The Kessler syndrome

Of all the human-made satellites in low-Earth orbit, 95% are space junk – rocket thrusters, derelict satellites, and fragments of debris from collisions and explosions. All this debris poses a potential threat to active satellites and the future of space travel if we don’t find a way to clean it up.

We’ve seen the dangers of space junk over the past year: in May 2021 a large, uncontrolled piece of debris from China’s Long March 5B rocket hurtled back to Earth, and in June the International Space Station’s robotic arm was punctured by a mystery piece of space junk in a “lucky strike” that thankfully didn’t impact operations or any of the seven astronauts aboard.

Then, to make matters worse, a Russian anti-satellite weapon test sent shockwaves through the space community when it was launched at a “dead” spy satellite, sending 1500 fresh fragments careening around in low-Earth orbit in November.

The need to reduce the amount of space junk in orbit is clear, and there has been incredible research done to find ways to do it, from tracking space debris with mobile observatories and radar, to recycling space trash into rocket fuel or removing it completely with hunter-killer satellites and lasers.

These are all tactics designed to avoid the Kessler Syndrome – the subject of the 2019 SCINEMA International Science Film Festival entry Kessler Syndrome: The Space Debris Problem. It describes the devastating domino effect that’s likely to occur when there are collisions in orbit, causing a runaway cascade of fallout that will continue to over-crowd our already congested low-Earth orbit.

The film explains the science behind the Kessler Syndrome and features astrophysicist Dr Donald Kessler, whom the phenomenon is named after. Read on to learn more about Kessler and how he came to predict the dire threat of space junk.

In Neal Stephenson’s 2015 novel Seveneves, he imagines that the Earth’s moon breaking into seven pieces. This being a work of science fiction, the event of course has dire consequences for everyone on the planet.

But if the idea of a cloud of space debris left over from the shattering of the Moon seems fanciful, think again. As of 26 May 2021, according to NASA, there are about 23,000 pieces of debris “larger than a softball” (greater than 10cm) orbiting the Earth, travelling at speeds over 28,000km/h – “fast enough for a relatively small piece of orbital debris to damage a satellite or a spacecraft”.

But wait, there’s more – lots more. Elon Musk, Jeff Bezos, Richard Branson and all those prospective space tourists (hello, William Shatner) should know that NASA estimates there are half a million pieces of debris the size of a marble or larger (up to one cm), and approximately 100 million pieces of debris about one mm and larger. There is even more micrometre-sized debris.

Of that, only 27,000 pieces of orbital debris, otherwise known as “space junk”, are being tracked by the US Department of Defense’s global Space Surveillance Network.

NASA established the Orbital Debris Program Office in 1979 at the Johnson Space Centre in Houston, Texas, to track the orbital debris environment “to support risk assessments for all NASA projects and programs” and to “evaluate risk of human casualties from satellite re-entries”.

The office came about largely as the result of a 1978 paper by American astrophysicist Donald Kessler, “Collision frequency of artificial satellites: The creation of a debris belt”, in the Journal of Geophysical Research: Space Physics.

The paper also gave rise to a phenomenon called “the Kessler Syndrome”, which Kessler described in a 2009 article as “an orbital debris term that has become popular outside the professional orbital debris community without ever having a strict definition”.

Kessler says he believes the term originated with a colleague, John Gabbard, who worked for North American Aerospace Defense Command (NORAD), which “maintained a catalogue of man-made objects in orbit, but did not maintain a breakup record of events in orbit”.

Man in suit at conference smiling
Retired astrophysicist and former NASA scientist Donald Kessler seen attending the European Conference on Space Debris at ESA in Darmstadt, Germany, on 18 April 2017. Credit: ESA/J. Mai

He says Gabbard may have come up with the name in an interview with Popular Science magazine. “The label stuck, becoming part of the storyline in some science fiction, and a three-word summary describing orbital debris issues.”

Kessler’s 1978 paper predicted that around the year 2000 the population of catalogued debris in orbit around the Earth would become “so dense that catalogued objects would begin breaking up as a result of random collisions with other catalogued objects and become an important source of future debris”.

He gave three reasons for concern. First, he said, “it was generally assumed that there were very few objects in orbit that were too small to catalogue, although there was no definition as to what limiting size was in the catalogue. The paper illustrated that even if this assumption were correct, future collisions between catalogued objects would produce a large amount of small debris fragments. This small debris population would be more hazardous to other spacecraft than the natural meteoroid environment immediately after the first collision.”

Next, he said each collision would also produce several hundred objects large enough to catalogue, increasing the rate that future collision breakups would occur – “resulting in an exponential growth in the collision rate and debris population”.

Finally, he said, “The only way to prevent this exponential growth was to reduce the number of rocket bodies and non-operational spacecraft left in orbit after their useful lifetime.”

Gabbard unofficially kept a record of major satellite breakup events, which later proved very useful in understanding the sources of smaller orbital debris. “John is known for his description of these events with a graph we now call a ‘Gabbard Plot’,” Kessler says.

Donald Kessler was born in 1940 and grew up in Texas. In 1962 he went to work for NASA at the Johnson Space Centre; three years later he earned a bachelor of science degree with honours in physics from the University of Houston.

In 1990, as NASA’s senior scientist for orbital debris research, he co-ordinated the agency’s orbital debris research program. Although he retired in 1996, the 81-year-old continues to advise various organisations and individuals on orbital debris issues.

It’s work worth doing, as NASA says that “even tiny paint flecks can damage a spacecraft when travelling at these velocities”.

“A number of space shuttle windows were replaced because of damage caused by material that was analysed and shown to be paint flecks. In fact, millimetre-sized orbital debris represents the highest mission-ending risk to most robotic spacecraft operating in low-Earth orbit,” NASA says.

In 1996, NASA reported that a French satellite was hit and damaged by debris from a French rocket that had exploded a decade earlier. And on 10 February 2009, a defunct Russian satellite collided with and destroyed a functioning US communications satellite.

The BBC recently reported an incident that occurred in March 2021 in which a Chinese military satellite, the Yunhai 1-02, launched in September 2019, was crippled after colliding with “a small piece of space junk – probably a piece of debris between 10-50cm wide – from the Zenit-2 rocket that launched the Russian spy satellite Tselina-2 in September 1996”.

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