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Leaving The Solar System

Swammerdami

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In photosynthesis, incoming light energy is converted to electrical energy (an "exciton") by "light-harvesting antennae" and bounces around until it transfers energy to the "reaction center." If the exciton bounced at random it would almost always dissipate as heat energy before chancing on the reaction center, but in fact the process is remarkably efficient. A group led by Graham Fleming at UC Berkeley has detected the signature of quantum tunneling in this initial stage of photosynthesis in green sulfur bacteria. The exciton is a superposition of states and, just as in a quantum computer, collapse occurs into a desirable state: specifically the state which allows the exciton's energy to be utilized by the following biochemical processes.

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.

The beam is composed of quantum particles whose distribution of destination probabilities can be cleverly manipulated, much as quantum computers or photosynthesis work. This allows the power-receiving antennae to be much smaller than would otherwise be necessary. This is not trivial, but we are positing a very advanced technology, right?

Infidels who wish details will first need to sign an NDA.
 

steve_bank

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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.
A common beam sensor is a quadrant detector. Four sensors arranged in a square. Relative intensity is used to sense position.

On the sail arrange sensors. Detect the center of the beam and radio back corrections. Complicated by the radio delay but fast response is not needed.

A quetion comes to mind, once you get going how do you stop?
 

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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.
The diffraction problem is the reason this scheme typically involves a many-kilometer-wide Fresnel lens several AUs from the laser. The spread angle beyond that will be determined by the aperture of the lens rather than the aperture of the laser. And yes, in a perfect world you'd use the shortest wavelength light you can get; but in practice you probably won't get a choice -- it will be forced by the reflective and transmissive properties of the sail material.

Certainly aiming will be a problem; but taking for granted that measurement and control capability will continue to improve steadily is just about the least speculative forecast of advancing technology we can make. Two hundred years ago, who'd have believed we could measure the distance to the moon with a precision of 1.1 mm?

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.
This is why solar sails are just for getting around the solar system -- for a starship you need the light sail to be laser-pumped.

That means that it will be VERY hard to get a reflection-powered spacecraft to go anywhere close to c.
Well, that's true even with a laser. The "Breakthrough Starshot" people are planning for 0.15c.
 

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As the target moves farther away you will need very good collimation to keep the laser spot on the sail.
A common beam sensor is a quadrant detector. Four sensors arranged in a square. Relative intensity is used to sense position.

On the sail arrange sensors. Detect the center of the beam and radio back corrections. Complicated by the radio delay but fast response is not needed.
I think it's unrealistic to expect the laser to be corrected to match the probe position -- if the probe ever drifts out of the beam the mission is over. It will be up to the probe to stay in the beam; the best the beam can do is stick very accurately to where it promised to stay so the probe knows where it's going to need to be.

A quetion comes to mind, once you get going how do you stop?
The most plausible-sounding option I've heard of is a magnetic parachute -- you're basically using the destination star's solar wind as a braking medium.
 

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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.
It only takes one very small particle to do a lot of damage to a ship. Between the stars, there is very little light so some sort of radar would be needed. The first problem is that you would need a very short wavelength say 0.01mm or shorter. Otherwise bigger particles would not be detectable. Suppose you are going at 1% of the speed of light and want at least 100 seconds warning. You would need to detect the particle at least 1 light second away. You can add a bit because you may not be looking at every piece of sky at the same time. Plus it might take a few seconds from receiving the echo to work out that the particle would hit the rocket. Several echos would be needed to work out the course of the particle, so detection even further out would be needed. This would require a lot of power for the flashlight.
The wavelength of the flashlight would also have to be one that is not heavily emitted by stars. Otherwise, the light of the stars would hide the particles.
 

skepticalbip

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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.
It only takes one very small particle to do a lot of damage to a ship. Between the stars, there is very little light so some sort of radar would be needed. The first problem is that you would need a very short wavelength say 0.01mm or shorter. Otherwise bigger particles would not be detectable. Suppose you are going at 1% of the speed of light and want at least 100 seconds warning. You would need to detect the particle at least 1 light second away. You can add a bit because you may not be looking at every piece of sky at the same time. Plus it might take a few seconds from receiving the echo to work out that the particle would hit the rocket. Several echos would be needed to work out the course of the particle, so detection even further out would be needed. This would require a lot of power for the flashlight.
The wavelength of the flashlight would also have to be one that is not heavily emitted by stars. Otherwise, the light of the stars would hide the particles.
Being able to detect the dust would be of little use at those velocities because there would be no time to avoid it unless it is an extremely long range detection. An ablation shield in front of the ship to absorb the impact (or a Star Trek "force field") would be needed.
 

steve_bank

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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.
Yes, equal and opposite reaction on the sail from a change in momentum of the particle, as in a tank of hot gas resulting in a pressure on the tank.'

The main problem is space is heat.

Work out the efficiencies and the energy need from the laser. Then work out how much radiator area is needed to get rid of the heat. You can look at info on the ISS coling system and estimate area vs power. Also in high power laser optics lenses get hot, they are not 100% efficient.

The space shuttle with its small electrical and human heat load could not stay up if the cargo doors could not open exposing the radiators.

If you are going to use solar panels how much area for the energy demand?

A+ for rthe idea I woud not have thought of it, the rest is engineering.
 

steve_bank

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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.
Newton’s Law Of Equal And Opposite Reaction

Imagine you are in a rectangular box in space. Fix a gun to the wall on one end. Pull the trigger and a bullet leaves the gun. As the bullet leaves the gun there is an equal and opposite reaction against the wall resulting in a net change in momentum of the box.

When the bullet strikes the opposite wall momentum is transferred to the box with an opposite signed magnitude(vector) resulting in a net zero change in momentum of the box.

Photons have momentum. Mount a solid state laser on a wall in te box. As a photon emerges from an atom in the laser an equal and opposite reaction transmitter through inter atomic forces through the laser material to the package to the wall. When the photon strikes the opposite wall the net chamge in momentum is zero in the box.
 

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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.

Actual terminal velocity for a solar sail is about .01c.

However, note that we are talking about laser-pumped sails--your math on the energy hitting the sail is irrelevant. The only limit on how much energy you can hit it with is how much it can take without damage.
 

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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.
A common beam sensor is a quadrant detector. Four sensors arranged in a square. Relative intensity is used to sense position.

On the sail arrange sensors. Detect the center of the beam and radio back corrections. Complicated by the radio delay but fast response is not needed.

A quetion comes to mind, once you get going how do you stop?

The feedback loop is too long to be of much value. The sail simply has to track the beam. However, that does nothing about collimation--you need to keep the laser spot down to the size of the sail or the energy just goes past, wasted.

There are three options to slow:

1) If you've already slowed to about .01c by arriving you can brake on the sail itself. This gives a transit above .01c but not by all that much.

2) A laser at the destination system--the best approach if possible.

3) Split the sail--keep the inner 30%, release the rest. Turn over, use the jettisoned ring to reflect the beam back at the part you retain. I have my doubts about holding the sail flat enough to pull this off.
 

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Between the stars, there is very little light


The wavelength of the flashlight would also have to be one that is not heavily emitted by stars. Otherwise, the light of the stars would hide the particles.

You can't have it both ways. If there's enough starlight to drown your detection beam, there's enough to see the thing you wanted to detect.
 

steve_bank

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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.
A common beam sensor is a quadrant detector. Four sensors arranged in a square. Relative intensity is used to sense position.

On the sail arrange sensors. Detect the center of the beam and radio back corrections. Complicated by the radio delay but fast response is not needed.

A quetion comes to mind, once you get going how do you stop?

The feedback loop is too long to be of much value. The sail simply has to track the beam. However, that does nothing about collimation--you need to keep the laser spot down to the size of the sail or the energy just goes past, wasted.

There are three options to slow:

1) If you've already slowed to about .01c by arriving you can brake on the sail itself. This gives a transit above .01c but not by all that much.

2) A laser at the destination system--the best approach if possible.

3) Split the sail--keep the inner 30%, release the rest. Turn over, use the jettisoned ring to reflect the beam back at the part you retain. I have my doubts about holding the sail flat enough to pull this off.

How about surfing gravity waves? Or a paddle wheel that dips into another another dimension?
 

steve_bank

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The laser can be aplitude modulated which can be used to electronically screen out everything else. A common technique in different forms to detect a light signal in the presence of sunlight.

Removing a DC offset from an AC signal.
 

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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.

Can a lightsail support a hover like that?
Divide the force of sunlight on a square meter of sail, 9.08e-6 kgm/s/s, by the gravitational acceleration of the sun, 5.83e-3 m/s/s, and you get the maximum mass of the sail material in order not to fall into the sun, 1.56e-3 kg. So as long as the sail material is lighter than about one and a half grams per square meter, yes, a light-sail can support hovering, assuming 100% reflectivity. 90% is more realistic. Some light goes right through a thin film. Scale the weight limit down accordingly. (This is calculated at the height of Earth's orbit, but since sunlight and gravity both follow the inverse square law, it's the same answer at all hovering heights.)
 

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Between the stars, there is very little light


The wavelength of the flashlight would also have to be one that is not heavily emitted by stars. Otherwise, the light of the stars would hide the particles.

You can't have it both ways. If there's enough starlight to drown your detection beam, there's enough to see the thing you wanted to detect.
I can have it both ways.
 

skepticalbip

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Between the stars, there is very little light


The wavelength of the flashlight would also have to be one that is not heavily emitted by stars. Otherwise, the light of the stars would hide the particles.

You can't have it both ways. If there's enough starlight to drown your detection beam, there's enough to see the thing you wanted to detect.
I can have it both ways.
Hey, it's hard to dispute such a pithy argument.
 

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The first space craft sent to another system will probably just do a fly-by and not decelerate at all (bar some gravitational maneuver). For colonization, you probably want to first send the smallest possible craft capable of harvesting resources and building an army of robots in the destination system. Then a craft can build more lasers that will both help decelerate incoming crafts as well as shoot new "seed ships" to new systems.

The catch is that the first craft will need to carry fuel for deceleration phase which means more mass and slower acceleration and average speed. The faster you want to go, the more dominant the mass of the fuel is going to be compared to the payload or the solar sail.
 
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