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At one point in the book after the resupply, when the Hermes is on the way back to Mars, Beck is giving a status report and says, w.r.t. life support:

Limping. We've been in space way longer than it was designed to handle. There are a bunch of filters that would normally be replaced each mission. I found a way to clean them with a chemical bath I made in the lab, but it eats away at the filters themselves.

So, they're consumable filters that the crew can reach (as well as the 'tween mission ship maintainers) and the actual lifespan (and presumably the expected lifespan as well) is less than two Mars round-trips. And, you know, life support!

Why didn't NASA send some up on the resupply ship? NASA oversight? (Or author oversight?)

  • I am sure there are bunch of things that can be replaced by the crew on a ship but every one of them may not have replacements on board for plenty of reasons (weight is one). Plus, as you have already noted, no one expected the ship to be there that long. – bobbyalex Feb 4 '16 at 2:26
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NASA may not have realized the Hermes was going to need them.

In is only inference that filters are designed to last two years; they may have been designed to last significantly longer than that. After the Earth gravity assist, the Hermes did something it was almost certainly not designed to do: it spent two to three month inside the orbit of Venus, when normally it would spend all of its time outside Venus orbit.

The ship, which produces enough heat that it has to actively radiate it to stay cool enough, would be spending a lot of time closer to the sun than it was presumably designed to handle. It would not be unexpected for things to go wrong, and sometimes the things that would go wrong would not be problems they would expect ahead of time. While there is no way to be certain, the filters may have been one of the unexpected problems.

Or maybe NASA just didn't think of it in time; they were in a bit of a rush, after all. A few oversights wouldn't be a huge surprise.

Edit: Holy run-on sentences, Batman!

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    Huh, I had no idea about where the Rich Purnell maneuver actually sent them. But based on your answer, I googled and found this reddit that showed it going well inside Earth's orbit. But notice that that animation shows the original flight time to be ~390 days while the rescue (total) flight time is ~900 days. And Beck says these filters are normally replaced after every mission. – davidbak Feb 4 '16 at 2:53
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    @davebak You're right, I'd forgotten that. Maybe they can't be replaced except while the ship is 'in dock'? Or someone didn't include them because (technically) the ship was still on the same mission... As far as the flight path, take a look at insidescience.org/sites/default/files/… image from InsideScience.org; it shows the orbit of Venus as well as Earth and Mars. (I hope I did the link right, there. No, I didn't.) – Kaine Feb 10 '16 at 3:25

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