Spacecraft Are Sprinkling the Stratosphere with Metal

Spacecraft Are Sprinkling the Stratosphere with Metal

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With life spans ranging from years to centuries, many of the satellites we launch into orbit around Earth come back down—roughly 200–300 objects annually. And though we have been keeping an eye out for this falling space debris, the stratosphere has been taking most of the hit, according to new research.

When spacecraft burn up in the upper atmosphere on reentry, metals making up the hardware don’t just disappear. “We have now found where they go: the stratosphere,” said atmospheric chemist Daniel Murphy at NOAA. Roughly 10% of the stratosphere’s large aerosol particles are contaminated with these metals.

Getting to the Stratosphere

The stratosphere’s lower reaches are 6–20 kilometers above Earth’s surface, making it difficult to study.

Satellites can measure the composition of the stratosphere by pointing a sensor at the horizon and measuring which wavelengths of light are absorbed at sunrise and sunset. “They get the big picture,” said atmospheric chemist Troy Thornberry at NOAA, who oversaw the project. To gather more detail, scientists can send instruments on high-altitude planes into the stratosphere.

Murphy and his colleagues turned to a specialized tool that measures the chemical fingerprints of individual air particles.

The researchers modified the instrument to withstand low air pressure and temperatures. (Flight crew wore space suits and breathed pure oxygen during the missions.) Attached to the nose cone of a high-altitude plane, a WB-57, the instrument flew more than 18 kilometers up into the skies over Alaska during several occasions earlier this year. Air in the stratosphere descends at Earth’s poles during the winter season, so high-latitude air samples reveal what’s happening at even higher altitudes than the plane can fly.

“Niobium and hafnium, to our knowledge, are not in any type of meteor that we’ve ever seen before, so we knew there was another source for them.”

As the WB-57 flew through the thin, cold air, the instrument swallowed individual particles. Once inside, each particle passed through two lasers that detected its size and speed and a third laser that vaporized it. The debris of ions was sorted into two mass spectrometers—one for positive ions and one for negative—to measure their mass and individual element components.

The team expected to find sulfuric acid and metals from the natural cosmic dust constantly bombarding Earth. After analyzing data from the 2023 mission, as well as data from a WB-57 mission above Houston, Texas, in 1998; a global atmospheric tomography mission in 2017; and a stratospheric investigation over Kansas in 2022, the researchers were surprised to see elements such as niobium and hafnium.

“Niobium and hafnium, to our knowledge, are not in any type of meteor that we’ve ever seen before, so we knew there was another source for them,” said atmospheric scientist Maya Abou-Ghanem, a coauthor on the study and a former postdoctoral scholar with NOAA.

A Polluted Stratosphere

When satellites reach the end of their life, many reenter Earth’s orbit. Gravity pulls them in fast, and friction against the atmosphere causes them to burn up into vapor, which eventually condenses onto other atmospheric particles.

An instrument is exposed in the nose cone of the WB-57 aircraft.
The instrument that measured the composition of the stratosphere was placed in the nosecone of the WB-57 aircraft. Credit: Gregg Schill, NASA

Engineers combine different types of metals to build each component of a spacecraft. When Murphy and his team analyzed their data, they identified more than 20 elements from spacecraft reentry that had the same combinations as those used in spacecraft. They also found more lithium, aluminum, copper, and lead from spacecraft reentry than from meteorites.

Nearly 10% of all large sulfuric acid particles, the main component of aerosols formed in the stratosphere, contained anthropogenic space metal hitchhikers. “That’s a big number, considering we didn’t know it would be abundant at all,” Abou-Ghanem said.

According to a collaborative report by McKinsey & Company and the World Economic Forum, the number of satellites orbiting Earth could triple in the next decade. At that rate, 50% of sulfuric acid particles could be contaminated, Abou-Ghanem said.

It is still unclear what these findings mean for the stratosphere and the life it protects. Satellite metals could affect the ability of particles to scatter and absorb light. And that could change how much sunlight reaches Earth and therefore affect Earth’s temperature.

“We now do things on a scale that has global consequences.”

Quantifying the human impact on the stratosphere is important because the number of satellites is likely to grow significantly in the coming decades, wrote Graham Mann, an atmospheric scientist at the University of Leeds who was not involved in the study, in an email. “The research is providing groundbreaking analysis,” he wrote. Scientists previously did not know how much human-caused pollution had made it into the stratosphere.

Murphy said the team believes this research is urgent. “We would like to understand these things before we put all these satellites into orbit, and not after.”

As researchers investigate the implications of these new findings, they want us to keep one thing in mind. “We now do things on a scale that has global consequences,” Thornberry said. “We just need to be mindful of that.”

—Molly Herring (@mollymherring), Science Writer

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Citation: Herring, M. (2023), Spacecraft are sprinkling the stratosphere with metal, Eos, 104, Published on 22 November 2023.
Text © 2023. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.

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