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I've just watched a Discovery Science program - Alien Encounters featuring SETI luminaries such as Jill Tarter, Frank Drake and Nick Sagan. A serious discussion on how to interpret ET messages coupled with a dramatised depiction of the social impact on Earth of an impending visit from ETs. My major point of confusion is that, within our solar system, the visitors head in a sunward direction to Earth powering a craft with a solar sail, powered from behind using very powerful lasers.

I have viewed some topics on Phys.SE on the powering of solar sails, and including 'how massless photons can have momentum'. My problem with the described system is;

1) This sounds like 'pulling yourself up by your bootstraps' - If the photons hitting the sail have sufficient momentum to push the sail along, would there not be an equal and opposite momentum of the photons leaving the laser - and thus cancelling out the thrust?

2) If the solar sail craft is heading sunwards, you are already fighting a 'reverse thrust' from the sun. The program postulated tapping energy sources such as dark energy to power the lasers (OK that's sci-fi at the moment, I admit). So if problem 1. above is wrong, the power source for the lasers would have to be immense as it would to exert more than double the sun's photonics pressure to move forward.

I thought the only sunward movement of a solar sail was possible only by an analog of land-sailing's 'tacking'. I know the TV program was speculative, but I hope this is not received as science fiction, as solar powered space probes are a reality.

To clarify, as I'm getting tangential responses to this. Is the issue I called problem 1. a complete bust? That is, powering a solar sail from behind with your own (laser) light source a non-starter, because of the 'bootstrap' scenario or another more scientific principle?

Attempting to load a screenshot of the craft as requested- Solar Sail craft

Judge Zeppelin
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  • Can you show a picture or video still of what their spacecraft looks like? – Kvothe Apr 02 '14 at 15:21
  • Probably the same way you sail a sailboat into the wind... – Kyle Kanos Apr 02 '14 at 15:24
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    @KyleKanos No, they discussed the solar tail tacking problem in detail on Space Exploration recently. No keel means no taking in the sailboat sense. To get close to the star, you just fall (you might want to turn the sail to kill angular momentum first, which is what I would call "tacking" in the context of a light sail). – dmckee --- ex-moderator kitten Apr 02 '14 at 15:29
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    Using a laser for propulsion is called a "photon drive" or "photon rocket" and you don't need (or want) a sail working with that. You just use the photon drive. Links: http://physics.stackexchange.com/questions/68041/best-em-photon-rocket-using-avalable-tech http://physics.stackexchange.com/questions/74013/rocket-propelled-by-a-giant-monochromatic-laser – dmckee --- ex-moderator kitten Apr 02 '14 at 15:29
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    @dmckee: Upon your comment, I searched for the discussion. The answers there seem to answer the OP's questions here. – Kyle Kanos Apr 02 '14 at 15:36
  • Perhaps they meant the lasers are mounted somewhere else like on Earth and aimed at the ship's sail? That would work. – user6972 Apr 02 '14 at 17:08
  • No. If I could get the image loaded, you could see an array of lasers firing from the craft onto the sail. As I mentioned earlier it makes no sense to me, as the rearward force from the lasers would balance the forward thrust on the sail - unless I'm missing something! – Judge Zeppelin Apr 02 '14 at 17:14
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    It'll work, the total momentum change at the laser is +p and at the sail is (ideally) -2p, but the sail can only approach perfect efficiency. Better to point the lasers in the direction you meant in the first place. – dmckee --- ex-moderator kitten Apr 02 '14 at 18:27
  • @dmckee. Could you expand on that a little. My laser reference books and mechanics reference books are on a shelf collecting dust. It would help to have a title to topic to zoom in on the right chapters to re-read. – Judge Zeppelin Apr 02 '14 at 22:02
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    You mount the laser to the structure of the craft at it spits a photon of momentum $+p$ toward the sail. The craft recoils with momentum $-p$. The photon hits the sail and is reflected back the other way creating a change in momentum of $-2p$, causing the craft to recoil also with momentum $+2p$. Total change to the photon: $-p$. Total change to the craft $+p$. Which is the same as if you had pointed the laser away from the sun in the first place. Then consider reflectivity and see that with the sail it is worse than using the laser directly for propulsion. Q.E.D. – dmckee --- ex-moderator kitten Apr 03 '14 at 01:09

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The most likely explanation in my opinion is that the programme makers have misunderstood a similar proposal, that might practically work, that is different in one specific, but very important, detail.

Imagine that the high powered laser was actually mounted somewhere in the alien home solar system (or somewhere similar) instead of being attached to the ship. The ship could then be accelerated away from the star system without needing to carry a big enough power source to do all the work itself. The fact that this acceleration happened to be towards our sun would be a coincidence.

There are complications around stopping again, though a well-timed gravity slingshot to reverse direction, followed by using the sail again to slow down, would in theory work, though you might have problems decelerating enough to match orbits with your desired target.

There is quite a lot more detail of this mechanism on the Space Exploration Stack Exchange site.

Community
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Bill Michell
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A solar sail could be a reflector - and as such you can sail in a bootstrapped direction (because the photons end up traveling in the opposite direction from the one you want to travel in). It would be very inefficient - because the solar radiation meanwhile is pushing back on your sail, and provides "drag". Turning the laser the other way and keeping your profile small would be somewhat better.

Now if you used the back of the solar sail as a means of capturing the sun's energy, could you then use that energy to power something? I realize that's not your question but it is an interesting one. The relationship between momentum and energy of a photon is given by $$E = pc$$

Looking at this equation, you see immediately that there is no way of taking the energy and repackaging it (either into photons of higher or lower energy) to increase the momentum for a given energy. However, if we look at massive particles (for example, Xenon atoms) the story changes.

At non-relativistic velocities, the momentum of a massive particle is a function of the kinetic energy according to

$$p = \sqrt{2mE}$$

This means that the momentum I get for the same energy goes up with the mass $m$ - a giant solar panel capturing energy from the sunlight could be used to generate the energy for an ion thruster.

Floris
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