Suborbital spaceflights that rely on rubber-based rocket fuel could shrink icecaps, alter the ozone layer and affect global temperatures, according to a new study.
Yet the study authors’ assumptions about the number of rocket launches per year and the chemistry of rocket exhaust have raised questions about their conclusions among space-tourism companies and climatologists not involved in the study.
Atmospheric scientists who performed the research probed the effects of belching ultrafine soot high into the stratosphere, where — unlike the troposphere below it — there isn’t rain and wind to quickly filter soot out of the air. Rubber-based rocket fuel burned with nitrous oxide is the preferred propellant of the burgeoning space-tourism industry, and chemists suspect such hybrid engines emit
sooty black carbon. Closer to Earth, the stuff has been shown to soak up extra radiation from the sun and contribute to climate change.
“This study was a natural extension of the climate-research community gaining a greater and greater appreciation of black carbon in terms of global radiative forcing,” said Martin Ross, an atmospheric scientist at The Aerospace Corporation in El Segundo, California, and leader of the research funded in part by his employer. “Soot is a very large issue in the troposphere,” Ross said, but its behavior isn’t well-understood at higher altitudes.
To model the effects of space tourist launches on the Earth’s atmosphere, Ross and his colleagues used the open-source Whole Atmosphere Community Climate Model Version 3, or
WACCM3, one of the most-advanced computer models available to study impacts to global climate.
They ran two
supercomputer-powered simulations for two weeks, one as a control and another modeling the impact of 1,000 suborbital flights per year for the next four decades. That many flights, according to the study, would annually deposit more than 1.3 million pounds of soot into the stratosphere.
“We looked at the stated business plans from corporations that are planning to build vehicles for space tourism,” Ross said. “If you go to their websites, they’ll say things like, ‘we plan to launch once per day.’ We found 1,000 per year is well within stated objectives of the industry.”
On average, according to the simulation, the soot pushed polar ocean temperatures up by 1.8 Fahrenheit degrees, melted 5 to 15 percent of sea ice and depleted 1 percent of tropical ozone (while boosting polar ozone by 6 percent).
“We’re not making any particular prediction about any system, just taking reasonable guesses at what soot from a hybrid rocket engine looks like and what the launch industry will do in the future,” Ross said. “When we put that into a gold-standard model, the effect on the Earth is surprisingly large. In short, we think black-particle carbon from rockets is something that deserves attention.”
Their assumptions may not be perfect, said Gerald North, an atmospheric scientist at Texas A&M University who was not involved in the study, but the measured effect is significant enough to warrant further investigation.
While they make assumptions about some unknowns, such as the behavior of soot at high altitudes, North said, “they’re careful in expressing this is not the last word” and are “inviting others to take a look.”
Ross and Michael Mills, an atmospheric chemist at the
National Center for Atmospheric Research in Colorado and a co-author of the study, said that’s precisely what the research team sees as the next step. In particular, getting a handle on what’s in the emissions of different types of rocket plumes.
“There are few direct high-altitude measurements of rocket plumes. We really need to get aircraft in those and get measurements of soot and other particles,” Mills said. “Until then, the sophistication of our models is limited.”
To do just that, Ross said The Aerospace Corporation is planning a workshop to bring together under one tent all the stakeholders in science, rocket engineering, space-tourism companies and the government agencies.
“We need to get these players together and exchanging ideas, then ask the policy people to figure out what to do, if anything, with the information,” Ross said.
“I think we and others in the industry welcome the opportunity to talk about all of these issues,” said George Whitesides, CEO of
Virgin Galactic, a space-tourism company that’s planning to use hybrid rocket engines. Whitesides wasn’t without reservations about the study and its conclusions, however.
“Frankly, I have to admit I wished they talked to us before putting out a paper, but that’s OK. Climate issues are deeply important to Virgin, and we take them very seriously,” Whitesides said.
Part of the reason the company chose the hybrid rocket design for its
SpaceShipTwo was “because of its significantly lower environmental impact than other designs.” Whitesides also said 1,000 space tourist launches per year is “guesswork,” because the industry is privatized and young.
“I think as we look at this more, we’ll find the impact will be far smaller than that set out in the paper,” he said. “In any case, I welcome the conversation.”
Whether or not peaceable collaborations ensue, both Ross and Mills expressed that carbon soot is something the nascent space tourism industry can’t ignore.
“This shows that a new kind and level of emission being deposited directly into the stratosphere could have a significant effect,” Mills said. “Companies need to proceed with developing their systems with full knowledge of consequences on the planet.”
Images: 1) Virgin Galactic’s SpaceShipTwo (center) attached to WhiteKnightTwo. Flickr/Jeff Foust. 2) The average predicted changes after one decade in the ozone layer (top) and regional temperatures (bottom) caused by 1,000 hybrid rocket launches per year for 40 years. Ross et al. 3) Average seasonal soot deposition, in grams per square meter, in the stratosphere predicted by Ross et al.’s simulation. Ross et al. See Also:
