MIT aerospace specialists have discovered that greenhouse gas emissions are altering the environment of near-Earth space in manners that, as time progresses, will diminish the quantity of satellites that can effectively function in that vicinity.

In a study published today in Nature Sustainability, the investigators reveal that carbon dioxide along with other greenhouse gases can lead to the contraction of the upper atmosphere. A layer of the atmosphere that is particularly noteworthy is the thermosphere, where the International Space Station and the majority of satellites currently orbit. As the thermosphere contracts, the diminishing density lessens atmospheric drag — a force that causes aging satellites and other debris to descend to altitudes where they will interact with air molecules and incinerate.

Consequently, reduced drag translates to longer lifespans for space debris, which will clutter valuable regions for many years and heighten the risk of collisions in orbit.

The team conducted simulations on how carbon emissions influence the upper atmosphere and orbital mechanics to gauge the “satellite carrying capacity” of low Earth orbit. These simulations anticipate that by the year 2100, the carrying capacity of the most sought-after areas could be diminished by 50-66 percent due to the influences of greenhouse gases.

“Our actions regarding greenhouse gases here on Earth over the last century are impacting how we operate satellites over the next century,” asserts study author Richard Linares, associate professor in MIT’s Department of Aeronautics and Astronautics (AeroAstro).

“The upper atmosphere is in a delicate condition as climate change disrupts the existing balance,” adds lead author William Parker, a graduate scholar in AeroAstro. “Simultaneously, there has been a significant surge in the number of satellites launched, particularly for providing broadband internet from space. If we fail to monitor this activity carefully and strive to reduce our emissions, outer space could become overly congested, resulting in more collisions and debris.”

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

Atmospheric decline

The thermosphere naturally contracts and expands every 11 years in response to the sun’s cyclical activity. When solar activity is diminished, Earth receives less radiation, causing its outermost atmosphere to temporarily cool and shrink before it expands again during periods of high solar activity.

In the 1990s, researchers speculated on how the thermosphere might react to greenhouse gases. Their initial modeling indicated that while these gases capture heat in the lower atmosphere, where global warming and weather phenomena occur, they radiate heat at considerably higher elevations, effectively cooling the thermosphere. With this cooling effect, the researchers predicted that the thermosphere would shrink, leading to reduced atmospheric density at elevated altitudes.

In recent years, scientists have measured alterations in drag on satellites, which has provided evidence that the thermosphere is contracting due to factors beyond the sun’s natural, 11-year cycle.

“The sky is quite literally falling — albeit at a rate measurable over decades,” Parker remarks. “We can observe this by how the drag on our satellites is being altered.”

The MIT team speculated on how this response will influence the capacity of satellites to function safely in Earth’s orbit. Currently, there are over 10,000 satellites navigating low Earth orbit, which refers to the region of space up to 1,200 miles (2,000 kilometers) above Earth’s surface. These satellites provide critical services like internet access, communications, navigation, weather forecasting, and banking. The satellite population has surged in recent years, necessitating operators to execute frequent collision-avoidance maneuvers to ensure safety. Any collisions that do occur can create debris that remains in orbit for decades or even centuries, increasing the likelihood of subsequent collisions with both aging and newer satellites.

“More satellites have been launched in the past five years than in the preceding 60 years combined,” Parker indicates. “One of the key questions we seek to answer is whether our current trajectory is sustainable.”

Congested orbits

In their latest study, the researchers modeled various greenhouse gas emissions scenarios for the next century to explore the implications on atmospheric density and drag. For each “layer,” or altitude range of interest, they subsequently analyzed the orbital dynamics and the potential risk of satellite collisions based on the number of objects within that layer. This approach allowed them to determine each layer’s “carrying capacity” — a term usually employed in ecological studies to define the number of individuals an ecosystem can support.

“We’re adapting the concept of carrying capacity to address the issue of space sustainability, aiming to understand how many satellites low Earth orbit can accommodate,” Parker elaborates.

The team evaluated multiple scenarios: one where greenhouse gas concentrations remain at their year 2000 levels and others where emissions adjust in accordance with the Intergovernmental Panel on Climate Change (IPCC) Shared Socioeconomic Pathways (SSPs). They discovered that scenarios with persistent increases in emissions would result in significantly diminished carrying capacity throughout low Earth orbit.

Specifically, the team projects that by the century’s conclusion, the number of satellites safely supported within altitudes of 200 and 1,000 kilometers could drop by 50 to 66 percent compared to a scenario maintaining year-2000 emission levels. Should satellite capacity be exceeded, even in localized areas, the researchers forecast that such regions could experience a “runaway instability,” or a series of collisions that generate enough debris, making it unsafe for satellites to operate there.

Their forecasts extend to the year 2100, but the team observes that certain layers of the atmosphere today are already becoming congested with satellites, particularly from recent “megaconstellations” like SpaceX’s Starlink, which consists of fleets of thousands of small internet satellites.

“The megaconstellation trend is emerging, and we’re demonstrating that due to climate change, our orbital capacity will be diminished,” Linares notes. “Moreover, in specific regions, we are nearing this capacity threshold even now.”

“We depend on the atmosphere to manage our debris. If the atmosphere undergoes changes, then the debris environment will also shift,” Parker complements. “We reveal that the long-term outlook on orbital debris is critically reliant on our efforts to curb greenhouse gas emissions.”

This research receives partial support from the U.S. National Science Foundation, the U.S. Air Force, and the U.K. Natural Environment Research Council.