Clearing space debris with lasers
Thirty-five years of poor housekeeping in space plus both deliberate and accidental spacecraft collisions have created several hundred thousand pieces of space debris larger than 1cm in diameter in low Earth orbit (LEO).1 Spacecraft-spacecraft collisions are on track to become the dominant source of debris. This runaway collisional cascading, predicted more than thirty years ago,2 threatens the use of LEO space (see Figure 1). At typical relative velocities of 12km/s, pieces 1cm in diameter can punch a hole in the International Space Station and a 100-gram bolt would be lethal if it hit the crew compartment. Large pieces of debris, such as one-ton spent rocket bodies, need to be removed, because they are a major source of debris when hit. But this is not enough. Small debris must also be removed: the chance that small items will damage one of our valuable space assets is 45 times as high as the chance of large-object collisions because of their much greater number.
Is there one way to do it all? Fifteen years ago, colleagues and I suggested3 laser orbital debris removal (LODR) as best suited for clearing both large and small debris (see Figure 2). LODR uses the impulse generated by laser ablation of the debris surface by a focused, pulsed ground-based laser to change the debris orbit and cause it to re-enter the atmosphere. We use a telescope to focus the laser down to a 30cm diameter circle on a target 1000km away. That's crucial for getting about 75kJ/m2 onto the target during a 5ns pulse (i.e., 15TW/m2), which creates a plasma jet. Even with the telescope, the beam spills over small targets, but it is still effective, slowing small debris items 10cm/s for each pulse. Only a few nanometers of surface are vaporized and the object is not melted or fragmented by the gentle ablation pulse. At a pulse rate of 10Hz and average power 75kW, the laser can slow targets up to 10cm diameter sufficiently in a single overhead pass that they re-enter the earth's atmosphere and burn up, because the amount of slowing required is only about 100m/s.
We have revisited the entire LODR design and found that advances since that article was written make it better than ever. In contrast with 1996, lasers and telescopes are now in service which meet the requirements for LODR. At Lawrence Livermore National Laboratory, a laser produces 500J pulses at 10kW for 10s. With diode rather than flash pumping, lasers now being designed will make the required 10kJ pulses at 10Hz indefinitely.4 Techniques for making light-weight segmented mirrors have already produced the 10m class mirrors we require, and 42m primaries with 984 segments are planned.5 More careful analysis of laser-target interaction and orbital mechanics have allowed us to calculate target shape effects,6 account for inefficiencies in the average induced momentum vector as the object spins, and realize that in many cases it is effective to push directly up on the object as well as against its direction of motion. Figure 3 shows the result of a typical deorbiting calculation. A complete description of the considerations involve in designing a pulsed LODR system is available online.7
Adaptive optics (AO) are necessary to correct for atmospheric scintillation effects. A new technique with the formidable title ‘Brillouin-enhanced four-wave mixing’ may simplify atmospheric phase correction8 by making automatic, passive correction possible.
What are the advantages of lasers for the ODR problem? Operating cost is the first. Energy costs 3¢/MJ on the ground, but any system designed to fly up and grapple or attach something to debris costs $10,000/kg to place in orbit. Agility is the second. The laser operates at the speed of light, and can sit and address any number of objects as they fly overhead (and enough do to make a single site effective), but an orbiting grappler must match its velocity to one object at a time and, as we mentioned earlier, this usually involves a velocity adjustment of 12km/s, limiting any one launch to two or three objects. Any mechanical technique capable of dealing with both large and small objects requires such a huge cross-section as to be impractical. For example, blocks of aerogel9 would need to be 50cm thick and would require a cross-section 174km2 in area10 to deal with all the debris in the same two-year time frame as LODR, and would cost $1trillion to place in orbit.
The main challenges are finding a target, tracking it and ‘look-ahead,’ which means pointing the laser at the spot where the target will be by the time the light gets there. Active acquisition is possible, in total darkness or in daylight, using the ‘pusher laser’ to illuminate the target,11 and the LODR mirror on Earth to collect the scattered light.
It's worth starting now. Even a small LODR system will be able to nudge large 1000kg objects by as much as 30cm/s in a single overhead pass to avoid anticipated collisions. We estimate cost per small object removed at a few thousand dollars, and that for large objects at about $1M each. This compares well with an estimated cost of $27M per large object for attaching deorbiting kits.12
In summary, we have analyzed all the major aspects of LODR, and concluded that it will work, even for large debris objects. An LODR system should provide the lowest cost per object removed among all approaches that have been proposed, and it is the only practical solution that can deal with both small and large debris. With LODR, target access is at the speed of light, redundant, and agile. LODR can handle tumbling objects, while mechanical grapplers cannot. The system has serendipitous uses aside from general debris clearing, such as preventing collisions, increasing the accuracy to which we know the location of debris from 1km to 1m, and controlling where large debris objects impact the Earth's surface.
We are excited about the next step in this program, which will be to do a demonstration of laser momentum transfer from a laser on the ground to a calibrated target in space. Development and construction of the LODR system offers the opportunity for international cooperation. Indeed, such cooperation will be necessary to avoid concerns that it is a weapon system and provide a framework for practical use.
Claude Phipps earned a PhD from Stanford University. Author of 240 publications and managing partner of Photonic Associates, LLC, he has made original contributions to the theory of laser interaction with surfaces. He edited Laser Ablation and its Applications (Springer, 2007) and originated the ground-based laser orbital space removal concept.