Why Couldn't a Laser be Used to Propel a Spacecraft?

Wow! 86 replies (so far) and none have really hit the mark. Aren't there any space cadets active here?

Laser propulsion for spacecraft is an active area of interest, both within NASA and in more speculative venues like JBIS (Journal of the British Interplanetary Society). There are two very different categories of system. One is for launching payloads from earth and for orbital operations near earth. That's of immediate interest, and you'll likely be hearing news releases about projects within the next few years. The other is for launching interstellar probes, and is purely speculative. More about that later.

The near term system uses lasers for beaming power to the spacecraft. The beamed power is used to accelerate reaction mass, producing thrust in the usual rocket manner. But because the power source is external, the exhaust velocity for the rocket engine is not limited by the chemical energy of its fuel. The velocity can be much higher. That allows smaller loads of reaction mass to deliver larger velocity deltas than is possible with chemical fuels.

This type of laser propulsion is similar to a nuclear rocket, but with power delivered by the laser beam rather than an on-board nuclear reactor. Extremely powerful lasers (multi-megawatt to gigawatt scale) are needed to launch useful payloads from earth. Practical technology for that does now exist. It's a by-product of military anti-missile laser research.

For interstellar probes, the technical requirements are quite different. The probes would be pulled by very large, gossamer light sails. Sunlight reflected from a light sail can produce enough force to accelerate a probe at milli-gee levels in earth-normal sunlight. A milli-gee doesn't sound like much, but it's 360 meters per second per hour. In one year, delivered steadily, it adds to over three million meters per second -- about 1% of light speed. The problem is that the intensity of sunlight falls off rapidly as the probe moves away from the sun. So instead of sunlight, the light sail would be pushed by the beam from a giant space-based laser. The system requires a kilometer-scale mirror to deliver a tight beam to the probe at distances approaching one light-year. The probe's acceleration phase would continue for about 10 years, at the end of which it would be at 10% of light speed.

The late Robert Forward championed this type of system, and wrote about it in his "Rocheworld" SF novels. In those stories, the propulsion laser mirror was hundreds of kilometers in diameter, so it could deliver a tight beam all the way to the star being visited. The probe carried a crew, and it had a "two stage" light sail enabling the crew vehicle to decelerate using light reflected back from the detached first stage. Forward was a distinguished aerospace engineer, and was scrupulous about the science in his fiction. He posited no new physics, although the scale of the systems he wrote about was off the charts.

Have you been actually reading the thread? Solar sails have been brought up and I presented a link showing the research work done on terrestrial lasers lifting an object.

Solar sails do not use laser light for propulsion, they use solar winds and photon pressure. Several solar sail probes have been utilized. The Ikaros probe from Japan is labeled an interstellar probe since it will sail out of our solar system. I think that is a misnomer since it will have no maneuverability outside of the heliopause into interstellar space.

Have you been actually reading the thread?

Not carefully, obviously. I noticed the thread, skimmed over the replies, saw that Europium had the physics of photon pressure right, but missed your "now" link to the YouTube video of Leik Myribo's work.

I don't think Myribo's approach is particularly promising. I think it's more likely we'll be seeing the type of system that Jordin Kare has been proposing. But that remains to be seen.

The simplest and most useful reply to the OP's question would probably have been to link to the (partially redundant) Wikipedia articles on Laser propulsion and Beam-powered propulsion. They're pretty good articles, though the references include a number of dead links.

If you want to quibble about terminology, you're right, "solar sail" is the wrong term. Call them "light sails" or "photon sails". Same physical structure, but using different photon source.

You also missed a good opportunity to ding me for slipping a decimal point. 360 mps per hour is 10 milli-g, not 1. 10 milli-g is still with the feasibility range for a laser-driven photon sail however. The thrust is 6.7 newtons per GW of illumination, if I haven't (again) messed up. For an acceleration of 0.01g (10 cm/sec^2), that's an accelerated mass of 67 kg / GW of illumination.

The problem that is missing from this discussion is that one cannot move away from your power source and continue to get the same power. Now a laser does produce well collimated light but fairly quickly any sail will become a very tiny target after leaving Earth orbit. One will be able to sail with the sun but one is not using a laser for propulsion.

You are quite right, the technical challenges for long-distance exploration are immense. If, for example, we focus a visible wavelength laser using an ideal optical system (telescope) of 10-km diameter, the beam diameter at 1 light-year would still be around 400-km. Maybe in theory it would be less unrealistic to use a coherent deep UV source - but the theoretical losses remain great, and the technological challenges unfathomable.

However, while we are sailing off on lights of ffancy, maybe we could send a number of intermediate optical systems at fractions of the velocity of the craft to refocus the light*. Once we've bitten on this bullet, the total area required for the optics becomes inversely proportional to the number of intermediate stages. (AT&T did quite a lot of work on beam control in semi-equivalent (static) terrestrial equivalents during the 1960s, but the work was shelved once low-loss optical fibres were demonstrated).

*Maybe the space cadets will tell us that this is already in the technical mix?

Fyz

So no beer and pretzels for the crew, huh.

I sure hope the tracking of the laser to the "indestructible" target of these high-powered laser beams (whether pulsed or cw) is spot-on, or should I say "dead"-on.

Thinking in regards to how long it would take to accelerate to really high relativistic speeds, one had best deploy the retro reflector about 1/2 way into the trip. Or not? If it is so hard to accelerate due to mass increase with velocity, would it not also be harder to decelerate? I would not want to be onboard attempting to figure this out, and then realise I goofed by a couple of light-years.

The momentum of a photon: P=h/λ, where h is Plank's constant and λ is the wavelength

The energy of a photon: E=hc/λ or E=cP. Energy = momentum x speed of light.

(ref: http://en.wikipedia.org/wiki/Photon)

By conservation of momentum, the momentum transferred to the light beam in one second equals the momentum delivered to the spaceship in one second

If you do the math it turns out that to accelerate a 1 kg mass at a rate of 1 meter/sec² requires 3x10⁹ watts of continuous power.

Because that would require a lot of batteries and batteries are heavy.