The Axiom-1 mission, which will send four private astronauts to the International Space Station, is the first of many planned by NASA to expand the ISS for commercial use as part of the so-called low-Earth orbit economy.
The commander of the Axiom-1 mission has stated unequivocally that this is not an example of space tourism, as the crew has been trained and the mission includes plans to conduct biomedical research.
Crew members—all men aged 52 to 71—are said to have paid a whopping US $55 million (£42.3 million) per ticket, an amount that would undoubtedly fund a formidable biomedical research program here on Earth. But, aside from the exorbitant ticket price, I’m concerned about the potential environmental consequences of such space excursions.
The mission is being carried out by a SpaceX Falcon 9 Block 5 rocket, with the crew aboard the Crew Dragon spacecraft at the apex. The rocket consists of two stages: a reusable booster that holds the majority of the fuel (about four-fifths) and returns to Earth for reuse; and a discarded second stage.
Before returning to Earth, the booster reaches an altitude of about 140 kilometers. The energy required to propel the spacecraft to the ISS is generated by a combustion reaction between rocket-grade kerosene and liquid oxygen, which emits environmentally hazardous byproducts.
Rocket launches and the return of reusable components emit air pollutants and greenhouse gases into multiple layers of the atmosphere. When compared to equivalent pollutants released at or near the Earth’s surface, these can linger for years in the middle and upper atmosphere. This is due to fewer chemical reactions or weather events flushing pollutants from the middle and upper layers.
The kerosene fuel used by SpaceX Falcon rockets is a mixture of hydrocarbons, which are made up of carbon and hydrogen atoms. These react with liquid oxygen to produce carbon dioxide (CO2), water vapor (H2O), and black carbon (soot) particles in the rocket exhaust.
CO2 and H2O are powerful greenhouse gases, and black soot particles absorb the sun’s rays very efficiently. As a result, all of these chemicals contribute to the warming of the Earth’s atmosphere.
Nitrogen oxides (NOx), reactive air pollutants, form during launch as a result of extremely high temperatures, causing a bonding reaction between normally stable nitrogen and oxygen molecules. As a result, NOx is also produced when the rocket’s reusable components return to Earth, as a result of the extreme temperatures produced by friction on their heat shields as they whizz through the mesosphere at 40–70 kilometers per hour.
When these particles come into contact with the stratospheric ozone layer, they convert ozone to oxygen, depleting the fragile sheath that shields the planet from the sun’s harmful UV radiation.
Although the total CO2 emissions from this launch will be small in comparison to the global aircraft industry, emissions per passenger will be around 100 times that of a long-haul flight.
Soot emissions are also much lower than those from the aircraft industry, but when released into the middle and upper atmosphere, soot has a 500-fold greater warming effect than at lower levels. This is due in part to the lack of clouds and few to no aerosols competing with soot to absorb the sun’s rays.
An Axiom co-founder compared the potential opportunities for creating industry and trade networks within low-Earth orbit to the early days of developing the internet, which is now a nearly universally accessible technology. If we apply that analogy to similarly high levels of access to the low-Earth orbit economy, rocket launches are likely to become far more common than the 146 launches achieved in 2021.
A scenario like this would significantly alter the Earth’s climate and jeopardize our significant progress in repairing the ozone layer. At the very least, research into the consequences of a thriving low-Earth orbit economy for our planet below is urgently needed.