How Dirty Is A Rocket Launch?
published during a waxing gibbous moon.
07/15/2016

rocket launch

Space Shuttle Atlantis is seen as it launches from pad 39A on Friday, July 8, 2011, at NASA’s Kennedy Space Center in Cape Canaveral, Florida on mission STS-135. Credit: Bill Ingalls

A rocket engine fires, releasing a bright burst of flame. The vehicle rises on a column of smoke; taking humans (or maybe a satellite) up and away into the heavens.

Few things are more thrilling than watching a rocket launch. But you might find yourself wondering about all that billowing white stuff—tinged with gray and sometimes black—coming out of the engine. What exactly is it, and just how safe is it for the environment?

Surprisingly, this isn’t an issue that’s been deeply studied. The few pieces of information that have been gleaned give both reasons to be glad and perhaps a bit concerned. Rocket launches are relatively clean and, for the time being, contribute only a fraction of a fraction to climate change. But as the price of launches comes down, and the launch rate goes up, the industry is likely to start making a larger impact. Private tourism could contribute an ever-increasing share of harmful carbon dioxide emissions into the atmosphere, while a huge rise in rocket launches might start to mess with the ozone layer. There are some suggestions that it will be environmental factors, not technological or economic limitations, that constrain our plans to reach for the stars.

There are many kinds of rockets and rocket fuel. Broadly speaking, engines can fall into five different categories. Liquid-burning engines are probably the cleanest, combining liquid oxygen and liquid hydrogen to produce a huge thrust. The big orange booster carrying NASA’s Space Shuttle was of this variety and it produced little more than water vapor, giving it a fairly small environmental impact. Solid rocket boosters—such as the kind that were attached to the side of the shuttle’s main tank—often burn aluminum perchlorate, producing hydrochloric acid and tiny pieces of aluminum as their exhaust. Not much is known about these particles’ effects on climate change; some researchers suggest they could reflect sunlight and help cool the planet while others point out that aluminum is a potent infrared absorber and might heat things up more.

rocket launch

A picture from the first launch of the rocket systems of SpaceShipTwo. Credit: Virgin Galactic

Hypergolic rockets are dirtier, using extremely reactive chemicals like hydrazine and nitrogen tetroxide that spontaneously ignite on contact with one another. These produce large amounts of nitrous oxides, which can react with other particles in the atmosphere to form nitric acid, a chemical that damages ozone. Some of the most polluting are kerosene rockets, which burn jet fuel and produce carbon dioxide and soot—two potent contributors to climate change. SpaceX’s Falcon launch vehicles combine kerosene and liquid oxygen, though the company has been developing a methane/liquid oxygen fuel as a replacement. Finally, there are what’s known as hybrid engines, which combine liquid oxidizers, nitrous oxide, and synthetic organic compounds like nylon or rubber and have larger soot emissions than even kerosene rockets. Virgin Galactic’s SpaceShipTwo uses such an engine; it’s rocket plumes are some of the darkest and most polluting.

Private spaceflight is a growing business and nobody yet knows what its full impact might be.

But so far the annual rate of rocket launches hasn’t created much environmental impact. Carbon dioxide originating from all rockets is a hundredth of that from the commercial airline industry, which is itself only a small percentage of total human carbon dioxide emissions. That could soon begin to change. Private spaceflight is a growing business and nobody yet knows what its full impact might be. Unlike for airplanes, pollution from rockets is injected directly into the stratosphere, where it can remain for a long time.

A widely-reported 2010 study suggested that 1,000 launches per year of something like SpaceShipTwo’s tourist jaunts to suborbital space might create a persistent layer of soot particles in the northern stratosphere. Soot—often called black carbon—is up to a million times more efficient at heating up the atmosphere than carbon dioxide and so this layer has the potential to cause significant changes in global atmospheric circulation and temperatures.

“Whereas natural aerosol injections into the stratosphere (e.g., volcanoes or surface fires) are episodic and decay within a few years, the continuous nature of rocket emissions means that the rocket stratospheric black carbon layer becomes a constant, asymmetric feature of Earth’s atmosphere for as long as the launches continue,” the researchers wrote in their paper.

If you happen to be a rich adventurer worried about the environmental damage of your space tourism, perhaps consider ditching the rocket. Within a few years, World View Enterprises plans to start offering a much greener alternative—balloon flights to the stratosphere to see the curvature of the Earth and the edge of space.

“No rocket engine is perfectly ‘green’ in this sense”

But climate change is not the only environmental impact rockets have. A more worrying concern comes from the way they affect the ozone layer, that protective covering floating between 20 and 30 kilometers above the Earth’s surface that keep out harmful ultraviolet radiation. Ozone is delicate stuff and highly reactive radicals—such as oxides of nitrogen, hydrogen and chlorine, all produced during combustion—have a tendency to break it apart. A single radical molecule can destroy up to 100,000 ozone molecules. Even the Space Shuttle’s seemingly benign H2O (which created hydrogen radicals) punched a tiny hole in the ozone layer during each launch, though natural processes repaired this within weeks.

rocket launch

“No rocket engine is perfectly ‘green’ in this sense,” wrote the same authors of the 2010 study a year earlier.

Until now, the impact of rockets on ozone depletion has been negligible. But with companies like SpaceX bringing down the price of a launch, we might soon see more and more activity in space. A two or three-fold increase in commercial launches could drive ozone loss up to dangerous levels within a couple decades. Rockets have never been part of the international agreements put in place in 1987 to maintain the ozone layer’s health. But “if weekly launches of the Space Shuttle were proposed now, it is not clear how the stratospheric protection community would respond,” the researchers wrote as part of their analysis. Regulation might be necessary to protect the Earth’s natural environment, putting a cap on the number of worldwide launches allowed each year.

It’s often said that images from the Space Age contributed directly to the environmental movement. Pictures of the entire fragile Earth spurred people to consider their impact on our globe. As we move into this new era, it’s important to keep thinking about our roles as caretakers. The stars may call us, but we have to remember that we still always have to come home.