Elon, Jeff or Richard - whose space rocket is most sustainable?

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“We don’t want to be one of those single-planet species, we want to be a multi-planet species,” Elon Musk, CEO of rocket company SpaceX, said last year.

While professional astronauts have been blasting into space for decades, SpaceX — and competitors like Jeff Bezos’ Blue Origin and Virgin Galactic, owned by Richard Branson — has revolutionised space travel for private citizens.

Both Blue Origin and Virgin Galactic have made manned, suborbital flights while in April, SpaceX sent private customers to the International Space Station.

And expansions appear in the cards. Virgin Galactic, for example, is aiming to launch 400 flights annually from each spaceport (like an airport for rockets), according to a CNBC report.

However with the potential for increased extra-terrestrial travel in the coming decades comes a host of questions on the impact of the space industry on the planet those astronauts would leave behind.

The “sustainability” of any rocket launch depends a lot on its fuel, according to Dr Eloise Marais, a geographer at University College London, who has collaborated with Nasa researchers and received funding from the European Space Agency, but never worked with SpaceX, Virgin Galactic or Blue Origin.

There’s plenty of variety in the fuels, she told The Independent. Blue Origin uses liquid hydrogen fuel; Space X uses kerosene fuel and Branson’s Virgin Galactic uses a hybrid solid/liquid fuel.

Some rockets also use solid rocket fuels or fuels from made from the chemical hydrazine.

Kerosene — similar to what’s used in portable camping stoves — is a carbon-based fossil fuel, meaning it produces carbon dioxide (CO2) when burned. The hybrid fuel used by Virgin Galactic’s VSS Unity also produces CO2, Dr Marais has previously written.

CO2, largedly caused by humans burning fossil fuels, is the biggest contributor to the global heating which is driving climate catastrophes.

Liquid hydrogen, on the other hand, isn’t a fossil fuel, so Blue Origin rockets produce water vapour instead. While also a greenhouse gas, its impact on climate change is more temporary, unlike long-lasting CO2, or relatively short-lived but potent methane.

But greenhouse gas emissions may not be the biggest concern when it comes to the impact of rocket launches on the environment.

Per passenger, many rockets produce more CO2 than airplanes, Dr Marais says, meaning an astronaut has a higher carbon footprint than a plane passenger. However since the number of rocket launches is currently miniscule compared to airplane flights, the overall CO2 impact of the former is small in comparison.

Martin Ross, an atmospheric scientist at the Aerospace Corporation, told The Independent that the main issue with rocket launches is other pollutants and particles that can be released.. That includes materials like black carbon, commonly known as soot, as well as nitrogen oxides, alumina and chlorine.

Dr Ross, who has worked with some space companies but declined to name specifically, noted that these pollutants don’t last particularly long in the atmosphere.

However as long as there’s continuous launches emitting these substances, they have the potential to alter the atmosphere’s chemistry. Rockets are unique, he points out, in that they have the ability to dump pollutants directly into the upper layers of the atmosphere.

Of particular concern is these pollutants’ potential to impact the stratosphere — the location of Earth’s ozone layer which protects the planet and its people from many of the sun’s ultraviolet rays.

The ozone layer has started to heal in the past three decades, ever since the Montreal Protocol, a global pact which banned several ozone-destroying chemicals, was signed in the late 1980s.

But spewing rocket pollutants into the stratosphere raises new concerns about ozone loss, Dr Ross says.

Rocket pollution in the upper atmosphere could have other effects, too. Hydrogen-fuel rockets release water vapor higher in the atmosphere than there is typically that much water, Dr Marais said, which could reflect incoming sunlight.

And soot in the stratosphere could cool the lower atmosphere by absorbing sunlight, she added. But while that may appear positive to tackling the climate crisis, that stratospheric soot could also damage the ozone layer.

Dr Marais says that kerosene, solid rocket fuel, hydrazine and the hybrid fuel used by Virgin’s rockets produce soot. All of the fuels, including hydrogen, could produce nitrogen oxides.

But Dr Ross said a lot of assumptions about rocket launches’ emissions and pollutants are based on models rather than actual measurements.

“We need to get into the stratosphere and measure those emissions directly,” he said.

SpaceX and Virgin Galactic did not respond to multiple requests for comment by The Independent.

A Blue Origin spokesperson said in a statement that the company’s New Shephard rocket had reusable parts, and that “during flight, the only byproduct of New Shepard’s engine combustion is water vapor with no carbon emissions”.

The company did not respond to specific questions about nitrogen oxide or concerns about other potential environmental impacts, such as ozone.

Overall the rocket fuel which appeared to do the least damage was liquid hydrogen, Dr Marais said, as it didn’t produce soot. However “there are consequences for all of these rocket fuels,” she noted.

“I suppose the punch line, really, is that all of these carry a penalty,” Dr Marais added. “There’s no clean fuel solution, currently.”