Space tourism could have big impact on climate

Space tourism could have major consequences for Earth's climate. New computer simulations suggest soot emitted by the rockets could raise temperatures at the poles, significantly reducing seasonal ice cover there, but uncertainty remains about the assumptions used in the study.

In the next few years, space tourism companies hope to start routinely flying passengers on suborbital space flights. Now, Martin Ross of the Aerospace Corporation in Los Angeles, California, and colleagues have performed the first detailed simulations of the flights' effects on Earth's climate.

They assumed a flight rate of 1000 suborbital trips per year, the number put forward in business plans for space tourism in 2020, and estimated the emissions from a rubber-burning engine like that planned for Virgin Galactic's SpaceShipTwo.

The researchers found that the effect of soot, which is incompletely burned fuel, would dwarf that of the carbon dioxide emissions from the launches. Soot readily absorbs sunlight, warming the atmosphere where it is abundant.

Above the weather

The 1000 annual launches would belch out about 600 tonnes of soot, or black carbon – less than today's output from airplanes and other sources. But plane soot occurs at low enough altitudes for rain to wash it out of the atmosphere in just days or weeks. Rockets expel the stuff at altitudes three times as high – in the stratosphere more than 40 kilometres above sea level. There, well above the weather, it can remain for up to 10 years.

To study the effects of black carbon emissions, Ross's team used a 3D simulation of Earth's climate. They assumed that all the black carbon is emitted over Spaceport America, a space tourism hub being constructed in New Mexico, US.

The researchers found that the black carbon caused temperatures to rise at the north and south poles. The increase was about 0.2 °C for most of the year, but peaked at around 1 °C during each hemisphere's winter. The extra warmth caused sea ice at each pole to melt, especially in Antarctica, where the area covered by ice shrank by as much as 18 per cent in the summer.

Ozone connection

Team member Michael Mills of the National Center for Atmospheric Research in Boulder, Colorado, says the team is still trying to understand exactly why the black carbon emissions would cause warming at the poles.

But the soot should warm air in the stratosphere, and this could strengthen currents there that carry air from the equator to the poles.

That would reduce the amount of ozone over the tropics and increase it over the poles – an effect seen in the simulation. That increase in polar ozone might be responsible for the polar warming, Mills says, pointing out that the converse has been observed on Earth – polar cooling is associated with decreases in ozone over Antarctica. The connection is probably due to the way ozone interacts with radiation, he adds, though researchers are still trying to understand the exact mechanism.

"It's not a pretty picture for the Arctic or Antarctic," says Charles Zender of the University of California, Irvine, who says the new study was "very carefully done".

Educated guess

Mills admits there is still uncertainty about the study's findings. He notes in particular that the team lacked data on how much black carbon would be emitted per flight by space tourism vehicles. The team assumed that Virgin Galactic's rubber-burning engine would emit 60 grams of black carbon per kilogram of fuel burned.

However, the team did not have access to measurements of black carbon emissions from Virgin Galactic's engines, or those of other space tourism companies, which plan to burn other types of fuel, such as kerosene.

Lacking emission measurements from rubber-burning rockets, the researchers extrapolated from data on kerosene-burning rockets. These suggested soot emissions of 20 to 40 grams of black carbon per kilogram. Rubber is expected to burn less cleanly, but it is not clear by how much – the 60 grams is an educated guess.

Newer and cleaner

Jeff Greason, CEO of XCOR Aerospace, based in Mojave, California, which is developing a suborbital tourism vehicle called the Lynx, questions the accuracy of the soot emission estimates used in the study.

The soot data they used for kerosene rockets appear outdated and "at least an order of magnitude higher" than other measurements suggest, Greason says.

Modern kerosene-burning engines like those used in the Lynx are cleaner, he says. "The soot generation is so low that it is difficult to measure it," he told New Scientist. "So much of the data out there seems to come from older rockets with more soot generation."

Journal Reference: Geophysical Research Letters (in press)