The complex, challenging problem of orbital debris
by Jeff Foust
|The recent attention given to falling satellites, though, potentially distracts from a bigger, longer-term issue: the growing hazard to satellites in orbit from debris.|
There were two problems with that story. One is that The Mail appeared to lift the story from the German magazine Der Spiegel; an English-language version appeared on the magazine’s web site the previous day. The version in The Mail copies all the key information and even some of the same quotes as the original Der Spiegel article, without attribution. The Mail’s major contribution appears to be sensationalizing the original German account, which only warned that debris from the satellite “may have destroyed buildings” and likely “would have resulted in human casualties”, without the city-wide destruction described in the tabloid’s headline.
The second is that The Mail’s depiction of the effects of ROSAT’s reentry were indeed sensationalized. Yes, having over a ton of metallic debris hit the ground at 450 km/h would likely have caused damage in an urban setting. But then, so would have a crashing airliner, which would have been far heavier and potentially travelling faster than that ROSAT debris. Yet no one would have suggested that an airplane crash would destroy a city.
The attention and headlines that ROSAT got are understandable, though, given that it seems like satellites are falling back to Earth routinely today. ROSAT reentered just a few weeks after NASA’s UARS satellite reentered over the Pacific, again out of harm’s way. And last month Russia’s Phobos-Grunt spacecraft reentered in the south Pacific off the coast of Chile after being stranded in a doomed, decaying orbit.
The recent attention given to falling satellites, though, potentially distracts from a bigger, longer-term issue: the growing hazard to satellites in orbit from debris. It’s commonly understood that orbital debris—aka “space junk”—is growing and posts a risk to spacecraft. However, at a panel on the topic held Friday in Washington by the Secure World Foundation, experts discussed the complexities and uncertainties associated with both the problem and how to deal with it.
“There are two big things” to know about orbital debris that aren’t commonly understood, said Darren McKnight, technical director with Integrity Applications Inc. and a member of a recent National Academies panel that studied NASA’s meteoroid and orbital debris program. One, he said, is that “natural perturbations in space are just as important as what we do.” The other, he said, is the “distinctly different” debris hazard between low Earth orbit (LEO) and geosynchronous Earth orbit (GEO).
While there is a distribution of debris as a function of altitude in LEO, the situation becomes more complex in GEO, he said, thanks to those natural perturbations. The Earth’s gravitational field is not uniform, he said, causing debris to collect in two places in GEO, at 75° east and 105° west longitude. “What I’m telling you is, if I’m at 74° east, eventually, space insurance may cost more money for me than if I’m at 150° east,” McKnight said, likening it to varying rates charged for auto insurance based on where you live.
There’s also a little-known variation by time of day because of gravitational perturbations, as objects line up in a “cosmic conga line”, he said. “The actual hazard is going to be 10,000 times greater at one time of day than another time of the day,” at some times of the year, he said. “You don’t hear about this.”
|“Although the space regime itself is different, SSA for the most part has not kept up,” said Weeden. “It’s still done primarily by the military for national security purposes.”|
Those variations by time of day and by longitude don’t exist in LEO, but there are changing effects of atmospheric drag, which McKnight said are linked to the Sun’s 11-year activity cycle, currently near its peak. The increased atmospheric drag during a solar maximum is partially responsible for the spate of recent satellite reentries, he said. “We have about nine times as many objects reenter on a daily basis during a solar max than in a solar min,” he said.
The dynamic and complex nature of orbits debris distribution makes it all the more important to keep track of debris, but efforts to improve what’s known as space situational awareness (SSA) have been difficult to implement even with the growing number of countries and organizations operating in space and relying on those satellites. “Although the space regime itself is different, SSA for the most part has not kept up,” said Brian Weeden, technical advisor for the Secure World Foundation. “It’s still done primarily by the military for national security purposes.”
“Good SSA requires a geographically distributed network of radars and telescopes,” he said. SSA, he noted, can be done unilaterally by any single nation, but only the United States comes closest to doing so with its Space Surveillance Network. Even that system, though, has gaps, such as a lack of coverage of the southern hemisphere. Other nations have even more limited coverage.
One solution would be some degree of international cooperation to exchange data and perhaps even create a single clearinghouse of SSA data, but progress on that front has been slow. Weeden noted there has been talk about the US sharing SSA data with some allies, but barriers to cooperation remain high. “There are still entities, different parts of the [US] government, that are very, very reluctant to share data, even when it’s in the best interest of the US,” he said. In addition, European nations have talked about pooling data but have run into problems among the militaries of the various nations and their differing data policies. “If the Europeans can’t do it among themselves, it doesn’t bode well for international cooperation.”
Collecting data is only part of the ultimate long-term solution for orbital debris, which involves mitigating the growth of additional debris and, eventually, cleaning up some of the debris. The biggest hazard, McKnight said, is posed by the biggest objects, which can fragment and create thousands of smaller objects. The National Academies study released last fall stated that future growth of the orbital debris population in LEO could be stopped if five large objects were removed from orbit each year, provided most of the objects launched followed existing guidelines to mitigate the creation of additional debris.
There’s been plenty of creative thinking in recent years about how to deal with orbital debris cleanup, from concepts like “bounties” for removing objects to innovative technical approaches, like electrodynamic tethers (see “Putting a bounty on orbital debris”, The Space Review, July 27, 2009). DARPA has shown an interest in this area as well with its recent “Catcher’s Mitt” study, although the agency has issued no updates following a symposium it hosted in late 2009.
|“We were doing a lot of good stuff, and then it was ruined in an instant,” McKnight said of the impact of the Chinese ASAT test and Iridium-Cosmos collision.|
McKnight suggested a more focused effort than simply deorbiting (or moving to other orbits) all large debris. He instead proposed something called “just in time collision avoidance” to deal with only those objects that pose a clear threat to active satellites. This would require improved SSA capabilities to detect a potential collision well in advance, offering enough advanced warning move one of the objects out of the way. “I believe that may potentially end up being more cost effective and analytically more sound,” than mass removal of large objects, he said.
“I think that the technology is probably ahead of the non-technical parts, the legal and policy issues,” when it comes to orbital debris removal, Weeden said. For example, an effort by one country to remove a debris object owned by another country could pose legal problems, since the laws of salvage in the ocean don’t apply to space, where ownership of a satellite or other object remains in place even after the object is no longer in service (see “Protecting Apollo artifacts on the Moon”, The Space Review, November 7, 2011). Weeden added that groundbased lasers could be a cost-effective way to deorbit debris, but their use poses some obvious policy issues.
For now, though, the focus is only trying to slow the growth of debris and better understating the effects debris has on satellites. “I don’t think we understand the degradation of our current systems over the last few decades well enough to make sure we make the right policy decisions,” McKnight said. “We need to continue what we’re doing with debris mitigation, redouble our efforts to make sure we don’t make it worse. However, we need to understand how much it’s affecting out current satellites.” One of the findings of the National Academies report is for NASA to devote more research to studying spacecraft anomalies to better understand how big the risk is to spacecraft from debris.
In the meantime, spacecraft operators need to be on their best behavior to limit the growth of debris. The rate of debris growth had been slowing for some time thanks to the implementation of guidelines and best practices, such as the venting of propellant tanks of spent rocket stages to prevent explosions. However, the Chinese ASAT test of January 2007, which created thousands of debris objects, changed all that, as did, to a lesser extent, the Iridium-Cosmos satellite collision two years later. “We were doing a lot of good stuff, and then it was ruined in an instant,” McKnight said.
McKnight said he isn’t worried in the near future about a runaway growth of debris: the so-called “Kessler syndrome” where debris creates more debris in a cascade until some orbits are rendered unusable. Instead, he’s worried that the growth of debris will remain linear, but at a steeper slope, causing more impacts with active satellites and shortening their average lifetimes. “If we don’t start being proactive in an incrementally aggressive way to control the problem, we are going to end up having to fix it and do multiple things” at a much greater expense, he said. That’s a tough case to sell in a tight budget environment, though, he said.
However, without some action, it’s possible future tabloid headlines won’t be about terror from the skies, but terror in the skies, as the debris problem only grows worse.