Virgin Galactic, a space tourism company, promises that anyone who can afford the US$200,000 fare and pass pre-flight medical exams and a two-day bird course will soon be able to look down on Earth’s majesty from 360,000 feet. One touted benefit: seeing the electric blue glow of our thin, fragile atmosphere will instil an unquenchable desire to protect the planet. But will these starry-eyed “future astronauts” irreparably damage it on the way up?
Not if you ask founder Richard Branson. “In fact,” he claims in a promotional video, “our system will be many thousands of times friendlier to our environment than any previous manned spacecraft.” Indeed, the company claims the carbon footprint of one of its suborbital flights will be less than that of a one-way flight from London to New York. (Supporting evidence was not provided, and interview requests directed to Galactic received no response.)
Three independent atmospheric scientists argue Galactic may be missing the point. In a paper recently published in Geophysical Research Letters, they highlighted something called black carbon — soot, really — that is distinct from carbon dioxide, the gas primarily associated with global warming. Darin Toohey, a professor at the University of Colorado’s atmospheric and oceanic sciences department and one of the paper’s authors, says black carbon absorbs shortwave radiation from the sun, causing the atmosphere to heat up.
The problem stems partly from the type of engine Virgin plans to use in SpaceShipTwo. Martin Ross, another of the paper’s authors, who works at the Aerospace Corp. in El Segundo, Calif., explains that hybrid motors favoured by space tourism companies are safer, cheaper to run, and quicker to refuel than the liquid or solid rocket boosters that have dominated rocketry thus far. These characteristics lend well to commercial flight, but one significant trade-off is that they spew black carbon. “Rocket engines in general are very sooty compared to jet engines, and we think these hybrids are the sootiest of all the hydrocarbon-based rocket engines,” Ross says.
Worse than what comes out the tailpipe is where it ends up. As SpaceShipTwo ascends, it will pass through the stratosphere, a narrow atmospheric layer found between 58,000 and 163,000 feet above sea level. Toohey says black carbon released there lingers far longer than it does in the troposphere where airplanes fly — years rather than days.
Based on stated plans of Galactic and other commercial space startups, the scientists built a model assuming 1,000 launches a year. This “would create a persistent layer of black carbon particles in the northern stratosphere that could cause potentially significant changes in the global atmospheric circulation and distributions of ozone and temperature,” they concluded. The warming impact could be 100,000 times greater than the carbon dioxide emitted by the same spacecraft.
As space transport moves from a state-sponsored activity to a commercial one, Ross and Toohey expect it will eventually become subject to emissions regulations. As for Branson’s remarkable environmental claims, Toohey isn’t buying. “Unless they can steer me toward something that’s published or examined and confirmed,” he declares, “I think it’s a wag.”