Opponents of Starlink often cite Kessler Syndrome as a risk of the constellation. I have never seen a single one of them account for altitude while doing so.
So, what is Starlink? I’ll try to summarize, there are hundreds of documents and I’m not going to go through them all. It started off as a constellation of 4425 satellites ranging from 1,110km to 1325km orbits.
(Source: SAT-LOA-20161115-00118 Technical Attachment)
Note: when opening FCC filing links, if clicking the link directly doesn’t open the file correctly, try opening the link in a new tab. I don’t know why their website is the way that it is. I will include a link to both the parent file and the specific document.
It was modified to be constellation of 4,408 satellites ranging from 540km to 570km.
(Source: SAT-MOD-20200417-00037 Technical Attachment)
At some point during this process, SpaceX also filed for a separate VLEO (Very Low Earth Orbit) constellation of 7,518 satellites ranging from 335km to 345km.
(Source: SAT-LOA-20170301-00027 Technical Attachment)
Finally, SpaceX also has filed for a second generation (Gen 2) constellation to go alongside the Gen 1 constellation. I can’t find out when it happened, but the VLEO constellation officially became part of the Gen 2 constellation at some point.
The Gen 2 constellation was an application for 27,612 satellites to serve around the Gen 1 constellation. Note that there are no Gen 2 satellite sin the 540-570km range where Gen 1 is slated.
The number of VLEO satellites also increased from 7,518 to 23,582.
(Source: SAT-LOA-20200526-00055 Technical Attachment)
This ended up being changed slightly. SpaceX proposed two different configurations, and as of January 7, 2022, selected Configuration 1 to pursue.
(Source: SAT-AMD-20210818-00105 Technical Attachment and Response from SpaceX to FCC)
So now we’re looking at 19,440 satellites in VLEO, 10,548 satellites in LEO, and the original 4,408 Gen 1 satellites. 34,396 satellites in total.
Altitude Matters
Now that we know how many satellites are going where, we can talk about orbital decay. Assuming a satellite completely fails, how long will it take to deorbit from atmospheric drag? Tory Bruno from ULA has provided us a handy, generic chart:
As we can see, generally speaking, orbital decay for the following altitudes falls into the following timeframes:
150km: Hours
250km: Weeks
~350km: Months
550km: Years
800km: Decades
1,000km: Centuries
When discussing space debris, decay time is paramount. Debris that will reorbit in hours, days, weeks, or even months isn’t much of a concern. That’s why there are so few objects in those very low orbits as seen here:
(Source: NASA report to the United Nations Office for Outer Space Affairs, 2011)
~800 km and higher is where we need to be extremely careful of adding more space junk. Not only is there already a large amount there, but it will take decades, or longer, for it to deorbit. Below 600km is already kept quite clean by atmospheric drag, and that is where Starlink satellites are set to go. 57% are set to go to 350km, where even if a satellite completely fails, it will deorbit in a matter of months.
Earlier, I said the time frames listed were generally speaking, and that is because of several factors. First, the spatial density of the satellite matters greatly. Second, the atmosphere expands and contracts along with solar minimums and maximums. Right now, everything is deorbiting more slowly since we are in a solar minimum. In the next decade, we will hit a solar maximum and objects will start to deorbit much more quickly.
As far as spatial density, Starlink has started putting sun shades on all of their satellites. You can think of it like a parachute when it comes time to deorbit, creating more drag and causing the satellites to deorbit more quickly. SpaceX has used NASA’s Debris Assessment Software to calculate the demise time for its satellites in each orbit. Again, this assumes full failure of the satellite. Every Starlink satellite is designed to propulsively deorbit itself at the end of its lifespan.
(Source: SAT-AMD-20210818-00105 Technical Attachment)
Kessler Syndrome is often portrayed in a dramatic, exagerrated fashion. This Twitter thread by Hugh Lewis explains a bit of how overblown the issue is, and why Starlink specifically is not a huge Kessler concern.