Using asteroids for water

Question

I have a planet that has no natural water bodies or sources. Is it feasible (assuming technology along the lines of the Earth Engines from "The Wandering Earth") to take asteroids from a belt/field and deliver water to this planet? Probably enough water to sustain a human population of 1 billion + agriculture.

This also probably ties in to "what is the best way to transport water" and ice density.

(I am new to this, please go easy on me)

Edit: (clarification) Earth engines are giant engines pointed towards the sky in a large array that are used to move the Earth out of orbit and to a new system.

Answer 1

Depends

For base assumptions (Since you haven't mentioned the specs of your solar system), we are going to take our Solar System for a start.

In case you are trying to mass mine water ice from asteroids in the Asteroid Belt, which is located between the orbits of Mars and Jupiter, you would be hard pressed to find water in the Asteroid Belt. It's a straight pain in the a** to find enough water to provide water for a single spaceship.

Based on this paper, there is only about 100-400 billion gallons of water in the asteroid belt. And thats not per asteroid, that's the TOTAL AMOUNT of water in the asteroid belt as a whole

400 billion gallons may seem grand enough. But only till you realize that the total amount of water on Earth is 326 quintillion gallons. The asteroid belt contains about half the amount of water present in Lake Huron. And that means each asteroid would contain a tiny miniscule proportion of water in them. Of course there are exceptions like Ceres, that contain a lot of water. But as I said, on average, it is as hard to scavenge (yes, scavenge) water in the asteroid, as to move a truck by using fireworks.

However, there is a lucky spot in the Solar system, that is rich and juicy (yes "juicy") with water- Pluto's crib The Kuiper Belt

If your humans have access to the Kuiper Belt, they are GODDAMN LUCKY.

Based on this paper's estimates, if you could gather the entire mass of the water in the Kuiper Belt, you can gather as much as 5 EARTH MASSES worth of water. This would rival GJ 1214b (An ocean world with 8 Earth masses) in terms of water present.

Your humans could send large mining probes (manned/unmanned), to mine out the asteroids/comets in the Kuiper Belt, to get a ton of water from each asteroid/comet. Since each comet (I am going to stop using "asteroid") is rich and juicy with water, your planet could easy get wet and drench to the brim with water quickly.

Happy mining comets!!!

Answer 2

You absolutely could. There's a lot of water in the asteroid belt. Most of it can be found in Ceres... it is estimated that Ceres may be as much as 25% water by mass. Given that it weighs asbout 9x10²⁰ kg, that gives you ~2.25x10²⁰ kg (or ~50 billion billion gallons) of water which is a generous share for each of the billion consumers.

That's small beans compared to Earth, which may as well be an ocean planet... Earth's seas have five times that much water alone, and water in the deeper layers of the planet could mass at least 10x that again. Your world might be a bit of a desert by comparison, but it at least wouldn't be dessicated. I'm not sure if you'd be able to form a natural water cycle, but if you kept the water in domes and tanks and so on you wouldn't have to worry about losses. Spare water might be parked in the form of ice at the sun-planet L2 point. I do have to wonder why a billion people would live somewhere with no water, but its your setting and not mine!

Now, moving even dwarf-planetary masses about is very far from trivial and neither is breaking them up for parts, but if you're handwaving in mechanisms capable of samely ejecting Earth from the Sun's gravity and driving it somewhere nice, then moving around something 10000 times smaller should be child's play. Completely dismantling Ceres and gently depositing it on the surface of a planet is left as an exercise to the reader, but again: if you can move the Earth then this sort of thing shouldn't present a significant challenge.

If your local asteroid belt is a bit dessicated, you don't have to go as far as the Kuiper belt or Oort cloud (which are a very long way away, so transport times are likely to be quite extended, and very large, so exploring and harvesting them will inevitably be awkward and slow). Once you get past the frost line of your local planetary system, you should find a fair amount of water given that its elemental constituents are some of the most abundant in the universe. Ceres is right on the edge of the Sun's frost line, so maybe in your setting such a convenient world isn't available, but gas giant moons are pretty packed with ice too. Ganymede is heavier than Ceres by a considerable amount and it is less dense implying that it may be as much as 50% ice by mass.

If breaking up something that big is too much of an engineering challenge then you may find that nature has done it for you in the form of something like Saturn's rings. This has less water than Ceres (about 1.5x10¹⁹ kg, but that's still 15 million tonnes of the stuff per person on your world) but it is mostly in the form of easily accessible lumps that may be many meters across for convenient collection. If you can shuffle moons around then bringing some ice moons to their Roche limit by moving them into a close orbit around their parent and creating some new ring systems might be a good way to split them up for harvesting and transport.

Answer 3

Look Down, Not Up

Planets are formed by the accretion of local elements. If your solar system has a significant amount of water in the asteroid belt, but not on the surface of an inner planet, it does not mean that your inner planet does not have water, but that the surface water has evaporated away over time. However, your primordial solar system would have had a relatively even mix of elements meaning that your planet should have similar amounts of subsurface water as you find in your asteroid belt. You just have to figure out how to mine and refine it.

Depending on the tectonic nature of your world, this will look like one of 3 things.

1. The cheapest and most probable option is that the planet has subsurface oceans that you can tap into with technology similar to a modern oil or fracking rig. Any planet that ever was tectonically active will likely have significant subsurface oceans; so, by the time you have a permeant settlement of anywhere near 1 billion people, you'll have had plenty of time to explore and survey your desert world for these subsurface oceans.
2. The planet is not, and never has been tectonically active. Here, any rocks below the plant's surface will contain large amounts of primordial minerals rich in oxygen and hydrogen that have never seen the blast of solar radiation to strip the water away. These minerals will form steam when smelted. Because there is not techonomic activity, it means that the subsurface rock is not being naturally smelted and recycled; so, they will contain significant amounts primordial water producing elements even if no liquid water can be found.
3. Your people rely on a hybrid model. When your colony is first founded, you will likely not know where to find sub surface oceans, and as you expand, your demand may exceed what you can get from them requiring you to smelt rock in addition to your deep water mining. In this case, smelting rock will be less efficient, but still doable.

Why this is better than asteroid mining

Moving mass across interplanetary distances is extraordinarily expensive. Even if you found a giant asteroid made of pure water, moving the trillions of gallons of water your population will need across this distance, and lowering safely down into an atmosphere/gravity well without it all just melting away is an unimaginable cost. Doing that with modern technology would cost thousands if not 10s of thousands of dollars per gallon of water.

In contrast, a gallon of water pumped from a deep subsurface ocean could be almost as cheap as water is today when pumped from Earth's own subsurface water reservoirs... though it could be a bit more expensive depending on what kinds of impurities you need to remove from it before its fit for consumption and/or agriculture. This puts the cost at somewhere between 5 and 8 orders of magnitude less energy per gallon than it takes to get your water from space on a world with Earth like gravity.

The second option is of course more expensive because you have to heat rock to its melting point. Depending on what minerals are native to your planet, expect to need to heat up rocks or sand to 600-1200°C to get water out of them. This is of course going to be several times more expensive than mining a subsurface ocean, but it's still probably going to be about 3-5 orders of magnitude cheaper than importing your ice from space.