Yesterday the History Channel ran a program entitled “Meteors: Fire in the Sky”. In it one of the ideas set forth in keeping an NEO (near earth object) from striking the earth was to use a laser to push the NEO slightly off course.
Can a laser beam actually move an abject? Wouldn’t it heat it up or simply cut through it all together?
I believe the idea is that the laser would vaporize the surface material and the pressure of this vapor would push on the rest of the body. A chemical rocket is basically just a method for creating a lot of high pressure gas in a confined space, and this laser technique would just try to create a hole in the rock/ice/cheese that acted like a rocket engine with an external heat source.
Lasers can move objects, in at least two different ways
1.) Light pressure. Light can exert pressure on objects. It’s usually pretty small over a given area, so typically you need large areas. That’s why prposed “light sails” using solar light pressure are so huge and so light. Lasers exert much more light pressure because they’re concentrated and unidirectional. Robert L. Forward suggested using lasers to propel light sails to the stars (he suggested this as a serious venture, I note. The idea has been used in science fiction already. See, for instance, Niven and Pournelle’s “The Mote in God’s Eye”.)
2.) Ablation reaction. If you shoot a high-power laser at something, especially a short high-power pulse, you rapidly heat up part of the surface and then heat up the resulting vapor. It blows off and expands. By Newton’s Laws, you get motion of the thing you blew it off of in the other direction. Lather, rinse, and repeat (Keep shooting pulses at the object) and you can build up quite a lot of momentum. This is the idea behind “Laser Propulsion”, only there you build the thing you’re aiming at to maximize the momentum you can get from your laser pulses. Yours truly speaks from experience – I worked on laser propulsion for a couple of years. The light pressure you can induce on a dime-sized target in a single brief pulse isn’t really huge. But the ablation effect will send that sucker flying. Do this often enough and you can loft a payload into orbit.
Ground to orbit is a big task for such ablation propulsion. Changing orbital paths out in space, where there’s nothing to scatter or attenuate your beam, you have clean shots over huge distances, and youdon’t have to fight gravity so aggressively is even easier. Laser Propulsion can be used more easily to changer orbits than to get things off the ground. Ity can also be used, as some have suggested, to clean up orbital “garbage” that threatens orbiting equipment. Or it can be used to redirect things you don’t want to come crashing down on you. I note that blowing bits off the surface is easy. Blowing up big chunks of matter is hard. “Armageddon” was right about at least that much.
SF writer Dean Ing and Rensselear Professor Leik Myrabo wrote a book on this circa 1985. Ing went on to fictionalize it in his novel “The Big Lifters”. See also Pournelle’s “High Justice” and Michael Kube-mcDowell’s “The Quiet Pools”, and Forward’s “Indistinguishable from Magic”
In rocket propulsion terms, laser propulsion (either by light pressure or ablation) has a very high specific impulse, I[sub]sp[/sub], which is a measure of its thrust per unit mass of propellant (especially with a light sail, where the propellant are the impinging photons) and the standard measure for the efficacy of a motor, but a very low overall thrust. Such engines are good in regions where gravitational influences are low; by maintaining a low thrust over a long timespan, they can achieve high eventual velocities. The biggest advantage is that they need not carry a power source, and in the case of a solar sail, a propellant. This reduces weight and complexity and reduces or eliminates limitations based upon carting around fuel/propellant mass. (Most of the mass of a chemical rocket is in lifting the weight of propellant until it is used.)
They are not, however, well suited to lifting against strong gravity fields or making rapid changes in velocity as they require massive energy throughput that is well beyond our technical capabilities; for this, we need propellant sources with large, tightly directed momentum. There are physical limitations to directing laser output through an atmosphere, too. The air and suspended particles tend to absorb the energy and distort the atmosphere, further disrupting and absorbing the beam. The more energy you pump the worse the effect, called thermal blooming, becomes. This is one of the big unresolved technical issues behind ground-based missile defense lasers.
Ablating away an incoming meteor and using the momentum of the vaporized material to push the object out of the interception path would be highly inefficient; assuming the target area on the meteor to be a flat surface the ablated material will spray in a hemispherical direction with only half of the momentum being normal to the surface, and no nozzle to accelerate and capture radial (to thrust) momentum. You’d also want to fire laterally against the meteor; shooting at it head-on isn’t going to accomplish much, unless you can significantly slow it down. On the other hand, you don’t have to send anybody out to intercept it and try to mount some kind of rocket or blast charge to it, so overall, it might cost less energy, and certainly less risk and complexity, to fire at it from afar.
Stranger
There’s no friction in space to overcome, so you can change an orbit as much as you want in whatever direction you want using whatever method you want… Provided you’re patient enough. If we discover a meteoroid that’s a month away from impact, we’re not going to be able to do much of anything beyond duck and cover (actually more along the lines of stockpiling food). If we find one that’s twenty years away from impact, we have considerably more options, which might include laser deflection (even perhaps including the time spent to construct the laser). So it’s with good reason that most of the effort currently put towards asteroid defense is based on attempting early detection. Our capabilities in that regard are still woefully inadequate, though: Some recent near-misses have actually only been detected after their closest approach to Earth.
So, say you had a spaceship with a big-ass chunk of concrete at the bottom. If you had a long boom with a laser at the end, pointing at the concrete, could you propel yourself along? Would this transgress any Newtonian mechanics?
No. But concrete wouldn’t make a very effective propellant. You’d want to use something readily absorbs the laser radiation and quickly reacts by ejecting heavy, fast-moving exhaust. Some kind of copper-, aluminum-, or magnesium-doped propellant would work best.
How about this; instead of a laser, you have a catapult or rail gun at the base of the capsule, which fires a charged mass up into an electromagnetic field at the base of the capsule. The capsule, which is no larger than the lifesystem and the field generator, is propelled by the transferred momentum. Of course, not only have to push the charge mass up to the capsule but provide enough energy to allow it to keep transferring momentum, though you can presumably absorb most of the residual momentum on the way back down. Og help you, though, if the capsule drifts off normal (as it will inevitably do as it assends to orbital altitudes) and you don’t have any way to achieve orbital velocities…but you don’t have to carry the mass of your propellant with you, which is a big advantage.
Hey, just 'cause it’s unworkable doesn’t mean we shouldn’t spend billions of dollars in program development.
Stranger
That is, no, it wouldn’t violate any Newtonian mechanics. It’s about as workable as a gnat-powered airliner, but it doesn’t violate any physical principles.
Stranger
What if there were a lot of gnats?
:dubious:
Yeah, didn’t think so.