On board a laser powered solar sail

[NineNinjas911]

Can an on board laser be used to propel a solar sail spacecraft if the laser is pointed at the sails ?
Would Newtons third law affect the laser and maybe prevent the ship from moving ?

Thank you for answering my very ignorant questions.
NineNinjas911

 

[Nik_2213]

Reflection from the sail cancels the scant laser thrust overall. Sail 'stops' when the guy-lines tension.
I suppose you could use the laser to help deploy and adjust the sail, which then reflects *solar* light.

 

[Nugatory]

Can an on board laser be used to propel a solar sail spacecraft if the laser is pointed at the sails ?

Not in any effective way. If the light is absorbed when it reaches the sail then there is no net thrust, because the force on the sail is equal and opposite to the force on the laser. If some fraction of the light is reflected then there will be some thrust, but never more than you'd get if you got rid of the sail and just pointed the laser out the back.

 

[anorlunda]

If the sail had near perfect reflection, it would be analogous to thrust reversers on an airplane jet engine.

You can create thrust with a laser (albeit inefficiently), and you can use a reflector to change the direction of that thrust.

However, the best solar sail proposals do not put the laser on-board the spacecraft . (Lasers and their power supplies are heavy.) They use lasers in orbit near the sun aimed at a solar sail spacecraft that has small mass. One imaginative proposal I saw in the 1970s had an aluminum sail the size of The Moon but weighing only 1 gram, plus a 2 gram payload, It could be accelerated to 0.3 c (by lasers orbiting the sun) before leaving our solar system. Of course, the next question is what can you do with a 2 gram payload?

 

[Filip Larsen]

A recent example of a laser propelled mission proposal could be Breakthrough Starshot [1]. I haven't looked at the technical details so I am not aware if this is just warm air or a solid concept that realistically can be carried out in the near future, or somewhere in between. Maybe someone here can add a comment on that.

[1] https://en.wikipedia.org/wiki/Breakthrough_Starshot

 

[anorlunda]

Breakthrough Starshot

Interesting. But the Wiki article says it is still R&D. They don't mention how it will be able to have enough transmit power to send pictures back to Earth. Also, how they punch so much laser power through the atmosphere. But it illustrates the very good concept that if we focus on spaceships weighing grams rather than tons, many things become easier.

 

[Filip Larsen]

They don't mention how it will be able to have enough transmit power to send pictures back to Earth.

They have a long list of essential challenges [1] that indeed seem to keep the final project goal in R&D for a some time to come. However, small scale validation or isolated parts of it, e.g. accelerating nanostructures with lasers, could potentially become feasible near-term. We have already have had a few actual light-sail experiments so it seems obvious that scaling down whenever possible should make access to experiments from actual LEO much more realistic. It is not hard to imagine the Starshot concept being useful on a much smaller scale also for say, cheap and fast remote "nano-sensing" of smaller asteroids (cheaper in the sense of price per asteroid visited).

[1] http://breakthroughinitiatives.org/challenges/3