Interesting new ideas about asteroid deflection are coming out of the University of Strathclyde (Glasgow), involving the use of lasers in coordinated satellite swarms to change an asteroid’s trajectory. This is useful work in its own right, but I also want to mention it in terms of a broader topic we often return to: How to deal with the harmful effects of dust and interstellar gas on a fast-moving starship. That’s a discussion that has played out many a time over the past eight years in these pages, but it’s as lively a topic as ever, and one on which we’re going to need a lot more information before true interstellar missions can take place.
Lasers and the Asteroid
But let’s set the stage at Strathclyde for a moment. The idea here is to send small satellites capable of formation flying with the asteroid, all of them firing their lasers at close range. The university’s Massimiliano Vasile, who is leading this work, says that the challenge of lasers in space is to combine high power, high efficiency and high beam quality simultaneously. He adds:
“The additional problem with asteroid deflection is that when the laser begins to break down the surface of the object, the plume of gas and debris impinges the spacecraft and contaminates the laser. However, our laboratory tests have proven that the level of contamination is less than expected and the laser could continue to function for longer than anticipated.”
Vasile believes using a flotilla of small but agile spacecraft, each with a highly efficient laser, is more feasible than trying to deflect an asteroid with a single, large spacecraft carrying a much larger laser. One benefit is that the system is scalable — add as many spacecraft as needed for the job at hand. The other is that you have the redundancy afforded by multiple laser platforms. The Strathclyde work is also investigating whether a similar system could be used to remove space debris by de-orbiting problematic objects to avoid potential collisions.
Erosion Shields on the Starship?
If lasers can be used to alter an asteroid’s trajectory, we need to consider their uses in clearing out the space ahead of futuristic space probes. That the interstellar medium itself was going to be a problem became apparent as researchers began to study starship deceleration concepts in the early 1970s. Get your vehicle moving in the range of 0.3 c and any grain of carbonaceous dust a tenth of a micron in diameter it encounters carries a relative kinetic energy of 37,500,000 GeV, according to Dana Andrews (Andrews Space) in a 2004 paper. How that kinetic energy is dealt with is clearly a major issue.
By the late 1970s, aluminum and then graphite had been considered as possible erosion shields, with the preference going to graphite, but in 1978 Anthony Martin reviewed the literature and suggested a beryllium payload shield be deployed on the Project Daedalus probe, which would be moving at .12 c. It would be quite a large object, some 9 millimeters thick and 32 meters in radius, and even it didn’t completely solve the problem, for Daedalus would, upon arrival, be moving into a still denser gas and dust environment around Barnard’s Star. Daedalus designer Alan Bond suggested additional shielding in the form of a cloud of dust deployed from the probe, which would vaporize larger particles before they could damage the vehicle.
Image: Diagram of the Project Daedalus probe, developed by the British Interplanetary Society in the 1970s. Note the beryllium shield at upper left. Credit: Adrian Mann.
Clearing Out the Path
We’re still not through, though. What about particles larger than dust grains, up to hailstones in size? We are now talking about collisions that would be catastrophic, and must turn from passive to active measures to tackle the problem. Gregory Matloff and Eugene Mallove have suggested using a light or X-ray laser or a neutral particle beam firing ahead of the ship to deflect any objects detected in its forward-pointing radar. The Project Icarus team has looked at creating a bumper out of graphene, as discussed in this blog entry, and coupling it with a laser defense:
What I’m interested in for shielding is making a large, low-mass “bumper” which cosmic sand-grains run into before hitting the craft. After passing through several layers of graphene the offending mass is totally ionized and forms a high-energy spray of particles, but particles that can now be deflected by the vehicle’s cosmic-ray defences (akin to the mag-sail, but smaller with a higher current) and safely diverted away from sensitive parts.
The notion seems an adaptation of Conley Powell’s 1975 work on shields that move ahead of the ship, trapping ionized material on impact within a magnetic field. The earlier Daedalus researchers found that Powell’s ideas resulted in less erosion than other methods then being studied. This is an interesting shield, one placed perhaps 100 kilometers ahead of the spacecraft. Moreover, it is not passive but can signal the vehicle when grains have passed through it without being ionized:
This causes a signal to be sent back to the vehicle which then activates its final layer of defence, high-powered lasers. In microseconds the lasers either utterly ionize the target or give it a sideways nudge via ablation – blowing it violently to the side via a blast of plasma. Such an active tracking bumper would need to be further away than 100 km to give the laser defence time to react, though 1/600th of a second can be a lot of computer cycles for a fast artificial intelligence. The lasers might use advanced metamaterials to focus the beam onto a speck at ~100 km, without needing to physically turn the laser itself in such a split-second. Highly directional, high-powered microwave phased arrays exist which already do so purely electronically and an optical phased-array isn’t a stretch beyond current technology.
All of which takes me back to the University of Strathclyde work on laser deflection, and makes me wonder whether laser technologies first deployed against asteroids in our Solar System may one day be used to protect our interstellar voyagers.
Anthony Martin’s paper is “Bombardment by Interstellar Material and Its Effects on the Vehicle,” Project Daedalus Final Report (Journal of the British Interplanetary Society, 1978): S116–S121. Alan Bond discusses in-system shielding in “Project Daedalus: Target System Encounter Protection,” S123–S125 in the same publication. The Dana Andrews paper is “Things to Do While Coasting Through Interstellar Space,” AIAA-2004-3706, 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Fort Lauderdale, Florida, July 11-14, 2004.