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I've just started watching the show and haven't read the books, so perhaps this is answered later than episode 4. There is a discussion in the very first episode where it's mentioned that Ceres used to be full of water and Earth and Mars took it. Why would Earth need water?

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    If nothing else it probably costs less to get water to Earth orbit from the asteroid belt than from the surface of Earth. – DavidW Jul 31 at 18:43
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    @drewbenn: actually it's probably not freshwater but briney water. I assume that either the original author didn't know that or assumed that water distillation technologies were advanced enough for it not to matter. However, if distillation were possible on such a large scale for Cerean water then why not just desalination ocean water? – Michael Stachowsky Jul 31 at 18:57
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    Haven’t read the novels, but seeing the series I got the impression oceans and others sources were so polluted they actually needed to import for human consumption, no desalination or viable mass-decontamination possible anymore. – Seretba Aug 1 at 1:00
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As long as you don't need it fast, it's much cheaper to ship water (or any other bulk material that's available) from the asteroid belt to Earth orbit than from the surface of the Earth. Just to get to LEO (200km) from Earth's surface is a minimum of 9.4km/s of delta-v, more if you're in a higher (more stable) orbit. LEO to Ceres averages around 9.5km/s of delta-v, also from a 200km LEO.

If you're in a higher orbit, or even moreso at one of the Lagrange points (I believe there suggestions of stations at L4 and/or L5) then the cost shifts; you're looking at a delta-v requirement of >12km/s from Earth's surface, but <7km/s from the Belt.

(Note that those are strictly delta-v requirements; losses due to atmospheric drag aren't included, nor is the cost of a rocket body/fairing to hold the water. From the Belt to L4/L5/GEO you just need to freeze a ball of water, wrap it in mylar and stick a small ion drive on it. Also, launching from the Earth requires being able to accelerate at least 15m/s^2 just to overcome gravity, whereas from the asteroid belt the required accelerations are a couple of orders of magnitude smaller. Smaller accelerations means less mass required for motors, and more efficient use of fuel.)

  • I love the answer but I'm not immediately convinced (unless, of course, the original author of the books simply needed a plot device). They have extremely high thrust engines, so much so that they need special technologies to avoid dying while using them, and fuel is never mentioned as an important resource. Also, they burn their engines constantly at high thrust, implying that they have extremely fuel efficient engines... – Michael Stachowsky Jul 31 at 19:19
  • They did not always had it. Epstein drives were invented when Mars was already partially colonized. Mars has water deposits, but probably not enough for full terraforming. – b.Lorenz Jul 31 at 19:34
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    @MichaelStachowsky I'll admit I'm not super familiar with the technological background of The Expanse so I just assumed they weren't firing off a fusion drive in an atmosphere. My bad if I'm wrong, but just in case I'll be watching from over there. Way over there. :) – DavidW Jul 31 at 19:50
  • @MichaelStachowsky They have high thrust engines but the details of the technology are not described. Maneuvering thrusters use water as reaction mass. It is possible that the Epstein drive consumes water. It is also possible that the engines used before the Epstein drive used water as a reaction mass and that much of Ceres water was consumed before the Epstein drive was developed. – krb Aug 16 at 15:51

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