The thermosphere usually drags space junk to its doom. As it thins, ruined orbits are a possibility
Earth’s atmosphere is shrinking due to climate change and one of the possible negative impacts is that space junk will stay in orbit for longer, bonk into other bits of space junk, and make so much mess that low Earth orbits become less useful.
That miserable set of predictions appeared on Monday in a Nature Sustainability paper titled “Greenhouse gases reduce the satellite carrying capacity of low Earth orbit.”
Penned by two boffins from MIT, and another from the University of Birmingham, the paper opens with the observation: “Anthropogenic contributions of greenhouse gases in Earth’s atmosphere have been observed to cause cooling and contraction in the thermosphere.”
The thermosphere extends from about 90 km to 500 km above Earth’s surface. While conditions in the thermosphere are hellish, it’s not a hard vacuum. NASA describes it as home to “very low density of molecules” compared to the exosphere’s “extremely low density.”
Among the molecules found in the thermosphere is carbon dioxide (CO2), which conducts heat from lower down in the atmosphere then radiates it outward.
“Thus, increasing concentrations of CO2 inevitably leads to cooling in the upper atmosphere. A consequence of cooling is a contraction of the global thermosphere, leading to reductions in mass density at constant altitude over time.”
That’s unwelcome because the very low density of matter in the thermosphere is still enough to create drag on craft in low Earth orbit – enough that the International Space Station requires regular boosts to stay in orbit.
It’s also enough drag to gradually slow space junk, causing it to descend into denser parts of the atmosphere where it vaporizes. A less dense thermosphere, the authors warn, means more space junk orbiting for longer and the possibility of Kessler syndrome instability – space junk bumping into space junk and breaking it up into smaller pieces until there’s so much space junk some orbits become too dangerous to host satellites.
Which is bad because we’re using low Earth orbit a lot these days for things like broadband satellites.
The paper warns we may need to revisit such plans soon.
“Modelled CO2 emissions scenarios from years 2000-2100 indicate a potential 50-66 percent reduction in satellite carrying capacity between the altitudes of 200 and 1,000 km.”
The good news is the paper notes that satellite makers know Kessler syndrome instability is a possibility, so often build collision avoidance capabilities that let them avoid debris.
The authors hope manufacturers and operators work together on many debris-reduction tactics, and that greenhouse gas emissions are reduced to keep the thermosphere in fine trim.
