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After their readjustment, the Hermes is off trajectory to return to Earth. It is remarked that the have very little liquid fuel left to spare. The main engine would take a very long time to put them back on a correct trajectory, and they'd still have to resort to cannibalism which defeats the purpose of the rescue mission.

How does the Hermes get back to Earth with the resources they have after the ending of the book?

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    Incorrect. They are on a cycler trajectory. – Deer Hunter Nov 1 '15 at 6:01
  • How so? You mean to say that their new trajectory just so happens to be a cycler trajectory? – davidtgq Nov 1 '15 at 7:01
  • He meant to say that the RPM put them on a trajectory that Sling-shotted them from EO to Mars, which then Sling-shotted them from MO to Earth, where they would then only need the month long deceleration maneuver – Benjamin Feb 12 '17 at 21:57
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According to the original novel, The Hermes has two engine systems; Attitude Thrusters (which use classical oxygen/hydrazine-based rocket fuel) and an Ionic-Thrust engine (which use electrostatically-charged argon to create thrust and is going to be used to get them home safely).

When it becomes clear that the ship has missed its rendezvous point with Mark, they use the attitude thrusters to close the gap. At no point is it suggested that the ion engines would run low on fuel, just that they would be ineffective, given their low rate of thrust:

“Vogel,” Lewis continued, “how far can we deflect in thirty-nine minutes with the ion engines?”
“Perhaps five kilometers,” he radioed.
“Not enough,”

This is explained in a little more detail a few paragraphs later:

“The launch missed badly,” Venkat said, looking past Mitch to the screens beyond. “The intercept distance was going to be way too big. So they’re using the attitude adjusters to close the gap.”
“What do attitude adjusters usually do?”
“They rotate the ship. They’re not made for thrusting it. Hermes doesn’t have quick-reaction engines. Just the slow, steady ion engines.”

Since the main ionic engines haven't dramatically affected their orbit (nor run low on fuel) it should be a vanishingly simple matter to use them to un-correct the earlier course-correction and keep them on their planned Earth-Mars cycler trajectory.

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  • Yes, that much I knew. Can you provide calculations based on information given in the book that Hermes can use ion thrusters to uncorrect for return to Earth before they run out of food? – davidtgq Nov 1 '15 at 9:39
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    @vandidant - No. I can't produce calculations. I can assure you though, that a deflection of less than 50 km isn't going to make anything but the most microscopic course-correction necessary in a 225 million km return journey. According to the novel, the Ion thrusters can create a deflection of 5km in less than 1 minute. That means that they only need to run them for 5 minutes to completely undo the reaction thrust maneuver and heck, getting closer to the planet would actually result in their getting back to Earth sooner, not later. – Valorum Nov 1 '15 at 9:41
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    You're right. I did some research on the Mars cycler, and it seems that no gravity assist is provided by Mars. To return to the original trajectory, it'd be relatively as simple as changing lanes on the highway. – davidtgq Nov 2 '15 at 2:34
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    I don't think Hermes was on a cycler trajectory. Based on what I've read about cyclers, in theory they don't require thrust, and they cycle indefinitely. In practice, of course, minor course corrections would be required. The Hermes went into orbit around both Earth and Mars. Otherwise the Ares III crew wouldn't have been able to reach Hermes 6 days after landing because it would be long gone. – Scott Whitlock Jan 31 '16 at 4:54
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    The Hermes was on a cycler orbit when it came back to get Watney. It did NOT go into orbit at that time. – JRE Mar 16 '16 at 10:08

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