It would be stupid to travel to Mars carrying fuel you’d need to return: There would be five times the mass, the weight, and the risk. So when the first humans who colonize the red planet want to return, they’ll need to mine Mars to fuel the journey back.

Fortunately, SpaceX CEO Elon Musk has some ideas for gassing up spacecraft on the red planet. On Tuesday, Musk shared his plans for fuel generation at the International Astronautical Congress in Guadalajara, Mexico. According to Musk, humans on Mars would use an upgraded form of propellant that can be produced using materials endemic to the red planet. It could slash the costs of traveling to Mars and make it more sustainable for humanity to one day colonize the planet and beyond.

SpaceX Mars Colony
Our journey to Mars shouldn't include the fuel we'd need to get back.

It’s all part of the Interplanetary Transport System, the Mars-bound space travel system unveiled by Musk.

“It would be pretty absurd to try to build a city on Mars if your space ships just kept staying on Mars and not going back to Earth, you’d have this massive graveyard of ships,” Musk said.

The ITS propellant system essentially moves the rocket fuel away from the previous kerosene/liquid oxygen combo used on current Falcon 9 rocket engines, and pivot toward methane and liquid oxygen. Across five main criteria — vehicle size, cost of producing, reusability, the ability to produce on Mars itself, and the ability to transfer it to and from the spacecraft using mobile propellant tankers — “methane was the clear winner,” said Musk. Kerosene is simply not cost-effective to produce in mass quantities, and the necessity to store hydrogen is energy-prohibitive.

methane spacex mars
Methane production on Mars

In particular, the type of methane/oxygen combo SpaceX is striving to use and can be stored safely at cold temperatures. In fact, Musk said the ability to produce methane at colder temperatures actually makes it most efficient as a fuel.

This cryogenic methane was used in SpaceXs first tests of its all-new Raptor engine which will be installed on the ITS booster.

According to Musk, the propellant should be pretty easy to produce on Mars given its carbon dioxide atmosphere and the water available in the vast water-ice reserves on the red planet. The limiting factor for producing this propellant is “the energy source,” said Musk, “which we think we can do with large solar panels.”

If the company can pull off a large shipment of efficient solar panels to the red planet — and if it also finds a way to easily harvest water from the planet’s ice stores — then there’s little reason to doubt methane could be produced on the ground right on Mars.

The problem is, those are two big feats. Musk’s presentation neglected to go into detail as to how exactly an ITS propellent-producing facility would work. And bear in mind — Musk thinks 50 to 60 percent of whatever electricity the solar panels are able to generate will go toward propellant production. That’s an insane amount of energy that cannot go toward other things that make day-to-day life in Mars safe and bearable.

Still, this is SpaceX. The company doesn’t do small dreams.

Photos via SpaceX, SpaceX/YouTube