In one of last year’s most popular movies, Gravity, moviegoers worldwide experienced cinematic beauty and gut-wrenching drama while rooting for Sandra Bullock to survive a dangerous encounter with space junk.
Unfortunately, viewers may have also come away from this film with two other impressions: First, the risk of a catastrophic collision in space is high. Second, the main culprit is irresponsible behavior. (In this fictional story, Russia initiates the problem by shooting a missile at a defunct satellite.) The fact is, as people talk more of space travel, we are not doomed to pervasive space junk collisions. However, what determines the danger level is the behavior of people on the ground.
Indeed, there have been several recent collisions in space, including a 26,000-mph accident in 2009 between an American communications satellite and an inactive Russian satellite. However, complex 3-D computer simulation models, which track every object larger than a softball in low Earth orbit, have revealed that the likelihood of a satellite experiencing a catastrophic collision with orbital debris during its operational lifetime is very small during the next 200 years.
These computer models, developed at NASA and elsewhere, run for only 200 years because of their immense complexity. They predict the likelihood of collisions at less than 1 in 1,000 in the most congested region of space, which is 900- to 1,000-kilometers altitude. This risk is negligible compared to the risks of mission-impacting failure due to electromechanical problems. Those occur at a rate of 10-20%.
Limiting Space Junk – A Tipping Point
Importantly, relatively inexpensive mitigation measures are available to limit the amount of space debris that does clutter up orbit paths. Many countries adhere to an international 25-year rule for post-mission disposal, where sufficient fuel is left in the tank at the end of a mission to maneuver the satellite to an orbit from which it will decay within 25 years. In addition, some satellites are capable of avoidance maneuvers in space in the event that a collision is imminent.
We developed a system of mathematical equations that is capable of predicting collisions for thousands of years into the future. We take an environmental sustainability approach and measure the maximum risk over all future time of a catastrophic collision to a satellite over its operational lifetime. Our model suggests that the greatest risk occurs in about 1,500 years. As in a disease epidemic, the chain reaction eventually burns itself out for lack of targets.
We find that this risk can remain manageable — less than 1 in 1,000 — if deorbit compliance to the 25-year rule is very high. However, we appear to be near a tipping point: Our model predicts that the maximum future risk is 1 in 1,000 if compliance is 98%, but increases to 1 in 100 if compliance is only 85%.
Our computations assume no irresponsible behavior, which brings us to the second impression of moviegoers. In fact, Russia has not recently tested an anti-satellite missile, even though a legacy of the Cold War is that the majority of large objects in space are old satellite parts from the former Soviet Union and the United States. However, China’s 2007 anti-satellite test created an enormous amount of debris in low Earth orbit, a piece of which destroyed a Russian nano-satellite last year. In 2008, the United States performed a similar operation, but the target was much lower and so the debris deorbited quickly.
Because the risk of a catastrophic collision increases by roughly 50% over the next 200 years, some have called for active remediation of the space environment, which means removing large inactive objects from space. Several technological approaches have been suggested for how to do this. Proposals include nets connected to 6-mile-long wire tethers to drag debris down, and balloons or robot-installed engines or lasers to alter objects’ orbit. But these are early days and no approach has been shown to be reliable and cost effective.
Immediate Standards Compliance Essential
In summary, current debris levels are sustainable without active debris removal only if 100% compliance with current voluntary standards is immediately initiated. That is, countries must follow end-of-mission maneuvers to deorbit, expend excess fuel that can cause explosions, and not shoot objects in space.
This may seem simple, but as an example, less than 90% of upper-stage rockets have been deorbited from low Earth orbit in each major space-faring nation in the last decade. Hence, research into active debris removal is needed to address uncertainties in noncompliance and possible future space wars. Research into developing protective shields to ward off junk also is important to address sub-catastrophic, more frequent collisions that routinely degrade solar panels and payloads (and that are not captured by models such as ours that ignore debris less than 10 centimeters in diameter).
Finally, it is essential to develop policies to assess noncompliance penalties and to share highest-quality-available orbital data among space-faring nations and corporations to anticipate avoidable collisions.
Cost of Space Access Could Increase
We are at a critical time when responsibility in space will determine the cost of access to space for generations to come. If we don’t very soon initiate and maintain relatively cheap and mundane measures, we will be forced to deploy much more expensive ones and thereby increase the cost of access to space. In addition to providing a safe environment for the future Sandra Bullocks in space, we need access to space for security and to maintain the benefits of satellites that we all take for granted.
Lawrence Wein is the Jeffrey S. Skoll Professor of Management Science at Stanford Graduate School of Business, and Andrew Bradley is a postdoctoral scholar in the Geophysics Department at Stanford School of Earth Sciences.