Leaving The Solar System

[Loren Pechtel]

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]

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.

EDITED TO ADD: Just read your second post which addresses this point. Nevermind.

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]

The Rutherford quote is controversial. Another version gives

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]

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*.

   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]

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*.

   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]

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]

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]

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*.

   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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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

I'll use the Sun's light at 1 AU as a comparison.  Solar constant - 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.

 

[skepticalbip]

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

Obviously.

 

[Bomb#20]

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.