Mars is also much lighter than Earth. Venus is less massive than Earth, but not by much.
Even so, Venus has had most of its hydrogen stripped away over time. Thanks to its high temperature, that also includes hydrogens formerly bound in heavier molecules, like water. The process continues, very slowly. Venus is still losing its hydrogen and helium, more quickly than Earth.
One terraforming plan for Venus involves shipping magnesium hydride there, decomposing it, and using both the H2 and the Mg to react with all the CO2 in the atmosphere to make soot, chalk, and water, reducing the surface pressure to about 3 atm of mostly N2. Nothing works without shipping hydrogen to Venus. But once it's there, it will take many millions of years to get stripped away again, because that happens so slowly, over geologic time scales.
I guess it depends how much magnesium hydride you find or can make there. It's a very long trip from the belt to Venus though.
And honestly, if you have the resources to mine global amounts of magnesium hydride from the belt and ship them all the way to Venus you have the technology to build a nearly unlimited number of space colonies that would be much much nicer to live on than even a terraformed Venus.
Yep, that's the problem with these ideas about terraforming. If you have the technology and energy to change the environment on a planet that much, it's really easier to just build O'Neill cylinders or some other large stations and rotate them so that you get perfect 1g gravity, and you can stick them in whatever orbit you want (i.e., close to Earth) so that they aren't far away and they're at just the right distance from the Sun so that they aren't too hot or cold.
I wonder if maybe future space dwelling people will look at living on planets like we do at living in caves.
What I feel would be missing for me is the scale and freedom of living on an earth like planet. It might be different if one is born and raised on a space station.
For "scale" and "freedom", theoretically, you could build as many O'Neill cylinders as you want (there's lots of resources in a star system), and make them quite large. What's easier about planets is that you don't have to have this technology or go to this trouble, but the problem is that in reality, there just aren't any habitable planets out there within a reasonable distance, except the one we're standing on right now. If we discovered warp drive or some alien stargates or something and could easily transport ourselves to countless distant star systems, that might be different, but right now it isn't.
That's the problem with Elon Musk's mars colony plan. There seem to be better alternatives closer to home. Even the moon would be better in many respects, but space stations make more sense.
Well to be fair, building an O'Neill cylinder is well beyond our current capabilities, and simply flying a craft to Mars and blowing up an inflatable dome is much easier and more practical given our current technology.
However, I do agree that this colonize-Mars idea seems a little silly. We have a large body (the Moon) that's only a few days away, instead of 6-18 months; why not start there? Once we get pretty good at mining on the Moon and nearby asteroids, we should get better at building some rotating space stations.
If you find any asteroid with a sufficient magnesium hydride (or any other useful substances that could help you terraform Venus), then changing its orbit so it goes and collides with Venus could be orders of magnitude simpler than building a space colony.
In the asteroid belt there are some so-called Kirkwood gaps [1]. The main one is at 2.5 AU from the Sun; an asteroid orbiting at that distance is in a certain resonance with Jupiter (and with the Earth too) that makes the orbit unstable (on large time scales). The orbit becomes more and more eccentric, until it crosses Mars's orbit, and in time it gets close to Mars and gets sling-shot somewhere. There is a good chance you can nudge an asteroid close to this or another Kirkwood gap, and do it in such a way as to obtain the exact Mars slingshot that the asteroid ends up crashing into Venus. How can you nudge an asteroid? By ablation, e.g. [2]. You can use either a large parabolic mirror that orbits the asteroid, or a powerful laser. What timescales are we talking about? Asteroids in the Alinda group [3] have an orbital period of 4 years; if we manage to perturb one's orbit and execute the Martian slingshot in 10 periods, that's roughly half a century, which is in line with the mission length of Voyager.
You are absolutely right. I checked wikipedia ([1] and [2]), and it turns out that the mass of whole asteroid belt is only about 6 times as high as the mass of the Venusian atmosphere, and half of this mass is taken by the top 4-5 asteroids, which arguably we wouldn't want to start flinging around the solar system. Moreover, most of the asteroids are far from the Kirkwood gaps (almost by definition), so if you want to nudge them you would lose a lot of mass to ablation.
I think the technology would be to send a dumb robot to an S-type asteroid, to mine it for silica-associated metals, very slowly, using solar power. The Mg and Ca dust pellets would be wrapped in aluminum foil and thrown at Jupiter, or solar-sailed to it. Another dumb robot would catch the packets, add H2 pumped up from Jupiter's upper atmosphere or electrolyzed from water on Europa or Callisto, and throw the hydride packet at Venus, where it would crash into the planet completely unattended and react in the atmosphere. At no point do those miners support human habitats. They can also throw other raw materials at Earth-Moon L4 and L5, for station construction.
After 1000 years of dumb robots throwing foil-wrapped-dust-balloons at each other, and at Venus, a ton at a time, Earth throws some algae and extremophile vent bacteria at Venus. Then, after another 1000 years, you maybe have a planet ready to be landed on without immediately being crushed and burned by the hellish sky.
This is not mutually exclusive with building in-space colonies, or with terraforming other celestial bodies.
And we can build vacuum-breathing space robots now. Building a space habitat that can support an Earth-life ecosystem indefinitely is currently beyond our means, if not our abilities. Space robots playing catch is easier.
Humans are really bad at maintaining big projects for any length of time, let alone thousands of years. This couldn't be a small project either. You'd better be slinging hundreds of tons at Venus every day if you want it to be done in just a thousand years.
Terrestrial Mg mining produces just shy of 1000 tons per day.
To terraform Venus's atmosphere with MgH2 in 1000 years, the robots in the belt and near Jupiter would have to process roughly 3.6e17 g per day, which is hundreds of billions of tons, hundreds of millions of times more than the entire Earth mines in a day.
You would need a lot of robots.
The project would have to be an AI capable of the following:
- Run for 1000 years without failure.
- Build robots that can build robot-building robots.
- Build robots that can build robots that mine S-type asteroids.
- Build robots that can build robots that mine hydrogen from Jupiter or its moons to make MgH2 and CaH2.
- Build robots that can build robots that patiently move bulk cargo around the solar system.
- Build robots that can build robots that electrolyze water in Venus orbit.
- Build robots that can build robots that defend and repair the project infrastructure.
- Build robots that can build robots that build and operate orbital shades, lenses, and mirrors.
- Fairly allocate the surplus by-products to human bidders.
The project would be a significant source of cheap aluminum, iron, nickel, silicon, sodium, potassium, and oxygen, as well as as much magnesium and calcium as is needed for ecosystem life support in space. Enough Ca and Mg to make all the bones of a billion animals is an insignificant fraction, compared to the amount required for terraforming.
The attention span of a human is already too short to spend 100 years moving tons of boring old rock to a rendezvous orbit. The goal has to be to set up a system that benefits humans to allow it to continue without interference, rather than one that requires cooperation.
It's the initial bolus of hydrogen to re-wet the Venus atmosphere and cool it off that's the megaproject. After that, the replacement rate is ridiculously low.
And if the lost hydrogen is never replaced, the terraforming-added hydrogen will still likely last longer than human civilization. Venus has been losing H2 and He for billions of years, and still has some left to lose. If we add enough H2 to precipitate all the CO2 out as mountains of C and oceans of H2O, and cool off the planet enough that the hydrogen stays in the water, the atmosphere will be fine for the foreseeable future.
One basic Venus atmospheric-terraforming equation is "2 CO2 + MgH2 -> C + MgCO3 + H2O", by multiple reactions at different temperatures and pressures. By mass, 4.6e23 g CO2(g) + 1.4e23 g MgH2(s) -> 6.3e22 C(s) + 4.4e23 g MgCO3(s) + 9.4e22 g H2O(l).
That produces an ocean about 12% the mass of Earth's, but requires roughly 4% the mass of the asteroid belt in MgH2, or a sphere of solid MgH2 with diameter 570 km--about the same volume as Pallas, Vesta, or Hygeia (but half the mass). Magnesium is the 12th-most abundant element in the solar system, and could be mined from olivine-plagioclase asteroids. (Calcium can also be used to transport H2.) About 17% of asteroids are S-type. Seems doable.
Alternatively, decompose the MgH2 to Mg and H2 in Venus orbit, and only send down the H2, sending the Mg back out to store more hydrogen. That's "CO2 + 2 H2 -> C + 2 H2O". By mass, that's 4.6e23 g CO2(g) + 4.2e22 g H2(g) -> 1.3e23 C(s) + 3.8e23 g H2O(l), which would make an ocean about 28% as massive as Earth's, but requires that some magnesium (or calcium) make several round trips just to carry H2, which is relatively light.
If you're going to do that, you could also just ship H2O to Venus, electrolyze it in orbit with solar power, drop the H2 down the gravity well, and ship the O2 back out to the comet wranglers for breathing gas. You'd need to move exactly as much water into orbit as you're eventually creating on the surface, so 3.8e23 g H2O. That's more mass to ship than the MgH2 answer, but at least we know that the water is easily found in comets, and on Europa and Callisto, and easily processed.
There's no reason why any of these should be mutually exclusive.
Not all at once. A ton at a time, for a long time. That was just an illustration of magnitude.
Also, you have to mine and refine the rocks, so that the pure metals can be turned into hydrides. Ceres wouldn't have enough hydrogen just as itself.
It could still work as olivine to carbonate weathering. Cut Ceres up into pea-sized gravel, and drop it onto Venus, then use a solar shade to cool off Venus enough that metal carbonates don't immediately calcine into metal oxides. The exposed rock would suck up some of that CO2. No ocean, though.
Venus has a very thick, hot and high pressure atmosphere. This both makes it hard to blow off and gives it a strong ionosphere which acts as a stand in for a magnetosphere.
How would you explain Venus having an atmosphere while not generating a magnetic field?