MIT aerospace engineers have discovered that rising greenhouse gas emissions are altering the near-Earth space environment in ways that will gradually reduce the number of satellites that can safely operate in this region.

In a study published today in Nature Sustainability, the researchers reveal that carbon dioxide and other greenhouse gases are causing the upper atmosphere to shrink. The thermosphere, a layer where the International Space Station and most satellites orbit, is of particular concern. When the thermosphere contracts, its density decreases, which reduces atmospheric drag—the force that typically pulls old satellites and space debris down to lower altitudes where they burn up upon re-entry.

With less drag, satellites and debris will remain in orbit longer, creating an overcrowded environment in critical regions and heightening the risk of collisions.

The team ran simulations to assess how carbon emissions impact the upper atmosphere and orbital dynamics, estimating the “satellite carrying capacity” of low Earth orbit. Their findings predict that by 2100, the capacity of popular orbital regions could decline by 50-66 percent due to the effects of greenhouse gases.

“Our behavior with greenhouse gases over the past century is affecting how we will operate satellites over the next century,” says Richard Linares, an associate professor in MIT’s Department of Aeronautics and Astronautics (AeroAstro).

Related: all about space junk

“The upper atmosphere is in a fragile state as climate change disrupts the status quo,” adds lead author William Parker, a graduate student in AeroAstro. “At the same time, satellite launches have surged, particularly for broadband internet from space. If we don’t manage this activity and reduce emissions, space could become too crowded, leading to more collisions and debris.”

The study also includes co-author Matthew Brown from the University of Birmingham.

The thermosphere naturally contracts and expands in response to the sun’s 11-year activity cycle. During periods of low solar activity, the Earth’s outer atmosphere cools and contracts before expanding again during solar maximum.

In the 1990s, scientists wondered how greenhouse gases might affect the thermosphere. Early models suggested that while these gases trap heat in the lower atmosphere, they radiate heat at higher altitudes, cooling the thermosphere. This cooling, they predicted, would cause the thermosphere to shrink, reducing atmospheric density at high altitudes.

In recent years, scientists have measured changes in drag on satellites, providing evidence that the thermosphere is contracting in response to factors beyond the sun’s natural cycle.

“The sky is literally falling, but at a rate that spans decades,” says Parker. “We can see this by how drag on satellites is changing.”

The MIT team explored how these changes might affect the number of satellites that can safely operate in Earth’s orbit. There are currently over 10,000 satellites in low Earth orbit—an area extending up to 1,200 miles (2,000 kilometers) above the Earth’s surface. These satellites provide vital services like internet, communications, navigation, weather forecasting, and banking. With the recent explosion in satellite launches, operators now perform regular collision-avoidance maneuvers to maintain safety. When collisions occur, debris can linger in orbit for decades or longer, heightening the risk of further collisions.

“More satellites have been launched in the last five years than in the preceding 60 years combined,” Parker says. “One of the key things we’re trying to understand is whether the path we’re on today is sustainable.”

In their study, the researchers simulated different greenhouse gas emissions scenarios over the next century to evaluate their impacts on atmospheric density and drag. For each altitude range, they modeled orbital dynamics and the risk of satellite collisions based on the number of objects present. This approach helped identify the “carrying capacity” of low Earth orbit, a term typically used in ecology to describe how many individuals an ecosystem can support.

“We’re translating the concept of carrying capacity to this space sustainability problem to understand how many satellites low Earth orbit can sustain,” Parker explains.

The team compared several emissions scenarios: one in which greenhouse gas concentrations remain at 2000 levels, and others based on future projections from the IPCC’s Shared Socioeconomic Pathways (SSPs). They found that increasing emissions would significantly reduce the capacity of low Earth orbit.

By the end of the century, the team predicts that the number of satellites safely accommodated in altitudes between 200 and 1,000 kilometers could decline by 50-66 percent compared to a scenario where emissions remain at 2000 levels. If satellite capacity is exceeded in any region, the researchers anticipate a “runaway instability,” where a cascade of collisions would generate so much debris that satellites could no longer operate there.

Although these predictions extend to 2100, some regions are already nearing capacity, especially those occupied by recent “megaconstellations” like SpaceX’s Starlink, which consists of thousands of small internet satellites.

“The megaconstellation is a new trend, and we’re showing that because of climate change, we’re going to have a reduced capacity in orbit,” Linares says. “In local regions, we’re already approaching this capacity.”

“We rely on the atmosphere to clean up our debris,” Parker adds. “If the atmosphere is changing, the debris environment will change as well. Our study shows that the long-term outlook for orbital debris depends critically on curbing greenhouse gas emissions.”

This research is supported, in part, by the U.S. National Science Foundation, the U.S. Air Force, and the U.K. Natural Environment Research Council.”

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Astronomers are worried about the launch of the latest satellite to Earth’s orbit, the Bluewalker 3. The new satellite which unfolded in Low Earth Orbit in November, measures about 700 square feet in size, about the same size as a studio apartment in New York City. The satellite is meant to be used for cellular communication, but the amount of light pollution it emits makes it harder for astronomers to see the night sky. At its peak BlueWalker 3, which looks like a Tetris block flying across the sky, is as bright as two of brightest stars we can see with our eyes.

Bluewalker’s bright light is about the same stars Procyon and Achernar, two of the brightest stars in the night sky. And this satellite is just the first of dozens of constellations of satellites planned for Low Earth Orbit.

“After BlueWalker 3 unfolded its 64m² array it is visible in both dark sky and urban skies, though in urban settings, this will be limited to when Bluewalker 3 passes overhead,” said Jeremy Tregloan-Reed, a co-author in the new study from the Universidad de Atacama in Chile.

But the appearance of the satellite is a growing concern as satellite groups like Elon Musk’s Starlink eject thousands of light-polluting satellites to our skies, all of which become space junk at the end of their days. Astronomers are outlining the problem of Bluewalker 3 as a wake up call as more and more companies like Starlink race to cover the world in high speed internet – even if Starlink can help Iranian dissidents.

Other companies, such as Amazon and OneWeb, have similar ambitions to launch satellites in space, and many countries are developing their own private communications constellations. Astronomers are very worried.

Watch the video of Bluewalker 3 below captured by astromers

A ground telescope tracks the BlueWalker 3 satellite in orbit on April 3, 2023. Dimmer Starlink satellites are also visible. Credit Marco Langbroek/Delft Technical University

“Large constellations of bright artificial satellites in low Earth orbit (LEO) pose significant challenges to ground-based astronomy,” the study’s authors write in the journal Nature.

Tregloan-Reed explains, among other problems, a large reflective brightness interferes with space telescopes like Hubble. When a satellite crosses the detector of a telescope, it often leaves a streak of light that can be difficult to remove.

Observations by the Hubble telescope have been affected by Starlink streaks because the telescope was in a LEO, sitting below that of the Starlink constellation.

While the researchers acknowledge that efforts are being made by the aerospace industry, policymakers, astronomers and others to mitigate the impact of such satellites, “the trend towards the launch of increasingly larger and brighter satellites continues to grow,” they write.

It’s sort of like asking the petroleum industry to regulate the use of fossil fuels.

BlueWalker 3, built by AST SpaceMobile, is a predecessor to a planned constellation of satellites nicknamed BlueBirds. AST SpaceMobile plans to make cellular broadband connections available to people who are currently without –– a solution that could help alleviate poverty or save lives in remote locations.

AST SpaceMobile BlueWalker 3’s array is so large, they say, so they can beam strong cellular coverage directly to phones on Earth. The satellite is comprised of small antennas which connect to existing smartphones. This is different than Starlink which requires a ground antenna or dish.

AST SpaceMobile has agreements and understandings with mobile network operators globally that have over 1.8 billion existing subscribers, including a mutual exclusivity with Vodafone in 24 countries. Interconnecting with AST SpaceMobile’s planned network including Vodafone Group, Rakuten Mobile, AT&T, Bell Canada, MTN Group, Orange, Telefonica, Etisalat, Indosat Ooredoo Hutchison, Smart Communications, Globe Telecom, Millicom, Smartfren, Telecom Argentina, Telstra, Africell, Liberty Latin America and others, the ability to offer extended cellular broadband coverage to their customers who live, work and travel in areas with poor or non-existent cell coverage, with the goal of eliminating dead zones with cellular broadband from space.

“We want to close coverage gaps in our markets, particularly in territories where terrain makes it extremely challenging to reach with a traditional ground-based network. Our partnership with AST SpaceMobile – connecting satellite directly to conventional mobile devices – will help in our efforts to close the digital divide,” said Luke Ibbetson, Head of Group R&D, Vodafone and an AST SpaceMobile director.

AST SpaceMobile issued a statement that the company is working on anti-reflective materials for the next generation of satellites and they they will avoid broadcasting in the US National Radio Quiet Zone. But the frequency sound waves they use are close to those used by astronomers today making interference an issue with ongoing and future studies.

The latest research on Bluewalker 3 was done by amateur and professional astronomers from Chile, the US, Mexico, New Zealand,Netherlands and Morocco.

Indigenous groups and the astronomy community are now invested in protecting the night skies: “We shouldn’t have progress at any cost,” Tregloan-Reed said. “It’s like building a brand-new development over a historical site. You can’t just do that. You have to protect these things.”

While astronomers don’t own the night sky they don’t want to see it taken by the highest bidder in the name of progress: “What we’d like to do is share the night sky and get the public to understand that this is a potential issue,” he said.

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The Humanity Star is a reflective satellite launched in January by Rocket Lab, a private New Zealand company. It was intended to remain in operation for most of this year, instead it will flame out this week. Rocket Lab founder Peter Beck said in a statement, “In the coming days, the Humanity Star will begin its final descent into the Earth’s atmosphere where it will burn up on re-entry, leaving no trace.”

The satellite, which looks like a disco ball, had no specific function, except as a spectacle. (Think of the Tesla roadster shot into space by Elon Musk’s SpaceX last February.)  The Humanity Star is made from carbon fiber and has 65 reflective panels that bounce sunlight back to Earth. The ball spins rapidly as it traverses the night sky, creating a blinking effect.

Think fast! The satellite was meant to encourage people to “think a little differently about their lives, actions and what is important for humanity,” according to a company press release. But according to the satellite’s own tracker, the object is losing altitude at a rate where it is now expected to re-enter Earth’s atmosphere on Thursday.

Richard Easther, an astronomer from Auckland University, said the earlier-than-expected re-entry is likely down to flawed modeling. “I’m guessing that the forecast was based on a regular sized satellite, and an object that is essentially a balloon will feel a lot more drag, more than the regular satellites that are sent up, (which resemble) a hunk of metal,” he told CNN.

Beck, pictured above, said, “My hope was to encourage people to linger looking at the stars and ponder our place in the universe. While the Humanity Star was a brief moment in human history, I hope the conversations and ideas it sparked around the world will continue to be explored.”

Both men mentioned Kiwi pride in that a company from New Zealand, a small, non-military power, was able to successfully complete the launch. Added Easther, “As a New Zealander I’m proud… (to see) homegrown hardware put in orbit.”

This writer is still marveling that TV shows about cake ever became popular.  Now I have to grapple with this new trend of expensive space litter. Thankfully, at least Israeli start-up is working to clean-up space (read more here).

Images courtesy of Humanity Star

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