An experimental jet that aerospace company Lockheed Martin is building for NASA as part of a half-billion dollar supersonic aviation program is a “climate debacle,” according to an environmental group that is calling for the space agency to conduct an independent analysis of the jet’s climate impact.  

The Public Employees for Environmental Responsibility (PEER), an environmental advocacy organization based in Silver Spring, Maryland, said supersonic aviation could make the aviation industry’s goal of carbon neutrality unobtainable. In a letter sent to NASA Administrator Bill Nelson on Thursday, the group called on NASA to conduct a “rigorous, independent, and publicly accessible climate impact analysis” of the test jet. 

“Supersonic transport is like putting Humvees in the sky,” PEER’s Pacific director, Jeff Ruch, said. “They’re much more fuel consumptive than regular aircraft.”   

NASA commissioned the X-59 Quiet Supersonic Technology (QueSST) in an effort to create a “low-boom” supersonic passenger jet that could travel faster than the speed of sound without creating the loud sonic booms that plagued an earlier generation of supersonic jets.

The Concorde, a supersonic passenger plane that last flew in 2003, was limited to speeds below Mach 1, the speed of sound, when flying over inhabited areas to avoid the disturbance of loud sonic booms. The QueSST program seeks to help develop jets that can exceed the speed of sound—approximately 700 miles per hour—without creating loud disturbances.  

However, faster planes also have higher emissions. Supersonic jets use 7 to 9 times more fuel per passenger than conventional jets according to a study published last year by the International Council on Clean Transportation.

NASA spokesperson Sasha Ellis said the X-59 jet “is not intended to be used as a tool to conduct research into other challenges of supersonic flight,” such as emissions and fuel burn. 

“These challenges are being explored in other NASA research,” Ellis said, adding that NASA will study the environmental effects from the X-59 flights over the next two years.

The emissions of such increased fuel use could, theoretically, be offset by “e-kerosene”—fuel generated from carbon dioxide, water and renewably-sourced electricity—the study’s authors wrote. But the higher cost e-kerosene, coupled with the higher fuel requirements of supersonic travel, would result in a 25-fold increase in fuel costs for low-carbon supersonic flights relative to the cost of fuel for conventional air travel, the study found.  

“Even if they’re able to use low carbon fuels, they’ll distort the market and make it more difficult for enough of the SAF [Sustainable Aviation Fuel] to go around,” Ruch, who was not part of the International Council on Clean Transportation (ICCT) study, said.

The ICCT report concluded that even if costly low-emissions fuels were used for supersonic jets, the high-speed aircraft would still be worse for the climate and could also harm the Earth’s protective ozone layer. This is because supersonic jets release high volumes of other pollutants such as nitrous oxide at higher elevations, where they do more harm to the climate and to atmospheric ozone  than conventional jets.

In their letter to Administrator Nelson, PEER also expressed concerns about NASA’s Urban Air Mobility program, which the environmental group said would “fill city skies with delivery drones and air-taxis” in an effort to reduce congestion but would also require more energy, and be more expensive, than ground-based transportation.  

“It’s another example of an investment in technology that at least for the foreseeable future, will only be accessible to the ultra rich,” said Ruch.

 NASA also has a sustainable aviation program with a stated goal of helping to achieve “net-zero greenhouse gas emissions from the aviation sector by 2050.”   The program includes the X-57, a small experimental plane powered entirely by electricity. 

NASA plans to begin test flights of both the supersonic X-59 and the all-electric X-57 sometime this year.