"SpaceX is deorbiting about one or two satellites daily, and that number is only going to grow. What that means for our planet isn't entirely clear, says Harvard astrophysicist and space tracker Jonathan McDowell. Even so, Starlink isn't the space junk risk that some other satellite operations are. McDowell commented on the massive volume of reentering Starlink satellites to science news site EarthSky last week."
"The Register followed up with McDowell on Monday to learn more about what that might mean for the health of the planet, and the possibility that we're rapidly approaching Kessler syndrome, a scenario in which so much debris clutters Earth's orbit that one collision can trigger a chain reaction of further impacts, potentially making parts of Earth's orbit unusable for satellites."
""Active satellite maneuvers to avoid collisions will help avoid Kessler," McDowell said in a phone conversation. "If they're successful. And that's a big if." The current strategy to de-orbit Starlink satellites, which operate in a low orbit below 600 kilometers, is to use the satellites' thrusters to move them to such a low orbit that they eventually catch drag in the atmosphere and burn up in what McDowell calls an "uncontrolled but assisted" reentry."
SpaceX currently deorbits about one to two Starlink satellites each day, with that rate expected to grow as constellations expand. Planned low Earth orbit constellations could total around 30,000 satellites, producing roughly five daily reentries given a five-year replacement cycle. Reentries use thruster-assisted lowering below 600 kilometers to catch atmospheric drag and burn up in uncontrolled but assisted reentries. Active collision-avoidance maneuvers and deliberate de-orbiting can reduce the risk of a cascading Kessler syndrome, but the success of those measures is uncertain and increasing satellite volume raises debris concerns.
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