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The dark exhaust seen streaming from the Merlin engines on a SpaceX Falcon 9 rocket that launched last October contains black carbon, which chemists say may play a role in ozone-depleting chemistry. (SpaceX via YouTube)

Thanks to Blue Origin, SpaceX and other space ventures, the skies could well be filled with rockets in years to come. But what will that do to the environment?

The short answer is, not that much right now. But as experts look to the years ahead, the answer gets as hazy as the air after a Falcon Heavy launch.

“Ten years ago, the amount of emissions was not great,” said Martin Ross, an engineer at the Aerospace Corp. whose research focuses on the effects of space systems on the stratosphere. “Today, the effect is still small, but it’s growing.”

Over the 60 years of the Space Age, the year-by-year tally of orbital launches has gone up and down, with the most active years coming during the height of the U.S.-Soviet space race and the start of the space shuttle era. Now the rate is on a sharp upswing, and if all of the plans being hatched by launch providers come to fruition, 2018 could well be a record year.

Ross focuses on ozone-depleting chemicals rather than climate change — even though most rocket emissions take the form of carbon dioxide and water vapor, two of the best-known greenhouse gases.

“The carbon dioxide emitted by rockets is absolutely and completely insignificant compared with aviation,” Ross explained. And aircraft, in turn, are thought to account for less than 10 percent of the total greenhouse-gas emissions linked to the U.S. transportation sector. (That’s compared with more than 80 percent for cars and trucks.)

Ross has been modeling how the chemistry of rocket exhaust affects the stratospheric ozone layer for years, and I talked about his research on Seattle’s KUOW public radio this week, in response to a listener’s question. Take a listen to the interview here:

KUOW: Will private rockets destroy the ozone layer?

There’s much more to come: Ross’ updated analysis is due to be included in the United Nations’ quadrennial assessment of atmospheric ozone depletion, mandated by the 1987 Montreal Protocol and due for publication by the end of the year.

The formal findings have to be held back for the U.N. report, but when I talked with Ross, he made clear that there are lots of questions yet to be answered about future impacts. “We don’t really have a good handle on it,” he acknowledged.

Ross does have a good handle on the chemistry involved, however, at least when it comes to the propellants that are currently in use.

Solid-fueled rocket motors, such as the side boosters that were used in the shuttle program or will be used for NASA’s heavy-lift Space Launch System, pose the knottiest problems, “in that the emissions are a more varied mix,” Ross said.

Those rockets emit black carbon (basically, soot) as well as alumina particles and chlorine-based compounds. All three types of emissions can promote ozone-destroying reactions, particularly when they’re injected directly into the stratosphere.

When it comes to kerosene-fueled rockets, such as SpaceX’s Falcon 9 and Falcon Heavy, black carbon is the primary concern. “Once those particles get into the stratosphere, they stay in there for three or four years,” Ross said.

Hydrogen-fueled rocket engines, such as the ones that were installed in the space shuttle orbiter or are used by Blue Origin’s New Shepard suborbital spaceship, pose far less risk to the ozone layer. Their exhaust consists primarily of water vapor (2H2 + O2 = 2H2O).

Ross’ past studies have shown that the impact on stratospheric ozone would be negligible even if the launch rate were to increase by a factor of 10. But there are two big reasons why he sees uncertainty in the years ahead.

One is that SpaceX, Blue Origin and other companies are planning a dramatic uptick in the launch rate, in part to deploy constellations of satellites that could eventually number in the thousands. Ross said the shift to rocket reusability — which SpaceX’s Elon Musk and Blue Origin’s Jeff Bezos see as a must-have — could change the way those launches are done.

“We’re going to start launching with less than a full payload because the prices will come down,” he said. “But reusability requires launching often. Total emissions will be expected to rise.”

The other wild card is the shift to methane as a rocket propellant. Both SpaceX and Blue Origin are developing methane-fueled rocket engines (the Raptor and the BE-4, respectively) that could set a new standard for commercial spaceflight. “We don’t know anything about how a methane rocket affects the ozone layer,” Ross said.

So, what is to be done? Ross doesn’t have a prescription … yet.

“It’s far, far premature to talk about any kind of change in the mix,” he said, “but because the industry is growing, it would be good to understand the implications of all these types of propellants.”

By the time crews start taking off for Mars, a decade or two from now, Ross and his successors hope to know how best to keep the skies of the home planet healthy.

Update for 4:30 p.m. PT April 25: A newly published Aerospace Corp. policy paper, written by Ross and James Vedda and titled “The Policy and Science of Rocket Emissions,” addresses the issue in depth. The paper calls on the global launch industry and its stakeholders to boost research into the environmental effects of rocket emissions, so that such emissions get the appropriate level of environmental regulation as commercial launches become more frequent.

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