Giving Asteroids a Laser Push

by Paul Gilster on February 28, 2007

At the University of Alabama at Huntsville, a team of scientists and engineers is looking into the possibility of identifying and deflecting Earth-endangering asteroids with lasers. Blake Anderton, an engineer at Raytheon Corp., wrote his thesis on the topic. From a UAH news release:

Anderton said his thesis discusses “a way to look at asteroids at maximum range, which means early detection.” According to his calculations, an asteroid could be characterized up to 1 AU away (1.5 x 10 to the 11 meters). Arecibo and other radar observatories can only detect objects up to 0.1 AU away, so in theory a laser would represent a vast improvement over radar.

The laser the group is working on may one day evolve into a system with asteroid-nudging capabilities. UAH’s Richard Fork, who has compiled forty years of experience with lasers, says the work goes back to research he and others performed in the 1980s at AT&T Bell Laboratories. Remote sensing is a short-term goal, but Fork says “My vision is that this system is the progenitor of the laser that could characterize and deflect asteroids.” And that would be a helpful addition to our toolkit indeed.

{ 6 comments }

Robin Goodfellow March 1, 2007 at 0:05

Laser detection maybe, but I don’t think the math adds up for laser deflection. Not in any reasonable fashion anyway. The lasers we have today just don’t have the power for this sort of thing. The problem is that the targets we really care about diverting are the largest objects with the shortest times to impact, while a laser system would work best at the opposite end of the spectrum, smaller objects with longer times to impact. Consider a 100m diameter asteroid headed for an impact with the Earth in, oh, 20 years. In order to diver it we would have to apply a continuous acceleration of around 3e-11 m/s^2. That may not seem like much but this is an object that weighs more than a million metric tonnes, so this translates into a necessary force of around 30 milliNewtons. Using photons as the propulsive force that works out to a required power of over 9 megawatts. Continuously. Every second of every day, for 20 years. It’s questionable whether such a huge usage of energy (in electric power terms, this would cost around $150 million with 100% conversion efficiency to laser light) would be a justifiable expenditure of resources, when the cost of a launch vehicle and spacecraft to rendezvous with the impactor and do something locally might be much less. It would work better for smaller objects and impactors with longer lead times, but so would many other lower cost methods (such as modifying the impactors albedo slightly through various methods). I don’t think this is a very feasible mechanism for asteroid deflection.

Especially when we already have all of the technology to deflect even very large asteroids with relatively short lead times. And merely need to integrate it into workable systems. Namely, using the x-ray flux of nuclear explosives to ablate an area on the surface of an asteroid in order to create enough thrust to push it off its impact course. A 100 kT nuclear weapon weighing a few hundred kg should be able to handle this scenario with a single nudge, for example.

Chris Wren March 1, 2007 at 4:29

Depends on the asteroid’s composition too. Solid metallic ores might respond to the ablation/thrust technique, but a lot of asteroids might turn out to be the “gravel beanbag” or fluffball variety, in which case we’re screwed, and the albedo method would probably be the best bet.

The first thing we really need to do ( apart from continuing to look for potential threats) is to get a decent survey of asteroid compositions. Wouldn’t hurt to schedule a landing on Apophus, for starters. It’s not much of a threat in 2036, but I believe it comes back in ’49 or so, possibly with an even closer trajectory.

Administrator March 1, 2007 at 8:58

Robin’s got a solid point here: “…the targets we really care about diverting are the largest objects with the shortest times to impact, while a laser system would work best at the opposite end of the spectrum, smaller objects with longer times to impact.” I’m with Chris on getting some asteroid composition surveys, and a landing on one of the nearby ones seems like a workable and needed mission.

Robin Goodfellow March 1, 2007 at 10:42

Ablation should work well enough regardless of composition, even on rubble piles. The force would be spread out over a large area and would amount to only a slight nudge overall.

Also, I should point out that in the long term there’s no way to avoid the need for a fast acting, large impactor diversion system. We can spend as much resources as we want cataloging the inner Solar System’s asteroid threat, but we will never be able to completely catalog all the objects in the Oort cloud. Even with futuristic technologies, the amount of time between when we will be able to spot an incoming long period comet and when it arrives in the inner Solar System (perhaps exactly where Earth will be) can only be on the scale of a few decades. No known technology will be able to eliminate the possibility of the nightmare scenario of a large (multi km diameter) comet headed towards Earth, spotted only 2 or 3 decades from impact. Realistically, in such scenarios we would need very high performance propulsion systems (e.g. NSWR, Orion, Laser pumped light sails, etc.) to rendezvous with the comet, combined with a very stout system for nudging the comet off of an impact course (there may be tricky ways to take advantage of the comet’s natural outgassing properties to do most of the work for us, though it might not be wise to rely on the composition of the impactor).

Adam March 1, 2007 at 13:51

A very hot gas-core reactor might be able to use the asteroid’s material as reaction mass. I wonder how much of a nudge is needed with 20-30 years lead-time?

ljk March 6, 2007 at 0:15

Astrophysics, abstract
astro-ph/0703085

From: Edward L. Wright [view email]

Date: Mon, 5 Mar 2007 19:29:24 GMT (163kb)

Comparing the NEATM with a Rotating, Cratered Thermophysical Asteroid Model

Authors: Edward L. Wright (UCLA)

Comments: 8 pages LaTex with 13 Postscript fiugres

A cratered asteroid acts somewhat like a retroflector, sending light and infrared radiation back toward the Sun, while thermal inertia in a rotating asteroid causes the infrared radiation to peak over the “afternoon” part. In this paper a rotating, cratered asteroid model is described, and used to generate infrared fluxes which are then interpreted using the Near Earth Asteroid Thermal Model (NEATM). Even though the rotating, cratered model depends on three parameters not available to the NEATM (the dimensionless thermal inertia parameter and pole orientation), the NEATM gives diameter estimates that are accurate to 10 percent RMS for phase angles less than 60 degrees. For larger phase angles, such as back-lit asteroids, the infrared flux depends strongly on these unknown parameter, so the diameter errors are larger; and real world complications such as non-spherical shapes have been ignored.

http://arxiv.org/abs/astro-ph/0703085

Comments on this entry are closed.