Theory evolves toward practice—high-speed
travel in the frictionless medium of space.

Professional Pilot Magazine asked me to contribute a prediction about the future of flight for the next century. Naturally, I wrote about solar sailing. Writing the article allowed me to step back a bit from our efforts to make this first solar sail flight happen and focus my thoughts on future applications for solar sailing -- uses within our solar system and beyond.

The promise of light sails enabling new missions of application and exploration is what motivates The Planetary Society and Cosmos Studios to reach for the stars.

-Louis D. Friedman

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Sailing by light is the only technology we currently know that might one day take us to the stars, and this is what motivates the high interest in solar sailing.

With Special Thanks to Cosmos Studios!

turn the Musik on, the most people know that i am a great Fan from Carl Sagan.

Greetings from Germany NRW
Ulli

Those solar sails - It must be remembered that solar radiation drops off as the square of the distance. For interstellar travel practically all the boost would be from inside the asteroid belt. I don't think one could be accelerated to a speed that would reach other stars in a reasonable time.

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Those solar sails - It must be remembered that solar radiation drops off as the square of the distance. For interstellar travel practically all the boost would be from inside the asteroid belt. I don't think one could be accelerated to a speed that would reach other stars in a reasonable time.

That's why they would need a boost from a space laser so they could keep accelerating beyond that point.

Those solar sails - It must be remembered that solar radiation drops off as the square of the distance. For interstellar travel practically all the boost would be from inside the asteroid belt. I don't think one could be accelerated to a speed that would reach other stars in a reasonable time.

That's why they would need a boost from a space laser so they could keep accelerating beyond that point.

I'm no expert at this so I'm probably wrong, but it looks like if you used a space baxed laser to accelerate the sail you would have to expend fuel to keep the laser in position. It seems that the emission of the laser beam from the laser would produce a thrust to the laser equivalent or even more than what you would impart to the spacecraft. This would mean that the laser would have to expend more fuel than the same fuel used on the spacecraft itself. One advantage to this would be that the laser would be in orbit where it could be refueled whenever needed and not need to carry all the fuel required for the trip onboard.

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Jim

Some people plan their life out and look back at the wealth they've had.
Others live life day by day and look back at the wealth of experiences and enjoyment they've had.

Those solar sails - It must be remembered that solar radiation drops off as the square of the distance. For interstellar travel practically all the boost would be from inside the asteroid belt. I don't think one could be accelerated to a speed that would reach other stars in a reasonable time.

That's why they would need a boost from a space laser so they could keep accelerating beyond that point.

I'm no expert at this so I'm probably wrong, but it looks like if you used a space based laser to accelerate the sail you would have to expend fuel to keep the laser in position. It seems that the emission of the laser beam from the laser would produce a thrust to the laser equivalent or even more than what you would impart to the spacecraft. This would mean that the laser would have to expend more fuel than the same fuel used on the spacecraft itself. One advantage to this would be that the laser would be in orbit where it could be refueled whenever needed and not need to carry all the fuel required for the trip onboard.

I do believe you're wrong on the 'reaction thrust' to the laser. I don't understand everything to do with light and photons either, but light does not act the same way as mass does. i.e. the 'equal and opposite reaction' is not in force for light. Light is generated as an energy wave, and as such, does not cause a reactive force in the opposite direction. When light reaches a solar sail, it is either absorbed or reflected. The energy from this process (and I don't completely understand it either) is transferred to the electrons within the solar sail, generating a push which causes the solar sail to accelerate. See Solar Sailing 101 for more information.

I do believe you're wrong on the 'reaction thrust' to the laser. I don't understand everything to do with light and photons either, but light does not act the same way as mass does. i.e. the 'equal and opposite reaction' is not in force for light. Light is generated as an energy wave, and as such, does not cause a reactive force in the opposite direction. When light reaches a solar sail, it is either absorbed or reflected. The energy from this process (and I don't completely understand it either) is transferred to the electrons within the solar sail, generating a push which causes the solar sail to accelerate. See Solar Sailing 101 for more information.

Ok, if all the force is produced by reflection off of the solar sail, then the same would happen to the laser (or at least the high power lasers I'm familiar with). I don't know much about the hene or other gas lasers, but I know in a ruby laser (and I'm assuming the others too) the laser beam is created by bouncing the light between one fully reflective surface and another partially reflecting surface. So it appears to me that there would be a force applied to the laser equal to the total force of the light hitting the totally reflecting mirror minus the force exerted in the opposite direction by the partially reflecting mirror.

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Jim

Some people plan their life out and look back at the wealth they've had.
Others live life day by day and look back at the wealth of experiences and enjoyment they've had.

I do believe you're wrong on the 'reaction thrust' to the laser. I don't understand everything to do with light and photons either, but light does not act the same way as mass does. i.e. the 'equal and opposite reaction' is not in force for light. Light is generated as an energy wave, and as such, does not cause a reactive force in the opposite direction.

In turn I believe you are wrong here. Photons carry momentum proportional to their frequency:

p = (h/c)*nu .

Since momentum is a conserved quantity, anything that emits a directed beam of EM radiation must experience a reaction force.

Note that precise tracking of spacecraft like Pioneer requires that not only reflected sunlight but also differences in the amount of heat radiating from various surfaces be taken into account. And these far-infrared photons carry much less energy than do visible ones.

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In turn I believe you are wrong here. Photons carry momentum proportional to their frequency:

p = (h/c)*nu .

Since momentum is a conserved quantity, anything that emits a directed beam of EM radiation must experience a reaction force.

Note that precise tracking of spacecraft like Pioneer requires that not only reflected sunlight but also differences in the amount of heat radiating from various surfaces be taken into account. And these far-infrared photons carry much less energy than do visible ones.

After I replied I happened to remember reading about the plasma or ion rocket. A statement was made saying the higher the exhaust velocity became the more efficient the system was. They said the ideal exhaust for a rocket of this type would be pure light since it would have the highest velocity possible. So if a beam of light didn't cause a reaction force, why would it even be considered for a rocket engine.

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Jim

Some people plan their life out and look back at the wealth they've had.
Others live life day by day and look back at the wealth of experiences and enjoyment they've had.

In turn I believe you are wrong here. Photons carry momentum proportional to their frequency:

p = (h/c)*nu .

Since momentum is a conserved quantity, anything that emits a directed beam of EM radiation must experience a reaction force.

Note that precise tracking of spacecraft like Pioneer requires that not only reflected sunlight but also differences in the amount of heat radiating from various surfaces be taken into account. And these far-infrared photons carry much less energy than do visible ones.

After I replied I happened to remember reading about the plasma or ion rocket. A statement was made saying the higher the exhaust velocity became the more efficient the system was. They said the ideal exhaust for a rocket of this type would be pure light since it would have the highest velocity possible. So if a beam of light didn't cause a reaction force, why would it even be considered for a rocket engine.

After doing some additional research (I said I didn't understand everything regarding this subject), I'm going to concede that there must be a reactive force against the source of the laser beam. Particularly based on this article and it's follow-up.

So, based on the article in Uli's original post, the space based lasers would need to be in an orbit about the distance of Venus from the sun and most likely would use their own solar sail to maintain their position while sending their laser beams to the interstellar ship.

Can they make lasers as fine as a microarcsecond? At Pluto's average distance, a solar sail 100 feet (30 meters) in diameter would subtend only about that angle. The atmosphere here would bend that beam far more than that. If they used as much fuel as was on the Saturn V rocket to accelerate that sail it would go some way to making it go fast but only if conversion from fuel to acceleration were efficient. Maybe they need a nuclear propulsion system (but maybe that had already been mentioned).

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Can they make lasers as fine as a microarcsecond? At Pluto's average distance, a solar sail 100 feet (30 meters) in diameter would subtend only about that angle. The atmosphere here would bend that beam far more than that. If they used as much fuel as was on the Saturn V rocket to accelerate that sail it would go some way to making it go fast but only if conversion from fuel to acceleration were efficient. Maybe they need a nuclear propulsion system (but maybe that had already been mentioned).

A solar sail (should actually be called a light sail, not solar) capable of powering an interstellar ship would need to be several kilometers across. Still a rather small target at Pluto's distance (and beyond).

Can they make lasers as fine as a microarcsecond? At Pluto's average distance, a solar sail 100 feet (30 meters) in diameter would subtend only about that angle. The atmosphere here would bend that beam far more than that. If they used as much fuel as was on the Saturn V rocket to accelerate that sail it would go some way to making it go fast but only if conversion from fuel to acceleration were efficient. Maybe they need a nuclear propulsion system (but maybe that had already been mentioned).

A solar sail (should actually be called a light sail, not solar) capable of powering an interstellar ship would need to be several kilometers across. Still a rather small target at Pluto's distance (and beyond).

If we are ever able to explore the far reaches of our solar system and return with human cargo--the propulsion system would likely be that of Star Trek vintage : matter/antimatter annilieation. This gives us 1000 times the energy of the classic atomic reaction and one Billion times the energy of a chemical reaction. We would have to learn how to produce large amounts of anti-matter and store it --maybe in magnetic (tokamac ?) style magnetic vessels. Just a fanciful thought about the future.

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