The Kessler syndrome is named after NASA scientist Donald Kessler, who described it in 1978: a cascade in which collisions between orbiting objects generate debris, which causes more collisions, which generates more debris, until certain orbital shells become effectively impassable. Kessler was describing a theoretical future. In 2009, that future became partially real when a defunct Russian Kosmos satellite collided with an operational Iridium communications satellite, producing roughly 2,000 trackable debris fragments and an unknown number of smaller pieces. The debris is still up there, slowly spreading.
The numbers are not comfortable. The US Space Surveillance Network currently tracks about 27,000 objects larger than 10 centimeters in orbit. Estimates of objects between 1 and 10 centimeters — large enough to be catastrophic for most satellites but too small to track reliably — run to hundreds of thousands. Objects smaller than 1 centimeter number in the tens of millions. At orbital velocities, even a 1-centimeter piece of aluminum carries the kinetic energy of a hand grenade. The International Space Station regularly adjusts its orbit to avoid tracked debris and has had astronauts shelter in the Soyuz as a precaution when a debris cloud passed close enough to be dangerous.
Why the problem keeps getting worse
The debris environment is self-reinforcing because the sources of new debris outpace natural removal. Atmospheric drag — the primary natural cleanup mechanism — eventually causes low-altitude objects to reenter, but the timescale depends strongly on altitude. Objects at 600 kilometers altitude reenter within a few years. Objects at 800 kilometers take decades. Objects at 1,000 kilometers and above stay up for centuries. Most of the intact defunct satellites that pose the greatest collision risk sit in the 700-900 kilometer range, where they will remain for 50 to 200 years without intervention.
Megaconstellations are making this harder. SpaceX's Starlink, Amazon's Kuiper, and similar programs are deploying thousands of satellites in low Earth orbit. The satellites are designed to deorbit within five years of end of life, which is better than earlier generations of spacecraft, but the sheer volume increases both collision probability and the number of objects that need to deorbit correctly. SpaceX has already lost some Starlink satellites to atmospheric reentry failures. At sufficient constellation density, even a low failure rate produces a meaningful accumulation of uncontrolled objects.
Astroscale and the capture problem
Removing debris is technically difficult in ways that are not immediately obvious. Defunct satellites and rocket bodies are tumbling, uncooperative, and were never designed to be captured. They have no docking ports, no cooperative sensors, no thrusters that respond to commands. Grappling with a tumbling multi-ton object in zero gravity without being thrown off or damaging both spacecraft is an unsolved engineering problem — or was, until recently.
Astroscale, a Japanese company, has been developing debris removal technology since 2013. Its ELSA-d mission in 2021 demonstrated magnetic capture of a client satellite equipped with a ferromagnetic docking plate — a controlled test in orbit, with a cooperative target. More recently, Astroscale's ADRAS-J mission has been conducting proximity operations around a defunct Japanese H-IIA rocket upper stage, imaging and characterizing its tumble rate and surface condition. The goal is to demonstrate the inspection phase that would precede an actual capture. A follow-on capture mission is planned.
ClearSpace-1 and the ESA contract
ESA awarded the first commercial debris removal contract in 2020 to ClearSpace, a Swiss startup spun out of EPFL. The mission, ClearSpace-1, targets a Vespa payload adapter — a 112-kilogram object left in orbit by a Vega rocket in 2013, sitting at about 660 kilometers altitude. ClearSpace-1 will use four robotic arms to grapple the Vespa, stabilize its tumble, and then deorbit both spacecraft together into a destructive reentry.
The mission has faced delays and cost overruns, partly from the technical difficulty of the capture problem and partly from the generic challenges of a startup developing novel space hardware. The target launch window has slipped into the mid-2020s. But the contract represents a real commitment from a major space agency to pay for debris removal as a service — establishing a precedent that treating orbital slots as a managed commons, rather than a free dumping ground, is worth paying for.
The economics of cleanup
The fundamental economic problem with orbital debris cleanup is that the people who benefit from clean orbits are not necessarily the people who pay for cleanup. A company that removes a defunct Russian satellite benefits all satellite operators, but has no way to collect payment from them. This is a classic commons problem, and it is why government contracts — like ESA's award to ClearSpace — are currently the only viable business model for debris removal. The alternative, an international liability and cleanup fund, would require the kind of multilateral treaty negotiation that has so far proved impossible to achieve for space debris specifically.
What is clear is that the cost of not acting will eventually exceed the cost of acting. The question is whether that point arrives before or after a cascade event that damages the orbital environment enough to make the economics obvious to everyone involved.
