Since the dawn of the space age, more than 20,000 objects larger than a softball have accumulated in Earth’s orbit. About 1000 of those objects are spacecraft that carry active payloads, serving many valuable missions for mankind. But the rest could best be called junk, the by-product of thousands of launches and routine spacecraft deployments, nearly 200 explosions, and several collisions. And this junk poses a serious problem.
Many years ago, early orbital debris researchers predicted that parts of Earth’s orbit could eventually become so crowded that accidental collisions would fuel a self-reinforcing boom in the hazardous debris population—even if we put a stop to future launches.
That runaway debris generation scenario, often called the Kessler syndrome, may seem far off. But in fact, the sheer density of derelict objects in orbit has already exceeded what many consider to be the mathematical point of no return.
In some of the most congested regions of low earth orbit, this point was actually passed more than 10 years ago, although the onslaught of chain-reaction collisions will likely take decades to pick up steam. As a result, the threat of this potentially catastrophic domino effect has remained largely invisible. We’ve seen only one bellwether: the violent collision in 2009 of an active Iridium communications satellite with a derelict Russian payload called Cosmos 2251.
That one accident created thousands of fragments big enough to be seen by ground-based radar antennas, as well as tens of thousands other pieces of debris that could damage satellites but are too small to detect and avoid. You might think such an unexpected and dramatic event would have spurred the aerospace community into action. But while the event did create some temporary interest and a slew of conferences and policy discussions, it didn’t result in meaningful change to the way orbital debris is handled.
How do we change our ways? A year ago, at NASA’s request, we, along with 11 other experts in space operation, policy, law, and risk management finished an assessment of the agency’s orbital debris and micrometeoroids programs for the National Research Council. NASA has led the way in many of the measurement, testing, analysis, and operations activities needed to address the problem. But we found the program is struggling to keep up with the expanding threat. And in the year since our report was published, little seems to have changed. NASA continues to fight a growing hazard with what has been, at best, a level budget.
DANGER ZONES: This graph shows the number of tracked objects (blue) and their total mass (red) in orbit as a function of distance above Earth’s surface (both the objects and their mass are counted in 20-kilometer-wide chunks). Those altitudes with the largest number of objects pose the greatest risk to satellites today. The overall mass determines what the riskiest zones will be in the future. That’s because the more massive colliding satellites are, the more destructive debris they’ll create. As this figure indicates, the space around an altitude of 780 km is currently the most hazardous. In the future, altitudes around 860 km and just below 1000 km may pose even more of a risk. Since Earth’s atmosphere is thinner at those higher altitudes, it will take longer for debris there to be dragged out of orbit. Click image to enlarge.
What NASA—and the world—needs is a clear mandate. Under NASA’s leadership, there has been a concerted effort to minimize mission-related debris. This is most commonly done by purging abandoned rocket bodies of excess fuel and limiting orbital lifetimes to 25 years after mission termination, by moving retired satellites to lower orbits to accelerate reentry. Over the years, satellite owners have also practiced collision-avoidance maneuvers, physically moving the orbits of active satellites to give large, detectable pieces of debris a wide berth.
But long-term models tell us there are really only two ways to reduce the threat of collisions in orbit. One is to actively remove debris from densely populated regions of space, drawing the large object population back down below the tipping point. The other is to perfect just-in-time collision avoidance for objects that can’t change direction under their own power, executing maneuvers (for example, by puffing gas into the path of a piece of debris) that could prevent collisions between two derelict objects.
At the moment, we don’t have the technology needed to pursue either of these options, and it’s still unclear what national or international agencies should be responsible for pursuing them. Alarmingly, it may take hundreds of billions of dollars or more to clean up the debris environment over the next century in order to ensure reliable space operations. There isn’t even a clear legal statute that precisely defines ownership and liability issues for derelict object removal operations, and any international effort will likely struggle against trade and export restrictions that can limit technology development and sharing.
Of course, these problems aren’t insurmountable. If we had to guess, we’d say the main reason why little action was taken after the 2009 collision is a lack of visibility. There is a long lag between the onset of the Kessler syndrome and the creation of a truly dangerous orbital environment. It will likely take decades for the products of that collision and future ones to begin to noticeably affect operational satellites (through, say, shorter operational lifetimes or more frequent collision-avoidance maneuvers).
In the end, we may find that the impetus for debris removal activities will be financial. For instance, the current on-orbit annual insurance rate is 1.5 percent. Underwriters currently consider debris to be only a fraction of that risk. However, it is estimated that there is already a sliver of low earth orbit where the lethal collision hazard to active payloads already exceeds 1.5 percent per year all by itself. That sliver could easily become a wide swath within a few decades.
The unfortunate fact is that if we wait too long, the cost of the remediating the near-Earth environment will balloon. An ounce of prevention is worth a pound of cure. The more we do to prevent large, trackable derelict objects from colliding in the near term, the less we’ll struggle to sweep out countless less detectable but still quite dangerous debris fragments in the long term. In the immediate future, the best thing we can do is make sure that all future launches adhere to debris mitigation guidelines and use the best technology available.
As a space-faring species, we’ve accomplished much since Sputnik. But without a substantial investment, both financially and intellectually, orbital debris is destined to become a serious problem for many Earth-orbiting spacecraft where the cost of cleanup may cripple the aerospace industry. The longer we wait, the more expensive and difficult it will be to turn the near-Earth environment around.
Darren McKnight is the technical director at Integrity Applications, which specializes in providing technical services related to such issues as space-system survivability for the U.S. government.
Donald Kessler, who is now retired, served as the chief scientist for NASA’s orbital debris program for 17 years.