How climate change is threatening satellites in orbit

Until now, space management tools have relied on technologies such as collision avoidance manoeuvres, debris removal, and improved tracking. But this research turns the tables: reducing greenhouse gas emissions is not merely beneficial, but essential for safeguarding space. By lowering these emissions, we help preserve the upper atmosphere's density and, in turn, protect the orbit from collapse.

Within the lower layers of the atmosphere (up to about 110km), the proportions of major gases remain nearly constant. The higher you go, however, and the more gravitational diffusion effects start to reduce the concentrations of heavier gases such as carbon dioxide.

Unwanted cooling

Physically, carbon dioxide is highly efficient at absorbing and re-emitting infrared radiation. In the troposphere, increased concentrations of this gas cause heat retention and a rise in temperature, known as the 'greenhouse' effect.

Above the troposphere, however, this absorption leads to thermal energy loss, resulting in cooling. In other words, carbon dioxide has the reverse effect at higher altitude. This cooling, in turn, causes the thermosphere to contract, resulting in a decrease in mass density at constant altitudes.

The thermosphere is naturally highly sensitive to variations in solar activity, which follows an approximately 11-year cycle: during solar maxima, the thermosphere expands; during solar minima, it contracts.

There can be volatility, for instance from geomagnetic storms triggered by solar flares causing temporary expansions in the atmosphere, increasing atmospheric drag on satellites. Yet what is most concerning is that the shrinkage caused by greenhouse gases is not temporary. Rather, it represents a permanent change, with long-term implications for mass density in low Earth orbit—and for satellites.

Atmospheric shrinkage

Studies over the past two decades show clear evidence of contraction and cooling in the thermosphere. After measuring temperatures and pressures there from 2002-21, NASA concluded that the cooling of the upper layers is progressing at a steady pace, reinforcing the hypothesis of atmospheric shrinkage.

These finding were further supported by data from the Halogen Occultation Experiment, which measured ozone, hydrogen chloride, hydrogen fluoride, methane, water vapour, nitric oxide, nitrogen dioxide, aerosol extinction, temperature and pressure from 1991 to 2005.

The decline in air density near Earth's orbit is both good and bad. First the good: a less dense atmosphere means that satellites experience less resistance as they orbit the Earth, allowing them to remain operational for longer and reducing the need for fuel consumption to adjust or maintain their trajectories.

Now the bad: the thinning atmosphere allows space debris (remnants of old satellites or rocket parts, travelling at around 10km per second) to stay in orbit for longer, rather than falling back towards Earth and burning up in the atmosphere as usual. This lingering leads to more debris, heightening the risk of these hugely high-speed collisions, and complicating efforts to predict or avoid them.

Clogging up space

Malfunctions, explosions, and collisions involving satellites have contributed significantly to the build-up of uncontrollable—and often untraceable—debris. In parallel, lower launch costs and advances in satellite technology mean that private companies such as Elon Musk's SpaceX control most of the active satellites orbiting the Earth, operating vast constellations.

In 1978, researchers Donald Kessler and Burton G. Cour-Palais demonstrated that an increase in debris could lead to an unstable growth pattern, where each collision generates more fragments, a phenomenon now known as Kessler Syndrome. The density of certain orbital regions, especially from 900km to 1,400km, has become high enough to approach this unstable state.

There is a growing realisation that the orbital environment is now less safe, so most new satellites are equipped with the ability to avoid collisions. In addition, the US Federal Communications Commission now requires companies to deorbit satellites within five years of their mission end date (it was previously 25 years), but there is a reliance on natural atmospheric drag to reduce orbital altitude, particularly for satellites lacking manoeuvrability.

Limiting launches

Calculating 'orbital carrying capacity' (i.e. the maximum number of satellites that can safely operate without the orbit) is ever more important, yet it requires consideration of several factors, such as the number of launches, radiofrequency distribution, object-tracking capabilities, operational standards, and overall dynamic equilibrium.

The solution to the problem is to reduce the number of satellites launched, to lower the probability of collisions. Without it, experts warn of a 'chain collision' or cascade effect, whereby each impact generates more debris and more collisions, which would soon render parts of space unusable.

Like the oceans or the air, orbital space is a common resource to mankind that requires regulation and equitable management by space actors, whether companies or countries. Regulatory frameworks must account for worst-case scenarios, particularly periods of low solar activity and heightened emissions, when orbital capacity is at its most fragile.

To calculate the safe orbital capacity, researchers simulated the impact of rising carbon dioxide concentrations on the density of the upper atmosphere, particularly within the thermosphere, while also accounting for variations in solar activity. They modelled the densities of different gases up to 1,000km and calculated how climate change influences these densities.

Following this, they modelled the inflow and outflow of space debris in orbit to compute the maximum number of satellites that could be deployed, calculating collision rates between objects based on their size, speed, orbital occupancy, and the debris removal rate.

A safe capacity

The researchers found that there are two equilibrium points for debris volume: one stable, where conditions remain manageable, and another unstable, where exceeding the threshold would result in a rapid escalation of debris. When these two points draw closer, the orbit becomes fragile and susceptible to total collapse.

Therefore, the safe capacity is defined as the maximum number of satellites that maintains a safe distance between them, preventing the orbit from descending into a chaotic debris field.

To increase the number of satellites that can be accommodated in low Earth orbit, satellites must be distributed carefully by altitude. Between 200km and 400km, the atmospheric density is higher, meaning debris is cleared more quickly, allowing a greater number of satellites. Between 600km and 1,000km, debris lingers for much longer, and collision effects last decades.

According to current models, the capacity of low Earth orbit could reduce by as much as a half or even two thirds by the end of the century based on current emission rates, particularly at altitudes between 200km and 1,000km. To ensure the sustainability of this orbit, urgent measures must be taken.

These include legislation to set maximum satellite limits per orbit, mandating the removal of decommissioned satellites, and imposing fines for orbital pollution. There also needs to be better coordination among space operators, and efforts to reduce terrestrial emissions. A new global space agency has been suggested, because low Earth orbit is not merely an empty expanse but a limited shared resource.

Ignoring its fragility and ploughing on regardless means that we will soon lose the ability to use it safely, threatening our communications, navigation, and instruments of scientific discovery, leaving us all the poorer for it.