Greenhouse Gases Will Reduce The Number of Satellites That Can Orbit Safely

Among all the consequences of climate change, one that few people have considered is the effect on space beyond the traditionally defined atmosphere, since it could seem logical that this region would be immune. However, there is no sharp point where the atmosphere stops. Instead, it slowly gets thinner and thinner as you go up, and traces are present far above the Karman line, usually considered to mark where space begins. 

Increased greenhouse gas concentrations cause the upper atmosphere to shrink, so hundreds of kilometers up, the particles get even sparser. This sounds like a win for space flight companies – no more cases of satellites unexpectedly coming down early because a solar storm causes the atmosphere to bulge. However, a few particles at the heights most satellites orbit is actually a good thing, because it helps clear out space junk.

Any atmospheric particles, no matter how diffuse, produce drag on orbiting objects. This causes their orbits to decay, bringing them into contact with the slightly thicker atmosphere below, accelerating the fall further.

For valuable satellites, this is annoying but can be addressed by occasionally boosting them into higher orbit, as is done for the ISS sometimes. Meanwhile, items that have passed their use-by date, or been chipped off satellites, experience the same effects. The largest may land dangerously, but most burn up harmlessly in the atmosphere, removing their threat to operating satellites, and people.

A more condensed atmosphere means space junk stays up longer, and therefore reduces the number of operating craft that can orbit without the danger of a setting off chain reaction of collisions, known as Kessler Syndrome. 

“More satellites have been launched in the last five years than in the preceding 60 years combined,” the study’s lead author, MIT graduate student William Parker, said in a statement. “One of key things we’re trying to understand is whether the path we’re on today is sustainable.”

“The sky is quite literally falling — just at a rate that’s on the scale of decades,” Parker said. “And we can see this by how the drag on our satellites is changing.”

Although this is clearly a negative for space pioneers, it’s easy to assume the effect would be so small as to be insignificant. Surely some trace gases don’t change atmospheric density that much?

However, when Parker and co-authors ran the numbers, they found something very different. “Our behavior with greenhouse gases here on Earth over the past 100 years is having an effect on how we operate satellites over the next 100 years,” co-author Dr Richard Linares of MIT said. 

Parker, Linares, and co-author Dr Matthew Brown of the University of Birmingham, UK, modeled what they call the “carrying capacity” of each shell around the Earth under the IPCC’s scenarios for greenhouse gas emissions. They estimate high emissions will reduce the number of satellites the most popular orbits – between 200 and 1,000 kilometers (124-621 miles) high – can hold in the year 2100 by 50-66 percent.

“In local regions, we’re close to approaching this capacity value today,” Linares said. The 900 and 1,400-kilometer (559 and 870-mile) shells in particular are now dangerously cluttered,

“The upper atmosphere is in a fragile state as climate change disrupts the status quo,” Parker said. “At the same time, there’s been a massive increase in the number of satellites launched, especially for delivering broadband internet from space. If we don’t manage this activity carefully and work to reduce our emissions, space could become too crowded, leading to more collisions and debris.”

At first glance, global warming might be expected to make the atmosphere expand not contract, since that is what (most) things do when they heat up. However, it’s important to remember the greenhouse effect does not create more heat, but rather traps it in the troposphere and oceans where we experience it. Meanwhile, the stratosphere and layers above cool down, causing them to shrink.

Satellite-launching nations and companies have at least three ways to address the problem. They could get serious about tackling greenhouse gas emissions or removing space junk, boost satellites into higher and more expensive orbits, or just ignore the whole issue until disaster strikes and thousands of satellites are damaged and destroyed. 

We’ll leave it to you to decide which is most likely, but it is worth noting the second path is not the easy compromise it might seem. Since higher orbits are further out of reach of atmospheric drag, satellites placed there will continue to orbit indefinitely unless deliberately brought down. The same goes for any bits that come off, particularly likely for places like space hotels where the human factor could lead to many accidents. Consequently, such locations will be filled up a lot more quickly than those currently in use, forcing subsequent satellites to be shifted ever higher.

“We rely on the atmosphere to clean up our debris. If the atmosphere is changing, then the debris environment will change too,” Parker said. “We show the long-term outlook on orbital debris is critically dependent on curbing our greenhouse gas emissions.”

The paper is published in the journal Nature Sustainability.