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In "The Martian", why did they need to catch the probe after they already changed the plan to not support Mark with food, but pick him up from the orbit.

Did they need the supply because the trip of the Hermes got prolonged? Did they say something about it in the movie?

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    What probe are you referring to? Do you mean the Chinese spacecraft?
    – Paulie_D
    Nov 16 '21 at 16:25
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    This excellent video explains the 3 trajectories of the Hermes during The Martian along with how long each one took ... the explanation in the movie was simply the crew discussing how long their voyage would be extended if they went back to Mars for the rescue - "they'll never let you back in space again" - "two years in space is enough".
    – davidbak
    Nov 17 '21 at 2:04
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    @davidbak excellent animation! Oh, Andy Weir's website :-)
    – uhoh
    Nov 17 '21 at 22:12
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    @uhoh - Yes, if you poke around in the parent directory containing that video you'll find other goodies ...
    – davidbak
    Nov 17 '21 at 22:17
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    In the book, they make a minor point about the backup plan in case the resupply launch fails. Not to go too much into the gnarly details, one of the Hermes crew would have been able to survive with the remaining supplies and.. other nutrients aboard.
    – Bass
    Nov 18 '21 at 14:38
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I did not understand why they still had to catch the probe after they already changed the plan to not support Mark with food, but pick him up from the orbit.

I'm assuming you are referring to the Chinese craft, Taiyang Shen.

They had two options...

  1. Send Watney enough food to last until Ares 4 on the Taiyang Shen,

  2. or send Hermes back to get him right now.

They could not do both as both required the use of the Taiyang Shen.

Either way, the supplies needed were on the Taiyang Shen so that's why they had to catch it... otherwise the Hermes crew would have starved.

This is clearly explained (almost verbatim) when Rich Purnell explains his plan to the NASA bosses.

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All of NASA's original planning was all about sending more food and supplies to Mars, to keep Watney alive for years before Ares 4 was scheduled to visit Mars. In particular, they needed to launch some supplies to Mars very soon or Watney would starve.

They rushed their own launch but botched it. Fortunately, the Chinese had 1 rocket capable of making it to Mars.

And then... Rich Purnell figured out a plan to slingshot Ares 3 off Earth's gravity, and send it back to Mars, and pick up Watney directly. Big problem: Ares 3 was out of food and other critical consumables, so sending it back without resupply would kill the entire crew and Watney too. So to do the Purnell plan, Ares needed resupply from the earth.

So that is why Ares 3 met the probe.

To answer your question.

Second problem: the slingshot maneuver is NOT an Earth orbit, and is much faster - a higher delta-v. That means common, normal spacecraft like Soyuz or Raptor that go to LEO all the time, would not be fast enough to catch up to Ares 3 to hand off the supplies.

So they needed a Big Forking Rocket. And the only rocket large enough to carry that much resupply payload, to that delta-v, that would be ready in time, was that same Chinese rocket.

Thus they could not do both missions at once. They only had 1 rocket. They had to decide to either resupply Watney, or resupply Ares and send them back for Watney.

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    Ares was the overall mission name, no? Ares 4 would have thus "gone" to Mars, not "returned" to Mars. The spacecraft the crew was on when orbiting Earth and returning to Mars w/ new supplies was the Hermes spacecraft (the references to "Ares 3" here are a bit confusing), not the Ares 3 spacecraft.
    – TylerH
    Nov 18 '21 at 16:38
  • @Tyler good point. Edited. I don’t want to get into The gory details of the Hermès spacecraft being reusable. Nov 18 '21 at 17:23

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