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Comment by jitl | Hacker News Reader

Where are you going to get the water? What makes water mass cheaper to orbit compared to nitrogen mass?

Hang out a big butterfly net and capture the dirty snowballs that float past.

There have been lots of new releases of frozen water discoveries on various bodies. Typically in craters that pretty much stay in shadow. Water doesn't seem to be that scarce. It's just not as abundant as on Earth

Going to the moon to get water and bring it back to earth orbit is going to be much more expensive than sending water from earth for a long time.

The image of the really big mass driver that looks like a train you get from O’Neill or Heinlein is obsolete.

The US Navy built a 2.5km/s railgun you could fit on a ship. A 3.5 km/s coilgun has to look like the Paris gun to be practical and be able to shoot at a high elevation to hit Earth-Moon L1/L2 or near-Earth space. A railroad car worth of material per day in 1kg or so increments seems plausible. A radiation shield for a deep space station or a simulated asteroid to test mining and manufacturing technology might compete with terrestrial materials.

O’Neill’s students never came up with believable catcher and I’ve yet to see one I like. I guess you could get to LEO if you could aerobrake but it looks tough because the outer atmosphere is always changing and a wild shot could trash your target, talk about a space junk problem.

The moon has at best a large glacier on it and my guess is Lunarians, if they could vote, would not want to export a gram of it but rather incorporate it into a circular economy.

you just let fleet of Boeing 702 go fetch couples of comets and asteroids into L4/L5, build Manhattan sized pressure cookers around each piles, allow contents to come to boil on solar heat, and boom, pressurized habitats

Closed-loop systems like on the ISS already reclaim water from humidity, sweat, and urine, and could be scaled up for larger habitats. Air works similarly: CO₂ is scrubbed, oxygen is regenerated from electrolysis, and trace gases are filtered. I realize you still need an initial stock of oxygen and nitrogen, but isn’t this a problem that could be solved eventually with science?

This is like saying “once we solve the problem, the problem will be solved”

Now you’re speaking my language! Let’s get started shall we?

After we solve it the problem won't exist anymore. No more deals.

>What makes water mass cheaper to orbit compared to nitrogen mass?

Nitrogen needs a high-pressure bottle which is wasted mass, whereas water you can send to orbit in thin plastic and fabric bags like they do on ISS.

Often on ISS they're actually shipping oxygen as water, but still the wasted mass of water being 11% hydrogen plus the wasted mass of the bag is more efficient than a high pressure gas cylinder.

For my baby Bernal sphere it is shipped either as LN2 or NH4. The large airspace needs a lot of it compared to the water inventory. O’Neill’s published designs except for the Stanford torus were much worse.

The water inventory of both N2 and H2O on Earth (0.2 cubic km of water per inhabitant) is extravagant. Ceres is roughly 50% water (like the generic outer solar system or interstellar object) and we could use all of it but it adds up to about the mass of our oceans, and that is most of the water inventory the asteroid belt.

I’ve looked at various large rotating structures up to the size of the Banks Orbital and concluded you could solve the structural problems if you could make a reliable journal bearing (structural parts don’t need to be spun) that could handle the velocity difference but no matter what you cannot afford tall mountains or deep oceans.

Agreed, liquid is probably the way to go.

Very tempting to swap nitrogen for helium, but AIUI the main downsides are that the speed of sound is higher so air will leak out from a breach faster, and there may be negative effects on flammability due to helium's high thermal conductivity and low specific heat.

Helium gas also leaks a lot, even when everything is normal.

Probably a naive question but would it possible for us to skim gasses from LEO for use in space stations?

Putting aside issues with capturing basically vacuum at 8km/s, by doing that you're going to lose impulse that you're going to have to regain, spending more mass in fuel+oxidizer than you gained in gathered gases, unless you use ion propulsion (which ISS doesn't). Also most of what you're going to capture at 400km is going to be atomic oxygen and helium, N2/N share is only a couple percentage points at that altitude.. [0]

[0] https://en.wikipedia.org/wiki/Heterosphere

Clearly you would not choose similar operational parameters and propulsion as the ISS when designing an atmospheric scooper. Not sure why you would assume so, honestly.

There have been proposals to use scoopers with structurally integrated electrodynamic propulsion ("tether without the tether"),[0] which would use zero propellant.

[0] https://forum.nasaspaceflight.com/index.php?topic=44542.msg1...

We already built LOX tanks into rockets. Just send up a tank of LN2. It doesn't need the strength of a bottle, you just need thermally insulating materials which can be very light.

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