Space agencies join forces to tackle problem of small debris in low Earth orbit

Defunct satellites, broken-up launch rockets, bags of rubbish and even satellites containing human remains for ‘space burial’ are hurtling above our heads in orbit. All these items are large enough to be tracked from Earth using radar. Nasa estimates that there are 23,000 large objects in low Earth orbit (LEO) although the true figure including untracked objects could be as high as 30,000.The amount of junk in space is at a tipping point, according to a 2011 report by the US National Research Council, which called for international regulation to tackle the situation. The possibility of collisions with debris puts the world’s infrastructure and essential activities at risk.

“We rely on space-based assets for many of the things that we take for granted, such as communications, major weather information and navigation,” says Professor Ray Sedwick, director of the University of Maryland’s Centre for Orbital Debris Education and Research. “While temporary loss of such conveniences is not a major issue, the biggest long-term issue is not being able to even replace failed assets due to an environment that may become too hazardous for system operation.”

All satellite launches are preceded by a screening to ensure that they are not in the path of large pieces of debris. When satellites suffer these collisions, they break apart, forming clouds of smaller pieces. Some fear that major collisions – such as that of a working US communications satellite with an inactive Russian satellite in 2009 – could result in uncontrollable chains of destruction.

There are an estimated 100 million pieces of small debris – including bits of paint or solidified droplets of coolant from Soviet-era nuclear satellites – in LEO. Due to the extreme speeds reached in orbit, even specks can cause damage to satellites: scratching windows, exposing thermal protection systems, damaging spacesuits and sensitive equipment.

“The smaller pieces of debris do not take out your mission in a dramatic fashion, but they still have the potential to cause a crack in your camera, for example,” says Stijn Lemmens, a member of ESA’s space debris team, “So the value of your mission is quite reduced.”

While the International Space Station (ISS) navigates around large pieces of debris to avoid devastation, smaller space junk cannot be tracked and avoided, so the spacecraft endures multiple small impacts, protected by its ‘Whipple shields’.

Small space debris is a patchy area of knowledge compared with large debris. Researchers studying small debris often piece together their understanding by investigating the scenes following collisions, such as by studying the impacts on space shuttle Endeavour’s heat shield after its launch. Now, however, two major space agencies have begun projects to improve our understanding of the composition and behaviour of small space debris.

In December, Nasa delivered a new space debris sensor to the ISS to track small debris impacts in real time. The device – an acoustic system of three layers of sensors – will collect data from impacts of pieces of space junk in the 0.05-0.5mm range to characterise their size, density and speed. According to Nasa, the project could help map smaller pieces of debris and inform the design of future sensors.

Meanwhile, the Japanese Aerospace Exploration Agency (JAXA) has announced that it is developing far more sensitive radar to detect space debris under 10cm in size. This would make the technology 200 times more sensitive than its predecessor, which monitors objects 1.6m and larger. The technology could enter operation from 2023, and would be paired with a separate detector designed by the country’s defence ministry.

Perhaps due to the difficulty involved with studying small debris, there are no technologically mature projects in place to capture this type of junk and remove it from LEO. Removing just a few vulnerable large objects, however, can play a valuable role in controlling the main source of small debris.

According to a report by the International Advisory Debris Committee, at least five large objects must be snatched out of LEO every year to stabilise debris growth. Suggestions for how this may be achieved range from a failed ‘space net’ trialled by JAXA to a ground-based ‘laser broom’ for ablating massive objects. These approaches either send the debris into a ‘graveyard’ orbit where it will not interfere with live satellites, or causes it to slow down until it falls into the atmosphere and burns up on re-entry.

ESA’s e.Deorbit programme aims to be the world’s first active debris removal mission, and is currently in the detailed design phase. This mission will send a satellite into a similar orbit to a massive piece of space debris, meet it, then capture and remove it from the LEO, most likely using a robotic arm. Mission project manager Robin Biesbroek says that while the mission will initially target a specific piece of ESA-owned debris, the sophisticated sensors and capture mechanisms could later be reused to remove other debris.

Meanwhile, the European Commission-funded removeDEBRIS [sic] project, involving 10 organisations, is preparing to launch an experiment to demonstrate more cost-​effective technology to capture space debris in LEO. This would embrace simple technologies such as nets or harpoons, rather than developing sophisticated robotic arms.

“Clearly everybody agrees it would be a nice idea to go there and start to remove some of this debris, i.e. the most ‘threatening’ ones. But this is too expensive, maybe there will be other priorities,” says Professor Guglielmo Aglietti, director of Surrey Space Centre and principal investigator. “So we are trying to see if this can be done in a cheaper way with simpler technologies that can fulfil a goal in the same way as more complex solution, [such as] a robotic arm.”

The removal of old debris from LEO is not enough to keep the skies clear. Dr Hugh Lewis leads the University of Southampton’s Astronautics Research Group, which uses computer modelling to understanding the impact of measures to handle space debris. He says there is “absolutely no question” that the best way to manage space debris is to remove spacecraft from LEO at the end of their life. The easiest way to achieve this is to use a spacecraft’s thruster systems and remaining fuel to lower its orbit so that it re-enters the atmosphere.

The ‘25-year rule’ – detailed in the US Government Orbital Debris Mitigation Standard Practices – recommends that the process of removing spacecraft from LEO should take no longer than 25 years. ESA and other organisations have similar guidelines, although the extra expense and complication of removing spacecraft from LEO means that the non-binding guidelines not always observed. For instance, after the Chinese government performed an anti-satellite missile test in 2007, successfully destroying a weather satellite and creating a record cloud of space debris, there were no formal sanctions applied.

“Other than placing them on the unofficial naughty list, there was really no fallout,” said Sedwick, commenting on the missile test. “One would hold out hope that the possibility [of a binding international agreement] does exist, but it is a very complex political problem. The majority of the debris is a result of the countries that have been space-faring for the longest so countries with newer space programmes tend to feel that putting tighter constraints on end-of-life requirements put them at a disadvantage relative to what [the US and USSR] had. And how [do we] fairly determine the cost burden of any particular solution?”

Dr Luisa Innocenti, who heads ESA’s Clean Space Office, compares the cluttering of space with the short-sighted pollution of Earth and exploitation of its resources; to continue to do so will prove a financial burden. “The responsibility not to pollute in the future is everybody’s, full stop,” she says. “As for cleaning [past satellites], it is extremely difficult to say. The position that we have in ESA is a different one. Instead of talking about responsibility we’re saying there might be a market for active debris removal […] so we’re not talking about responsibility, we are talking about opportunity.”

Some of the technologies under development at ESA, she says, could be used for other operations on orbiting satellites, such as by refuelling satellites to extend their working lives.

The issue of mitigation – and crucially, how to enforce it – becomes more pertinent with the planned launches of a number of satellite constellations. These constellations consist of huge populations of satellites spread out above the Earth. Samsung has proposed a constellation with 4,600 satellites, and SpaceX is looking at a staggering 12,000. For comparison, there are just 31 Global Positioning System (GPS) satellites in orbit, of approximately 1,400 operational satellites.

Just one constellation launch is a “massive step change”, says Lewis, whose team at the University of Southampton has explored the potential impact of these constellations. As more objects enter the orbital environment, he says, it is reasonable to assume that risk will increase.

“If the operators are not responsible then there are big problems with the environment. If they are responsible and if they go beyond what is currently expected with respect to space debris mitigation then things do look as though they’ll be okay.”

There is reason to be optimistic; by October 2017, SpaceX filed a document with the US Federal Communications Commission proposing space debris mitigation plans that go beyond the 25-year rule, proposing that the satellites deorbit far more quickly. These constellation satellites could move into a ‘disposal orbit’ from which they can re-enter the atmosphere approximately just one year after they complete their missions.

A widespread move towards such strong mitigation by the world’s satellite operators could be what it takes to ensure that our space environment is protected.