----- Original Message ----- From: "leaking pen" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Tuesday, June 19, 2007 12:27 AM Subject: Re: [Vo]:Space tractors seem ineffective
> that would be great michel, if photon pressure was what was being > discussed. I believe the discussion was gravity. Thanks Leaking I had realized that, I was simply adding another possibility to David's alternative suggestions. My worry is that schemes involving sending a ship to meet the incoming object will waste a lot of precious time (months), requiring an enormous push in the end for not having acted early enough. Regarding the photon pressure scheme, I never took the time to do a back of an envelope calculation to (in)validate it, I'll try to do that now. What's the solar mirror area required to exert a 1 N push on an incoming asteroid? Photon momentum is photon energy divided by speed of light c, so thrust provided to the target by relected photons will be 2*P/c where P is the total beam power, say P/c for a real asteroid which isn't flat and totally relective. So a light beam power of the order of 3*10^8 W is needed for each Newton of thrust. If this must be provided by near-earth mirrors reflecting sunlight, a total mirror area of 1.4 (orientation factor assuming 45° reflection on average) times 3*10^8 W divided by 10^3 W/m2 (incident sunlight power) is needed, that's about 4*10^5 m2, i.e. 40 hectares of mirror area. Sounds playable to me. We might even manage with steerable Earth-based mirrors. Comments/Corrections welcome. Michel > I agree with you David, even at twenty tons, i dont think there would > be much force. of course, remember that the larger the object being > pulled in that way, the more force transferred, so at a point that may > in fact be more viable once the pulled source gets large enough. > plus, remember, it works slowly over time, building up, and as the > article states, it doesnt matter WHAT your pulling. or how compact it > is, how dense, ect. > > On 6/18/07, Michel Jullian <[EMAIL PROTECTED]> wrote: >> Using photon pressure from space mirrors focussed the Archimedes way would >> allow to act early while the asteroid is still very far away, and the >> earlier you push the less push is needed to deviate the orbit sufficiently, >> which would make up for the relative feebleness of the effect. >> >> Michel >> >> ----- Original Message ----- >> From: "David Jonsson" <[EMAIL PROTECTED]> >> To: <[email protected]> >> Sent: Tuesday, June 19, 2007 12:02 AM >> Subject: [Vo]:Space tractors seem ineffective >> >> >> > http://arxiv.org/abs/astro-ph/0509595 >> > >> > The article says that a 20 metric tonnes heavy space vehicle should be used >> > as a gravity pull on asteroids to move them from dangerous orbits. >> > >> > Wouldn't it be better to ram the asteroid or use dielectric influence to >> > achieve forces on it? Gravity seems so weak in comparison. >> > >> > David >> > >> >> > > > -- > That which yields isn't always weak. >
