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<\/a>Credit: NASA<\/a>.<\/p>\nAlthough Earth\u2019s atmosphere looks pretty peaceful if you\u2019re gazing at it from a space station in LEO or from a commercial airliner at cruising altitude, it\u2019s 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.<\/p>\n
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.<\/p>\n
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\u2019s 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<\/a>.<\/p>\n This incinerator feature of the atmosphere is also very useful when it comes to the question of where the trash goes, whether it\u2019s 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 \u2018out of sight, out of mind\u2019 approach, which is understandable as long as the volume of waste is still relatively small.<\/p>\n Running The Numbers When a human-made object disintegrates in the atmosphere, it\u2019s 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.<\/p>\n 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\u2019s oceans.<\/p>\n This provided the perfect natural experiment. Batteries onboard satellites contain lithium, and because it\u2019s relatively scarce in the atmosphere, it makes a great marker for the effects of satellites burning up on re-entry.<\/p>\n In the article<\/a> by Robin Wing et al., as published in\u00a0Communications 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.<\/p>\n Most notable perhaps was that the plume of atomic lithium was being detected at the same altitude of 100 km, after advecting<\/a> 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.<\/p>\n 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\u2019t usually composed of intricate collections of metal alloys, rare earths and composite materials, but generally more boring things that we\u2019d generously call \u2018rocks\u2019 or \u2018gravel\u2019, with the occasional iron variant mixed in.<\/p>\n 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\u2019s atmosphere will do to it and consequently life in Earth\u2019s biosphere.<\/p>\n Potential Impact<\/p>\n 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<\/a> of the ozone layer. This layer is found primarily in Earth\u2019s stratosphere and is essential for blocking harmful ultraviolet radiation which would otherwise irradiate the surface, in particular UV-C.<\/p>\n Although it\u2019s currently projected that the ozone will have completely regenerated by 2045, a worrying 2024 research letter<\/a> by Jos\u00e9 P. Ferreira et al. from the American Geophysical Union (AGU<\/a>) with accompanying press release<\/a> 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.<\/p>\n Using an atomic-scale molecular dynamics simulation they found that a typical 250 kg satellite upon its fiery demise in Earth\u2019s atmosphere releases about 30 kg of aluminium oxide nanoparticles. These may remain in the atmosphere for decades, meanwhile acting as a catalyst<\/a> for chlorine activation and thus ozone depletion.<\/p>\n With currently projected mass of mega-constellation satellites burning up in the atmosphere, we\u2019d 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.<\/p>\n This is just one potential impact that we might see as we keep adding all of these foreign substances to the atmosphere. Fortunately there\u2019s nothing that says that we cannot have all our satellites and still dodge these issues.<\/p>\n Reduce, Reuse, Recycle<\/p>\n 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\u2019re beginning to realize that tossing our waste into the atmosphere may not be such a good plan, albeit it largely for financial reasons.<\/p>\n For many decades, it\u2019s been accepted that rockets and satellites are effectively disposable, single-use items. Even the infamous STS (\u2018Shuttle\u2019) program didn\u2019t really push it much past \u2018intense refurbishing\u2019. Only recently has it become fashionable to reuse rockets and capsules, with the SpaceX Falcon 9 rocket\u2019s 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.<\/p>\n
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<\/a>The five basic layers of the atmosphere. (Credit: NOAA<\/a>)<\/p>\n<\/a>Credit: NOAA<\/a><\/p>\n<\/a>What can be done? Back in 2020 we covered<\/a> Northrop Grumman\u2019s Mission Extension Vehicle (MEV<\/a>), which provides a way to latch onto an existing satellite and provide propulsion as well as other functionality when the target\u2019s own resources have become exhausted. In 2021 MEV-2 docked with Intelsat 10-02<\/a> to push it back to a geosynchronous orbit, extending its life by five years.<\/p>\n