An artist’s illustration of operational and defunct satellites crowding Earth’s geostationary orbit, 35,786 km above ground. Image: ESA/ID&Sense/ONiRiXEL, CC BY-SA 3.0 IGO.
In 1989, the Soviet Union launched a navigation satellite called Kosmos 2004 from one of its northern cosmodromes. Twenty years later, in an unrelated event, China launched a rocket called CZ-4C, to put a military reconnaissance satellite in space. Other than their military utilities, these two space-bound objects had nothing in common. But unbeknownst to anyone, their destinies were going to intersect.
On October 13 this year, LeoLabs Inc., a Silicon Valley company, alerted the world to a dangerous collision event. The company employs a collection of ground-based radars that allow it to track and identify objects of various sizes in low-Earth orbit – which is any orbit above 100 km and below 2,000 from Earth’s surface.
In a tweet, the company identified two objects that were on a collision curse – Kosmos 2004 and a part of the CZ-4C rocket. According to its pre-collision analysis, the now defunct Cold Soviet-era satellite and a discarded stage of the Chinese rocket had a 1% to 20% chance of colliding in space, some 991 km above Antarctica, on October 16.
We are monitoring a very high risk conjunction between two large defunct objects in LEO. Multiple data points show miss distance <25m and Pc between 1% and 20%. Combined mass of both objects is ~2,800kg.
Object 1: 19826
Object 2: 36123
TCA: Oct 16 00:56UTC
Event altitude: 991km pic.twitter.com/6yWDx7bziw
— LeoLabs, Inc. (@LeoLabs_Space) October 13, 2020
The next day, LeoLabs updated its probability calculation to greater than 10%. The combined mass of the two objects approximated to be 2,800 kg, and they were approaching each other at a relative speed of 14.7 km/s. This scenario represented a large amount of potential collision energy in the event of a crash, even if it posed no threat to anything on the ground. More importantly, the intensity of the collision would likely produce small debris that would burn off when it reentered Earth’s atmosphere. So why bother?
So a more accurate estimate of the potential collision energy, in the centre of mass frame, is 60 plus or minus 5 gigajoules, or about 14 tonnes of TNT equivalent. The main uncertainty is the mass of the CZ-4C third stage, which I have at 2000 plus or minus 500 kg.
— Jonathan McDowell (@planet4589) October 15, 2020
It is bothersome because such collisions typically generate debris that, while it would burn up on reentry, actually tends to hang around in orbit for a long time first. In an interview with ScienceAlert, Alice Gorman, a space archaeologist in Australia, called this event “one of the potentially worst accidental collisions that we’ve seen for a while.”
Apart from the intensity of the crash itself, a collision between two large objects like Kosmos and CZ-4C is also likely to produce lots of small pieces, and could single-handedly increase the amount of space debris occupies in low-Earth orbit by a few percentage points. The high potential energy also means the pieces are flung out into space, and without any drag, they whiz around in orbit at those high speeds. They’re practically bullets to satellites and space-walking astronauts.
As space gets more crowded this way, we get closer to a tipping point, called the Kessler syndrome. NASA scientists Donald Kessler and Burton Cour-Palais posited in 1978 that if the debris cloud surrounding Earth becomes dense enough, one collision event could lead to a cascade of subsequent collisions, like a domino effect, that produce even more debris. Eventually, sections of Earth orbit could be rendered entirely unusable.
In the 1970s, Kesssler’s and Cour-Palais’s thesis was more in the realm of informed foresight backed by technical rigour. But today the Kessler syndrome is a very real possibility. Every time there is an accidental collision in orbit, orbital debris is one of the major concerns of every stakeholder, but especially spaceflight agencies like NASA and the Indian Space Research Organisation (ISRO).
In May 1963, the Massachusetts Institute of Technology placed 480 million tiny copper needles, each about 1.7-cm long, in orbit to create an ‘artificial ionosphere’ that would help the US military’s radio communications. This isn’t considered to be a debris-generating event, but the needles as such considerably cluttered space and are obsolete today thanks to advances in communication technology.
A second significant event took place in 2007, when China launched an anti-satellite missile to blow up a test satellite. The event generated many thousands of pieces of debris that also dispersed around the satellite’s orbit as well as spread up and down to other orbits. A 2007 analysis estimated that only 6% of the debris will burn up in Earth’s atmosphere in 10 years, and only 21% in 100 years.
Only two years later, there was another significant collision event — between Russia’s commercial Iridium 33 satellite and military Kosmos 2251 satellite. According to the US Space Surveillance Network, the event had been responsible for more than 2,000 debris fragments in orbit by July 2011. As of January 2016, SpaceNews reported that 1,505 pieces were still in orbit and hadn’t yet reentered the atmosphere. This was the first instance of two satellites colliding, instead of one satellite and some debris.
A decade later, in March 2019, India’s Defence Research and Development Organisation (DRDO) emulated China, launching an anti-satellite missile to shoot down a purpose-built satellite that ISRO had placed in low-Earth orbit. DRDO chief G. Sateesh Reddy had said all the debris would burn up in the atmosphere by May. But six months later, at least 30 fragments were still found to be in orbit, plus how many ever smaller fragments that ground-based systems couldn’t track.
As for the potential Kosmos and CZ-4C collision: 10 minutes after the time of closest approach, LeoLabs tweeted that CZ-4C had passed over its radar station in New Zealand as a single object without debris. The company followed it up with an estimate of the miss distance – about 11 metres.
No indication of collision. 👍
CZ-4C R/B passed over LeoLabs Kiwi Space Radar 10 minutes after TCA. Our data shows only a single object as we’d hoped, with no signs of debris.
We will follow up in the coming days on Medium with a full in-depth risk assessment of this event!
— LeoLabs, Inc. (@LeoLabs_Space) October 16, 2020
Even though other experts have come up with more conservative estimates, there is no denying that this was a near-hit. We avoided disaster this time but there is no avoiding the potential for such collisions in future. Space junk is not going anywhere. The European Space Agency combines observations with statistical models to list the number and sizes of objects that make up this junk. The data unequivocally suggests that we are only generating more debris with time.
Some spaceflight agencies today require new missions to include de-orbiting manoeuvres that will ensure a satellite that has reached the end of its life doesn’t hang around in orbit, and falls back down to get burnt. SpaceX’s planned fleet of 12,000 Starlink satellites, to facilitate internet access around the planet, is one example. While these measures are good, they’re not enough. We need to pollute space less but we also need to clean up what’s already there. Many space missions shed multiple stages in the course of their journeys, and each stage is a potential piece of debris and/or the source of more pieces. The CZ-4C object is an example.
Researchers have already begun to develop some tools and techniques that could help. For example, Surrey Satellite Technologies has developed a net to capture and remove debris from orbit. The European Space Agency has developed e.Deorbit – a ‘debris grabber’ – that is planning to launch in 2025.
But more broadly, we don’t have any reliable way of mitigating this problem except hope that spacefaring countries listen to their better angels and that the technologies we develop hold out. This is also why, when a potential collision is impending, space scientists and engineers around the world can only wait with bated breath, and react to the best of their ability.
Tipping points are fraught with uncertainty. There is no saying when we reach one.
Ronak Gupta is pursuing a PhD in fluid mechanics at the University of British Columbia, Vancouver.