For many decades humankind has entertained the notion that we can maybe tweak the Earth’s atmosphere or biosphere in such a way that we can for example undo the harms of climate change, or otherwise affect the climate for our own benefit. This often involves spreading certain substances in parts of the atmosphere in order to reflect or retain thermal solar radiation or induce rain.

Yet despite how limited in scope these attempts at such intentional experiments have been so far – with most proposals dying somewhere before being implemented – we have already embarked on a potentially planet-wide atmospheric reconfiguration that could affect life on Earth for centuries to come. This accidental experiment comes in the form of rocket stages, discarded satellites, and other human-made space litter that burn up in the atmosphere at ever increasing rates.

Rather than burning up cleanly into harmless components, this actually introduces metals and other compounds into the upper parts of the atmosphere. What the long-term effects of this will be is still uncertain, but with the most dire scenarios involving significant climate change and ozone layer degradation, we ought to figure this one out sooner rather than later.

Nobody Hears You Burn In Space

Although Earth’s atmosphere looks pretty peaceful if you’re gazing at it from a space station in LEO or from a commercial airliner at cruising altitude, it’s actually constantly being assaulted. Everything from radiation to meteoroids, as well as the occasional asteroid are constantly making an attempt at inflicting real harm. This ranges all the way up to another mass-extinction event, but a meteoroid will settle for at the very least flattening another forest or inconveniencing a home owner.

Fortunately the atmosphere provides another feature beyond allowing us to not suffocate: by providing strong friction, the resulting high temperatures and intense plasma formation tend to burn up any object that tries to enter it at high velocity.

A less extreme form of this comes in the form of aerobraking, which is what spacecraft use to reduce their velocity relative to the planet; by creating enough friction in the atmosphere to shed kinetic energy, yet not heating up the spacecraft’s exterior to the point where things begin to melt, is incredibly helpful if one wishes to avoid having to resort to Plan B, being the violence of lithobraking.

This incinerator feature of the atmosphere is also very useful when it comes to the question of where the trash goes, whether it’s literal trash from the International Space Station, or things like discarded rocket stages and fairings, all the way to satellites that have reached their end of life stage. Yet much like the medieval solutions to waste disposal, the theme here is very much an ‘out of sight, out of mind’ approach, which is understandable as long as the volume of waste is still relatively small.

Running The Numbers

When a human-made object disintegrates in the atmosphere, it’s reduced to its base compounds, after interaction with the super-heated plasma that forms around said object. With the commonly used aluminium, for example, this means the production of aluminium oxide.

By far the largest amount of mass that will be burning up in the atmosphere over the coming years is formed by LEO internet constellations such as Starlink, which have a cumulative mass of over 10,000 tons. In addition, the second stage of the Falcon 9 rockets that are currently used to launch Starlink v1 and v1.5 satellites into LEO also burns up in the atmosphere. Recently, such a Falcon 9 stage suffered a mishap that caused it to disintegrate over Europe, rather than the typical trajectory over remote parts of Earth’s oceans.

This provided the perfect natural experiment. Batteries onboard satellites contain lithium, and because it’s relatively scarce in the atmosphere, it makes a great marker for the effects of satellites burning up on re-entry.

In the article by Robin Wing et al., as published in Communications Earth & Environment, the upper atmosphere measurements by a resonance lidar in Germany allowed for a ten-fold increase in atomic lithium to be measured after the stage had disintegrated near Ireland at an altitude of 100 km. Air currents subsequently dispersed the atomic debris over the rest of Europe.

Most notable perhaps was that the plume of atomic lithium was being detected at the same altitude of 100 km, after advecting for 1,600 km, placing ablation and dispersal in the mesosphere and lower thermosphere (MLT). Normally this plume would be dispersed far away from instruments, making it a fortuitous event from a scientific perspective that it could be measured like this.

Lithium is just one tracer for the debris plume, but there are many other metals. Here also lies the issue with comparing purely the mass of asteroids and rocket stages burning up in the atmosphere versus meteoroids and asteroids doing the same. The latter aren’t usually composed of intricate collections of metal alloys, rare earths and composite materials, but generally more boring things that we’d generously call ‘rocks’ or ‘gravel’, with the occasional iron variant mixed in.

As noted by Robin Wing et al., this feature makes artificial sources relatively easy to distinguish from natural ones. Since within the next decades re-entering satellites are projected to match or exceed 40% of natural meteoroid influx, the question remains of what these substances hanging around in Earth’s atmosphere will do to it and consequently life in Earth’s biosphere.

Potential Impact

Back in 1987 the Montreal Protocol was signed. This banned the use of chlorofluorocarbons (CFCs) after it was found that the large-scale release of CFCs into the atmosphere from refrigeration systems and other sources had resulted in a significant depletion of the ozone layer. This layer is found primarily in Earth’s stratosphere and is essential for blocking harmful ultraviolet radiation which would otherwise irradiate the surface, in particular UV-C.

Although it’s currently projected that the ozone will have completely regenerated by 2045, a worrying 2024 research letter by José P. Ferreira et al. from the American Geophysical Union (AGU) with accompanying press release suggests that the massive rise in satellites burning up in the atmosphere over the coming decades could add so much aluminium oxides to the atmosphere that it could revert this ozone layer regeneration process.

Using an atomic-scale molecular dynamics simulation they found that a typical 250 kg satellite upon its fiery demise in Earth’s atmosphere releases about 30 kg of aluminium oxide nanoparticles. These may remain in the atmosphere for decades, meanwhile acting as a catalyst for chlorine activation and thus ozone depletion.

With currently projected mass of mega-constellation satellites burning up in the atmosphere, we’d be looking over 360 tons of aluminium oxides per year being added. As a catalyst, these aluminium oxides would not be used up, but would keep depleting the ozone layer as fast as the input products (ClO or Cl) are added.

This is just one potential impact that we might see as we keep adding all of these foreign substances to the atmosphere. Fortunately there’s nothing that says that we cannot have all our satellites and still dodge these issues.

Reduce, Reuse, Recycle

The central issue here is that we have always treated the atmosphere similarly to the way that early medieval cities treated the local waterways. In their case it only took a few cholera- and other assorted epidemics to realize that maybe it was best to not use the waterways both for waste and drinking water. Similarly, we are now at a point where we’re beginning to realize that tossing our waste into the atmosphere may not be such a good plan, albeit it largely for financial reasons.

For many decades, it’s been accepted that rockets and satellites are effectively disposable, single-use items. Even the infamous STS (‘Shuttle’) program didn’t really push it much past ‘intense refurbishing’. Only recently has it become fashionable to reuse rockets and capsules, with the SpaceX Falcon 9 rocket’s first stage currently being the world-leader when it comes to partial reuse. Unfortunately its second stage still is burned up, as we saw with the analysis by Robin Wing et al.

What can be done? Back in 2020 we covered Northrop Grumman’s Mission Extension Vehicle (MEV), which provides a way to latch onto an existing satellite and provide propulsion as well as other functionality when the target’s own resources have become exhausted. In 2021 MEV-2 docked with Intelsat 10-02 to push it back to a geosynchronous orbit, extending its life by five years.

This is an example of on-orbit satellite servicing, which can take many forms. At its most basic it will just drag a satellite to a specific orbit, but it can also entail actual servicing, refueling and repairs. This was actually one of the concepts behind the Shuttle, with the Hubble Telescope being serviced and upgraded during a number of missions.

Unfortunately with the STS program’s in-orbit repair feature remaining mostly a pleasant dream due to the high cost of such a mission, we may one day see satellites being refueled and repaired by robotic systems. Although fully reusable rockets seem to be just around the horizon with SpaceX Starship and kin leading the way, we can only hope that we can soon figure out a way to make it cheaper to just repair a satellite than to toss it and launch a new one.