Anyone who has seen the 2013 film Gravity has a grasp of the hazards of “space junk,” the literally millions of bits of relict debris from past space missions that still orbit Earth. Could high-energy lasers help clean up this orbital wreckage? New modeling from a team of German scientists suggests that the answer might be yes—but only if society can live with an increase in risk attributable to some individual pieces of the stuff (Opt. Eng., doi: 10.1117/1.OE.56.1.011007).
Not science fiction
Gravity notwithstanding, space debris isn’t science fiction, but a growing problem for near-Earth missions, particularly as an increasing number of countries put new probes into space. The U.S. National Aeronautics and Space Administration (NASA) currently tracks more than half a million items of such orbital debris, the size of a marble or larger, and says that there are “many millions of pieces” of additional debris too small to be tracked.
The debris consists largely of the relics of old, abandoned space missions—decaying satellites or rockets that have slowly broken apart or collided in orbit. Traveling at speeds of 8 to 15 kilometers per second, this orbital flotsam can indeed, if not avoided, rip through vehicles such as the International Space Station and other manned and unmanned probes. And avoiding the proliferating debris has become an increasing problem. Indeed, there’s a real worry that, if the mess isn’t cleaned up, new low-Earth-orbital missions, ranging from weather satellites to orbiting telescopes, could become impossible to launch.
A laser solution?
For years, high-energy lasers have been floated as one possible way to sweep out Earth’s orbital neighborhood. The idea is that laser-induced ablation of material on the orbiting junk leads to a recoil and momentum transfer that can nudge the debris to a lower orbit and, eventually, cause it to burn up in Earth’s atmosphere. Analytical modeling has suggested that such laser nudging, coupled with extremely high-resolution optical detection to pick out orbiting objects, might well constitute part of the solution to space junk.
The problem with this earlier modeling, according to researchers Stefan Scharring, Jascha Wilken and Hans-Albert Eckel of the German Aerospace Center in Stuttgart, is that these studies have largely been premised on analog models using simple hypothetical debris shapes, such as spheres, cubes, or flat plates, that are optimally oriented with respect to the laser beam. Real space junk is much more irregular, heterogeneous and randomly oriented, putting its real-world behavior completely out of the range of analog modeling.
It’s complicated
To get a more rigorous answer to how space debris might respond to a high-energy laser push, the German team developed a sophisticated numerical model to the simulate momentum coupling in detail. The model and accompanying experiments, including a Monte Carlo simulation on the predictability of outcomes from zapping space debris with lasers, was designed to take into account irregular shapes as well as variations in orientation and in laser fluence across the target surface.
The results suggest that the effects of lasers on space debris are, to say the least, highly unpredictable. In particular, the orientation of the object with respect to the laser made a big difference in the change in the object’s trajectory, introducing lateral momentum components that were very difficult to call in advance. And there were substantial variations in the actual amount of momentum coupling—and, thus, potential debris-removal efficiency—depending on object shape and orientation relative to the beam.
Still a place for a “laser broom”
Notwithstanding these complications, the work did suggest, according to the researchers, that laser-based removal of space debris has “great potential.” The Monte Carlo simulations, involving half a million different conditions, did show a tendency for laser nudging to be broadly effective in a range of object shapes and orientations, albeit with a healthy dose of uncertainty.
What that means, the study concludes, is that high-energy laser ablation can still represent an important part of eliminating the space-junk problem—but only if the laser solution isn’t viewed as a deterministic one, and if society is prepared to accept the idea that, for some pieces of space junk, zapping with lasers could make things worse. “A probabilistic assessment of the concept is necessary,” they write, “taking into account … the risk to transiently push space debris into an orbit that is even more dangerous than before.”
Yet in the long haul, they think that’s a risk worth taking. Drawing a homely analogy, the scientists write that “even if the trajectory of dust particles is unpredictable when sweeping the floor, the space is promised to be cleaned in the long run.”