Elon Musk is proposing a ton of audacious things to get to Mars before the 2020s are over. But perhaps the most striking feature of his plan is the simplest. He's not just sending people to Mars; he's planning on bringing them back.

At this point, every journey to Mars has been a one-way trip. NASA is only just now planning a rover for a 2020 launch that will gather samples for return to Earth—how we're going to get the small collection of samples back hasn't yet been specified. By contrast, Musk intends to return everything: the people, the ship, and presumably any souvenirs that clear customs. That intention is going to require radically rethinking the approach.

One of the key things that will have to change is what our hardware does once it gets there. So far, all our equipment has been designed to sample the chemistry that's present (though that will change on the 2020 rover—we'll have more on that in an upcoming story). Musk's plan envisions creating a chemical factory on the red planet, one that makes all the fuel needed to get back off the surface and return a ship to Earth.

The basic chemistry of making fuel is simple. The challenges of doing it on Mars are rather profound.

A recipe for methane

The engines of the Interplanetary Transport System are being designed to burn a fuel—methane—with Mars in mind. While there have been indications that there are methane sources on the Red Planet, readings of the atmosphere made by the Curiosity rover indicate that whatever's producing them is erratic and ephemeral. All indications are that the gas doesn't survive long in the atmosphere, either, so harvesting the gas is out of the question. That means fuel has to be made from ingredients that are present and easy to get at. That's been thought through as part of the process of choosing methane in the first place.

Methane is simply a carbon atom bonded to four hydrogens. The plan is to produce it using what's called the Sabatier reaction, well understood chemistry that dates back over a century. It involves reacting carbon dioxide with hydrogen, which breaks up the carbon-oxygen bonds to form water and methane. The reaction is energetically favorable thanks to having water as an end point, but it requires energy, pressure, and a catalyst to work, thanks to the stability of the carbon dioxide.

For the purposes of this article, we'll assume that Musk can get an essentially infinite supply of solar panels to Mars. No, owning Solar City won't help; he'll want to use high-efficiency gear that's too expensive to slap on a house but will provide the most electrons per weight. Luckily, these can be bought from other sources. More panels would mean faster production, but they don't fundamentally change the chemistry.

To the extent that Mars' thin atmosphere has a lot of anything, it has a lot of carbon dioxide. There's quite a bit of hydrogen available, as well. It just happens to be in the form of water, which would have to be split to release the hydrogen. Again, this is well understood chemistry, and it simply requires a catalyst and some energy input. Once the full production process is up and running, at least some of the hydrogen from the Sabatier reaction ends up as part of water, which can be cycled back to make more hydrogen.

The other noteworthy thing is that the end products of burning methane are carbon dioxide and water, the same as the starting ingredients. Thus, the processes used to make methane in the first place should produce enough oxygen to power the engines. Of course, people are going to want some of this to breathe, as well, so at least the water-splitting reaction might need to be run in excess. Or, there are ways to extract oxygen from carbon dioxide, and there's also going to be a small bit in the atmosphere that's used to supply the CO 2 . In any case, the basic chemistry works out.

Raw materials

Pulling carbon dioxide out of the atmosphere is a relatively simple process. In fact, it's about as simple as pulling in the atmosphere, which is more than 95 percent carbon dioxide. Water, by contrast, is a bit more of a challenge.

There's a tremendous amount in the polar ice caps. But it's not clear that anyone would actually want to go to the poles, in part because of the incredibly harsh conditions there. The poles would also limit the amount of solar power available, putting a crimp in the production process. At least some of that water ice is carried by the winds to more temperate regions, enough to create thin frosts and a weak water cycle. But it's not clear if that's enough to power the levels of chemical production this plan needs.

There is an additional source, part way between Mars' equator and its polar ice caps: glaciers. "Numerous glacier-like forms have been identified in the mid-latitudes of Mars, and within recent years the acquisition of radar sounding data has revealed that the features are chiefly composed of water ice," says a recent paper that takes an inventory of Mars' ice. It calculates that there are over 100,000 cubic kilometers of water ice in Mars' mid-latitudes—that's a lot of potential rocket trips. Musk's presentation specifically referred to "water mining," so it's clear this is what he's targeting (although he lists Mars' total water inventory, including the poles).

This still makes the landing sites geographically limited—you'd have to drop near one of them. This rules out the equatorial regions, but the glaciers are widely scattered throughout the remainder of the mid-latitudes. Of course, the landing sites would seem to demand a preliminary lander to survey the ice and ensure it's accessible enough and high enough quality to risk an entire ship (and its occupants) by landing there. With the right site, however, this should work.

Chicken and egg

Really, the only place where things run into trouble comes in the form of a series of competing priorities. Musk's presentation stated, "First ship will have a small propellant plant, which will be expanded over time." A small propellant plant, however, means it'll take more time to produce enough fuel for the return trip. And more time either means more supplies to keep everyone fed while that propellant gets made, or it means fewer passengers.

The obvious solution would be to send a small crew and lots of hardware to a landing location ahead of any major passenger trips, and that's likely what will be in the works. That would allow the plant to expand with the very first passenger-carrying trips. It would also provide the chance to pre-produce lots of fuel before the second ship lands.

While this makes sense, it involves committing increasing amounts of infrastructure to a single location. Which means committing future landings to the same spot on Mars (and the obvious risk of creating a "hive of scum and villainy" like Mos Eisley). Which is a bit limiting.

More problematic is the fact that the resources at the location will also be limited. Glaciers are big, but they're not infinite. Either you're going to have to repeat the process elsewhere at some point, or you'll need to design the entire chemical plant so that it could be packed up and moved.

None of this is insurmountable. It may just be that we have to accept some level of having Martian infrastructure become a stranded asset as travel continues. But from the chemistry perspective at least, Musk's plan should work for the initial exploration, pending a few choices about whether to make fuel in advance of the first passengers.