Virgin Galactic proudly touts the fact that each of the passengers who will fly into sub-orbital space on its SpaceShip2 will emit less carbon dioxide than a typical air passenger on a flight from New York to London. But some scientists say carbon dioxide emissions are irrelevant to measuring the greenhouse gas footprint of the nascent space tourism industry. The big threat from the scaling-up of space travel, they say, comes from something called black carbon—a type of particulate matter that, when hurled into the stratosphere, builds up for years, absorbing visible light from the sun. According to one study, black carbon emitted into the stratosphere by rockets would absorb 100,000 times as much energy as the CO2 emitted by those rockets.
"There's one issue and it's simple: you don’t want to put black carbon in the stratosphere. Period," says Darin Toohey, a professor of atmospheric and oceanic sciences at the University of Colorado, Boulder. Industry insiders say otherwise. Who’s right?
Black carbon should be familiar to anyone who’s ever idled behind a diesel truck or sat by a wood stove: it’s what makes soot black. Formed from the incomplete combustion of fossil fuel, biofuel, and biomass, it is emitted directly into the atmosphere and absorbs about a million times more energy than CO2. According to one study, it is Earth’s second largest contributor to climate change, after carbon dioxide. The reason black carbon doesn’t wreak more havoc on the environment is that it has a short lifetime in the lower atmosphere—precipitation washes away black carbon emissions from planes and other sources within a matter of weeks.
Not so in the stratosphere, which begins as low as 5 miles above the Earth and rises up to about 31 miles. Rockets need to scream through the stratosphere to the point 62 miles above the sea level, where space is conventionally said to begin. They are also the only direct source of human-created compounds above 12 miles. Because there is no rain or other atmospheric factors to wash out the black carbon in the stratosphere, black carbon would linger for 5 to 10 years or more. Moroever, rockets produce over 1,000 times more black carbon per unit of fuel than standard aircraft.