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SpaceShipTwo in glide
SpaceShipTwo, air launched over New Mexico and powered by a hybrid rocket engine, could deposit climatically significant amounts of carbon soot in the stratosphere, according to a recent study. (credit: Mark Greenberg/Virgin Galactic)

Climate change and suborbital spaceflight

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Talk about a bit of a downer. On Friday, October 22, several hundred people gathered in the desert of southern New Mexico to formally dedicate the runway at Spaceport America, the new facility being developed to host spaceflights by Virgin Galactic’s suborbital SpaceShipTwo, which put in an appearance attached to its WhiteKnightTwo carrier aircraft (see “A spaceport takes shape”, The Space Review, October 25, 2010.) The event’s atmosphere was one of anticipation of the potential economic, scientific, and societal benefits of such flights for the region and the world. That same day, though, came an announcement from the American Geophysical Union (AGU) that such flights could, in fact, be damaging to the environment.

According to a paper in press in the AGU journal Geophysical Research Letters (GRL), the problem with such launches is with the black carbon soot emitted by rocket engines. That soot, deposited in the stratosphere, could have a significant effect on the atmosphere should space tourism and other applications of commercial suborbital vehicles generate significant demand for flights. One model, assuming 1,000 suborbital flights a year from a single spaceport, found that the resulting soot could cool the latitude band around the spaceport by 0.7°C while warming Antarctica by 0.8°C. “The response of the climate system to a relatively small input of black carbon is surprising,” one of the study’s authors, Michael Mills of the National Center for Atmospheric Research, said in the AGU statement.

“The response of the climate system to a relatively small input of black carbon is surprising,” said Mills.

While the timing of the AGU release and the Spaceport America event was coincidental (as confirmed the following week by the AGU public information manager Peter Weiss), many reports linked the two events. “Soot from commercial space flights at the New Mexico-based Spaceport America could dramatically hasten global climate change,” noted an article in the New Mexico Independent. The linkages between suborbital spaceflight and climate change, though, may be more complex and less certain than some of the reports about the study indicated.

Models and approximations

The GRL paper, at its heart, examines what happens when a volume of black carbon, in the form of soot from the exhaust of rocket engines, is injected into the stratosphere in large quantities. The paper uses a “gold standard” computer model called the Whole Atmosphere Community Climate Model (WACCM) to measure the effects of the insertion of black carbon from 1,000 suborbital flights a year over 40 years, all launching from the same location, roughly associated with Spaceport America.

The model finds that most of the black carbon stays in a band in the stratosphere between 25 and 45 degrees north latitude. “The continuous nature of rocket emissions means that the rocket stratospheric BC [black carbon] layer becomes a constant, asymmetric feature of the Earth’s atmosphere for as long as launches continue,” the paper notes. That affects temperatures not just in that latitude band but globally, which the paper attributes to factors ranging from movement of subtropical jets to regional changes in cloudiness and oceanic heat content, among other factors. The model also shows that heating at the poles from the atmospheric model also affects sea ice levels, particularly in Antarctica: an average annual loss of 5%, peaking in the summer at 18%.

All this led the authors to conclude that suborbital spaceflight would have a significant effect on global climate. “Taken together, these results demonstrate that the accumulation of BC from the modeled suborbital rocket fleet launched 1000 times per year (or orbital rocket equivalent) will influence global climate about as much as the world’s fleet of subsonic aircraft,” the paper concludes. Such emissions “clearly cross a threshold to be considered a human-influenced climate impact of global importance.”

“There has never been a systematic effort to collect in situ black carbon data from a hydrocarbon rocket plume,” said Ross.

That conclusion, however, is based on a number of assumptions that went into the model that are open for interpretation. Perhaps the biggest is just how much black carbon rocket engines produce. The paper used as a model input the amount of soot hybrid rocket motors, like that which will power SpaceShipTwo, produce. Such motors, the authors argue, are dirtier than liquid oxygen (LOX)/kerosene engines because of lower oxidation rates for carbon particulates in the engine plume. For the paper the authors assumed a black carbon “emission index” of 60 grams per kilogram from a hybrid engine, a “provisional, though plausible, value”; LOX/kerosene engines have a lower index, of about 20–40 grams per kilogram.

However, all such estimates for black carbon production by rocket engines are just that: estimates. “There has never been a systematic effort to collect in situ black carbon data from a hydrocarbon rocket plume,” Martin Ross of The Aerospace Corporation, lead author of the GRL paper, said in an email interview. “Bottom line: the soot EI [emission index] of rocket engines is rather poorly known and is one of the most significant factors in the overall uncertainty.”

Another assumption of the model is the launch mode for such suborbital vehicles. The GRL paper assumed an air-launched system, like SpaceShipTwo, where all the emissions occur in the stratosphere. That is important in the model since soot deposited in the stratosphere has a much longer lifetime than in the troposphere. “Because the stratosphere is largely decoupled dynamically from the troposphere and emissions accumulate there, the concern is stratospheric emissions,” Ross said.

However, most other suborbital vehicle designs under active development—such as those by Armadillo Aerospace, Blue Origin, Masten Space Systems, and XCOR Aerospace—are not air launched, but instead launch from the ground, meaning much of their emissions don’t take place in the stratosphere. “A good rule of thumb is that one third of a ground launched vehicle’s exhaust is deposited into the stratosphere,” Ross said.

Another model assumption is that all the launches take place from a single site, Spaceport America. While the spaceport’s backers in New Mexico might not mind such a high level of flight activity (about three launches a day), such a high level of launch activity would likely be spread among a number of spaceports at different latitudes. Ross said that launch site latitude could be an important factor in the how the climate responds to the injection of soot into the stratosphere. “It could well be expected that the earth system response is controlled to a large degree by launch site latitude,” he said. “This is one of the important variables that need to be examined in future model runs.”

“Our results really are to be taken as a general indication of the sensitivity of the atmosphere to soot injected into the stratosphere, rather than as a detailed prediction of the atmosphere’s response to a particular system,” Ross concluded. “Our input was for a generic ‘suborbital’ system with order of magnitude system parameters.”

Industry response and future study

The GRL paper effectively concluded that a vehicle that looks very much like SpaceShipTwo, flying regularly from Spaceport America, could have an effect on climate that would be on the same scale as all of commercial aviation. That runs very much at odds to Virgin Galactic’s efforts to minimize the environmental impact of its system.

“The commercial spaceflight industry is deeply committed to protecting the environment and the priceless planet that we call home,” CSF president Alexander said.

“The carbon output of a SpaceShipTwo flight into space will be slightly less than a business class ticket on Virgin Atlantic from London to New York,” Will Whitehorn, president of Virgin Galactic, said at the Spaceport America runway dedication ceremony, reiterating previous statements by the company on its emissions. “When you compare that to something like the Shuttle or Soyuz, the carbon output is negligible.” He concluded that the system “has a long-term future, even in a carbon-constrained environment, if that comes about.”

What form that carbon will take isn’t clear, but if it’s in the form of carbon dioxide, it may be insignificant compared to the effects of the carbon soot. The GRL paper found that the effect from carbon dioxide from rocket emissions in its model was smaller than that from the black carbon by a factor of 140,000.

At the spaceport ceremony, Whitehorn did indicate that Virgin was looking at an alternative to the rubber solid fuel used in its hybrid engine. “With SpaceShipTwo we do have the option to use that [rubber] again, but we also have the option to use basically recycled nylon,” he said. “This would have a very, very minimal carbon output.”

A week after the AGU press release, the industry trade group the Commercial Spaceflight Federation (CSF) issued a response, including a fact sheet that addressed what it considered some of the shortcomings in the paper. For example, the assumption used in the GRL paper of 10,000 kilograms of fuel per launch may be too high, with SpaceShipTwo actually using a third less propellant. The CSF paper also argued that the black carbon emission index of LOX/kerosene engines of 20–40 grams per kilogram cited in the paper “is derived from measurements of an obsolete launch vehicle with 1950’s engine technology.” Modern LOX/kerosene engines, the organization claimed, could be factor of ten cleaner.

Ross acknowledged the uncertainties in emission indices for various types of rocket engines, and said there was a need for additional data. “In order to reduce the uncertainties in EI, we will need to execute a modest effort of measurements of plume composition at different altitudes for different engines,” he said. Such work, he said, would be similar to efforts in the 1990s to measure emissions of chlorine and alumina from solid rocket motors to measure their effect on the ozone layer. “This research can be done now, with existing instrumentation and aircraft, and with a very modest budget over a modest period of time.”

Ross added that he is in the opening stage of organizing a workshop at The Aerospace Corporation on the topic of rocket emissions and their effect on the atmosphere. That workshop, he said, “will gather all of the stakeholders to discuss and work out what the data needs are from a policy and technical perspective, educate the stakeholders from each others’ perspectives, and determine who can do the data collection and model analysis that needs to be done.”

“The commercial spaceflight industry is deeply committed to protecting the environment and the priceless planet that we call home,” CSF president Brett Alexander said in the organization’s statement. “Indeed, the ‘overview effect’ of traveling to space is well-known. Those who are blessed with seeing our fragile blue marble from space always return with a reinvigorated passion to protect it.” The industry, though, may be in the position of having to demonstrate that the benefits of such flights outweigh any adverse environmental impacts they might create.