Mega-constellations and mega-debris
by Jeff Foust
|“The concern for mega-constellations is not about the spacecraft themselves, but rather, it is the potential for debris generation from the explosion of or collisions involving the mega-constellation spacecraft,” said Liou.|
The second trend is growing interest in so-called “mega-constellations” of satellites in low Earth orbit. OneWeb, for example, is developing a constellation of initially 648 satellites in LEO to provide broadband communications services globally. Other proposed systems, also primarily in the communications realm, are even larger: Boeing has filed with the US Federal Communications Commission (FCC) plans for a satellite system of between 1,400 and 3,000 satellites, while SpaceX is studying a system of about 4,000 satellites.
At last month’s International Astronautical Congress (IAC) in Guadalajara, Mexico, a panel discussion examined the implications of such mega-constellations on the orbital debris environment. The implication from the session’s title—“Projection and Stability of the Orbital Debris Environment in the Light of Planned Mega-Constellation Deployments”—suggested that those systems could exacerbate the orbital debris problem. But even as one company argued it would take steps to be a good citizen in LEO, it was clear that those measures alone won’t be enough to alleviate the problems those systems could create.
The threat posed by orbital debris, particularly objects too small to track—and thus maneuver to avoid—is not academic. On August 23, an object about one centimeter across struck ESA’s Sentinel-1A Earth observation spacecraft. Before-and-after images provided by a camera on the satellite showed a ring about 40 centimeters across on one solar array, created when the particle hit the back of the array.
“Since August 23rd, nobody else can tell better than ESA that we are operating in a risky environment,” said Holger Krag, head of ESA’s Space Debris Office, at the panel. The impact, he said, caused a partial power loss and a “significant momentum exchange” to the spacecraft, although the long-term effects on the spacecraft are minimal. Analysis of the direction and velocity of the impact led ESA to conclude the object was a piece of debris, and not a micrometeoroid.
“In some sense, we were lucky, because we can continue the mission without major constraints,” he said. “But if the object had arrived a few microseconds later, it would have hit the main body with certainly more significant consequences.”
Debris experts are worried about an increase in collisions as the number of spacecraft increase, creating even more debris. “The concern for mega-constellations is not about the spacecraft themselves, but rather, it is the potential for debris generation from the explosion of or collisions involving the mega-constellation spacecraft,” said Jer-Chyi Liou, NASA’s chief orbital debris scientist.
The US and many other nations have adopted guidelines for mitigating the generation of new orbital debris. Those measures include recommendations that spacecraft in LEO be deorbited no more than 25 years after the end of their mission, so they don’t pose a risk for debris-generating collisions.
For satellites low enough in LEO, that deorbiting can be accomplished naturally, through atmospheric drag. Above about 650 kilometers, though, drag alone is not enough to deorbit a typical satellite within the 25-year guideline. Those spacecraft need to lower their orbit, typically through on-board propulsion, in order to deorbit within 25 years.
|“But if the object had arrived a few microseconds later, it would have hit the main body with certainly more significant consequences,” said Krag of the object that hit Sentinel-1A.|
Krag said simulations of the debris environment indicate that at least 90 percent of LEO satellites need to deorbit within 25 years in order to control to growth of debris. The record is discouraging, though: he said today only about 20 percent of satellites above 650 kilometers even attempt to deorbit. That practice, if continued by a mega-constellation, could result in a catastrophic increase in debris: too much, he said, for even his models to handle. “If I had done the simulation with 20 percent instead of 90 percent, that wouldn’t even work with our numerical tools. It would be overloaded with the amount of debris,” he said.
At those higher altitudes, the debris that would be created would be long-lived because of the lack of atmospheric drag. Liou said that many mega-constellations are planning satellites for altitudes of 1,100 to 1,200 kilometers. “Any debris generated in that region will exist for thousands of years,” he warned. “We need to be extra-sensitive about the potential for collision cascading in that region.”
Those concerns have put constellation developers like OneWeb on the defensive. Tim MacLay, director of mission systems engineering for OneWeb and the one person on the panel representing constellation developers, said his company is very aware of the orbital debris risks and is taking steps to mitigate the development of orbital debris.
“OneWeb designed its mission to minimize collision risks,” he said. That includes, he said, inserting the satellites after launch into an initial orbit above the International Space Station but below many other operational satellites. Those satellites will eventually move up to an altitude of 1,200 kilometers, which, despite concerns about the longevity of debris created there, is relatively unpopulated today.
MacLay said the satellites will provide accurate position information, which OneWeb will make available to other satellite operators as well as the US Air Force’s Joint Space Operations Center (JSpOC) to aid in analyses to calculate any potential collisions. The spacecraft will also have “a high degree of collision avoidance agility,” he said, to maneuver in the event of a collision risk.
OneWeb also plans to swiftly deorbit its satellites at the end of their lives. “At OneWeb, the ability to deorbit is specified as the highest reliability function in the spacecraft, even above the payload itself,” MacLay said. Each spacecraft will have enough fuel to lower its perigee “all the way to reentry” at the end of their lives, “and are committed to being out of orbit within five years of decommissioning.”
That emphasis on reliability is important, Liou said, calling “high design reliability” of spacecraft one of the most important factors in mitigating debris creation. “A second key element, which is applicable to all space missions, is post-mission disposal,” he said.
Even if OneWeb and other mega-constellation operators take strong efforts to avoid collisions and other debris-generating events, their development will still pose challenges for efforts to avoid satellite collisions in general, as the number of operations satellites dramatically increase.
Liou noted that while there are currently about 1,400 operational satellites in Earth orbit, mega-constellations could increase that to as many as 7,000 to 8,000. “It can be a non-trivial challenge for JSpOC to extend the conjunction assessment coverage to the mega-constellation spacecraft,” he said.
|OneWeb founder Wyler, MacLay recalled, “said that he doesn’t want it on his tombstone that he messed up space.”|
It’s also not clear if that responsibility will remain with JSpOC in the long term. At the direction of Congress, the Department of Transportation recently completed a report assessing whether the FAA could take on the role of providing “safety-related” space situational awareness data, like collision warnings. While the Air Force would continue to collect the bulk of the data, the FAA would take over the role of analyzing it and providing collision warnings. That report concluded the FAA could handle that role, provided it obtained legislative authority to do so.
In any case, both debris experts and mega-constellation developers are aware of the additional risks that hundreds or thousands of new satellites could pose for a region of space where the amount of debris continues to grow even with the current population of satellites.
MacLay, near the end of the panel, invoked the name of OneWeb’s founder, Greg Wyler. “From the very beginning, Greg Wyler has recognized the environmental issues associated with launching constellations,” he said. “In one of his interviews, he said that he doesn’t want it on his tombstone that he messed up space.”