If Elon Musk has his way, humans will be going to the red planet—and soon. Over the past few days, our senior space-master Eric Berger has analyzed Musk’s Interplanetary Transport System plans in detail , and our chief scientist Dr. John Timmer has examined the science of how to generate resources on Mars . But Musk’s thoughts for getting to and living on Mars—both the nitty-gritty details and also the glossed-over blank bits—still come across to many folks as science fiction. It’s all well and good to talk about building an enormous rocket and a self-sustaining Martian colony, but it’s another thing entirely to do it.

So who better to discuss the differences between science and science fiction than an actual science fiction author—one who studied up for years on the problems of surviving on Mars before crafting a cracking good story about how exactly it might work? And as it happens, we know the perfect such person: author Andy Weir, whose best-selling novel The Martian (and the subsequent Ridley Scott movie) covers much of the same ground Musk wants to cover—though Musk is dreaming on a much larger scale.

“In-situ resource generation”

Though Musk and SpaceX envision a long-term plan that involves thousands of launches to Mars, the company wants to bring most of those spacecraft back to Earth—otherwise, as Musk explained, we’d end up with a gigantic “spacecraft graveyard” littering Mars. This leaves us with two options: the Martian spacecraft have to bring their return fuel with them, or they have to generate it somehow on Mars (“in situ,” as the terminology goes). Since every gram of mass going to Mars must be paid for in fuel ( including the mass of the fuel itself ), generating the fuel on Mars is highly preferable to hauling all that fuel with you and paying the mass penalty for it.

Musk’s rockets are methane-powered, and, as John Timmer discusses in detail, creating methane on Mars actually isn’t complicated. Take some carbon dioxide from the atmosphere, mix it with hydrogen (which you can crack out of water molecules, which Mars has in surprising abundance), add energy, pressure, and a catalyst, and boom, you’ve got methane and water.

“It turns out that Mars is very cooperative when it comes to the Sabatier reaction,” said Weir in a long conversation earlier this week with Ars. “All you need to do it is carbon dioxide, water, and energy. And presumably you’re bringing some energy source with you if you’re going to colonize Mars—like either a reactor or just tons and tons of solar panels, though the correct answer is reactor.”

Life, the universe, and rocket engines

In The Martian’s fictional Ares missions, future-NASA elected to send tanks of hydrogen along in its ascent vehicles, effectively pre-staging the gas rather than having to extract it from the environment. “But I wrote that at a time when the common belief was that Mars had very little water,” said Weir. “Since then, we’ve learned, thanks to Curiosity, that Mars is riddled with water. You can literally just scoop up the dirt and desiccate it and you’ll get—” here he paused, glancing up for a moment to recall the factoid, “—there’s thirty-five liters of water for every cubic meter of soil, so all the hydrogen you could hope for. It’s all there.”

Musk didn’t eschew boldness (or possibly madness) with the rocket SpaceX wants to use for its Mars missions, either. The planned Interplanetary Transport System calls for a launch vehicle currently referred to as the “BFR” (which ostensibly stands for “Big Falcon Rocket”—that’s their story and they’re sticking to it), the first stage of which uses a massive cluster of forty-two Raptor engines to deliver almost four times the thrust of the Saturn V Moon rocket’s first stage.

The reasons for choosing to go with a whole bunch of smaller engines versus a few massive ones are myriad, but there are also risks—primarily around the complexity of the plumbing and controls to make that many separate thrusters work. The only other example of a launch vehicle with something approaching that many engines is the ill-fated Soviet N1 booster, which packaged thirty NK-15 kerolox engines into its first stage. Four N1 boosters flew from Baikonur between February 1969 and November 1972, and all four exploded. Three of the explosions were either related to or directly caused by the complicated KORD system that managed the N1’s bouquet of engines. (KORD is an acronym for “Control of Rocket Engines”— “KOntrol Racketnykh Dvigateley” or Контроль ракетных двигателей.)

But comparisons between the underfunded N1 and the potential failures awaiting Musk’s ITS BFR aren’t necessarily valid or useful. We asked Weir how Musk’s idea of slapping forty-two engines on a rocket made him feel.

“It makes me feel good,” he replied. “The more engines you have, the more safe points of failure you have. In other words, if, you know, if four engines go out during the ascent when you have forty-two of them, then you just burn the other thirty-eight a little longer. If four of your five Saturn V engines go out during ascent, then you’re really, really super-fucked.”

“The complexity that the Soviets ran into with the N1 was all about control systems, and nowadays we have computers that can handle that sort of thing….So, yeah, I think lots of little engines is a good approach,” he laughed. “The only question is how much mass you end up putting onto the booster to have all of those—to have a whole bunch of engines as opposed to one big one. One big one might be a more mass-efficient method of acquiring the same thrust than the equivalent number of small ones.”