When Elon Musk founded SpaceX in 2002, he envisioned a greenhouse on Mars, much like the one later depicted in the 2015 blockbuster The Martian. Soon his fantasy shifted from a small-scale botanical experiment to a vision of a self-contained Martian city. In a speech given at the 67th International Astronautical Congress in 2016, he made his point. “History will branch off in two directions. One path is to stay on earth forever, and then eventually there will be an extinction event,” Musk says. “The alternative is to become a space civilization and a multiplanetary species, which I hope you will agree is the right way to go.”
Although Musk later clarified that the extinction event he was referring to could take place millennia (or even eons) in the future, conditions on earth today are becoming increasingly dangerous for human beings. Deadly heat waves, food insecurity and catastrophic natural disasters are some of the dangers we face as the planet continues to warm. Unfortunately, the red planet is very far from becoming a viable alternative home. As we measure carbon dioxide concentrations in parts per million on earth, The atmosphere of Mars contains 96% CO2.just one of a litany of logistical nightmares Martian colonists would have to overcome.
Read more: Why didn’t humans reach Mars?
In a perfect world, Musks’ dreams of extraterrestrial civilization could coexist with the ecological values that have driven companies like Tesla’s solar program. But while SpaceX’s aspirations are in space, its operations have an undeniable impact at home. Unlike a Tesla sports car, SpaceX’s rockets are not propelled by electricity, they burn kerosene.
Carbon emissions from space launches are dwarfed by other sources of greenhouse gases, but they could have an outsized impact on the climate. The reason is a particular product of rocket propulsion: carbon black. These tiny pieces of crystalline carbon atoms have a short lifespan in the atmosphere, but strongly absorb sunlight. On the Earth’s surface, black carbon from burning diesel, coal and wood poses a threat to the environment and public health, especially in developing countries. But in the upper atmosphere, rocket engines are the only source of black carbon. For years, scientists have warned that these emissions could have unpredictable effects on the climate. Yet research on the subject has been extremely slow.
“We identified the black carbon problem in 2010,” says Darin Toohey, an atmospheric scientist at the University of Colorado at Boulder. “The story comes and goes, but the core cast remains the same.”
shifting winds
In 1985, a group of atmospheric researchers led by Pawan Bhartia presented a terrifying satellite image to a room full of scientists, policymakers and journalists at a conference in Prague: There was a gaping hole in the stratospheric ozone layer directly above Antarctica. The culprits were a group of chemicals used by refrigerator manufacturers called chlorofluorocarbons, or CFCs. Barely two years later, the Montreal Protocol was signed by 46 countries; over the next decade, CFCs were phased out by industry worldwide. Today, ozone levels are slowly rebounding.
But space travel could again endanger the ozone layer. Black carbon is an excellent greenhouse gas, excellent in the sense that it absorbs sunlight very well and converts it into heat. As rockets pass through the upper atmosphere, they increase temperatures in their wake. Currently, there are too few space launches for this effect to be very pronounced. But Toohey warns that consistent launches, like those needed to populate a Martian city, could pose a problem.
“The effect is to cause a slight temperature gradient between where the black carbon is heating things up and other parts of the planet that aren’t launching rockets,” he says. “You end up with a change in the winds in the stratosphere and the mesosphere, which may not seem like much, but these winds move ozone from one part of the planet to another.
In a research project now over a decade old, Toohey and his colleagues modeled the atmospheric outcome of a scenario where 1,000 rockets were launched each year. What they found is striking: stratospheric ozone levels are projected to change by 1% in the tropics and up to 6% at the poles. “You’re not creating a hole in the ozone layer, just changing things by the same amount,” Toohey says. “These are the same numbers that started the whole Montreal Protocol.”
In a historical article from 1995, dermatologist Frank De Gruijl estimated that even a 1% change in stratospheric ozone could increase the prevalence of skin cancer by 2%. As with many environmental issues, the public health cost of emissions poses an ethical dilemma for those tempted by the prospect of space colonization. “What is the most important life? Toohey asks. “A billionaire astronaut or someone in Bangladesh?”
Uncertain results
Ultimately, skin cancer may be just one of the many problems that arise from increased space launches. Many other rocket compounds emitted from rockets have yet to be studied. Even shifting ozone concentrations could have effects beyond the obvious. Although they haven’t yet proposed a cause, Toohey and his colleague’s model also showed a significant change in the amount of seasonal sea ice in the Arctic and Antarctic.
“What matters to me is not whether the sea ice has increased or decreased,” Toohey says. “It’s that such a small change in atmospheric ozone has such a large effect.”
Although research on the global effects of space travel is still extremely limited, there is enough to know that we don’t know much yet. While research into space travel itself may be more appealing to terraforming enthusiasts like Musk, it must come with an understanding of its impacts.
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