Can Gravity attract attention to the orbital debris problem?by Jeff Foust
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| Does Gravity raise awareness of a real problem or unhelpfully exaggerate it? |
The awards are likely to keep the spotlight on Gravity, nearly three months after its theatrical release, well into the new year. The movie has had something of a polarizing effect on space professionals: some have appreciated its ability to convey the spaceflight experience, albeit a dramatic one, while others, most famously astronomer and science communicator Neil DeGrasse Tyson, criticized it for its many inaccuracies (see “Gravity and reality”, The Space Review, October 7, 2013.) Bill Gerstenmaier, NASA associate administrator for human exploration and operations, summed up that gap in movie perceptions when he described at a conference last month seeing the movie with someone outside of NASA: “My non-space colleague is crying, and I’m saying this violates the law of physics.”
One aspect of Gravity in particular that has attracted interest—and concern—in the space community is its depiction of orbital debris. A Russian ASAT test sets off a chain reaction of debris, precipitating the movie’s drama. Orbital debris, including that created by ASAT activities like China’s January 2007 test, is a real concern to governments and companies that operate satellites. But does the movie raise awareness of a real problem or unhelpfully exaggerate it?
At a panel session titled “‘Gravity’ in Real Life: Legal and Political Implications of an Accident in Space,” held by the Secure World Foundation in Washington earlier this month, participants argued that while orbital debris is a real concern, how it was portrayed in the movie, technically and otherwise, was highly exaggerated. “The good news is that the exact sequence of events portrayed in the movie has zero chance of ever happening,” said Darren McKnight, technical director at Integrity Applications Inc. “The bad news is that space debris is a problem.”
The triggering event for the movie is that a single ASAT test instantly triggers a cascade of debris that takes out every satellite. It’s based on a concept called the Kessler Syndrome, where the amount of orbital debris in an orbit reaches a critical density above which debris collisions cause runaway growth in the amount of debris, rendering that orbit unusable.
However, the Kessler Syndrome is not something that would manifest itself in a matter of minutes, as it does in Gravity. “The Kessler Syndrome is a mathematical singularity. If you look at equations of the critical density, we’ve already passed it,” McKnight said. “But it’s going to take decades for it to manifest itself.”
A single ASAT test would not be as catastrophic as the one depicted in the movie, McKnight said, citing the evidence from China’s 2007 test. That test broke up a 750-kilogram satellite, FY-1C, into 3,000 objects large enough to track. None of those pieces, McKnight said, have collided with other tracked objects to date. “The sky isn’t falling quite yet,” he said.
During the panel session, McKnight walked through a model of the same collision at the same altitude as the International Space Station (the FY-1C collision took place at 750 kilometers, more than 300 kilometers higher than the ISS.) Over time, the debris cloud expands into a shell with a much lower density of debris than the original cloud. Initially, the odds of encountering the debris cloud immediately after collision are extremely low—4×10-11 just 10 seconds after the ASAT impact—although if you hit the cloud, he said, the odds of colliding with debris are very high.
| “The Kessler Syndrome is a mathematical singularity. If you look at equations of the critical density, we’ve already passed it,” McKnight said. “But it’s going to take decades for it to manifest itself.” |
Over time, the odds of encountering the debris shell grow, but the debris is dispersed, and much of it has reentered. Six months after collision, McKnight calculates the probability of a piece of debris colliding with the ISS is at 4×10-6 per orbit. In other words, the probability of an impact is actually higher (but still very low) six months later than at the time of the impact—the reverse of what was implied by the movie—and the probabilities were for only a single impact, which he said was far more likely than multiple impacts, let alone the shotgun spray of debris that crippled and later destroyed the ISS in the movie.
Another curious factor about Gravity is the fact that it is a Russian ASAT weapon that triggers the debris cascade. “The Russians, who have their own cosmonauts on the International Space Station, decide, just on the spur of the moment, to launch a direct ascent anti-satellite weapon,” said Marcia Smith, founder and editor of SpacePolicyOnline.com. Russia and the former Soviet Union, she noted, never tested such an ASAT, although the Soviets tested other kinds of ASATs. “Why did the moviemakers pin this on Russia?” (Later discussion during the panel session suggested that blaming the test on China could have caused the Chinese government to ban the movie there; Hollywood filmmakers have in the past looked to avoid antagonizing Chinese censors in order to gain access to that country’s lucrative market.)
The movie, noted George Washington University professor Henry Hertzfeld, feeds on the belief held by many that orbital debris is a bigger problem than it actually is today. “The perception that we have from the media, from films like this, and a number of studies, is that space is crowded with debris. But in reality, it really isn’t,” he said. Commercial satellite operators, he said, deal with much bigger risks, such as launch failure and manufacturing defects. “We have not had a significant economic loss from an accident” caused by a collision with debris.
Hertzfeld added, though, that despite this limited effect from debris collisions today, now is the time to put into place mechanisms to adjudicate disputes should a future collision take out an operational satellite. “We need before-the-fact binding agreements” regarding how to arbitrate such disputes, something that he said could be incorporated into the licensing process for commercial satellites. “If we don’t plan today, we’re not going to get there, because once something happens, there’s too much at stake for one side or the other to come out with a balanced and logical approach.”
If Gravity has exaggerated the threat posed by orbital debris, can it still be a useful tool in raising awareness of the real dangers of orbital debris and, perhaps, help gain momentum for measures like binding agreements or improved efforts to track orbital debris, as Hertzfeld also suggested? Panelists were skeptical it would have much positive benefit.
“An awful lot of people have asked questions about whether this could happen, and that leads to a teachable moment,” said Michael Simpson, executive director of the Secure World Foundation. “We’re at that point where there’s a teachable moment, but we haven’t really designed the curriculum yet.”
“I do wonder whether it will do more good or harm,” said Hertzfeld. “Is this the most pressing thing that we should be advocating for in space?”
| “We’re at that point where there’s a teachable moment, but we haven’t really designed the curriculum yet,” Simpson said. |
Smith said the movie could create latent interest in the topic that would re-emerge after a real collision, analogous to the effects movies like Deep Impact and Armageddon on concerns about asteroid impacts that reappeared after the Chelyabinsk meteor in February. “Maybe it’s not doing anything today, but the next time there’s an Iridium-Cosmos collision, or something like that, people will think about this movie and wonder if you could really have something as dramatic,” she said, referring to the early 2009 collision between a defunct Russian Cosmos satellite and an operational Iridium satellite. “In that case, the movie probably did more harm than good, because it really could not have happened like that.”
McKnight said that he’s seen a surge of media interest in the orbital debris issue since the movie’s release, “but I don’t think there’s been any substantial difference on the politics of it.” He also said that when Gravity is released on DVD (scheduled for late February), it will include a documentary on orbital debris that features interviews with him and Donald Kessler, who first proposed the orbital debris cascade scenario that bears his name.
McKnight added that, while the movie’s physics might be “average” at best, and its influence on raising awareness of real orbital debris concerns limited, it should not dissuade people from going to see it. “Go see Gravity, okay? It’s awesome,” he said. “But do not base any funding decisions on it. Do not think that you know debris now because you’ve seen the movie Gravity.”