I saw the movie and my Kerbal bell rang when the Hermes, at the fastest point in its trajectory, slingshoting around Earth back to Mars, rendezvous with the supplies probe.

Well, everywhere I read says that the movie/book is supposed to have its orbital mechanics right, and that someone actually wrote a program to verify the stuff and such. If so, how did the just launched supply probe from Earth's atmosphere, with only a few minutes of acceleration, matches the speed (within a few m/s) of Hermes accelerating for months?

Am I missing something here? Did Hermes decelerate? If so, what was the point of the slingshot? Did the rendezvous take place enroute between Earth-Mars, so that the probe had time to accelerate (and could make it there by itself)? If so, why did Hermes have to return?

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    I came here looking for an answer precisely to the same question. After all, if the probe could rendezvous with Hermes, which was on an escape trajectory, then the probe could simply escape earth by itself, and fly to mars. Why did it not? Oct 23, 2015 at 13:58
  • @JoachimBreitner The Hermes uses it's ion engines for constant small acceleration. So while the supply capsule definitely needs to be on an escape trajectory to rendezvous with Hermes, it's not a trajectory that will let it reach Mars as quickly as the Hermes itself will.
    – Saiboogu
    Jan 30, 2016 at 23:16
  • I don't understand why was the question downvoted, it is a perfectly valid question.
    – Apollo
    Apr 27, 2017 at 11:17

2 Answers 2


Keep in mind that the Hermes uses an Ion propulsion system, which produces very small amounts of thrust, 2mm/s^2. The advantage of this is that it uses very little fuel (gas) and so can be almost constantly in use.

The probe is using conventional chemical engines which has the advantage of producing lots of thrust in a very short period of time.

This is a link to a Talk at Google with the author showing the orbital mechanics, though he doesn't discuss the rendezvous with the probe. Andy Weir talking about The Martian

  • And the probe was also using a relatively massive engine which had been intended to bring a large heavily shielded exploritory vessel all the way (if I remember correctly) within Mercury's orbit.
    – kaine
    Oct 6, 2015 at 13:08
  • And also with the ion engines running constantly for months, Hermes would have plenty of time to adjust course so that it passes Mars at the best time, place, and velocity for it to rendezvous with the Ares.
    – RichS
    Apr 27, 2017 at 6:17

Agree, both slingshot rendezvous were technically unrealistic, the Earth slingshot rendezvous moderately unrealistic, the Mars slingshot rendezvous terribly unrealistic.

For the Earth rendezvous, the supply probe was on a rocket capable to sending the supplies direct to Mars, so it would have the energy to get the probe to the rendezvous velocity. Probably the rocket didn't have time to get to speed to rendezvous near earth, but a realistic rendezvous could well be a short distance from Earth.

The second rendezvous at Mars is not realistic. The MAV is not capable of escape velocity from Mars. Some commentary call it a sub-orbital vehicle, but that is absurd, because its primary purpose is to rendezvous with the orbiting Hermes. Even by stripping the capsule to nothing, the rocket could not realistically match the speed of the Hermes in flyby.

The only way I have been able to think the catch-during-flyby scenario could work would be if the Hermes were to hook Watney on a shock-absorbing energy absorbing tether. Conceivably, such a thing could be made starting with rope. Then, the Hermes would only need to pass close to the spacewalking Watney, get the tether to him, and connect it to him quickly. Their relative velocities could then be quite different. Dangerous, yes.

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