SOURCE: Scientific American
DATE: November 6, 2020
SNIP: The space industry is growing and innovating at a pace not seen since the days of the moon landings. Fifty years ago, nearly everything related to space was a government-sponsored project. In 21st-century space, launch vehicle and satellite finance are most often bottom-line corporate investments or public-private partnerships.

Untethered from government leashes, the global space industry looks and operates increasingly like global aviation.

Sustainability has not much been a concern for space systems development. Just like their jet engine cousins, rocket engines emit a variety of gases and particles into the atmosphere that can have regional and even global consequences. Even so, launch vehicle environmental impacts are typically disregarded by comparing jet and rocket fuel consumption in an overly simplified way.

The argument goes like this: Rockets burn only 0.1 percent of the fuel that aircraft burn each year and are therefore only 0.1 percent of the emissions problem that aviation presents. But this is a case of false equivalence. Careful understanding of every phase of space flight shows that space emissions can impact the atmosphere in ways that are wholly different from, and in some cases larger than, aviation emissions.

Unlike with aviation, every layer of the atmosphere sees space industry emissions. While jet emissions into the troposphere are quickly washed away to the surface by precipitation, rocket emissions into the stratosphere clean away only slowly. Stratospheric emissions accumulate year over year, adding up exhaust from all of Earth’s launches and reentries over the past four or five years. In fact, the fragile ozone layer resides in the stratosphere, near where rocket emissions accumulate.

Rockets famously display brilliant exhaust plumes. Hydrocarbon-fueled rocket engine “flame” is mostly the incandescent glow of soot particles oxidizing in the hot plume. Soot production in rocket engines is complicated and not very well understood. Soot forms in fuel-rich combustion chambers, fuel-cooling nozzle walls and turbopump gas generators, and is partly consumed in the hot plumes. Jet engines have none of these complexities and burn very clean compared to rocket engine. Some types of hydrocarbon-fueled rocket engines emit hundreds of times more soot for each kilogram of fuel burned than do their jet engine cousins. And jets only occasionally fly in the stratosphere; rockets fly there every launch.

What is the concern about soot in the stratosphere? Black carbon soot (BC) very efficiently absorbs sunlight. The absorbed energy is transferred to surrounding air so that BC acts as a heat source, warming the stratosphere, which can in turn slightly change the circulation of the global atmosphere. And since ozone concentration is inversely proportional to temperature, a warmer stratosphere equates to depletion of the ozone layer. Is the BC emitted by the current global fleet of rockets great enough to have a significant impact on the global atmosphere? We do not yet know. The required climate models are only now being assembled. BC soot emission by hydrocarbon-fueled rockets, and its global impacts remain a mystery.

SRM plumes are even more brilliant than hydrocarbon-fueled plumes. White hot alumina droplets leaving the nozzle are the source for the SRM “flame. As with the chlorine gas emission, SRM plumes diffuse and eventually mix into the global atmosphere so that rocket alumina particles are found in random stratospheric air samples from equator to poles. In the 1990s, researchers discovered how ozone-destroying chemical reactions occur on the surface of SRM alumina particles, but alumina’s significance as a source of ozone depletion is not known. SRMs emit ozone-destroying chlorine gas, too, of course, and the double-sided nature of SRM ozone depletion remains poorly described. The 2018 World Meteorological Organization (WMO) Ozone Assessment acknowledged the large knowledge gaps and noted that further research is “warranted.”

Contrary to many media stories about the latest space junk reentry spectacular, space junk returning to Earth does not “disappear” upon reentry. Some parts of derelict spacecraft will survive reentry and reach the surface. However, most of the reentering mass vaporizes into a hot gas that quickly condenses into a spray of small particles. Thus, as with launch, bright plumes mean particle production. Unlike the chemically simple particles from launch, particles from reentering space junk will be a zoo of complex chemical types. Particles from vaporizing propellant tanks, computers, solar panels and other exotic materials will form around an 85-kilometer altitude then drift downward, accumulating in the stratosphere along with launch’s soot and alumina. Reentry is as much of an “emission” as launch.

The growing LEO megaconstellations, with thousands of satellites in each constellation, use reentry vaporization as the satellite end-of-life disposal mechanism. Once these constellations are deployed, hundreds of tons of nonfunctioning satellites will be “brought in” for disposal every year. Most of this mass will become particles in the middle atmosphere. Very little is known about reentry dust production, the microphysics of the particles and how reentry dust could affect climate and ozone.

The particles emitted by rocket launches and space debris reentries cause much larger changes in atmospheric chemistry, dynamics and radiation than rocket CO2 emissions. For the space industry, the “carbon footprint” is a complicated story that is yet to be appropriately defined.