A long exposure showing a string of SpaceX StarLink satellites over Florence, Kansas, this month. Photo / AP
A long exposure showing a string of SpaceX StarLink satellites over Florence, Kansas, this month. Photo / AP
Changes to the atmosphere caused by carbon dioxide emissions could increase the amount of debris that stays in orbit.
It's easy to compare the space junk problem to climate change. Human activities leave too many dead satellites and fragments of machinery discarded in Earth orbit. If left unchecked, space junkcould pose significant problems for future generations — rendering access to space increasingly difficult, or at worst, impossible.
Yet the two may come to be linked. Our planet's atmosphere naturally pulls orbiting debris downward and incinerates it in the thicker lower atmosphere, but increasing carbon dioxide levels are lowering the density of the upper atmosphere, which may diminish this effect. A study presented last month at the European Conference on Space Debris says that the problem has been underestimated, and that the amount of space junk in orbit could, in a worst-case scenario, increase fiftyfold by 2100.
"The numbers took us by surprise," said Hugh Lewis, a space debris expert from the University of Southampton in England and a co-author on the paper, which will be submitted for peer review in the coming months. "There is genuine cause for alarm."
Our atmosphere is a useful ally in clearing up space junk. Collisions with its molecules cause drag, pulling objects back into the atmosphere. Below 480km above the Earth's surface, most objects will naturally decay into the thicker lower atmosphere and burn up in less than 10 years.
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In the lower atmosphere, carbon dioxide molecules can rerelease infrared radiation after absorbing it from the sun, which is then trapped by the thick atmosphere as heat. But above 95km where the atmosphere is thinner, the opposite is true. "There's nothing to recapture that energy," said Matthew Brown, also from the University of Southampton and the paper's lead author. "So it gets lost into space."
A piece of debris that strayed from the Earth-orbiting Space Shuttle Challenger in February 1984. Photo / Nasa via The New York Times
The escape of heat causes the volume of the atmosphere, and thus its density, to decrease. Since 2000, Brown and his team say the atmosphere at 400km has lost 21 per cent of its density because of rising carbon dioxide levels. By 2100, if carbon dioxide levels double their current levels — in line with the worst-case scenario assessment by the Intergovernmental Panel on Climate Change — that number could rise to 80 per cent.
For space junk, the implications are stark. More than 2,500 objects larger than 10cm in size currently orbit at or below an altitude of 400km. In the worst-case scenario, increased orbital lifetimes of up to 40 years would mean fewer items are dragged into the lower atmosphere. Objects at this altitude would proliferate by 50 times to about 125,000.
Even in a best-case scenario, where carbon dioxide levels stabilise or even reverse, the amount of space junk would still be expected to double. Brown thinks a more probable outcome is somewhere in between, perhaps a tenfold or twentyfold increase.
The research is "very important work," said John Emmert, an atmospheric scientist at the US Naval Research Laboratory in Washington, DC, who has studied atmospheric density loss. However, Emmert says more research is needed to understand the severity of the problem — with the impact of the sun's solar cycle also known to be a major factor in atmospheric density changes.
The findings may also pose challenges for regulators and satellite operators, especially SpaceX, Amazon and other companies seeking to build megaconstellations of thousands of satellites to beam internet service down to the ground from low Earth orbit.
Just last month, for example, the US Federal Communications Commission approved a request from SpaceX to decrease the orbits of nearly 3,000 satellites in its Starlink constellation, reasoning that atmospheric drag would naturally sweep up dead satellites and debris in a reasonable amount of time.
Starlink satellites viewed from the International Space Station in 2020. Photo / Nasa via The New York Times
Research by Brown and his team suggests that assumption may be flawed.
An FCC spokesman said that most of its applicants currently used Nasa's Debris Assessment Software to predict lifetimes of satellites in low Earth orbit. "We do not know at this time if there are any plans to change that program to address the changes in atmospheric composition predicted in the paper," he said. "The FCC periodically reviews its rules and regulations and updates them consistent with developments in the marketplace and in scientific knowledge."
SpaceX did not respond to a request for comment.
Lewis said that he suspected that some of the modelling, however, relies on outdated data, and that more needed to be done to actively remove satellites and debris from orbit rather than relying on the passive atmospheric effect. "Operators have to make this aspect of the mission a priority," he said.
Even a moderate increase in lifetimes for large constellations could pose significant problems. "If SpaceX's spacecraft reenter passively in 10 or 15 years, would you argue that's good enough?" Lewis said. "Given the fact that it's a large constellation, lots of people would say probably not."
