Fresh from being named Time magazine's Person of the Year, Elon Musk has said that SpaceX is starting a program to draw carbon dioxide from the atmosphere to make into rocket fuel. So far details are scant, to say the least, but Musk has noted this ideal “Will also be important for Mars”, where atmospheric carbon dioxide represents one of the few things that could be used to power the trip home.
“SpaceX is starting a program to take CO2 out of atmosphere & turn it into rocket fuel,” Musk tweeted on Monday. “Please join if interested.”
How people are supposed to “join” is not clear. That's just one of the many questions relating to the work that are yet to be answered.
The rush of joy-flights to the edge of space has prompted criticism of the damage such missions are doing to the atmosphere, even from other one-percenters. Unlike Blue Origin or Virgin Galactic, SpaceX is somewhat shielded because the assistance it is providing to NASA's research benefits more than a tiny fragment of a percent of humanity.
Nevertheless, the immense amounts of carbon dioxide released every time a SpaceX rocket takes off doesn't sit well with Musk's efforts to wean ground-based transport off fossil fuels through Tesla. So Musk's recent tweet was not entirely out of the blue.
The value of the idea for Mars missions isn't disputed. If a round-trip to another planet needs to carry the fuel to take off again and make the journey home, it's going to be a lot heavier on the outward journey, requiring still more fuel and creating a vicious circle. Much better to make as much as possible from raw materials available at the destination. On Mars, that almost certainly means some of the carbon dioxide that makes up 96 percent of the Martian atmosphere. Practicing on Earth would seem essential; any carbon dioxide removed from the atmosphere would be a bonus.
On the other hand, making rocket fuel out of thin air economically enough to power SpaceX's current launch schedule, let alone its big plans, won't be easy. Neither Musk, nor the SpaceX website, have provided details of whether that is even what is intended, let alone how they might achieve it.
The closest thing to clarification we have is Musk tweeting “Yup” in response to an engineer who referred to the Sabatier reaction in which carbon dioxide and hydrogen are combined to make methane, a potential rocket fuel, and water.
As plenty of people noted in response, the conversion of carbon dioxide to fuel requires the input of a lot of energy – much more than can be used on release. Cheap electricity is a necessary input for the idea, making it matter less if the conversion efficiency is low, but not sufficient on its own.
If the plan is to use the Sabatier reaction it comes with the disadvantage that any methane that escapes unburnt will have a warming effect many times larger than the carbon dioxide used to make it. That's not a problem for escaping fuel on the journey between planets, but could be on lift-off. Alternatively, carbon dioxide could be converted to liquid fuel. Whether any of these options is better than simply using hydrogen or another carbon-free fuel directly remains to be seen, but those with ideas on how to do it apparently have a standing invite from Musk to contribute.