All (it) garbage ?: How climate change threatens space travel

It could actually be a reason to be happy: The lifespan of multi-million dollar satellites will probably be significantly longer in the future.

At least that is what a study by Matthew Brown of the University of Southampton in Great Britain and his colleagues published in early April in the Journal of Geophysical Research: Atmospheres suggests.

The reason is the increase in CO₂ in the atmosphere, which changes its interaction with objects in orbit.

But not only commercial, military and scientific satellites will benefit.

The length of stay of the “space junk” accompanying human activities in Earth orbit will also be extended. Space junk is understood to mean disused satellites that can no longer be steered, empty rocket stages and many, often only centimeter-sized pieces of junk. These not only threaten the satellites that are still active, but also space travelers. NASA and ESA track around 17,500 objects with sizes of ten centimeters or more at orbit heights of 2000 kilometers or less, the so-called “Low Earth Orbit”, or LEO for short. Only 2300 of them are active satellites. The ISS is also circling at an altitude of around 400 kilometers and thus in the LEO zone. Every collision of a satellite with space debris can have fatal consequences.

Fortunately, the thin residual atmosphere ensures that space is “self-cleaning” up to a height of a few 100 kilometers: the low but noticeable friction with the air causes all objects to fall back to earth one by one. Only at higher altitudes do they remain without further action for many thousands of years or even forever. The density of the so-called thermosphere at the border with vacuum fluctuates with the activity of the sun - and evidently changes as a result of man-made climate change.

Once again, it is the carbon dioxide blown into the air to blame: as a greenhouse gas, the CO₂ converts solar radiation into long-wave infrared radiation.

In deeper layers of the atmosphere near the ground, this thermal radiation is quickly absorbed, which leads to the well-known global warming of the atmosphere.

It looks completely different in the much thinner high atmosphere: Here the infrared radiation escapes almost unhindered into space.

The result is a strong cooling - and thus a decrease in air density in the area of ​​the lower earth orbit.

Shaky self-commitment by space players

This effect has been known for years. However, following the analysis by Brown and his colleagues, its impact on space travel has been dramatically underestimated. The researchers simulated the expected decrease in atmospheric density up to an altitude of 500 kilometers using a computer model into which they fed various scenarios of the increase in CO₂ published by the Intergovernmental Panel on Climate Change (IPCC) in its fifth assessment report. The results are remarkable: in the worst case outlined by the IPCC, the density at an altitude of 500 kilometers would decrease by a full 80 percent by the year 2100.

The less CO₂ gets into the atmosphere in the coming decades, the smaller the effect, but even in the most favorable scenario to be expected, the atmospheric density in the LEO will noticeably decrease in the coming decades - and the lifespan of satellites and scrap will be extended accordingly. Brown and his team estimate that even if the 1.5-degree climate target of the 2015 Paris Agreement were reached, objects in the LEO will be circling around 30 percent longer than they were in 2000.

For Holger Krag, head of ESA's “Space Debris Office” in Darmstadt, the study is also relevant for space travel against the background of increasing demands on the LEO by private space companies: “Many satellites from large constellations are currently positioned just over 500 kilometers above sea level, here If necessary, it would have to be checked whether these satellites might have to be disposed of even deeper into the atmosphere after operation than before. "This area was not considered in Brown's work, but here the consequences would be particularly drastic:" At heights of 800 kilometers is the natural Railway life in the range of a few hundred years. If the described effect also applies here, it would have noticeable consequences for long-term development.“Krag refers to the self-commitment of the aerospace players to actively dispose of their disused satellites in the future if necessary, for example by directing a spacecraft into the atmosphere with the help of rocket engines at the end of its service life - if it can still be controlled. "The technical requirement is that all space objects must have left the low earth orbit at the latest after 25 years."

The US company SpaceX, which has already sent around 1,600 satellites of its "Starlink large constellation" to the LEO since 2019 and is planning several tens of thousands of them, has so far relied on the earth's atmosphere as a disposer and assumes that uncontrollable satellites burn up in the atmosphere after five years at the latest. Such calculations will probably have to be updated in the light of the new findings.