Despite challenges with supercooled liquid hydrogen leading up to the Nov. 16 launch, the mission successfully utilized the universe’s lightest and most abundant element to produce enough energy to power four engines with 2 million pounds of thrust. The other main by-product was water vapor.

That’s good for planet earth, especially when compared with rocket launches that rely on a popular alternative: kerosene-based propellant. In the case of SpaceX, a single Falcon 9 flight emits about 336 tons of carbon dioxide—the equivalent of a car traveling around the world 70 times—according to John Cumbers, a former NASA synthetic biologist and CEO of SynBioBeta.

Burning liquid hydrogen is far from perfect: Producing it is usually a fossil-fuel–intensive process. But green hydrogen projects are attempting to improve its sustainability through the use of renewables.

NASA has been using liquid hydrogen for fuel for decades and will likely keep doing so, both because of its efficiency and because Congress has mandated that the Artemis mission use space-shuttle engines, which were designed around hydrogen use. But it is just one of many entities in the space sector today, and private companies are expected to play a growing role in space tourism and satellite launches.

Environmental considerations are becoming increasingly important as space travel grows. Cumbers projects that humans will launch 10 times as many rockets over the next 10 to 20 years as we do right now. 

Last year, the global space economy hit $469 billion—growing 9% from 2020, according to the Space Foundation. In the first six months of this year, a record 72 successful rocket launches put 1,022 identified spacecraft into orbit, more than the total number placed in the first 52 years of the Space Age, the foundation said.

Aviation accounts for about 2.5% of all carbon emissions on earth, while rocket launches contribute a much smaller fraction than that. But as the new space race unfolds and more rockets are being launched than ever before, the quest to find sustainable rocket fuel is well underway.

The race to create green rocket fuel

While green rocket fuel isn’t a planet saver, it’s one small step humankind can take toward achieving a net-zero future.

There are several efforts afoot to produce rocket fuel in a more environmentally friendly way. Energy startup Green Hydrogen International is developing a green hydrogen project in South Texas. Researchers at the German Aerospace Center are working on a fuel that only produces nitrogen, oxygen, and water when heated. Canadian company Hyox is developing technology for production of net-zero aviation fuel and rocket propellants that will use low-cost solar power and electrolysis to produce methane and kerosene, both of which can propel rockets into space.

New York–based Air Company is working with direct air capture, a technology that removes carbon dioxide from the air. The company mixes that CO2 with hydrogen made with renewable energy to create Rocket Propellant-1 (RP-1), the type of kerosene used by Space X’s Falcon 9 rocket. The process to make the kerosene is not only carbon negative but could one day be replicated on Mars—where the atmosphere largely consists of carbon dioxide—to make fuel for return trips. Thus, the fuel won’t have to be carried from earth for missions to the red planet.

The company’s process removes about 2.8 kilograms of CO2 from the air per liter of RP-1, while traditional production with fossil fuels emits more than three kilograms of CO2 per liter, Air Company CEO Gregory Constantine said.

Black carbon and other emissions

Producing rocket fuel in an environmentally friendly way is just part of the battle. Kerosene and methane fuel also deposit black carbon—or soot from combustion—into the upper atmosphere, where it warms the air for years.

To eliminate the black carbon problem, rockets can burn hydrogen, but that is technically challenging to deal with because it has to be kept very cold. The Artemis program has been plagued by liquid hydrogen issues. Meanwhile, methane can be a better option, because it produces less black carbon than kerosene and can be stored at a higher temperature.

“Liquid methane is where everyone is going,” said Matt Oehlschlaeger, an aerospace engineering professor at Rensselaer Polytechnic Institute.

There is still a ways to go before rocket fuel that is created with environmentally friendly methods becomes widespread.

We believe that these technologies should be applied to more pressing problems, namely industry, before a relatively low emitter like rockets.

—Mo Islam, head of research at Republic Capital, an early investor in SpaceX and an investor in Axiom Space

“Renewable fuels, no matter the application, are still very much in their infancy,” said Mo Islam, head of research at Republic Capital, an early investor in SpaceX and an investor in Axiom Space. “We believe that these technologies should be applied to more pressing problems, namely industry, before a relatively low emitter like rockets.”

Islam said many stakeholders need to align to expedite the development of renewable fuels.

“We need coordination between policymakers, technologists, and businesses to properly fund R&D on these fuels,” he noted. “Once we have a proof of concept, we can begin to think about implementing it at scale—first into existing terrestrial processes, then eventually into rockets.”

The cost of direct air capture will also need to go down before that technology can enter the mainstream. For direct air capture RP-1 and methane-based fuels to become cost equivalent with their fossil-based counterparts, the cost will have to decline from $650 to $1,000 per ton of carbon dioxide to $100 per ton, per Hyox CEO Glenn Martin.

“This is where the technology curve is headed,” he said.

Lower costs, greater demand

Meanwhile, the cost for rocket launches is also coming down. When adjusted for inflation, the cost for heavy launches into low earth orbit has fallen from $65,000 per kilogram to $1,500 per kilogram since the 1960s, according to an analysis of Center for Strategic and International Studies data by McKinsey. The main drivers are computer-aided design, 3D printing, reusable components, new commercial launch providers, and an increase in launch frequency.

Lowering costs could boost demand for satellite-based services, making the need for sustainable rocket fuel more urgent. Some of those satellites can assist in fighting climate change, even if they require emissions to launch.

“Satellites in space can be used to track the trends of water resources on earth or to detect emission leaks that can be used to reduce emission sources,” noted Conor Tomac, a process engineer specializing in gas, fuels, and chemicals at Black & Veatch. 

Specialized earth-observation satellites have been able to identify previously unknown methane super-emitters, helping facilitate mitigation, said Jory Bell, general partner at Playground Global, a venture capital firm focused on frontier technologies.

“Using space to observe and understand the earth,” Bell said, “will be a key part of cleaning up the climate mess we’ve gotten ourselves into.”