Atmosphere

Spaceflight is Polluting the Atmosphere with Metal

Humans can’t seem to interact with the environment at all without fouling it in some way. From plastic bags in the ocean’s deepest regions to soot on Himalayan glaciers, our waste is finding its way into Earth’s most difficult-to-reach places.

Now, we can add metals in the stratosphere to this ignominious list.

Researchers at Purdue University’s College of Science found metals in aerosols high in Earth’s stratosphere. Those metals are altering the chemistry in our planet’s atmosphere in ways we don’t fully understand.

The researchers presented their results in a paper in the Proceedings of the National Academy of Sciences. It’s titled “Metals from spacecraft reentry in stratospheric aerosol particles.” The lead author is Daniel Murphy from the Department of Earth, Atmospheric, and Planetary Sciences at Purdue University.

The research team attached instruments to the nose of aircraft that flew over 17.7 km (11 miles) above the Earth’s surface. They detected significant quantities of over 20 chemical elements, including lithium, aluminum, copper, and lead. Nothing in nature can explain their presence, and the team concluded that they come from vapour boiling off of the surfaces of spent boosters and other space debris as it re-enters the atmosphere and burns up.

“Understanding our planet is one of the most urgent research priorities there is.”

Professor Dan Cziczo, Purdue University

One of the researchers is Professor Dan Cziczo, head of the Department of Earth, Atmospheric, and Planetary Sciences at Purdue’s College of Science. Cziczo is an internationally recognized expert in atmospheric science.

Dan Cziczo stands in front of NASA’s ER-2 High-Altitude Science Aircraft. Image Credit: Purdue University photo/John Underwood

“We are finding this human-made material in what we consider a pristine area of the atmosphere,” said Cziczo. “And if something is changing in the stratosphere — this stable region of the atmosphere — that deserves a closer look.”

The stratosphere is where Earth’s ozone layer is. The ozone layer protects the Earth’s surface and its inhabitants from the Sun’s harmful UVB radiation. UVB radiation is what gives us sunburns, and it plays the largest role in causing skin cancer. Disrupting it is not a good idea.

The ozone layer in the stratosphere shields life on Earth from most UV-B and UV-C, the most harmful varieties of ultraviolet radiation. Image Credit: NASA

The ozone layer protects Earth, but it also has its own protectors: large sulfuric acid particles. But these particles can also act as seeds for the ozone-depleting sulphate aerosol layer under the right conditions. The study found that almost 10% of these particles contain aluminum and other metals. What does this mean for the ozone layer?

That may depend on how much metal is aloft in the stratosphere.

To find out, the researchers flew two specially-equipped aircraft that can reach high altitudes. One was a WB-57 airplane that sampled the atmosphere 11.8 miles (19 km) above the ground in Alaska, and the other was an ER-2 aircraft that flew over the continental United States.

The WB-57 aircraft pictured here is particularly unique because it can fly at high altitudes, up to 60,000 ft. Image Credit: NASA

Though the stratosphere doesn’t enter into most peoples’ minds on a regular basis, scientists have been studying this important region for decades. It’s filled with meteoric particles from meteors that burn up in the atmosphere. But with more launch activity, the nature of those particles is changing.

“Shooting stars streak through the atmosphere,” Cziczo said. “Often, the meteor burns up in the atmosphere and doesn’t even become a meteorite and reach the planet.” The meteors’ burned-up remnants form ions that persist in the atmosphere. Initially, they form a very hot gas. But, eventually, the gas cools and condenses into molecules that fall into the stratosphere.

“The molecules find each other and knit together and form what we call meteorite smoke,” Czizco said, and that smoke can be measured. “Scientists recently started noticing that the chemical fingerprint of these meteoritic particles was starting to change, which made us ask, ‘Well, what changed?’ because meteorite composition hasn’t changed. But the number of spacecraft has.”

One of the challenges in this research is differentiating sulfuric acid particles containing meteoric metals from ones containing spacecraft metals. This figure from the study shows spectra from a particle with only meteoric metals (top) compared to one from a particle containing both meteoric metals and spacecraft metals (bottom.) Notice the Aluminum (AL,) Lithium (Li,) and Copper (Cu) in the bottom spectrum. In the bottom spectrum, only elements that are significantly enhanced are tagged. Also, the presence of Niobium (Nb) is an “unequivocal signal of ablation from specific rocket body structures,” according to the authors. Image Credit: Murphy et al. 2023.

The number of spacecraft being launched is growing rapidly. In fact, 2022 was a record year for spacecraft launches, with 180 successful launches. Unlike in the past, many of these launches were from private companies, and there’s no sign of it slowing down.

Most launches don’t only loft their payload into orbit and beyond; they also produce debris, especially spent boosters, that fall back through the atmosphere to Earth. Defunct satellites also deorbit, adding to the problem. On their way down, they burn up, and that puts metals into the stratosphere. All that unnatural metal could change the atmosphere, and scientists don’t yet understand how or how much or for how long.

This video shows Japan’s Hayabusa spacecraft burning up on re-entry. Credit: NASA

“Just to get things into orbit, you need all this fuel and a huge body to support the payload,” Cziczo said. “There are so many rockets going up and coming back and so many satellites falling back through the atmosphere that it’s starting to show up in the stratosphere as these aerosol particles.”

This is an alarming trend, considering how our industrial activities damaged the ozone layer in the past. Chemicals called chlorofluorocarbons (CFCs) were emitted in large quantities, and they were the primary culprits in damaging the ozone. Eventually, countries around the world established regulations regulating their use, and over time, the ozone layer repaired itself.

That took a lot of effort. Scientists had to understand the ozone, politicians had to develop agreements not to use it, and industry had to develop alternatives. None of that was easy, and the issue of metals in the stratosphere is equally confounding.

This research doesn’t arrive at any firm conclusions on the effect these metals have. It does mention some possibilities, though. They could affect the nucleation of ice particles or of nitric acid trihydrate (NAT) above the poles. A paper from 2000 suggested that the “… presence of NAT particles enhances the potential for chlorine activation with subsequent ozone destruction in polar regions, particularly in early and late winter.”

They also write that “novel stratospheric chemistry” is possible as more metals accumulate in the stratosphere. “Until the perturbations caused by such aerosols are better understood, they represent a growing uncertainty for the stratospheric aerosol layer,” the researchers write. As we continue to inject metals into the upper atmosphere, there are “unknown consequences for inclusions and ice nucleation.”

This is all on top of growing evidence that rocket exhaust itself is damaging the ozone layer.

Will we need to change how we do things to protect the ozone layer, just like we did in 1987 when we developed the Montreal Protocol? Maybe. But if we do, we’re only in the first phase: understanding the problem scientifically.

“Changes to the atmosphere can be difficult to study and complex to understand,” Cziczo said. “But what this research shows us is that the impact of human occupation and human spaceflight on the planet may be significant — perhaps more significant than we have yet imagined. Understanding our planet is one of the most urgent research priorities there is.”

Evan Gough

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