbit until the planned departure time? Or did Hermes have abilities which allowed it to take disadvantageous routes?

It seems like it would affect the story if Hermes had remained for awhile...

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Harper - Reinstate Monica
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    Burn more fuel, or burn more food. Choices, choices... (I'm of the unfounded opinion they it didn't matter, probably didn't leave immediately but soonish, because either way, Watney had zero way to talk to them or reach them). – Radhil Feb 01 '18 at 18:35
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    Even if it took them a few days or weeks to set off for Earth, so what? Watney has no radios capable of contacting the relay satellites (that will later be used so the ground-imaging satellites on the far side of Mars can beam orbital photos back to NASA all day) so he can't contact the Hermes, and the Hermes has no way of "scanning" the ground for Watney (since this is hard sci-fi and there aren't scanners), so Radhil is right: it doesn't matter to the plot. – Kevin Fee Feb 01 '18 at 18:50
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    But surely Hermes has a telescope of some kind. That and some curiosity would seal the deal. Of course NASA is sometimes short of curiosity, like no one aimed a telescope at Colombia's tiles after there was a question, and several agencies had the ability. – Harper - Reinstate Monica Feb 01 '18 at 18:56
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    Would the heartbroken crew that just saw their fellow get bulldozed by debris really be all that curious to check in on his corpse? C'mon. Don't be the guy that wants to "fix" the plot. It never helps. – Radhil Feb 01 '18 at 19:04
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    @Harper Why would the Hermes have a telescope? Especially one large enough to resolve the ground. That would be entirely useless. What good would it do in Earth orbit, in Earth-Mars transit, or even in Mars orbit? We already have ground-scanning satellites in orbit of both bodies. A telescope on the Hermes would be extra weight that needs to be accelerated (thus costing fuel) for no reason. – Kevin Fee Feb 01 '18 at 19:04
  • @Radhil "Burn more fuel, or burn more food." Presumably, they had enough food to last them until the scheduled departure time. Probably more, since there's one less mouth to feed. Unless some of the food was left on Mars? – Acccumulation Feb 01 '18 at 19:50
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    @Acccumulation - they didn't pack and leave, they evac'd, so yeah, food was left, Watney used it. Procedures like the 24 hour abort in Valorum's answer exist in case crap happens (which it did), supplies are still manageable. – Radhil Feb 01 '18 at 19:55
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    Hermes crew could stretch a 6 month food supply to 7 months. Watney did. – Harper - Reinstate Monica Feb 01 '18 at 20:55
  • This is a great question. Good to see it was actually explained somewhat. – Z. Cochrane Feb 02 '18 at 01:08

3 Answers3

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In the source novel we learn that it's standard operating procedure for the ship to leave orbit within 24 hours of dusting off. Presumably their low-consumption ion engines allow a straight shot back to Earth regardless of the date of departure.

I had no way to talk to Hermes. In time, I could locate the dish out on the surface, but it would take weeks for me to rig up any repairs, and that would be too late. In an abort, Hermes would leave orbit within twenty-four hours. The orbital dynamics made the trip safer and shorter the earlier you left, so why wait?

Even assuming that they were looking (with telescopes) at the Hab site, there's simply nothing Mark can achieve in the short time he has available that would attract their attention. There's also a pretty solid chance that they'd already left during the hours in which he was unconscious.

Valorum
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    The kind of ion engines described for the Hermes combined with a nuclear energy source frees the Hermes from the tyranny of Hohmann orbits and one-every-two-years departures. They could leave any time. – Mark Olson Feb 01 '18 at 19:14
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    @MarkOlson They clearly didn't even need to fuel it between missions, as they just up-n-decided to do an Ares 3-and-seven-eighths mission without even telling NASA. – Harper - Reinstate Monica Feb 01 '18 at 19:50
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    @Harper They had a resupply mission, and used the gravitational slingshot around Earth to save on fuel, that's why Rich Purnell needed to use a supercomputer to determine the precise course and maneuver to manage it within the fuel constraints. NASA always puts in some margins on their fuels, just in case, plus they weren't lugging around the mass of samples and an extra crew member, so that gave them more fuel to work with. Remember when they got to Mars they had to discuss just how much fuel they had for maneuvering to meet with Watney when the plan didn't work. – Kevin Fee Feb 01 '18 at 20:02
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    Asking to learn. What is SOP ? – atakanyenel Feb 01 '18 at 21:28
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    @atayenel Standard Operating Procedure – aherocalledFrog Feb 01 '18 at 21:29
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    +1, but I don't think "standard operating procedure" is the right term, because I don't think the quotation is stating as a general fact that Hermes would leave so soon; rather, it's saying that during the 30 days that this mission was scheduled to be on Mars, the orbital dynamics were best at the start of the 30 days and worst at the end, such that during this mission Hermes would leave very soon after any abort. – ruakh Feb 02 '18 at 02:15
  • Wouldn't the fuel used for maneuvering be separate from the main ion engine? That engine had very little actual thrust; it worked on constant acceleration over a long time. – Nyerguds Feb 02 '18 at 11:43
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    By the way, I think the piece in bold in this answer should actually be The orbital dynamics made the trip safer and shorter the earlier you left, since that's the part that really answers the technical side of the question. – Nyerguds Feb 02 '18 at 11:47
  • @Fattie sorry, I only saw the movie (who's Tom Bombadil?) I'm well aware of the resupply being a critical factor, but I thought that was food, not fuel. I do recall the resupply module Did Not Like fluids so I assumed everything on it was dry goods. – Harper - Reinstate Monica Feb 02 '18 at 16:25
  • Upvoted Ruakh's comment because it contains something really key: the whole 30-day mission was timed on the basis of an earth-mars-earth round trip window, the 30-day mark being the decided-upon cutoff, so that the earlier you leave, the better. – CCTO Feb 02 '18 at 17:23
  • @MarkOlson - purely on the Hohmann transfer issue - note that a few weeks either way makes no difference -> https://space.stackexchange.com/questions/24738/whats-the-reasonable-window-in-days-for-a-mars-earth-hohmann-transfer – Fattie Feb 02 '18 at 18:45
  • @Harper What makes you think the resupply module doesn't like fluids? The original resupply mission, to send food to Watney on Mars, failed because the protein bars liquefied unexpectedly, which caused the center of mass to go off the center of thrust. But the Taiyang Shen resupply mission that rendezvoused in near-Earth Earth-Mars transit was a full Hermes resupply, designed to carry whatever fluids and supplies that were needed, but this was easier since it didn't have to deal with the whole "hitting mars" and "landing" things. – Kevin Fee Feb 02 '18 at 19:30
  • @Fattie: Agreed. I don't recall hearing anything about the specific impulse of the Hermes' engines, but it has to have been huge. Hermes could run under constant boost for years, apparently, and when you can do that, transit time scales roughly with the square root of the distance and everything changes. (I hope to see the day!) – Mark Olson Feb 02 '18 at 19:53
  • @MarkOlson I definitely think a big honking veritable starship like that is the way to go, so you aren't chained so helplessly to fuel and thrust math, and can change the mission if somethink horks up, as it always does, it is space! – Harper - Reinstate Monica Feb 02 '18 at 20:41
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The killer issue wasn't the return fuel - it was the MDV / MAV.

There was only one Mars Descent Vehicle (MDV), and I'm guessing it was used to help build the habitat after it landed. The MDV doesn't get much mention in the book, with the focus on a need for a replacement Mars Ascent Vehicle (MAV).

Once the crew of the Hermes used that MAV, there was no return for them. End of mission, no point in hanging around - 24 hours for NASA to plot the return vector and you're homeward bound.

Even if they had telescopes, and had seen a body (or Watney alive) there was no spare MDV to return to the surface. They probably couldn't even do a food drop, as getting things to land intact on Mars atmosphere is tricky.

In fact, as Hermes was the only vehicle capable of making the trip, any delay in their departure would have just delayed their return, with potentially fatal results.

Alan Campbell
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    The question is not why they did not wait (to rescue Watney), but how they could leave orbit earlier than planned. Their mission schedule certainly did not include such an early return. – Dubu Feb 02 '18 at 08:54
  • What @Dubu says is true, but it's unfortunate that the question has become muddled and everyone is asking / answering different things. Note that the OP (bad OP!) confusingly asks about telescopes onboard etc, which has no connection to the core question "could leave orbit earlier than planned". – Fattie Feb 02 '18 at 15:48
  • @Dubu if you put the question that way - then I'm going to assume a 30 day "window" [or however long the planned excursion was] of optimum time to start return voyage. There would be some margin for error, but the later you start the smaller the error margin becomes. With the disappearance of Watney and an aborted mission, starting the return trip early would then actually minimise fuel expenditure. – Alan Campbell Feb 03 '18 at 11:07
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The simple answer to the (apparent) actual question is

"In “The Martian”, did the Hermes leave immediately or wait for awhile?"

Yes, in the movie, they did leave "immediately".

That's the answer.

  • the "exact amount of time before they leave" is simply not detailed in the movie. Why would it be? It's a movie, he's utterly dead: so they leave. As an audience we simply interpret it as "He was dead so they left the scene." It's utterly clear that they leave "quickly" (like, "within a few days"). The entire, total, point of the movie and book is that "They think he's dead so they leave."

  • Even in the book, there's like one throwaway sentence (in the whole book) dealing with the details of when, exactly, they leave. Note that it would make no difference, at all, if Hermes left within a minute, a day, an hour, a week or even a month.

  • Note that of course they had utterly no way to get up and down to him. Why stay even if, amazingly, they learned he was alive? Once they pressed the "go" button on the ascender, it was all over for him.

Again, precisely when they leave is not especially discussed in the movie, and I don't see any reason it should be. We "just accept" that (bizarrely) their incredibly advanced comms systems incorrectly thought him dead; we "just accept" that (surprisingly) jpegs can be sent to Hermes, we "just accept" that (ridiculously) at the end there was conveniently a hatch to blow on Hermes' nose, and so on.

The question here seems to be asking/implying numerous other questions,

  • why didn't they spend more time, in orbit, checking if he was dead?
  • wouldn't they have (various) technology, from orbit, to be able to check if he was dead?
  • what is the detailed nature of Mars orbital spy telescopes?
  • ditto for Mars orbital radio, radar etc etc devices?
  • assuming it was government policy in such cases to just leave, why would that policy be so?
  • with the putative fictional Ion Engines, were they using a Hohmann transfer?
  • if using a Hohmann transfer from Mars, what window do you have?
  • if not using a Hohmann transfer, what would be their windows of departure?
  • why was it limited to a 30 day mission? does that relate to departure windows or was it unrelaed?
  • how does the movie plot compare to the book plot?
  • in the book specifically, what are the answers to every one of the above questions?
  • would Hermes be able to stretch food and what is the detail of that?
  • numerous questions about Hermes return transfer
  • etc etc

I'm totally in favour of discussing those issues, but I guess here on SO the SO philosophy is that those should be separate questions.

Fattie
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    Not only does this not answer the question in any meaningful way but it also contains a bunch of questions that should be asked as new questions on here and possibly on Space:SE – Valorum Feb 02 '18 at 16:26
  • hi @Valorum ! (1) scroll to top of this page (2) read the question :) :) – Fattie Feb 02 '18 at 16:27
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    I read the question and I also read your answer. Your question doesn't answer the question asked, except in the most banal ("It's a movie, we don't need to look into it too deeply") kind of way. – Valorum Feb 02 '18 at 16:29
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    Also, calling The Martian an "action movie" with events that the audience should "just accept" displays, frankly, an utter lack of understanding of the kind of fiction it is. The movie might have taken some liberties, but it still very much is one of the few works that invite rigorous analysis. – Santa Feb 02 '18 at 17:17
  • I don't always like your stuff but I really like this answer. You're right, my question is fairly simple (the only followup question I aimed to infer was "why") but it asks so many more. Discovering Watney was down there whilst Hermes was still in orbit would not short circuit the story, but make it a very different adventure. Could they get Watney to the Ares 4 site before Hermes starves? (And why, oh why put the sites so far apart?) – Harper - Reinstate Monica Feb 02 '18 at 17:25
  • I would like to emphasize that I, personally, always ask more and more questions inside questions here on SO :) :) My point is not "You shouldn't do that Harper". My point is "I'm with you - but I suppose SO would rather you don't do that - screw SO right?" :) :) – Fattie Feb 02 '18 at 17:33
  • that's not logical @Santa. in a movie or book not everything is explained in detail. Note that in the book, this issue is only addressed in passing in literally 11 words. By all means questions like "so, let's discuss these interesting unknowns in the plot" ... are terrific. But this is not that sort of question, the OP just wants to know did they or didn't they leave quickly. – Fattie Feb 02 '18 at 17:35