January 10th, 2021 by Jennifer Sensiba
Zero-emissions vehicles are great for the environment, and Tesla makes a lot of them. Elon Musk also owns SpaceX, so people are naturally going to lump them together in their minds. When someone sees a rocket launch, it’s obviously not a zero-emission event. As Jerry Lee Lewis says, “Goodness gracious, great balls of fire!”
Unfortunately, this leads some people to conclude that all of the emissions from a rocket launch must cancel out all of the emissions saved from a Tesla. I’ve been wanting to cover this for a while and dispel that myth*, but was waiting for some more solid word on what SpaceX’s plans really were for this. With one of Elon Musk’s recent tweets, we now know that zero-emissions rockets are definitely coming.
Indirectly, an electric rocket is possible by using electricity to convert CO2+H2O —> CH4+O2. This will be needed on Mars even for early missions & Earth too over time.
— Elon Musk (@elonmusk) January 7, 2021
First, let’s unpack what SpaceX’s emissions look like at present. Existing and mature SpaceX rocket designs use their Merlin engines. Like many other launch companies and government entities, Merlin engines use RP-1 fuel, which is basically a more refined version of the fuel most jet engines and oil lamps use (kerosene). They mix it with liquid oxygen and combust it, and out comes not only CO2 and H2O, but a lot of pollutants like hydrocarbons, carbon monoxide, and oxides of nitrogen.
For newer rockets, like the Starship and Super Heavy, SpaceX is using its newer Raptor engines. While there are many ways that Raptors are better than Merlins, one of the biggest differences is that they burn methane (CH4) instead of RP-1. Methane can be combusted like RP-1, but its molecules are far simpler than kerosene. When you combust a methane molecule (CH4) with two oxygen molecules (O2), you only get carbon dioxide and water in the exhaust, with none of the other contaminants.
This alone is a big advantage to using methane, but as Elon Musk points out in his tweet, it’s possible to make methane rockets even cleaner. Putting water vapor out doesn’t cause much harm with climate change, but all that carbon dioxide is still an issue.
By capturing carbon dioxide (CO2) from the atmosphere where the rocket left it behind, you can make up for what the rocket did. You then take the carbon dioxide, add some water, and use the Sabatier Reaction to put the methane back together. You can then use the methane to power the next rocket.
The result? Zero net emissions.
There’s a catch, though: it takes electricity. You need a metal like nickel to act as a catalyst, and you need a lot of electricity to make the carbon dioxide and water into methane. The good news is that it doesn’t matter where the electricity comes from, so it’s possible to use solar power, hydropower, wind power, or another clean source. So, if you play it right, you’re still at zero emissions!
It’s likely that the first place you’ll see SpaceX use this will be on Mars. On Earth, it’s easy to get methane from natural gas. After all, it’s the main ingredient. Using Sabatier to make methane is an expensive process compared to buying natural gas, so it doesn’t make a lot of sense just yet. The current goal is to get the company to Mars and set up a colony, which is going to be an expensive undertaking.
There aren’t any natural gas pipelines on Mars, though. If you want fuel for Starships, you’ll need to either take a lot of the stuff to Mars or find a way to make it there. It’s a long, long trip to Mars. Even light, the fastest thing we know of, takes between about 4 and 20 minutes to get there. The long trip and extra launches would make it prohibitively expensive to get natural gas there.
On the other hand, Mars’ atmosphere is mostly carbon dioxide, and there’s plenty of water in the polar ice caps. It’s also possible to use solar panels or other electricity sources on Mars to power the Sabatier reaction, so it’s a really good way to come up with fuel for return trips to Earth or even trips to other places in the solar system.
The biggest news with Elon Musk’s tweet is that SpaceX eventually plans to produce fuel this way on Earth. We also have plenty of carbon dioxide and water here, but we can’t afford to keep putting more carbon in our atmosphere in the long run. Fortunately, not doing that is part of the future plan.
*Editor’s note: Emissions from SpaceX rocket launches are actually surprisingly low. See these two articles for more on that:
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