## Off to the stars........

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### Off to the stars........

Hawking and others - micro space craft.
Question: Newton's law of action / reaction. If we use the enormous energy source of numerous power stations to propel the craft, surely the 'reaction' will be to shift the earth in orbit or rotation?
With apology from a 'beginner' remembering ( incorrectly ? ) a bit of science from school..
criky

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### Re: Off to the stars........

Physics is not my strong point! I would say the only mass involved will be the (small) mass of the spacecraft leaving the earth. The amount of energy beamed away from the earth towards the spacecraft will impart no 'reaction' to the earth. Its rotation and orbit are assured!

I'm happy to be corrected by a physicist on this point !
I use an 11" Celestron SCT (CPC 1100) on an equatorial wedge, housed in a 2.2m Pulsar observatory. I use a ZWO ASI1600MC and Canon 500D for imaging.
Aratus

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### Re: Off to the stars........

I think you are right. But it does beg the silly question (which is permitted). Surely if we do shine a torch or fire a very high energy microwave beam then it feels as though there should be some re -action. Certainly a gun will react but I'm not sure what we are actually firing with light and m/w.

Also do I recall correctly that firing photons in space could be a method of propulsion which would give weight ( mass ) to my original question.

Or perhaps I've been watching too much Star Trek.
criky

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Joined: Wed Apr 13, 2016 12:42 pm

### Re: Off to the stars........

Again with the caveat that I'm not a physicist - I understand that photons effectively have no mass. Therefore there will not be a re-action of an object shining a beam of light, no matter how powerful.
I use an 11" Celestron SCT (CPC 1100) on an equatorial wedge, housed in a 2.2m Pulsar observatory. I use a ZWO ASI1600MC and Canon 500D for imaging.
Aratus

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Joined: Tue Jan 12, 2016 2:17 pm
Location: East Lincolnshire

### Re: Off to the stars........

Aratus wrote:Again with the caveat that I'm not a physicist - I understand that photons effectively have no mass. Therefore there will not be a re-action of an object shining a beam of light, no matter how powerful.

Although the photon doesn't have mass, it does have momentum ( = h/λ, where λ is the wavelength of the photon), so there IS a reaction - which is why light sails work.
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Gfamily2

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### Re: Off to the stars........

There you go!
It is good to have an expert on board.

Going back to the original question - I take it then that any such momentum will not affect the orbit or rotation of the earth. - or will it?
I use an 11" Celestron SCT (CPC 1100) on an equatorial wedge, housed in a 2.2m Pulsar observatory. I use a ZWO ASI1600MC and Canon 500D for imaging.
Aratus

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Joined: Tue Jan 12, 2016 2:17 pm
Location: East Lincolnshire

### Re: Off to the stars........

Aratus wrote:There you go!
It is good to have an expert on board.

Going back to the original question - I take it then that any such momentum will not affect the orbit or rotation of the earth. - or will it?

Not noticeably - the momentum of the Earth is so much bigger than the momentum given off by the lasers.
Also, the laser on the earth will circle through 360 degrees in azimuth as it points towards the nano-probes over the 12 month cycle, so quite a lot of any theoretical change in momentum would be cancelled out.
Actually, the best place for a laser would be in orbit - so the momentum transfer due to the laser could be cancelled out by having a small ion-drive pushing in the opposite direction.
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Gfamily2

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### Re: Off to the stars........

Really interesting - thank you. So the answer is 'yes' but very little - which I suppose is what I anticipated.
The energy requirements for accelerating the nano probes is apparently enormous and equivalent to quite a few nuclear power stations according to the press I read. I'm guessing from his knowledgeable answer that Gfamily2 has taken this into account in his answer.
Not quite understanding the 12 month firing being an answer. The target will not be following earth rotation but will be on a straight trajectory to its destination. Therefore laser pulsing will be in the same direction with reaction being a constant opposite.
But it does beg the question that some very clever mathematics would be needed to make sure we don't go whizzing off in the opposite direction. Don't want to get this wrong do we?
Splendid - many thanks…..
criky

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Joined: Wed Apr 13, 2016 12:42 pm

### Re: Off to the stars........

criky wrote:Really interesting - thank you. So the answer is 'yes' but very little - which I suppose is what I anticipated.
The energy requirements for accelerating the nano probes is apparently enormous and equivalent to quite a few nuclear power stations according to the press I read. I'm guessing from his knowledgeable answer that Gfamily2 has taken this into account in his answer.
Not quite understanding the 12 month firing being an answer. The target will not be following earth rotation but will be on a straight trajectory to its destination. Therefore laser pulsing will be in the same direction with reaction being a constant opposite.
But it does beg the question that some very clever mathematics would be needed to make sure we don't go whizzing off in the opposite direction. Don't want to get this wrong do we?
Splendid - many thanks…..

Let us suppose that the laser is used to project towards Alpha Centauri.
In June, the laser (red arrow) will be pointing south at midnight - so the reaction impulse (blue) on the Earth is (in principle) directed towards the Sun, and at right angles to the Earth's orbit, yes?
3 months later, the earth will have moved 90 degrees around the sun, so the impulse will be along a tangent to the Earth's orbit - in effect, pushing the Earth faster in it's orbit.
3 months later again (in December) the laser will be pointing in the opposite direction to the way it was in June - so the reaction would tend to be pushing the Earth away from the Sun.
By March, the Earth has moved another 90 degrees around, so the reaction impulse to the laser is in the opposite direction to the Earth's movement.
orbit and impulse.png (12.96 KiB) Viewed 3002 times

So, overall, the effect on the Earth's orbit is largely neutral.
Which is what I meant.
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Gfamily2

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### Re: Off to the stars........

Aha. I understand. Thank you for taking the time to fully explain plus diagram. I was thinking of the earths rotation rather than orbit around the sun. Also there will be phased pulse firing rather than an attempt to constantly target the probe. So overall neutral with a little bit of an earth jiggle until cycle complete. I am now much more confident.
Carry on..........
criky

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Joined: Wed Apr 13, 2016 12:42 pm