7 July 2018
According to the European Space Agency’s newly published annual space environment report, the total mass added to orbit in 2017 was 558.4t, nearly half of which – 261t – went to the low-Earth orbits up to 2,000km that are crucial for Earth observation. Meanwhile, 77.5t re-entered the atmosphere; almost all of that – 72t – came back from LEO.
To get a sense of what’s good and what’s junk, note that of the 261t that went to LEO, 188.4t were payloads and 71.3t were rocket bodies, just about equal to the mass re-entered – so the net result was that, in 2017, for every ton of payload delivered to LEO, those precious orbits also got a little less than half a ton of debris.
Whether this evolution of the space environment constitutes a debris crisis depends on the time scale that one is concerned about. Items re-entered would have been everything from old rocket bodies and fairings to very low orbit payloads and debris such as fragments from explosions or collisions or lens covers and other mission-related discards. Aerodynamic drag makes the lower reaches of LEO self-cleaning on the 25-year time scale prescribed for de-orbiting dead missions.
But higher up, items remain in orbit for a very long time – a satellite, rocket body or fragment at 800km will remain in orbit for decades, and at 1,000km the time for orbital decay is normally a century or more.
ESA’s press release – simply headed “Latest report on space junk” – notes: “This year’s report includes how the number of objects, their total mass and the area they cover has changed over time until 2017 – and it may come as no surprise that in each of these measures the numbers are increasing.”
Or, it adds: “As of the end of 2017, it was determined that 19,894 bits of space junk were circling our planet, with a combined mass of at least 8,135 tons – that’s more mass than the entire metal structure of the Eifel Tower.”
The numbers, alas, are numbing. Do we have a problem? Some perspective came from a roundtable discussion of the debris challenge at the Toulouse Space Show during the last week of June. In a keynote address to kick off the session, Jorge Del Rio Vera, scientific affairs officer at the United Nations Office for Outer Space Affairs, said of debris: “We have a problem. It may be too late already.”
Del Rio Vera estimates the societal and economic benefits of activity in space to be worth $320 billion. But put in another context, that value may be much greater; without space, he said, it would be “very hard” to monitor progress toward the UN’s 2030 agenda for sustainable development.
Striking a similar note of either cautious optimism or tentative pessimism, Christophe Bonnal, a space debris expert at France’s CNES space agency, answered the is-it-too-late question thus: “We don’t know.” But in perhaps a nod to the pessimism side, he noted that there is at the moment a major collision in orbit every 10 years – but the orbital situation is changing. The first of several planned mega-constellations – 900 OneWeb satellites to deliver affordable internet access globally – will see its first launch this year.
And, observed Bonnal, those 900 satellites will be flying at 1,200km, an altitude where they could persist for centuries. The US Federal Communications Commission, he added, expects 20,000 satellites to launch in the next 10 years, to high altitudes where they will be stable for 1,000 years. So why, he said, did OneWeb choose 1,200km? Because that orbit has the lowest debris density.
Another change coming is to our understanding of the debris problem. Today there are some 30,000 items bigger than a fist, 20,000 of which are well-known and catalogued. But better radar for tracking debris is on the way, which should take detection down to items of 2-3cm, from 10cm today – which increases the mathematical risk of collision by an order of 10, said Bonnal.
Ultimately, he said, if from now we do all the things we’re supposed to do to prevent creating more debris – such as deorbiting rocket bodies after their mission, designing missions without incidental debris, and building satellites with collision-avoiding manoeuvrability and deorbiting capability – everything could be ok. But, he said, that assumes nothing goes wrong.
Things, of course, do go wrong – and so a point of reasonable concern is the sheer number of satellites going into orbit. Luciano Anselmo, a researcher with Italy’s ISTI research institute, told the conference that the current launch rate of a few hundred new satellites per year is very different than thousands. So, operating procedures will have to change, and ISTI is contributing to work on specific guidelines for mega-constellations. But, he stressed, this is a difficult task because there must be full consensus; guidelines are rigid, but consensus means they have a big influence on national laws and practices.
Meanwhile, said Anselmo, that job of forging a consensus on how to act for clean space is made more difficult by the fact that different models of debris accumulation and propagation turn in very different forecasts. And, he added, the past decade has seen good intentions when it comes to compliance with the passivisation practices which are intended to prevent explosions of ageing or retired satellites by measures such as discharging batteries. But “effective implementation is not so good” and there are still many explosions annually – of old satellites built before the passivisation rules were established but also of newer spacecraft which can suffer failures of batteries or other components.
Mega-constellations of reasonably large satellites are one aspect of the sheer numbers problem, but another is cubesats, whose low cost and increasing capability recall the disposable consumer electronics creating a terrestrial waste problem. Benoit Charnot is an engineer and business development officer at Innovative Solutions In Space, or ISIS, the Netherlands cubesat and nanosatellite maker which also brokers launches. One of those brokerage jobs was to deliver 88 of Planet’s “Dove” Earth observing three-unit cubesats to India for its February 2017 PSLV flight, which set the record for payloads orbited from a single launch: 104. Those cubesats, Charnot insists, are not contributing to the debris problem: “We take it very seriously. We want to be responsible. But everybody is watching us.” The solution, he said, is to “launch low”, because Doves – and other cubesats brokered by ISIS – have no propulsion system. Natural drag from residual atmosphere at 600-620km will cause re-entry within the 25-year guidelines.
Low-Earth orbit may be self-cleaning, but a quarter-century feels like a long time given the volume of material being launched. And, the risk from smaller pieces of debris is not trivial. Tiago Soares, from ESA’s Clean Space office, showed a picture taken by an onboard camera of one of the solar panels on Copernicus programme’s Sentinel-1A, which was hit by a millimetre-sized particle in August 2016. The resulting damage cut the available power by 10%; fortunately, he said, the spacecraft – flying in a 700km Sun-synchronous orbit – has enough margin that its mission has not been affected. But, added Soares, the reason the risk of impact is high in useful orbits is because they are so much used!
The message, perhaps, to take away was well summed up by Bonnal, who observed that new rules on clean space operations do not apply retrospectively, and it takes years, maybe 10, to develop and launch a satellite. France’s national law on space debris mitigation was passed in 2010, so it won’t in principle be fully effective until the coming decade.
Meanwhile, engineers and satellite operators had best take seriously the challenge of managing orbital space. As ESA puts it: “As vast and empty as space may be, it is not an infinite resource.”