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Comment by adastra | Hacker News Reader

But the goal isn't to just colonize any old planet. It's to colonize a place we can successfully terraform. Mars is the clear winner in that respect, and it's not close.

Mercury needs basically a planet's worth of water and atmosphere to be imported. Mars on the other hand could be (we hope) good to go. The only question for Mars is whether there are abundant amounts of nitrogen locked up in the soil. But even if not, the amount of stuff we'd need to import by bombarding the planet with asteroids would be orders of magnitude less than for Mercury.

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I think the article makes the case that you do not need to import water, it is supposed to already be there. If you have a ready supply of hydrogen and oxygen (which indications are Mercury does) and an incredible amount of energy that can easily be converted you reduce your import burden substantially. This has always been my doubt about Mars, is the energy available that we need to get anything done over there? (I also agree that Nitrogen availability is a serious issue for any food production, regardless of the strategy, and this is likely a Mercury weak pont.)

Terraforming is not the goal, it is a means to an end of making a survivable planet that does not required resources from earth. Limiting your options to terraforming is not required and may not be desirable.

I'd say the goal is to find a survivable planet that can support billions of people. In that sense, the amount of water ice that is in a couple of permanently shadowed craters is really negligible. And on Mercury you're talking about only a narrow zone around the poles that you can support people if you excavate massive amounts of rock underground.

If you're talking about the energy required on Mars to change the atmosphere on a planetary scale, is there a source that says that would be a limiting factor? None of the research on this that I've seen considers it to be an issue. Mars receives about 1/3 the incident sunlight that Earth does, measured at vacuum. But the amount of that incident sunlight that will reach the surface will be higher on Mars due to the thin atmosphere and lack of cloud cover.

Perhaps the eventual goal is billions of people on another planet, I think the medium term goal is just a large enough gene pool to ensure survival of the species. You do not need billions for that. The truth is that even if you want billions you do not necessarily need a whole lot of space for them, if you examine the total area of arable land on earth and how many people you can pack into a city the space requirements are not as large as one might first assume if you have climate controlled growing conditions and a 100% urban population.

I have read a lot of material about the energy required to sustain some sort of colony on Mars but I have never read anything estimating the kind of energy strategy required for terraforming or building a substantial colony. This is why I have doubts about it, I do not see it being properly considered. For example most of the terraforming schemes involve the generation of greenhouse gases, creating a more opaque atmosphere and nullifying the thin atmosphere advantage. So now you have a sun-starved version of earth without the repository of hydrocarbons to dig up and burn off that earth has. I'm no expert though, this is a fairly uneducated opinion.

"Terraforming" is just a concept. We have no idea how to do it and whether it's even possible. And even if it were, it would take hundred of years and would need constant life support from Earth for that time. Hardly a good bet either...

Mars has no magnetosphere which makes it a non-starter due to radiation unless you live underground or in heavily shielded dwellings.

If you have to do that you might as well start at either the Moon or my personal favourite Europa - a vast ocean of water kept warm by tidal forces underneath a thick protective crust of ice. Power would be a challenge, but there might be a way to harness either those tidal forces or the radiation from Jupiter.

Mars is a non-starter? I think you've received some bad information.

Average radiation on the Mars surface is 10-20 rem/year. With a thicker atmosphere it will be less. And for people spending 12-15 hours a day indoors in shielded dwellings it will be a lot less. It's not a showstopper at all.

The low gravity may not be enough to hold a thick atmosphere, no?

Not for more than 100 million years or so. But then, Earth will also become uninhabitable within the next few hundred million years.

If you want to colonize Europa, you'll have to dig a lot deeper than on Mars or the moon. Cosmic ray dosage on the lunar surface is around 30 rem/year. On Mars, it averages 50.

On Europa, it's around 500-600 rem per day. That's a fatal dose.

Indeed it is. Colonizing the surface of Europa would be madness without serious shielding. What's the dose under a 200m thick ice sheet though?

I believe the goal is to survive.

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