# Leaving The Solar System

##### Member
We have discussed this previously in depth.

The bottom line is that you cannot carry enough energy to get anywhere useful in human-scale times.

Gathering energy on the way simply isn't enough.

Human beings (as human beings) are not going to colonize any other solar system. Human beings will go extinct on Earth, not even having permanent sustainable colonies in the solar system.

****

The more interesting problem is that people REALLY believe in a star-trek-like future... Their belief is extremely passionate and quasi-religious. These same people are HUGE supporters of science, technology and living rationally.

#### Bomb#20

##### Contributor
The energy produced by the breaking down of the atom is a very poor kind of thing. Anyone who expects a source of power from the transformation of these atoms is talking moonshine. - Ernest Rutherford

#### skepticalbip

##### Contributor
The energy produced by the breaking down of the atom is a very poor kind of thing. Anyone who expects a source of power from the transformation of these atoms is talking moonshine. - Ernest Rutherford
Yes, Ernest Rutherford died before the development of nuclear weapons and nuclear power plants so maybe he was a bit overly pessimistic about 'future science'.

#### Bomb#20

##### Contributor
Funny story about that. Rutherford made his moonshine remark in a speech on Sept. 11, 1933. Leo Szilard had his "Doc Brown" moment, when he mentally invented the atomic bomb while crossing a London street, on Sept. 12, 1933.

#### Loren Pechtel

##### Super Moderator
Staff member
We have discussed this previously in depth.

The bottom line is that you cannot carry enough energy to get anywhere useful in human-scale times.

Gathering energy on the way simply isn't enough.

Laser-pumped lightsail. No need to carry energy.

I have my doubts about the reliability of splitting the sail for deceleration, but you can send a robotic factory that enters the target system slow enough to brake on it's own, it then builds a laser that can decelerate incoming starships.

Human beings (as human beings) are not going to colonize any other solar system. Human beings will go extinct on Earth, not even having permanent sustainable colonies in the solar system.

1) You think we won't figure out how to extend the human lifespan??

#### steve_bank

##### Diabetic retinopathy and poor eyesight. Typos ...
You could run across the ship and bump into one end, then walk back and do it again. It will also serve as exercise.

What does an advanced space traveling ET call a metal ship filled with humans?

Canned tuna fish.

#### rjh01

##### Member
The Laws Of Thermodynamics would seem to say resources like O2, H20, and human waste can not be recycled indefinitely.
No, they don't say that at all,
Earth has been recycling O2, H20 for hundreds of millions of years.

One other problem. There will be collisions between tiny dust grains and the spacecraft. The energy of these dust grains, at this speed, is huge. They are also almost impossible to detect.
Impossible? just put a flashlight in front of the ship and see reflection.
The dust particles would be very small and hard to spot at a large enough distance to avoid. Remember you will also have the background stars that would be emitting more light than a dust grain.

#### barbos

##### Contributor
The Laws Of Thermodynamics would seem to say resources like O2, H20, and human waste can not be recycled indefinitely.
No, they don't say that at all,
Earth has been recycling O2, H20 for hundreds of millions of years.

One other problem. There will be collisions between tiny dust grains and the spacecraft. The energy of these dust grains, at this speed, is huge. They are also almost impossible to detect.
Impossible? just put a flashlight in front of the ship and see reflection.
The dust particles would be very small and hard to spot at a large enough distance to avoid. Remember you will also have the background stars that would be emitting more light than a dust grain.
Small particles are perfectly visible, not individual but clouds,
And distance is easily calculable by measuring time to travel back.

#### Bomb#20

##### Contributor
Laser-pumped lightsail. No need to carry energy.

I have my doubts about the reliability of splitting the sail for deceleration, but you can send a robotic factory that enters the target system slow enough to brake on it's own, it then builds a laser that can decelerate incoming starships.

Human beings (as human beings) are not going to colonize any other solar system. Human beings will go extinct on Earth, not even having permanent sustainable colonies in the solar system.

1) You think we won't figure out how to extend the human lifespan??

For that matter, we can already build artificial hearts, so how long can it be before we know how to build an artificial womb? Your robotic factory can be pre-stocked with a freezer full of in-vitro-fertilized embryos. This way we won't need humans to travel any faster than robots. If it takes 10,000 years to reach Alpha Centauri, no big woop, so it takes 10,000 years. If the human race wants interstellar extinction insurance, we must learn patience.

#### bilby

##### Fair dinkum thinkum
Laser-pumped lightsail. No need to carry energy.

I have my doubts about the reliability of splitting the sail for deceleration, but you can send a robotic factory that enters the target system slow enough to brake on it's own, it then builds a laser that can decelerate incoming starships.

Human beings (as human beings) are not going to colonize any other solar system. Human beings will go extinct on Earth, not even having permanent sustainable colonies in the solar system.

1) You think we won't figure out how to extend the human lifespan??

For that matter, we can already build artificial hearts, so how long can it be before we know how to build an artificial womb? Your robotic factory can be pre-stocked with a freezer full of in-vitro-fertilized embryos. This way we won't need humans to travel any faster than robots. If it takes 10,000 years to reach Alpha Centauri, no big woop, so it takes 10,000 years. If the human race wants interstellar extinction insurance, we must learn patience.
The problem being that most of the people who dream of humans visiting other stars are really dreaming of themselves visiting other stars.

When the transportation system has the side effect of ensuring that all those who get there are strangers, people's enthusiasm tends to wane.

#### steve_bank

##### Diabetic retinopathy and poor eyesight. Typos ...
I think Jimmy Higgins said it, we are adapted to Earth not space.

Lasers do not need on board power supplies? The energy required to accelate a mass by some ampount is required, there is no way around it. Doen't matter what the propulsion is. From the plots I posted even a 1g constant accertion for 1kg quickly becomes impossible. More so for a large spaceship.

#### bilby

##### Fair dinkum thinkum
I think Jimmy Higgins said it, we are adapted to Earth not space.

Lasers do not need on board power supplies? The energy required to accelate a mass by some ampount is required, there is no way around it. Doen't matter what the propulsion is. From the plots I posted even a 1g constant accertion for 1kg quickly becomes impossible. More so for a large spaceship.
A laser driven spacecraft needs an onboard power supply in exactly the same way that the solar panels on your roof need you to have the Sun in your house.

#### steve_bank

##### Diabetic retinopathy and poor eyesight. Typos ...
As you move away from the Sun energy density in W/m^2 goes down by 1/r^2. Inverse Square Law. Out from the solar system it is nothing.

Lasers were looked at to lift a space levator and there was a small scale demonstraion.

If you have a solar sail what would the force per unit area or pressure be where the laser beam hit the sail? How hot would it ge?

Imagine a platform in space. On one end is a sheet vertical. Fixed to the plate is a laser. The laser shines on sthe heet. Newton's 3rd Law wouldseem to say there will be an equal and opposite reaction through the mounts to the plate. Like standing on the plate and pushing against the sheet. Make a fist and start punching the sheet and the net change in momentum is zero. Or throw baseballs at the sheet.

#### Jayjay

##### Contributor
I don't think we, as in biological humans, will ever leave the solar system. At least not in space ships. By the time technology is there, we'll have superhuman AIs, mind uploads, and far faster ships already colonizing and building infrastructure on other solar systems. Hauling a human meat bag, let alone a human society, all that distance is a bit like sending a horse to the moon with a catapult.

#### Bomb#20

##### Contributor
Imagine a platform in space. On one end is a sheet vertical. Fixed to the plate is a laser. The laser shines on sthe heet. Newton's 3rd Law wouldseem to say there will be an equal and opposite reaction through the mounts to the plate. Like standing on the plate and pushing against the sheet. Make a fist and start punching the sheet and the net change in momentum is zero. Or throw baseballs at the sheet.
The sail you push on isn't fixed to the laser. The sail isn't fixed to anything but the payload. So it accelerates away from the laser.

Yes, that puts a reactive force on the laser, which will accelerate away from the sail with equal and opposite momentum change, unless there's some other force on the laser. So you'd engineer the thing to have another force on the laser. The obvious arrangement would be to add a solar sail fixed to the laser. So the sun pushes on the solar sail (and also powers the laser), the solar sail pushes on the laser, the laser pushes on the starship's sail, and the laser beam pushes the laser back away from the starship toward the solar sail, balancing the forces on the laser, so the laser stays still. The net overall effect is that the starship accelerates away from the home star and a bazillion photons bounce off the two sails, back in the general direction of the home star. Their collective momentum away from the destination star is what balances the starship's momentum toward the destination star.

#### Jayjay

##### Contributor
Imagine a platform in space. On one end is a sheet vertical. Fixed to the plate is a laser. The laser shines on sthe heet. Newton's 3rd Law wouldseem to say there will be an equal and opposite reaction through the mounts to the plate. Like standing on the plate and pushing against the sheet. Make a fist and start punching the sheet and the net change in momentum is zero. Or throw baseballs at the sheet.
The sail you push on isn't fixed to the laser. The sail isn't fixed to anything but the payload. So it accelerates away from the laser.

Yes, that puts a reactive force on the laser, which will accelerate away from the sail with equal and opposite momentum change, unless there's some other force on the laser. So you'd engineer the thing to have another force on the laser. The obvious arrangement would be to add a solar sail fixed to the laser. So the sun pushes on the solar sail (and also powers the laser), the solar sail pushes on the laser, the laser pushes on the starship's sail, and the laser beam pushes the laser back away from the starship toward the solar sail, balancing the forces on the laser, so the laser stays still. The net overall effect is that the starship accelerates away from the home star and a bazillion photons bounce off the two sails, back in the general direction of the home star. Their collective momentum away from the destination star is what balances the starship's momentum toward the destination star.
The conversion efficiency of energy from sunlight to laser is probably so bad, that there are better ways to keep the local laser in place than a light sail. If the laser platform is close to the sun, you can resupply it with a propellant from time to time. I suppose there could be a chain of lasers along the way, using e.g. nuclear fusion for power, but even then, it's probably more efficient to use that energy source to shoot ions or something else in the opposite direction to counteract the force from the laser, than it would be to have another laser somewhere in the inner solar system.

Besides, assuming that the laser is powered by solar panels, having some of that real estate be taken by a solar sail that just does nothing except block out the sun seems very impractical.

#### bilby

##### Fair dinkum thinkum
Imagine a platform in space. On one end is a sheet vertical. Fixed to the plate is a laser. The laser shines on sthe heet. Newton's 3rd Law wouldseem to say there will be an equal and opposite reaction through the mounts to the plate. Like standing on the plate and pushing against the sheet. Make a fist and start punching the sheet and the net change in momentum is zero. Or throw baseballs at the sheet.
The sail you push on isn't fixed to the laser. The sail isn't fixed to anything but the payload. So it accelerates away from the laser.

Yes, that puts a reactive force on the laser, which will accelerate away from the sail with equal and opposite momentum change, unless there's some other force on the laser. So you'd engineer the thing to have another force on the laser. The obvious arrangement would be to add a solar sail fixed to the laser. So the sun pushes on the solar sail (and also powers the laser), the solar sail pushes on the laser, the laser pushes on the starship's sail, and the laser beam pushes the laser back away from the starship toward the solar sail, balancing the forces on the laser, so the laser stays still. The net overall effect is that the starship accelerates away from the home star and a bazillion photons bounce off the two sails, back in the general direction of the home star. Their collective momentum away from the destination star is what balances the starship's momentum toward the destination star.
The conversion efficiency of energy from sunlight to laser is probably so bad, that there are better ways to keep the local laser in place than a light sail. If the laser platform is close to the sun, you can resupply it with a propellant from time to time. I suppose there could be a chain of lasers along the way, using e.g. nuclear fusion for power, but even then, it's probably more efficient to use that energy source to shoot ions or something else in the opposite direction to counteract the force from the laser, than it would be to have another laser somewhere in the inner solar system.

Besides, assuming that the laser is powered by solar panels, having some of that real estate be taken by a solar sail that just does nothing except block out the sun seems very impractical.
The object of the exercise is to eliminate the need for the spacecraft to carry fuel.

Once that problem has been solved, then we can start worrying about efficiency and practicality; Though these latter issues don't need to be considered at all if our sole objective is to demonstrate that the inability to carry sufficient fuel need not be a show-stopper.

The physical possibility of powering a spacecraft from a source that need not be carried with the spacecraft eliminates the argument "it's impossible to carry sufficient fuel to travel interstellar distances under constant acceleration". It's not necessary to do this efficiently, as once we can use power from sources not traveling with the spacecraft, the power available becomes enormous - we can harness entire stars if necessary.

Nobody's arguing that it would be practical or easy - just that it's not physically impossible.

#### steve_bank

##### Diabetic retinopathy and poor eyesight. Typos ...
Imagine a platform in space. On one end is a sheet vertical. Fixed to the plate is a laser. The laser shines on sthe heet. Newton's 3rd Law wouldseem to say there will be an equal and opposite reaction through the mounts to the plate. Like standing on the plate and pushing against the sheet. Make a fist and start punching the sheet and the net change in momentum is zero. Or throw baseballs at the sheet.
The sail you push on isn't fixed to the laser. The sail isn't fixed to anything but the payload. So it accelerates away from the laser.

Yes, that puts a reactive force on the laser, which will accelerate away from the sail with equal and opposite momentum change, unless there's some other force on the laser. So you'd engineer the thing to have another force on the laser. The obvious arrangement would be to add a solar sail fixed to the laser. So the sun pushes on the solar sail (and also powers the laser), the solar sail pushes on the laser, the laser pushes on the starship's sail, and the laser beam pushes the laser back away from the starship toward the solar sail, balancing the forces on the laser, so the laser stays still. The net overall effect is that the starship accelerates away from the home star and a bazillion photons bounce off the two sails, back in the general direction of the home star. Their collective momentum away from the destination star is what balances the starship's momentum toward the destination star.
A recoiless laser?

#### steve_bank

##### Diabetic retinopathy and poor eyesight. Typos ...
In scifi movies they just turn on a light bulb at the rear of the spaceship.

#### Loren Pechtel

##### Super Moderator
Staff member
For that matter, we can already build artificial hearts, so how long can it be before we know how to build an artificial womb? Your robotic factory can be pre-stocked with a freezer full of in-vitro-fertilized embryos. This way we won't need humans to travel any faster than robots. If it takes 10,000 years to reach Alpha Centauri, no big woop, so it takes 10,000 years. If the human race wants interstellar extinction insurance, we must learn patience.

Vernor Vinge, Longshot.

#### Loren Pechtel

##### Super Moderator
Staff member
I think Jimmy Higgins said it, we are adapted to Earth not space.

Lasers do not need on board power supplies? The energy required to accelate a mass by some ampount is required, there is no way around it. Doen't matter what the propulsion is. From the plots I posted even a 1g constant accertion for 1kg quickly becomes impossible. More so for a large spaceship.

Laser-pumped lightsail has no lasers on the ship. The logical place for the laser is around Mercury--it's going to be huge, it needs gobs of power and you have to deal with the competing forces of the sail effect of the solar panels and the thrust from the beam--from a practical standpoint that means it must either be on the surface (night becomes a big issue) or in an "orbit" that can be shifted to a forced orbit to balance the forces involved. (Which will be constantly changing as Mercury goes around the sun.)

#### Loren Pechtel

##### Super Moderator
Staff member
As you move away from the Sun energy density in W/m^2 goes down by 1/r^2. Inverse Square Law. Out from the solar system it is nothing.

Which is why ordinary lightsail is limited to about .01c. By then you're too far from the star to get meaningful acceleration.

With laser-pumped lightsails you can use big enough lenses to compensate.

Lasers were looked at to lift a space levator and there was a small scale demonstraion.

If you have a solar sail what would the force per unit area or pressure be where the laser beam hit the sail? How hot would it ge?

Imagine a platform in space. On one end is a sheet vertical. Fixed to the plate is a laser. The laser shines on sthe heet. Newton's 3rd Law wouldseem to say there will be an equal and opposite reaction through the mounts to the plate. Like standing on the plate and pushing against the sheet. Make a fist and start punching the sheet and the net change in momentum is zero. Or throw baseballs at the sheet.

Actually, this produces thrust--after the light is reflected by the sheet it goes off into space. This effect has actually been measured in practice--there was a longstanding mystery about the Voyager probes being slightly off from where all the math said they should be. They finally figured out that it was radiation from the heat of the RTGs they carry.

#### Loren Pechtel

##### Super Moderator
Staff member
Yes, that puts a reactive force on the laser, which will accelerate away from the sail with equal and opposite momentum change,
Nope, the sail gets twice the effect--the photons hit it and bounce back, as opposed to the laser simply emitting them.

#### Jayjay

##### Contributor
Imagine a platform in space. On one end is a sheet vertical. Fixed to the plate is a laser. The laser shines on sthe heet. Newton's 3rd Law wouldseem to say there will be an equal and opposite reaction through the mounts to the plate. Like standing on the plate and pushing against the sheet. Make a fist and start punching the sheet and the net change in momentum is zero. Or throw baseballs at the sheet.
The sail you push on isn't fixed to the laser. The sail isn't fixed to anything but the payload. So it accelerates away from the laser.

Yes, that puts a reactive force on the laser, which will accelerate away from the sail with equal and opposite momentum change, unless there's some other force on the laser. So you'd engineer the thing to have another force on the laser. The obvious arrangement would be to add a solar sail fixed to the laser. So the sun pushes on the solar sail (and also powers the laser), the solar sail pushes on the laser, the laser pushes on the starship's sail, and the laser beam pushes the laser back away from the starship toward the solar sail, balancing the forces on the laser, so the laser stays still. The net overall effect is that the starship accelerates away from the home star and a bazillion photons bounce off the two sails, back in the general direction of the home star. Their collective momentum away from the destination star is what balances the starship's momentum toward the destination star.
The conversion efficiency of energy from sunlight to laser is probably so bad, that there are better ways to keep the local laser in place than a light sail. If the laser platform is close to the sun, you can resupply it with a propellant from time to time. I suppose there could be a chain of lasers along the way, using e.g. nuclear fusion for power, but even then, it's probably more efficient to use that energy source to shoot ions or something else in the opposite direction to counteract the force from the laser, than it would be to have another laser somewhere in the inner solar system.

Besides, assuming that the laser is powered by solar panels, having some of that real estate be taken by a solar sail that just does nothing except block out the sun seems very impractical.
The object of the exercise is to eliminate the need for the spacecraft to carry fuel.

Once that problem has been solved, then we can start worrying about efficiency and practicality; Though these latter issues don't need to be considered at all if our sole objective is to demonstrate that the inability to carry sufficient fuel need not be a show-stopper.

The physical possibility of powering a spacecraft from a source that need not be carried with the spacecraft eliminates the argument "it's impossible to carry sufficient fuel to travel interstellar distances under constant acceleration". It's not necessary to do this efficiently, as once we can use power from sources not traveling with the spacecraft, the power available becomes enormous - we can harness entire stars if necessary.

Nobody's arguing that it would be practical or easy - just that it's not physically impossible.
I'm not arguing against an interstellar spacecraft with a light sail being pushed by a laser; that makes perfect sense. But there is no point in having a light sail on the laser. It stays in the solar system, and it can either carry fuel / propellant or be restocked if necessary. Or replaced: I would imagine multiple lasers would be better than one anyway, for redundancy purposes.

#### steve_bank

##### Diabetic retinopathy and poor eyesight. Typos ...
Yes, that puts a reactive force on the laser, which will accelerate away from the sail with equal and opposite momentum change,
Nope, the sail gets twice the effect--the photons hit it and bounce back, as opposed to the laser simply emitting them.
Stand in water and push on a boat and it moves. Throw a ball at the sail and the boat will nudge. Stand on the deck and push and it goes nowhere. Throw a ball at the sail and there is an opposite reaction transmitted to the boat by friction through your feet.

If the photons orginate in another inertial frame like the Sun then momtum will be trasfered to the sail. NASA tried an experiment with a solar sail and I believe it failed for some mechanical reason.

When machine guns were first put on helicopters in the VN War, long forward bursts actualy slowed down the helecopter.

Recoiless large caliber rifles mounted on vehicles use springs to absorb the reaction force keeping the vehicle from going backwards.

I'm guessing you would be better off pointing the lasers backwards for propulsion.

#### Bomb#20

##### Contributor
The sail you push on isn't fixed to the laser. The sail isn't fixed to anything but the payload. So it accelerates away from the laser.

Yes, that puts a reactive force on the laser, which will accelerate away from the sail with equal and opposite momentum change, unless there's some other force on the laser. So you'd engineer the thing to have another force on the laser. The obvious arrangement would be to add a solar sail fixed to the laser. So the sun pushes on the solar sail (and also powers the laser), the solar sail pushes on the laser, the laser pushes on the starship's sail, and the laser beam pushes the laser back away from the starship toward the solar sail, balancing the forces on the laser, so the laser stays still. The net overall effect is that the starship accelerates away from the home star and a bazillion photons bounce off the two sails, back in the general direction of the home star. Their collective momentum away from the destination star is what balances the starship's momentum toward the destination star.
A recoiless laser?
Steve, I understand you have poor vision. But if you're going to argue with people you need to try to read what they write and try to understand it. I didn't propose a recoilless laser.

This would all be easier if I could just draw a picture, but I'm no artist. To simplify: the sun pushes the laser away, and the laser pushes its own beam away, and the recoil from the laser beam pushes the laser back toward the sun, and the two forces pushing on the laser cancel each other out. Nothing is recoilless. Momentum is conserved. The other end of the laser beam pushes on the starship. The laser stays here, next to the sun. The starship flies away to Alpha Centauri.

Stand in water and push on a boat and it moves. Throw a ball at the sail and the boat will nudge. Stand on the deck and push and it goes nowhere. Throw a ball at the sail and there is an opposite reaction transmitted to the boat by friction through your feet.
...
I'm guessing you would be better off pointing the lasers backwards for propulsion.
Yes, you already made that argument, and we already understood you. But you say it over and over, because you keep making the same incorrect assumption about what design other people are proposing. Nobody except you is talking about putting a propulsion laser on a starship. Take that picture out of your head.

#### Bomb#20

##### Contributor
I'm not arguing against an interstellar spacecraft with a light sail being pushed by a laser; that makes perfect sense. But there is no point in having a light sail on the laser. It stays in the solar system, and it can either carry fuel / propellant or be restocked if necessary. Or replaced: I would imagine multiple lasers would be better than one anyway, for redundancy purposes.
Well sure there's a point in it -- it makes restocking the propellant unnecessary. Higher up-front costs, lower ongoing costs. Both solutions are feasible, and if you know enough that you can calculate that economic tradeoff hundreds of years in advance of the technology being ready, hey man, will you be my investment advisor?

#### Jayjay

##### Contributor
I'm not arguing against an interstellar spacecraft with a light sail being pushed by a laser; that makes perfect sense. But there is no point in having a light sail on the laser. It stays in the solar system, and it can either carry fuel / propellant or be restocked if necessary. Or replaced: I would imagine multiple lasers would be better than one anyway, for redundancy purposes.
Well sure there's a point in it -- it makes restocking the propellant unnecessary. Higher up-front costs, lower ongoing costs. Both solutions are feasible, and if you know enough that you can calculate that economic tradeoff hundreds of years in advance of the technology being ready, hey man, will you be my investment advisor?
Where are you going to put the laser, that pushes the laser, that pushes the spacecraft? And what's going to keep that laser in one place?

It's not really an economic tradeoff, but physical one. It's less efficient to keep a laser in place with another laser, than it would be to... say, just have two lasers on the same platform pointing to opposite directions. Which itself would be kind of silly if you have access to better sources of propulsion.

Too many lasers! Unless there is a nuclear war in the future and cats develop the ability to shoot lasers out of their mouths.

#### Loren Pechtel

##### Super Moderator
Staff member
I'm not arguing against an interstellar spacecraft with a light sail being pushed by a laser; that makes perfect sense. But there is no point in having a light sail on the laser. It stays in the solar system, and it can either carry fuel / propellant or be restocked if necessary. Or replaced: I would imagine multiple lasers would be better than one anyway, for redundancy purposes.

Something has to be done to keep the laser in place. Rockets would be quite a problem to refuel--remember that the laser is going to be pushed half as hard as the probe, that's a lot of fuel! Thus you either need a gravitational tether (ie, Mercury) or a lightsail tether.

I think you'll also need to do something to handle the final focusing lens. Thinking about it I can't see how it was handled in Flight of the Dragonfly even though he was careful with the numbers otherwise.

#### Loren Pechtel

##### Super Moderator
Staff member
Yes, that puts a reactive force on the laser, which will accelerate away from the sail with equal and opposite momentum change,
Nope, the sail gets twice the effect--the photons hit it and bounce back, as opposed to the laser simply emitting them.
Stand in water and push on a boat and it moves. Throw a ball at the sail and the boat will nudge. Stand on the deck and push and it goes nowhere. Throw a ball at the sail and there is an opposite reaction transmitted to the boat by friction through your feet.

If the photons orginate in another inertial frame like the Sun then momtum will be trasfered to the sail. NASA tried an experiment with a solar sail and I believe it failed for some mechanical reason.

When machine guns were first put on helicopters in the VN War, long forward bursts actualy slowed down the helecopter.

Recoiless large caliber rifles mounted on vehicles use springs to absorb the reaction force keeping the vehicle from going backwards.

I'm guessing you would be better off pointing the lasers backwards for propulsion.

I don't see your point at all.

Put the laser on the ship and you have to power them somehow. Leave the lasers behind and they can use solar power and you're not faced with the tyrrany of the rocket equation. You also get the benefit of twice the momentum out of the same amount of power.

#### Loren Pechtel

##### Super Moderator
Staff member
I'm not arguing against an interstellar spacecraft with a light sail being pushed by a laser; that makes perfect sense. But there is no point in having a light sail on the laser. It stays in the solar system, and it can either carry fuel / propellant or be restocked if necessary. Or replaced: I would imagine multiple lasers would be better than one anyway, for redundancy purposes.
Well sure there's a point in it -- it makes restocking the propellant unnecessary. Higher up-front costs, lower ongoing costs. Both solutions are feasible, and if you know enough that you can calculate that economic tradeoff hundreds of years in advance of the technology being ready, hey man, will you be my investment advisor?
Where are you going to put the laser, that pushes the laser, that pushes the spacecraft? And what's going to keep that laser in one place?

It's not really an economic tradeoff, but physical one. It's less efficient to keep a laser in place with another laser, than it would be to... say, just have two lasers on the same platform pointing to opposite directions. Which itself would be kind of silly if you have access to better sources of propulsion.

Too many lasers! Unless there is a nuclear war in the future and cats develop the ability to shoot lasers out of their mouths.

I think you misunderstand the situation. Nobody is proposing a laser to keep the laser in place.

Note that at one point we are talking about laser-pumped lightsails and another we are simply talking about lightsails. The distinction matters!

The station-keeping lightsails are conventional lightsails using the sun. They hold the laser in place that is firing into deep space to accelerate the starship that is too far out to get meaningful propulsion from the sun. I think a gravitational tether to Mercury would probably be a better option at least at first--but if you have enough starships you'll need to have separate facilities and there's nothing else down there to anchor them to, it will have to be lightsails.

#### Jayjay

##### Contributor
I'm not arguing against an interstellar spacecraft with a light sail being pushed by a laser; that makes perfect sense. But there is no point in having a light sail on the laser. It stays in the solar system, and it can either carry fuel / propellant or be restocked if necessary. Or replaced: I would imagine multiple lasers would be better than one anyway, for redundancy purposes.

Something has to be done to keep the laser in place. Rockets would be quite a problem to refuel--remember that the laser is going to be pushed half as hard as the probe, that's a lot of fuel! Thus you either need a gravitational tether (ie, Mercury) or a lightsail tether.
But if you have a light sail, what's pushing it? If it's another laser, you either need fuel for that or an infinite regress of lasers. If it's the sun, it limits the lasers power and takes up space from solar panels. I suppose you could have combined sail and solar panel... or separate panels that farther away that transfer power via microwaves or lasers.

I think you'll also need to do something to handle the final focusing lens. Thinking about it I can't see how it was handled in Flight of the Dragonfly even though he was careful with the numbers otherwise.
I must have missed what this is about.

#### Swammerdami

Staff member
The Rutherford quote is controversial. Another version gives
Baron Rutherford of Nelson said:
Anyone who says that with the means at present at our disposal and with our present knowledge we can utilize atomic energy is talking moonshine.

(But just as Josephus' writings were corrected posthumously to make Jesus a Messiah, so might a Rutherford-loving editor have wanted to make this great man seem less short-sighted? )

The energy produced by the breaking down of the atom is a very poor kind of thing. Anyone who expects a source of power from the transformation of these atoms is talking moonshine. - Ernest Rutherford
In the long list of great and famous atomic physicists, Lord Rutherford probably belongs in the #1 or #2 slot. Thus I feel obligated to come to his defense! He made this statement in 1933 but Joliot-Curie didn't announce the discovery of artificial radioactivity until 1934. (OTOH it was Rutherford himself who predicted the existence of neutrons — key ingredient for atomic fission — in 1920, and Rutherford's disciple James Chadwick announced their discovery in 1932.)

Some years later, Niels Bohr — perhaps #1 on afore-mentioned list — was also dismissive of power-producing chain reactions at first (though he changed his view with new discoveries, e.g. the concept of neutron moderators).

#### Bomb#20

##### Contributor
Where are you going to put the laser, that pushes the laser, that pushes the spacecraft? And what's going to keep that laser in one place?

It's not really an economic tradeoff, but physical one. It's less efficient to keep a laser in place with another laser, than it would be to... say, just have two lasers on the same platform pointing to opposite directions. Which itself would be kind of silly if you have access to better sources of propulsion.

Too many lasers! Unless there is a nuclear war in the future and cats develop the ability to shoot lasers out of their mouths.
Hey, isn't that guy with a cat in your video Greg? That guy is totally an alien.

Okay, it looks like people are going to make me sit down and prove what I thought was a thoroughly uncontroversial claim*.
Code:
   Alpha Centauri

__________           <- Starship's light-sail (going up)
+               <- Starship
/          \          <- Laser light reflected back from laser-pumped light sail (going down)

/     |     |     \

|     |

|     |            <- Focused laser beam (going up)
(Not to scale. Of course none of the diagram is to scale.  But this bit is really, really not to scale.)
|     |

|     |

<======>            <- Focusing lens (stationary)

\   \  |  |  /  /

\ \ |  | / /           <- Laser beam (going up)

\V/
L                <- Giant Space Laser!!!  What could be more awesome than a Giant Space Laser!!!?
___   A  ___           <- Solar sail that holds laser in place (stationary)
--S--              <- Solar cells to power laser
E
/      R        \        <- Sunlight reflected back from solar sail (going down)

|     |     |     |

|     |     |     |      <- Sunlight (going up)

|     |     |     |

|     |     |     |

Sun

(* I.e., my claim that I'm no artist.)

#### Jayjay

##### Contributor
Where are you going to put the laser, that pushes the laser, that pushes the spacecraft? And what's going to keep that laser in one place?

It's not really an economic tradeoff, but physical one. It's less efficient to keep a laser in place with another laser, than it would be to... say, just have two lasers on the same platform pointing to opposite directions. Which itself would be kind of silly if you have access to better sources of propulsion.

Too many lasers! Unless there is a nuclear war in the future and cats develop the ability to shoot lasers out of their mouths.
Okay, it looks like people are going to make me sit down and prove what I thought was a thoroughly uncontroversial claim*.
Code:
   Alpha Centauri

__________           <- Starship's light-sail (going up)
+               <- Starship
/          \          <- Laser light reflected back from laser-pumped light sail (going down)

/     |     |     \

|     |

|     |            <- Focused laser beam (going up)
(Not to scale. Of course none of the diagram is to scale.  But this bit is really, really not to scale.)
|     |

|     |

<======>            <- Focusing lens (stationary)

\   \  |  |  /  /

\ \ |  | / /           <- Laser beam (going up)

\V/
L                <- Giant Space Laser!!!  What could be more awesome than a Giant Space Laser!!!?
___   A  ___           <- Solar sail that holds laser in place (stationary)
--S--              <- Solar cells to power laser
E
/      R        \        <- Sunlight reflected back from solar sail (going down)

|     |     |     |

|     |     |     |      <- Sunlight (going up)

|     |     |     |

|     |     |     |

Sun

(* I.e., my claim that I'm no artist.)
Sorry, I had misunderstood earlier that the sail on the laser platform would also be pushed by another laser, not sunlight. My bad.

Three points:

The laser platform is likely to be in orbit around the sun, and not always aligned exactly between the sun and the spacecraft. So it can stay "stationary" (in context meaning in stable orbit as if it was not firing the laser) only in very special cases and additional thrust is still needed.

Is sunlight going to give enough "push" to counteract the laser? Ideally, the laser should be VERY powerful indeed, drawing power from a very large array of solar panels.

And lastly, if there are going to be a lot of solar panels on the platform, the light sail would take up precious real estate from the solar panel array. Unless you combine the two, but in that case the efficacy of the light sail is probably going to be compromised a bit because it can't be a fully reflective surface.

#### Bomb#20

##### Contributor
The station-keeping lightsails are conventional lightsails using the sun. They hold the laser in place that is firing into deep space to accelerate the starship that is too far out to get meaningful propulsion from the sun. I think a gravitational tether to Mercury would probably be a better option at least at first--but if you have enough starships you'll need to have separate facilities and there's nothing else down there to anchor them to, it will have to be lightsails.
I probably shouldn't underestimate the brilliance of NASA's choreographers, but it seems to me using a gravitational tether to a planet is a lose. The laser beam has to be kept pointed at the starship with extreme precision -- an error of one arcsecond translates into moving the beam about a hundred million miles away from the starship when it's a couple light years out. Doing that while the laser itself is swinging around the sun, and continually reorienting itself to stay aimed at the focusing lens, and constantly adjusting the lens to compensate, strikes me as a few too many moving parts. Having everything but the starship held stationary sounds a lot easier to get right.

#### Bomb#20

##### Contributor
Three points:

The laser platform is likely to be in orbit around the sun, and not always aligned exactly between the sun and the spacecraft. So it can stay "stationary" (in context meaning in stable orbit as if it was not firing the laser) only in very special cases and additional thrust is still needed.
That's what Loren had in mind; but I think it would be simpler and more reliable not to have it in solar orbit at all, but to hold a fixed position relative to the sun. That means the solar sail will have to fight solar gravity as well as the laser's recoil, but I think that can be handled simply by making the sail bigger.

Is sunlight going to give enough "push" to counteract the laser? Ideally, the laser should be VERY powerful indeed, drawing power from a very large array of solar panels.
Sure. Conversion efficiency of lasers is pretty poor. A lot more photons are hitting the solar panels than are coming out the front of the laser.

And lastly, if there are going to be a lot of solar panels on the platform, the light sail would take up precious real estate from the solar panel array. Unless you combine the two, but in that case the efficacy of the light sail is probably going to be compromised a bit because it can't be a fully reflective surface.
This scheme doesn't work as long as real estate is precious. Space is big, rocket payloads are small -- it's having to fold everything up for launch that makes real estate precious. When we're building stuff in space and mining asteroids for materials with self-reproducing robots, making sails and solar panels by the thousand square kilometer should become economically viable.

#### bilby

##### Fair dinkum thinkum
Where are you going to put the laser, that pushes the laser, that pushes the spacecraft? And what's going to keep that laser in one place?

It's not really an economic tradeoff, but physical one. It's less efficient to keep a laser in place with another laser, than it would be to... say, just have two lasers on the same platform pointing to opposite directions. Which itself would be kind of silly if you have access to better sources of propulsion.

Too many lasers! Unless there is a nuclear war in the future and cats develop the ability to shoot lasers out of their mouths.
Okay, it looks like people are going to make me sit down and prove what I thought was a thoroughly uncontroversial claim*.
Code:
   Alpha Centauri

__________           <- Starship's light-sail (going up)
+               <- Starship
/          \          <- Laser light reflected back from laser-pumped light sail (going down)

/     |     |     \

|     |

|     |            <- Focused laser beam (going up)
(Not to scale. Of course none of the diagram is to scale.  But this bit is really, really not to scale.)
|     |

|     |

<======>            <- Focusing lens (stationary)

\   \  |  |  /  /

\ \ |  | / /           <- Laser beam (going up)

\V/
L                <- Giant Space Laser!!!  What could be more awesome than a Giant Space Laser!!!?
___   A  ___           <- Solar sail that holds laser in place (stationary)
--S--              <- Solar cells to power laser
E
/      R        \        <- Sunlight reflected back from solar sail (going down)

|     |     |     |

|     |     |     |      <- Sunlight (going up)

|     |     |     |

|     |     |     |

Sun

(* I.e., my claim that I'm no artist.)
Sorry, I had misunderstood earlier that the sail on the laser platform would also be pushed by another laser, not sunlight. My bad.

Three points:

The laser platform is likely to be in orbit around the sun, and not always aligned exactly between the sun and the spacecraft. So it can stay "stationary" (in context meaning in stable orbit as if it was not firing the laser) only in very special cases and additional thrust is still needed.

Is sunlight going to give enough "push" to counteract the laser? Ideally, the laser should be VERY powerful indeed, drawing power from a very large array of solar panels.

And lastly, if there are going to be a lot of solar panels on the platform, the light sail would take up precious real estate from the solar panel array. Unless you combine the two, but in that case the efficacy of the light sail is probably going to be compromised a bit because it can't be a fully reflective surface.
Meh. Just build a Dyson Sphere.

Coat the inside surface with solar panels, and put your giant space laser array on the outside.

Job done. Easy.

For a given value of 'easy'.

#### Bomb#20

##### Contributor
Having everything but the starship held stationary sounds a lot easier to get right.
I should add that for a solar sail to hover over the sun instead of orbiting it, the sail material has to be so thin that the ratio of the light pressure to the weight is greater than 1.0. As I understand it, all the solar sail experiments in space so far have used sail material too thick for this, but solar sail material thin enough for hovering has been fabricated in labs.

#### lpetrich

##### Contributor
There are two very big problems with this propulsion scheme:

1. Dilution of the beam by diffraction.

2. Poor feedback for aiming.

Diffraction is from the wave nature of light. It makes a spread angle of roughly (wavelength)/(aperture) radians.

I will consider blue light (500 nm) and the largest telescopes in operation (10 meters). That gives 5*10-8 radians or 10 milliarcseconds. Over 1 AU (average Earth-Sun distance, 150 million km), this gives a beam spread size of 7.5 km.

This also means that the beam must be aimed to within that difference in direction, and that will be *very* difficult.

So I'm completely skeptical about that proposed propulsion method.

#### steve_bank

##### Diabetic retinopathy and poor eyesight. Typos ...
Collimating a laser beam is generally not a problem. The more difficult problem is heat in the collimating optics, absorption. At high power levels even reflective mirrors can heat up.

A monster space laser would inevitably be made into a wapon.

#### bilby

##### Fair dinkum thinkum
Collimating a laser beam is generally not a problem. The more difficult problem is heat in the collimating optics, absorption. At high power levels even reflective mirrors can heat up.

A monster space laser would inevitably be made into a wapon.
Any technology that can get you into space can be used as a weapon.

Shit, I am struggling to think of any technology of any description that couldn't be used as a weapon.

#### steve_bank

##### Diabetic retinopathy and poor eyesight. Typos ...
Adding the laser does not need to be narrowly collimated. Assuming no loss from absorption in dust the total power is constant across the wavefront as it expands. As long as the diameter of the spot is smaller than the sail all the power goes into the sail.

A retro reflector was placed on the moon to laser range distance. You can probably look up power and spot size.

#### Loren Pechtel

##### Super Moderator
Staff member
I think you'll also need to do something to handle the final focusing lens. Thinking about it I can't see how it was handled in Flight of the Dragonfly even though he was careful with the numbers otherwise.
I must have missed what this is about.

By Robert L. Forward. He was a hard sci-fi author and that book involves a laser-pumped lightsail starship. Thus it's got a decent amount of detail and an appendix that dives deeper into the numbers. (There are other books in the series but they are all set at the destination. They have their lightsail to get around but no laser pump.)

#### Loren Pechtel

##### Super Moderator
Staff member
The station-keeping lightsails are conventional lightsails using the sun. They hold the laser in place that is firing into deep space to accelerate the starship that is too far out to get meaningful propulsion from the sun. I think a gravitational tether to Mercury would probably be a better option at least at first--but if you have enough starships you'll need to have separate facilities and there's nothing else down there to anchor them to, it will have to be lightsails.
I probably shouldn't underestimate the brilliance of NASA's choreographers, but it seems to me using a gravitational tether to a planet is a lose. The laser beam has to be kept pointed at the starship with extreme precision -- an error of one arcsecond translates into moving the beam about a hundred million miles away from the starship when it's a couple light years out. Doing that while the laser itself is swinging around the sun, and continually reorienting itself to stay aimed at the focusing lens, and constantly adjusting the lens to compensate, strikes me as a few too many moving parts. Having everything but the starship held stationary sounds a lot easier to get right.

If the laser is stationary it's going to have to be supported by something or fall into the sun.

#### Loren Pechtel

##### Super Moderator
Staff member
Collimating a laser beam is generally not a problem. The more difficult problem is heat in the collimating optics, absorption. At high power levels even reflective mirrors can heat up.

A monster space laser would inevitably be made into a wapon.

The Kzinti Lesson

And note that it's not just reaction drives. The Lensman series didn't use reaction drives but Jarvenon can tell you the drive was weaponized. (That is, the object that used to be Jarvenon.)

#### Loren Pechtel

##### Super Moderator
Staff member
Adding the laser does not need to be narrowly collimated. Assuming no loss from absorption in dust the total power is constant across the wavefront as it expands. As long as the diameter of the spot is smaller than the sail all the power goes into the sail.

A retro reflector was placed on the moon to laser range distance. You can probably look up power and spot size.

As the target moves farther away you will need very good collimation to keep the laser spot on the sail.

#### lpetrich

##### Contributor
Am I the only one who is doing any calculation here?

I'll use the Sun's light at 1 AU as a comparison. - at solar-activity minimum, it is 1361 watts per square peter, and at maximum, 1362 W/m2.

I'll use the minimum value. To get the solar-radiation pressure, I divide by c, giving 4.54*10-6 pascal or 4.54*10-11 bar.

Ignoring reflection, a solar sail with 1 gram per square meter, 1 micron thick for density of water, will have an acceleration of 4.54*10-3 m/s2 Reflection will increase this number by at most a factor of 2, and reradiation by less than that.

Since the solar flux obeys the inverse-square law, the terminal velocity is sqrt(2*a0*r0) where the acceleration is a0 at distance r0. It is 36.9 km/s ignoring reflection and 52.1 km/s for a perfect reflector.

That means that it will be VERY hard to get a reflection-powered spacecraft to go anywhere close to c.

#### Bomb#20

##### Contributor
If the laser is stationary it's going to have to be supported by something or fall into the sun.
Certainly. It's supported by the station-keeping lightsail.