Bush Cancels trip to Pluto.

[KrazyHorse]

As soon as I submitted my post I knew that was coming.

[Roman]

What's the equation for transfer of momentum from light hitting a solid object with a mass? I haven't done any physics for 2 years now, so I am forgeting more and more.

[Roman]

I just read in one of the links provided that to create an acceleration of 1g for 1kg using light would reguire 3*10^9W !!! This is the power output of three medium sized nuclear power stations - devoted exclusively to accelerating one kg and this assumes an almost perfect reflective surface. I am now very sceptical of this mode of propulsion.

[Dauphin]

That is why you would have a "launch" mirror placed on a heavy body, (e.g the Moon). If you have photon shuttling back and forth between the launch mirror and the satellite being propelled you only need to provide that energy once. It is not outside the bounds of possibility. This is what is done in testing the recession of the Moon from the Earth, and was used to test GR (the laser signals were sent to Venus satellites and back)

Once the light is Dopplered too much to be sufficiently reflected then it is absorbed or "recycled".

[Roman]

I had not thought of that. That's a very clever idea Big Crunch. But how difficult would it be to track a probe with such a mirror so that it constantly points at it? How quickly would significant Doppler shift manifest? AND above all, what is the equation for the transfer of momentum from a photon to a solid mass?

[TCO]

Originally posted by KrazyHorse
Just read it. I don't know; first off, I have no idea how you'd build a 0.1 kT fission bomb. Secondly, I'm really surprised the pusher platewould withstand hundreds of detonations of size ~20 kT. Third, I wouldn't advocate launching a rocket using nuke explosions; this sounds sort of wacko. Maybe a few flights would be okay using this method, but it doesn't sound like a long term solution. Fourth, given that the chances of a catastrophic failure of any launch vehicle in existence is ~1/50, it seems sort of crazy to put...umm...~500 lbs of fissile material on board such a lunch vehicle. I really don't know what the impact would be on Earth. Would the module containing the bombs be left intact or scattered for a couple of thousand miles along its flight path?

That being said, it's worthy of furthe study at least. Dyson is sort of known as an incurable blue-sky optimist isn't he?

But the interesting thing is that it's existing technology. The issues are pretty much common-sense engineering ones. In particular, the materials issue you mentioned. I've been to the Test Site while they were still doing tests (underground of course). Their are some pretty amazing things they can do in terms of working with nuclear explosions.

Why so hard to beleive that they could desing a plate to take the impacts. That stuff is pretty easily tested and worked on as part of nuclear weapons research. Of course, most of the results are classified. But it should be pretty easy to envision some of the experiments.

Please note: I'm not an advocate for this. Or an Orion "nut". I just think it's fascinating. Sorta like how the SR-71 is the fastest airplane and it's 4 decades old. There is some cool **** out there. Like the Fulton Skyhook, etc.

[TCO]

I'm not so sure solar cells are the answer. What are you going to do with the energy. What efficiency will you have (it isn't anywhere near 100%). and will it be able to handle the power densities you need?

[Roman]

As I understood it, you would use mirrors, not solar cells.

[Roman]

Could some resident physicist post an equation for the transfer of momentum from photons to matter with mass? Thanks.

[KrazyHorse]

Originally posted by Roman
Could some resident physicist post an equation for the transfer of momentum from photons to matter with mass? Thanks.

Assuming probe is moving non-relativistically (v/c < 0.01)

Well, a photon of frequency "f" carries momentum hf/c (h is planck's constant; ~6.6*10^-34 Js IIRC and c is the speed of light; 3*10^8 m/s) and energy hf. Given a massive (mc^2 >> hf) body, the momentum transfer from the photon is simply hf/c for absorption and 2hf/c for reflection.

[Roman]

Originally posted by KrazyHorse


Assuming probe is moving non-relativistically (v/c < 0.01)

Well, a photon of frequency "f" carries momentum hf/c (h is planck's constant; ~6.6*10^-34 Js IIRC and c is the speed of light; 3*10^8 m/s) and energy hf. Given a massive (mc^2 >> hf) body, the momentum transfer from the photon is simply hf/c for absorption and 2hf/c for reflection.

Thanks.

[TCO]

Originally posted by Big Crunch
That is why you would have a "launch" mirror placed on a heavy body, (e.g the Moon). If you have photon shuttling back and forth between the launch mirror and the satellite being propelled you only need to provide that energy once. It is not outside the bounds of possibility. This is what is done in testing the recession of the Moon from the Earth, and was used to test GR (the laser signals were sent to Venus satellites and back)

Once the light is Dopplered too much to be sufficiently reflected then it is absorbed or "recycled".

What percent of the photon energy is given to the vehicle on each bounce? I would think that if you are relying on mulitple bounces you will have some real issues here with beam spreading.

[Roman]

Originally posted by GP
What percent of the photon energy is given to the vehicle on each bounce? I would think that if you are relying on mulitple bounces you will have some real issues here with beam spreading.

That depends on the reflectability of the mirror.

With regard to problems, I agree especially the spreading of the beam and the targeting of the spacecraft despite movement of both the spacecraft and the mirror seem daunting.

[TCO]

What does it depend on? To what extent? Give me a bench mark. Anyway, I'm asking for an individual photon. Isn't the collision elastic in that extreme? Anyway assume a faceted single crystal of Ag that has been micro polished and was cut on the 111 plane.

[Roman]

Well, assuming a perfectly reflective mirror I dare say the energy conversion is 100% efficient if Big Crunches idea of recycling the light (using a second mirror and bouncing the light back and forth between the spacecraft and the mirror) is used.