Using Lasers to Get Moving

In the chain of crazy ideas of how to get to space and how to get from one planet to another, there is an idea to use lasers. Actually, there are a couple of ideas on how to do this.  This is the first of a two-parter where I talk about this idea.  The first part will cover one project that has actually gotten off the ground (literally) and an idea on getting to Mars, while the second post will look at interstellar travel with lasers.

The first idea on using lasers makes a tiny bit of sense.  It is called Lightcraft (get it – light and craft?).  The general idea with this is that you have an object that has a very specific shape on the bottom side. Then you shoot a laser at it and the shaped bottom focuses the laser so that it superheats the air that is touching the object.  The air then is propelled away from the object, resulting in a net thrust that is towards the top of the object.

Interesting, eh?  They have actually tested this with some very shiny objects that are about the size of a fist and are pretty light. Here is a picture:


That is actually almost real size, too! These little things have flown about 75 meters into the air.  That is not, um, unimpressive, I guess. There are several problems with this technology, since it is hard to keep the Lightcraft pointed in the right direction and keep the laser pointed directly at it, and all sorts of other things. My guess is that they have not had the right public relations people and the large amount of funding that is needed to take a project like this from the tiny prototype stage to anything of real size.

Recently, another team has also been working on using lasers to move things about in the solar system.  This idea with this team is to use very high powered lasers in a similar way as we would use the sun and a solar sail.

A quick aside on solar sails (boy, I really need to write a post about solar sails):  When light hits an object, it actually imparts a super, super small amount of momentum.  When you feel the sun beating down on you on a very bright days, it is totally because it is actually beating down on you. Well, technically it is, but in reality, the amount of force on you by sunlight is less than a paperclip put on you.  Like, way less.  But, if you were out in space, and you had a huge reflective “sail”, the light would shine on it and impart a very small force – something like a pound for a sail that is about 1 km². But, imagine if you could turn the brightness of the sun up by a factor of 100. Or 1,000. Or 1,000,000! Then you could get some real force to act on your spaceship!

So, the general idea with using lasers is that you could have a reflective surface on your ship that you would point a really really really intense laser at.  This would impart a large force on the ship and accelerate it. The beauty of this plan is that the lasers all would need to be powered here on Earth, so we could generate it using a nuclear power plant or hydroelectric or even good old-fashioned coal.  The ship could be very small, since it wouldn’t need a lot of fuel to accelerate it, since that power is coming from Earth.

In the article that I linked to above, the researchers say that they could envision getting to Mars in 3 days using this type of technology. Please excuse me if you heard a cough that subtly masked my slight doubt of this claim.

The first (and most obvious) issue with this is that you would have to have some sort of a laser system that would be on Mars to slow down the ship.  So, you would have to build something like a nuclear power plant on Mars. I am sure that this is not really likely to happen soon, since we are so successful at building them here in the United States (sarcasm). But, there are probably less regulations on Mars, so it should be easier. But then there is the whole getting all of the (highly radioactive) materials to Mars to actually build the plant.  Well, any ways, we will get there eventually!

Ok, so now that we have a laser system on Mars and a laster system on Earth, how much acceleration would we need to get to Mars in 3 days?  Well, we would accelerate for half the distance and then decelerate for the other half of the distance.  If we make a very simple approximation that the acceleration is constant, the problem is easy to solve.  Let’s assume that Mars and Earth are the closest they can be together, which is 0.3 AU, or about 45 million kilometers. We need to accelerate through about half that distance in about 1.5 days. Do a little math and we get that the acceleration needs to be a constant 5.3 m/s², which is about half of the acceleration of Earth on the surface.  This is extremely reasonable!

The problem with this is that the power from the laser falls off as the distance squared. This means that the acceleration that the laser system could supply would have to start off extremely large, then would fall to almost nothing, or that the power that is consumed by the laser would have to start off relatively small, and would have to increase dramatically.

Let’s think about how high-powered of a laser you would have to have in either case. I am going to simplify the problem significantly, since I am a relatively simple person. The sun, for reference, exerts about 4.5667e-6 Newtons of force per meter squared of area. This is an incredibly small force! Like, really, really, really small.  In order to exert that much force, the energy in that light is about 1350 Watts, which is a LOT of energy.  So, this idea is not very efficient at all!

Let’s say that we want to send something to Mars that is a 100 kg, or about 220 lbs. This is an extremely small satellite. If we want to accelerate it at a rate of 5.3 m/s², like the example above, we would have to use 530 Newtons. If we had a sail hooked up to this object that was, say, 100m by 100m (about the size of a football field), how much force would the sun exert on it?  About 0.0457 Newtons.  That is not much! And that is taking about 1350 W, as described above.  So, we would need a laser that is about 11,600 times more powerful than the sun to give us our 530N of force.  That would require a 15.7 Mega-Watt laser.  And this would only accelerate it at the 5.3 m/s² for a little while, since the distance between the laser and the satellite would increase and the received power from the laser would decrease.

Let’s say that the laser delivered the 15.7MW (or 530N of force) at a distance of about 10 Earth radii away from the surface of the Earth (I had to choose a distance, and this was quite arbitrary, but whatever). If you wanted to continue to accelerate the satellite at 5.3 m/s² all the way to the halfway point between Earth and Mars, the power of the laser would have to increase by a factor of about 500,000 times while it was shining on the craft. This means that in order to accelerate it all the way to the halfway point, the laser would have to be a 7,800,000 MW (7.8 Tera-Watt) laser, and would have to fire (ramping up in intensity) for about 36 hours.

Practical?  I don’t know.  This website talks about a 2,000 TW laser that was fired for 1 pico-second (not very close to 36 hours). Another website talks about getting a 10 TW laser that fires for about a femtosecond (that is also pretty short), but fits on a desk.

Where could we get the power? Well, if the sun delivers 1,350W of power per 1m x 1m area, then we would need about 5,800 km² of solar panel area to get that much energy.  Oops, solar cells are not perfectly efficient (more like 25% efficient), so we would need about 23,000 km² of area, which is about 150 km by 150 km of solar cells. This is about the size of New Jersey.

Anyways, the idea is that power on Earth is very cheap, while getting that power into space is really painful.  So, it is ok to take a HUGE hit on efficiency to accelerate something up to enormous speeds in space using Earth-based systems, instead of trying to haul some sort of chemical rocket engine up to space. In fact, chemical rockets will never get us to another star, so it is a non-starter. But, that is a conversation for next week (I promise!)




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