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# Laser Sailing

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In this article, they talk about a tiny wafer-like sail that could be powered by an orbital laser. Unfortunately, they don't give any specifics or details. How would this work exactly?

It also made me start thinking about the lasers. If you shoot a laser in space, does it travel backwards from the force? Photons don't have mass, but they have energy. Wouldn't Newton's third law apply here? Wouldn't this mean that an orbiting laser would have its orbit slowly decay without some compensation from thrusters or something?

There are tons of other questions too...What about space debris? How do you aim a laser at a small craft that is light years away? How much amplification would be needed at such distances?

The article says that we could send it to Alpha Centauri in 15 years. Since AC is ~4 lya, that would mean it would travel faster than .25c. Is that really feasible?

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In this article, they talk about a tiny wafer-like sail that could be powered by an orbital laser. Unfortunately, they don't give any specifics or details. How would this work exactly?

This article seems to me to be a lot of updated discussion of an idea proposed by the late Robert Forward is papers and books such as his 1995 book Indistinguishable from Magic. In 1985, Forward proposed sending many tiny robotic probes to interesting nearby extrasolar stars by accelerating them with Earth-orbiting solar powered microwave lasers, with would be financed by the lasers then being turned to the Earth’s surface to provide commercial power. He called this idea Starwisp. I believe he actually patented in the hope it would be done within his lifetime and make him rich and famous, mindful of speculation that Arthur Clarke should have patented his 1945 idea for geosynchronous satellites.

Being 30 years old, there’s a lot of commentary on how it could or could not be done, much by Geoffrey Landis – the Wikipedia article has a reasonably good synopsis of it.

Key features of the Starwisp idea include

• The probes are only pushed about as far as the orbit of Mars, at which point it’s no longer possible to aim the microwave beam precisely enough. They’re accelerate strongly, though, at 2+ gs, so could reach speeds greater than 0.01 the speed of light, c.
• The probes don’t have their own power supplies. When they reach their destination, a less powerful microwave beam is sent to them to power their sensors and communication systems, allowing them to collect data and radio it back to earth.
• A lot – hundreds or thousands – of them could be mass-produced and flown, so the choice of target destinations is simple: all of them. Forward was especially interested in discovering and exploring nearby brown dwarf sub-stellar objects.

The article says that we could send it to Alpha Centauri in 15 years. Since AC is ~4 lya, that would mean it would travel faster than .25c. Is that really feasible?

Forward wasn’t, I recall, as optimistic, expecting Starwisp missions to take many decades, but not many centuries.

The equations (non-relativistic, OK for estimates of speeds only a small fraction of c) for time and speed for a constant acceleration and distance are

$t= \sqrt{\frac{2d}{a}}$

$v= \sqrt{2da}$

So here are some examples:

a        d        v        t
(m/s/s)  (AU)     (c)      (days)
10       0.5    0.0041   1.42
24       2.5    0.0142   2.05
100       2.5    0.0289   1.00
2500       6      0.2238   0.31
100      50      0.1292   4.48
360      50      0.2451   2.36
150     120      0.2451   5.67

So to get the needed speed, you’d need to do something like

accelerate the probe at 36 gs to the aphelion of Pluto,

at 15 gs to the heliopause,

or at 250 gs to the orbit of Jupiter.

None out outside of the realm of the reasonably possible, but none look easy, either.

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• 2 weeks later...

Interesting reply, Craig. I seem to recall the Starwisp idea (perhaps from these forums many years ago as I have not read Forward's book - will have to check that out).

250 gs to Jupiter in about 8 hours seems crazy to me. How powerful are these microwaves?

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• 2 weeks later...

A ground based laser would not work. To quote:

"To concentrate as much laser power as possible onto the reflector array, we must ensure that the beam leaving the telescope is as collimated (parallel, non-diverging) as possible. We use a laser both because we can get ultra-short pulses of light from a laser, and also because the light from a laser is extraordinarily directional—not diverging the way a flashlight, or even searchlight, would. Even so, the turbulent atmosphere distorts the beam, imparting a divergence of about one arcsecond (sometimes more). One arcsecond is 1/3600th of a degree, or the angular size of a quarter about five kilometers (about 3 miles) away. At the distance of the moon, this angle translates to 1.8 kilometers (just over a mile). Though this is large compared to the size of the reflector (most of the light is wasted—never hitting the reflector), it is still a challenge to point and maintain the laser beam on this tiny patch of the moon."

As to space based lasers, E E "Doc" Smith in one of his SF books several decades ago pointed out that even if a space based heat ray is 99% efficient, it will be the devil to get rid of the cumulative effects in a space craft of heating from that 1% each time the weapon is fired.

As I said elsewhere, perhaps a solar sail in a solar orbit for years, continually building up speed till it reaches target speed, then leave the Sun's orbit and as needed, use a conventional propulsion from there.

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A ground based laser would not work.

Which is why everything mentioned in this thread describes space-based lasers!

Proposals to do this involve both visible and microwave radio lasers (masers). The advantage of microwave laser propulsion is that, despite them being lower-energy, the reflector carried by the ship can be lower mass for a given area than a visible light reflector. They also can easily generate electricity from the microwave beam, so they don’t need to carry their own power supply systems. One of the best know designs for this kind of spacecraft is the Starwisp, which I wrote at lot about in post #2.

Though this thread has been mostly about small unmanned artificially-pushed space sailships like the Starwisp, the same person who proposed the Starwisp, Robert Forward, also described a large, optical laser pushed manned spacecraft, in his 1984 novel RocheWold. The best online description of this system (which had the ability to accelerate both away from an, unusually, toward the source of the beam) is here.

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...

It also made me start thinking about the lasers. If you shoot a laser in space, does it travel backwards from the force? Photons don't have mass, but they have energy. Wouldn't Newton's third law apply here? Wouldn't this mean that an orbiting laser would have its orbit slowly decay without some compensation from thrusters or something?...

Yes, laser beams -and all electromagnetic radiations- have momentum.

...The radiation pressure from sunlight is very weak. However, that produced by laser beams can be enormous (far higher than any conventional pressure which has ever been produced in a laboratory). For instance, the lasers used in Inertial Confinement Fusion (e.g., the NOVA experiment in Lawrence Livermore National Laboratory) typically have energy fluxes of 1018 Wm-2. This translates to a radiation pressure of about 104 atmospheres!

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