If space tourism ever becomes big business, as plenty of well-heeled backers hope, the danger of the enterprise might not be confined to those who book a ride to the edge of space. A robust suborbital spaceflight industry could deposit enough soot in the stratosphere to cause significant global climate change, according to a new study.

Although the study, set to be published in Geophysical Research Letters, does not mention spaceflight companies by name, the authors seem to have Richard Branson's Virgin Galactic and its competitors in mind. The researchers modeled climate effects from hybrid motors, the type used by Virgin Galactic, launching repeatedly from southern New Mexico, where Virgin Galactic has a 20-year lease at Spaceport America, a publicly funded commercial spaceport.

Hybrid motors using synthetic solid hydrocarbons as a fuel and nitrous oxide as an oxidizer would inject black carbon soot into the stratosphere, explain the study's authors, Martin Ross of the nonprofit Aerospace Corporation, Michael Mills of the National Center for Atmospheric Research, and Darin Toohey of the University of Colorado at Boulder.

If the space tourism industry matures to the point that 1,000 hybrid-powered suborbital flights depart annually, those trips would deposit roughly 600 metric tons of soot into the stratosphere each year. Over decades of launches, those emissions would form a persistent and asymmetric cloud over the northern hemisphere that could impact atmospheric circulation and regional temperatures far more than the greenhouse gases released into the stratosphere by those same flights.

In a 40-year climate model incorporating rocket soot, ozone concentrations decreased at the tropics and increased at high latitudes. In midlatitudes of the northern hemisphere, where the carbon cloud would effectively act as a sunshade, temperatures fell by about 0.4 degree Celsius. That might sound like a welcome offset to the effects of rising greenhouse gas concentrations, which are expected to raise average global temperatures in the coming decades, were it not for the fact that rocket soot also had the effect of warming both poles in the researchers' climate model, boosting temperatures there by nearly a full degree C in local winter.

The specific climate outcomes rest on complex modeling and a series of assumptions that will need refining, specifically regarding the amount of carbon in hybrid exhaust and the interplay between climate changes caused by stratospheric soot and those caused by future increases in global greenhouse gases independent of tourism launches. Nevertheless, the researchers warn, "rocket emissions on this scale clearly cross a threshold to be considered a human-influenced climate impact of global importance."

Photo from a Virgin Galactic glide test: Mark Greenberg/Virgin Galactic