MadSci Network: Physics

Re: how do lasers move things without burning them?

Date: Wed Aug 11 07:57:52 2004
Posted By: Guy Beadie, Staff, Optical sciences, Naval Research Lab
Area of science: Physics
ID: 1090944034.Ph

Hi, David.

  Good question.  Seems odd, doesn’t it, that you could actually move 
things with light?  Light is a form of energy, however, so if you can 
convert the light energy into kinetic energy (energy due to motion) then 
it makes sense.  I’ll mention how it can be done, but I’ll also point out 
problems people have encountered – it’s a good way to extend an idea into 
how one actually applies it.

  Looking through the web for information, I’ve come across two different 
ways that people use light to generate motion of a vehicle.  They are each 
based on using the light energy to ignite a propellant, which provides 
thrust to the vehicle.  In other words, the ignition of the propellant 
generates a bunch of fast-moving molecules.  By setting up the explosion 
to occur right next to a shield on the vehicle (capable of withstanding 
the nearby explosion), all the molecules which “bump into” the shield push 
the vehicle away from the blast.

  The two different methods refer to two distinctly different propellants –
 in one method the propellant is actually the air around the vehicle, 
while in the other the propellant is a layer of combustible material 
coating the shield itself.

  I find the first one interesting, if only from the standpoint that it’s 
neat to realize people can actually blow up enough air with light to move 
a real object.  It’s done by making the shield into a highly-reflective 
mirror – actually a bunch of small spherical mirrors.  Light energy can 
propagate freely through the atmosphere if it’s in a big enough beam.  For 
example, think of all the solar energy passing through the air on a sunny 
day.  If, however, you take a big magnifying glass outside, it’s easy to 
see the effect of the concentrated solar light – it’ll burn most 
anything.  That’s the effect of the spherical mirrors on the launch 
vehicle.  When a large laser beam from the ground hits the mirrors, the 
energy is focused down into a bunch of very tiny spots near the mirrors.  
The light is so intense in these spots that it actually ignites the air, 
creating the thrust needed to propel the vehicle.

  Using air as a propellant also simplifies the launch process: you get to 
use free fuel anywhere you go … almost.  I say ‘almost’ because it’s 
pretty clear you can’t use this mechanism if you’re interested in putting 
something up into the vacuum of space.  Without any air, well, you’re not 
going to get very far.

  The second method involving a propellant coated onto the shield is, in 
my admittedly non-expert opinion, probably more practical.  I say this 
because 1) with tailored propellants you can certainly find a material 
that will combust at lower light energies (reducing the size of the laser 
you need to drive the propulsion), and 2) every molecule which blows off 
the shield will contribute to thrust, while only half the molecules from 
an air explosion will start off towards the shield to contribute to the 
vehicle’s motion.

  Although at this level the idea is a pretty simple one to communicate, 
the practical engineering involved with deploying a real vehicle is very 
complex.  That’s because of several factors including (but not limited 
to):  a) the laser power needed per pound of vehicle, b) keeping the laser 
beam focused and tracked onto the vehicle, and c) steering the vehicle.

(a) Laser energy.  As you might expect, the transfer of optical to kinetic 
energy isn’t all that efficient.  In other words, it takes a lot of laser 
energy to impart even a little kinetic energy.  For example, in upcoming 
tests announced at

the program participants mention that they hope to send 50 gram objects 
300 meters into the air with a 10 kilowatt laser.  NASA and the Air Force 
speculate that with a megawatt-class laser (1 million watts) they could 
send about a kilogram weight into earth orbit.  Now, peak powers of a 
megawatt are no problem for lasers on very short time scales, but a 
sustained megawatt averaged over minutes is.  This restriction is serious 
enough that it’s unlikely anyone will field such a laser any time soon.  
Conclusion: a kilogram won’t be laser-sent into orbit anytime in the next 
decade or two unless they find a way to do it with less-powerful lasers.  
[Tech. note: the previous statement applies if (as I suspect) the megawatt 
will need to be contained in a high-quality spatial mode.  
Megawatt “flashlights” could probably be fielded in the near future, but 
their beam focusing performance would be so poor that I’m guessing they’d 
be inapplicable to this problem.]

(b) Laser focusing & tracking.  Not only will a high-power laser system 
need to keep pointing towards the vehicle as it gets accelerated through 
its path, it will need to continually adjust its focal range to keep as 
small a spot as possible at the target.  At kilometer ranges, atmospheric 
turbulence and available beam-steering optics limit the minimum size of a 
laser spot to about a meter across or so.  At tens or hundreds of 
kilometers out, the laser spot gets even bigger.  These spot sizes are 
only achieved, however, if the transmission optics are continually 
adjusted throughout the vehicle’s flight.  This adjustment must also 
account for wind and other atmospheric conditions in real time.  As 
difficult as all this sounds, however, the importance of these problems in 
other areas of science has spurred solutions.  It’s merely a matter of 
incorporating these tremendously complex (and expensive!) elements into 
the launch process.

(c) Vehicle steering & stability.  Here’s yet another problem – with the 
laser giving you a big dumb push from a single spot, how do you manage to 
steer the craft to where you want it to go?  As an example, the earliest 
flight demos I saw (on a NOVA program or some such years ago) showed great 
results when the vehicle was guided by a wire.  As soon as the vehicle 
cleared the wire, however, it tipped to one side or another off the laser 
beam and promptly fell to the ground.  Do not judge the viability of the 
idea by this anecdote: the early tests were to measure conversion 
efficiency not steering.  I bring it up only to spur the thought of, “Just 
how would you steer one of those things??”

Other references:

For a somewhat different take on the laser propulsion schemes:

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