When India’s Prime Minister Narendra Modi proudly announced that his country successfully shot down one of its Low Earth Orbit (LEO) satellites with an anti-satellite missile in March, many in the global space community were quick to condemn the action. Though considerably less damaging than the infamous 2007 Chinese anti-satellite missile test, the Indian demonstration created a cloud of debris fragments, some of which might remain in orbit for years.
More clutter in orbit is exactly the last thing the space community needs. According to the European Space Agency (ESA) Space Debris Office, there are currently about 30,000 out of control objects larger than 10 centimeter (cm) in diameter hurtling around the Earth, including defunct satellites and fragments generated in collisions and in-orbit explosions. In addition to that, ESA’s scientists estimate a further 900,000 fragments larger than 1 cm and a staggering 130 million pieces larger than 1 millimeter (mm).
The smaller objects are impossible track. Still, they have the ability to destroy a satellite.
“These collisions occur at impact speeds of around 40,000 kilometer (km) an hour,” Holger Krag, head of ESA's Space Safety Program Office, tells Via Space. “A 1 cm object has the ability to terminate a mission. If a satellite is hit by something larger than 10 cm, the impact will not only terminate the mission but also generate a huge number of fragments.”
In August 2016, a fragment only a few millimeters in size hit the Earth-observing Sentinel-1A satellite, operated by ESA as part of the European Union (EU)-funded Copernicus program. Although the fragment created a 40 cm hole in the spacecraft’s solar panel, the spacecraft was able to make up for the resultant power loss, and the mission could continue without changes. Krag, however, admits the impact could have been far worse.
“If we were just two or three milliseconds earlier, we would have had an impact into the main body and that definitely would have had an effect,” he says. “We would have probably lost one of the instruments or, if the tank was hit, we might have lost the whole mission.”
Krag says that most satellites would experience a collision with an object about a millimeter in size during their lifetime. Depending on the area of the impact, such a collision could degrade the satellite’s performance. A collision with a larger, 1 cm, object could happen about every few hundred years per each satellite. With thousands of satellites in orbit, such an incident is statistically likely to occur every few years. A collision with a 10 cm object might happen every five years, according to Krag. Smaller fragmentation events are, however, far more frequent.
“Most of the debris pieces that we have in space are a result of explosions,” says Krag. “We have had more than 200 break-up events of rocket stages and satellites that happen because the spacecraft are left in space for too long after the mission ends and suffer from the aggressive space environment as a result. Residual fuel and pressure in the tanks can then lead to a break-up.”
Such an event, Krag says, happens about five times per year. And so, the amount of space debris fragments is gradually rising. The math is simple — the more objects in space, the greater the likelihood of collisions. And since the number of objects in space is expected to rise greatly as mega-constellations of small satellites are moving from plans to reality, the experts worry about the future.
Since the launch of the Soviet Sputnik 1 in 1957, around 8,400 satellites have been lofted into space. Out of the nearly 5,000 still orbiting the Earth only about 2,000 are operational. Over the past few years, new space companies have filed plans to launch a combined 14,000 satellites. Even if not all of these satellites make it into orbit, the environment around the Earth is without a doubt set to become much busier.
Fortunately, space industry stakeholders seem to agree on what needs to be done.
“I think that in general the space community wants to protect the environment and keep it safe,” Mike Martinez, vice president of space engineering at Maxar, which owns DigitalGlobe, told Via Space.
“But I am concerned about the possible impact of mega-constellations and I think that all of us who are launching into space need to be good space citizens and work together collaboratively.”
Mike Safyan, director of launch at Planet, agrees: “In general, I think the industry is well aware that debris is a concern. We can’t fall asleep at the wheel and we see companies taking more and more responsibility for the direction.”
ESA’s Krag agrees that the global space industry has the right intentions. He is, however, worried whether the success rate of the debris mitigation measures will be high enough. As deorbiting maneuvers are performed at the end of a mission — with the spacecraft structures already degraded by the intense space environment — failures are common.
Guidelines put together by the Inter-Agency Space Debris Coordination Committee (IADC), which comprises members of world’s leading space agencies, require spacecraft operators to remove residual fuel from their satellites, discharge batteries and vent pressure from the tanks upon the mission end in order to prevent explosions. Further, spacecraft operators are obliged to deorbit their spacecraft within 25 years. This might be an easy task for operators whose spacecraft fly at altitudes below 400 km, which are generally considered self-cleaning thanks to the intense atmospheric drag. The further from the Earth the spacecraft orbits, the more difficult it becomes to dispose of it by bringing it into the atmosphere for re-entry.
“Unfortunately, we are not very good at implementing these debris prevention measures,” says Krag. “Only 60 percent of the objects, that’s our observation, manage to do it properly. When it comes to the constellations that expect to launch more than 10,000 spacecraft within a very short period of time, our fear is that if they don’t have a higher success rate in removing their spacecraft than everybody else, we will have a disaster. Sixty percent of 10,000 means that there could be several hundred of objects that could get stranded and fail to get disposed properly.”
Even with a disposal success rate of 95 percent, he added, tens of defunct satellites could be left behind, creating risk to other assets.
“The more these activities increase, the more reliable the systems need to be to keep the environment at a reasonable level,” Krag says, adding that with the commercial pressures the constellation operators are likely to face, it might be difficult for them to achieve such stellar technical performance. The focus, he says, needs to be on minimizing the failure rate at higher altitudes.
“At 400 km, an object would not stay in space longer than one year,” says Krag. “At 600 km, it would probably not remain there longer than 25 years. At 800 km, because the atmospheric drag is much weaker, it would stay for 200 years. And at 1000 km, it would stay forever.”
Many of the commercial players agree that it is necessary to raise the bar and put forward new guidelines and strategies to prevent a future space environment crisis. A group of companies including Maxar, OneWeb, and Iridium proposed a set of practices to ensure long-term usability of the space around the Earth, which goes well beyond the current IADC guidelines.
The five recommendations, according to Martinez, include a requirement for large constellations to not overlap in altitude, an obligation to investigate a root cause of an in-orbit failure before launching further satellites, and a requirement for spacecraft operators to be able to control the flight path of their satellites and avoid debris. The companies also believe that all spacecraft launched into orbits above 400 km should be moved to a lower altitude at the end of the mission to ensure the object deorbits completely within 25 years. All satellites should be disposed of reliably at the end of a mission in order not to pose a risk to other satellites and the deorbiting should be done in a way that does not put at risk people and property on the ground.
Maxar set an example with their QuickBird and Ikonos satellites that were both decommissioned in 2015. The controllers were able to take QuickBird all the way down to the atmosphere from its original 450 km altitude. The orbit of Ikonos was lowered from 678 km to below 400 km in order for the spacecraft to naturally re-enter within the 25-year limit.
“When we plan for the satellite lifetime to be ten years, we need to have enough propulsion so that we have enough for the ten years as well as the deorbit,” says Martinez. “The cost goes up because of the added propellant. But it is very important if we want to ensure that we continue operating in a safe environment.”
Safyan agrees that more stringent requirements are needed to ensure long-term sustainability of space operations. To ensure that technical failures don’t thwart the removal of the satellites from critical orbits after the mission’s end, Safyan proposes in-orbit tests to be carried out at lower altitudes. The satellites would subsequently raise their altitude using on-board propulsion.
“That would really help mitigate the failure on arrival scenario, which is a potential problem when you are building constellations of tens or hundreds of satellites,” he says.
He further suggests that spacecraft should be designed with grappling features in order to be easily collectable by potential future active debris removal missions.
“The satellite industry is evolving very quickly, activity is proliferating, and we need those regulations to keep up and address scenarios that really weren’t realistic 20 years ago,” Safyan says. “Currently, we have the 25-year deorbit rule but that’s really the worst-case scenario. We need a more nuanced approach. For example, rocket stages shouldn’t be left behind for 25 years when they were only needed for a couple of hours.”
He calls on the launch industry to accept responsibility as much as the satellite operators.
“The most dangerous pieces of space debris that are up in the orbit right now are spent rocket bodies,” he says. “These are usually very large space objects that often have energetic propellant still on board so a collision with an upper stage that is packed with hydrazine is a very different scenario compared to two cubesats colliding inertly.”
ESA together with the World Economic Forum and MIT recently introduced a Space Sustainability Rating (SSR) concept that would enable engineers to calculate the potential environmental impact of each mission.
“It would be like a label that you have on your refrigerator, which allows you to see what that object would mean for the environment,” explains Krag. “For example, a massive satellite flying at 1,000 km with a low reliability would be very crucial while a small satellite in just 200 km is not that big of an issue.”
The rating system would make the impact measurable and allow regulators to request changes to be made either to the satellite or its mission in order to achieve an acceptable level of risk.
“We would like to have a concept that would allow us to compute it,” says Krag. “The regulators could, for example, request lowering of the altitude, or it would have to come with a much higher reliability so that we can be very sure that it can be disposed after the mission.”
Some Earth Observation (EO) companies are concerned that their future might be affected by clashes over spectrum. According to William Hosack, CEO of Orbital Micro Systems — which is developing a constellation of next-generation weather-forecasting satellites — the pressures by telecommunications operators to acquire ever more spectrum could in the future interfere with orbital operations.
“For us in microwave, if we want to observe certain molecules or atomic radiances in orbit, they only happen at very specific frequencies and we are very concerned about governments selling spectrum to the telecommunications operators that is very close to Earth observation frequencies,” he says.
Following the launch of the first batch of 60 satellites of SpaceX’s Starlink constellation in May, some astronomers raised concerns about the possible light pollution the constellation of up to 12,000 satellites could cause.
SpaceX lowered the altitude of the constellation from the originally proposed 1,150 km to 550 km, which is positive from the perspective of space debris. But, it makes the satellites — with their rather large light-reflecting solar panels — clearly visible from the Earth even with the naked eye. Their brightness could possibly interfere with astronomical observations by the next generation of super sensitive ground-based telescopes, such as the Large Synoptic Survey Telescope (LSST). Some experts have also voiced concerns the Starlink satellites might interfere with radio observations. VS