# QuestionSolar sail acceleration

#### JLivingstonSG

Hi.
What if we build hundreds of large solar reflectors that can then focus sunlight on a solar sail space craft.
How many square miles of sun reflectors would be needed to accelerate a spacecraft at 1 g, on lets say 100 tons to 90% of the speed of light in lets say 3 years ?
Regards from jlivingstonsg

#### JLivingstonSG

What if you build a Dyson sphear and make large parts of it as a sun reflector to accelerate sun sail spacecrafts ....

Interesting ?

#### JLivingstonSG

I got this answer in another forum
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The force exerted by light is F = P/c (where P is the power) for absorption, and 2P/c for perfect reflection.
You need P = mgc/2 which, for just 1 kg, is a little less than 1.5 Gigawatts.
At the distance of the earth, sun intensity is around 1.3-1.4 kW/m^2, so you need around one million square meters.
And you’ll likely fry the solar sail, because you don’t have perfect reflection.
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Interesting?

MagI
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#### Helio

I'm not following that solution all that well.

With R being the distance in AU from Sun, it's easy to get the force per unit area from the Sun using the equation:
9e-6 /R^2, which gives N/m^2 From here.

[This assumes a perfect reflection efficiency]

Knowing the force, you can easily get the acceleration for a given mass (a = F/m).

But this force is reduced in accord with inverse square law.

Also, at more relativisitc speeds, the photons have less energy due to redshift, also affecting accelaration.

[I tried p = 2E/c, where p is momentum and E is energy (J). But that gives Joules/m, not Joules-sec as needed for force. So I'm forgeting something, obviously.]

Another solution found is using:

F = 2*L*a* cos() / (4*pi*r^2 * c), where r is the radius of the sail; L is the luminosity of the Sun; a is the sail area. Cos() is 1 when the sail is perpendicular to light flux.

JLivingstonSG

#### iconoclast

BANNED
Hi.
What if we build hundreds of large solar reflectors that can then focus sunlight on a solar sail space craft.
How many square miles of sun reflectors would be needed to accelerate a spacecraft at 1 g, on lets say 100 tons to 90% of the speed of light in lets say 3 years ?
Regards from jlivingstonsg
Hundreds of large solar reflectors ARE a big solar sail. You dispense with the small sail in the concentrated reflected light.

#### JLivingstonSG

Hundreds of large solar reflectors ARE a big solar sail. You dispense with the small sail in the concentrated reflected light.
Can you explain what you mean?

#### iconoclast

BANNED
Can you explain what you mean?
A solar sail IS a solar reflector. A big shiny piece of material that reflects light, and uses the recoil for propulsion - a solar sail. A solar reflector does the same thing. It acts exactly like a solar sail. They are the same thing.

#### JLivingstonSG

Yes.
So to build these large reflectors they need to be attached to large heavy objects like moons.

#### iconoclast

BANNED
Yes.
So to build these large reflectors they need to be attached to large heavy objects like moons.
OK. You really should explain your configuration if you want to be understood. It appears you want to build a huge reflector near the earth to power a solar sail, to a distance of almost light years. WILL NOT WORK. Even at a distance much less than Mars, let alone a light year, your solar sail will be infinitely small and impossible to hit with any fraction of your reflected light. So your answer is your configuration would not work. And you should think this configuration through before you use the people on here as your own private calculators to compute your ill-defined and unworkable problem.

#### JLivingstonSG

OK.
You really should explain your configuration if you want to be understood. It appears you want to build a huge reflector near the earth to power a solar sail, to a distance of almost light years.
WILL NOT WORK.
Even at a distance much less than Mars, let alone a light year, your solar sail will be infinitely small and impossible to hit with any fraction of your reflected light.
And you should think this configuration through before you use the people on here as your own private calculators to compute your ill-defined and unworkable problem.
This can be used to both trips between our planets as to other stars.

I understand that there is problems with this idea with technology of today.
But we will have artificial intelligens with exponential IQ development, in some years.
And we do not know what kind of technology can be developed in the next 1oo to 1000 years.

So I believe it is a good idea to see what this idea with hundreds of reflectors,
spread out in our solar system, can turn in to.

Many people did not think it would be possible to measure gravitation waves.
Now we do.

And one way to get to speeds close to the speed of light is by not taking the energy on to the spacecraft.
The weight will be to large.

So one way that is possible is to reflect sunlight to a sun sail space craft because that solve the problem with no energy on the space ship.

So lets say we build hundreds of reflectors in our solar system and focus it on sun sail space ships to get to our planets,
in reasonable times and also focus on how to possibly accelerate the sun sail ship fast enough,
before it is to far away and to smal to focus the light beam on.

Lets say that this sun sail ship release some kind of lenses,
after its path, that focus the light beam from all the reflectors,
as it moves out to the distance star.

So all the reflectors just have to aim att the first lens.
Then the beam will be aimed to the next lens, and the next and so on,
until the beam reach the sun sail ship.

Lets not say "IT WILL NOT WORK"

Lets instead say how do we solve this problem.....

Regards from Sweden
MagI
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TacovilleMC

#### iconoclast

BANNED
This can be used to both trips between our planets as to other stars.

I understand that there is problems with this idea with technology of today.
But we will have artificial intelligens with exponential IQ development, in some years.
And we do not know what kind of technology can be developed in the next 1oo to 1000 years.

So I believe it is a good idea to see what this idea with hundreds of reflectors,
spread out in our solar system, can turn in to.

Many people did not think it would be possible to measure gravitation waves.
Now we do.

And one way to get to speeds close to the speed of light is by not taking the energy on to the spacecraft.
The weight will be to large.

So one way that is possible is to reflect sunlight to a sun sail space craft because that solve the problem with no energy on the space ship.

So lets say we build hundreds of reflectors in our solar system and focus it on sun sail space ships to get to our planets,
in reasonable times and also focus on how to possibly accelerate the sun sail ship fast enough,
before it is to far away and to smal to focus the light beam on.

Lets say that this sun sail ship release some kind of lenses,
after its path, that focus the light beam from all the reflectors,
as it moves out to the distance star.

So all the reflectors just have to aim att the first lens.
Then the beam will be aimed to the next lens, and the next and so on,
until the beam reach the sun sail ship.

Lets not say "IT WILL NOT WORK"

Lets instead say how do we solve this problem.....

Regards from Sweden
MagI
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I may as well ask you how long it would take 10 Disney fairies to push my spaceship to a star. No mention of size of ship, which star, and you answer that future tech will invent fairies. The problem is not solvable. You would be diffraction limited on delivering light to an infintisimally small area, and the tech problems ARE insurmountable. All of a square miles reflector would need to be positioned to atomic accuracy, and the position would not even be known. Just for starters. No future tech is going to do this. And to say lets instead solve the problem is infinitely ludicrous. Stop trolling.

#### Pogo

Even if you built an earth-size mirror, it could deliver less than 0.0001 that of what the sun itself delivers, minuscule. You would have to built a sun-size mirror to double the amount of light delivered, and would likely fry the sail as mentioned above. Sunlight, not being coherent, would spread out within a few AU, rendering it kinda useless. Laser light would remain coherent much farther. So, does Amazon sell a sun-size variable laser?

TacovilleMC

#### JLivingstonSG

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Lets play with this idea...

So lets say we build a partly dyson sphere, at the mercury orbit distance from the sun
and the inside is full of reflectors, that can be aimed to solar sail space ships.
That mean that much of the suns light, can be aimed at a spaceship, to give it a 1 g acceleration
for hopefully a long enough time, to get to a speed close to the speed of light.

Is this technology possible to get us to other stars?

JLivingstonSG from Sweden

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TacovilleMC

#### TacovilleMC

I'm just thinking, how would you slow it down, once it reached it's destination? Tons of gravitational assists, or just let it go forever?

Could be an interesting idea for unmanned probes, but i doubt it would be the solution for interstellar travel.

#### TacovilleMC

It's not a solution to anything, this guy is just putting together random sentences that he's heard in the popular literature, and claiming that future tech developments will make it all possible, we just need faith and try, even circumventing basic physics laws like diffraction, angular size etc.
I'm certainly not saying that it is likely, especially anytime soon, but it is not entirely impossible. I'm just considering that technology often advances by overcoming things thought to be impossible.

#### Helio

I'm certainly not saying that it is likely, especially anytime soon, but it is not entirely impossible. I'm just considering that technology often advances by overcoming things thought to be impossible.
Yes, and there was a somewhat recent article here proposing the use of lasers, of immense power, to push a very lightweight and tiny probe to about 0.2c, which would allow it to arrive at Alpha Centauri in about 21 years. It would not be able to slow down, of course, but it could, given a host of other steps, send us images.

It should be noted that we can't combine regions of sunlight into a concentrated beam of light. Optics, including mirrors, don't work that way. That doesn't mean mirrors reflecting sunlight couldn't be finely adjusted to shine on the moving probe, but it sure would be tricky to make that work, IMO.

TacovilleMC

#### JLivingstonSG

---
What if it is possible to develop a reflecting material ,
that in a parabola, reflect different light frequency just right, so that most light frequencies is then reflected
and focused at a specific focus point, far away.
Maybe it is possible to adjust this reflecting material (with volt/amp?)
so that it can be adjusted to reflect every light frequency just right to be able to focus most light frequencies
from the sun to the space ship.

and so on....

MagI
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#### iconoclast

BANNED
I'm certainly not saying that it is likely, especially anytime soon, but it is not entirely impossible. I'm just considering that technology often advances by overcoming things thought to be impossible.
No. It is entirely impossible. Technological advances cannot overcome the fact that diffraction exists and does not allow this to happen, for example. Transmitting light over these interstellar distances is diffraction limited. This is a fundamental law of physics - no amount of ingenuity or technical advance can overcome it.

#### TacovilleMC

No. It is entirely impossible. Technological advances cannot overcome the fact that diffraction exists and does not allow this to happen, for example. Transmitting light over these interstellar distances is diffraction limited. This is a fundamental law of physics - no amount of ingenuity or technical advance can overcome it.
Einstein said the same thing about black holes, and look how that turned out.

JLivingstonSG

#### Helio

---
What if it is possible to develop a reflecting material ,
that in a parabola, reflect different light frequency just right, so that most light frequencies is then reflected
and focused at a specific focus point, far away.
Maybe it is possible to adjust this reflecting material (with volt/amp?)
so that it can be adjusted to reflect every light frequency just right to be able to focus most light frequencies
from the sun to the space ship.
Yes, this is possible, but extremely difficult since in only a few days at 1g, the ship will be billions of miles away and extremely difficult to focus upon.

I'd bet it would be far more favorable to focus on the use of lasers or propulsion device to save both in expense and trouble.

As a woman entrepreneur once told me a few decades ago about a challenging project, "It's too much squeeze for the juice."

TacovilleMC

#### TacovilleMC

Yes, this is possible, but extremely difficult since in only a few days at 1g, the ship will be billions of miles away and extremely difficult to focus upon.

I'd bet it would be far more favorable to focus on the use of lasers or propulsion device to save both in expense and trouble.

As a woman entrepreneur once told me a few decades ago about a challenging project, "It's too much squeeze for the juice."
Just gonna copy/paste that quote onto my pc real quick, don't mind me...

Helio

#### iconoclast

BANNED
Einstein said the same thing about black holes, and look how that turned out.
Einstein never said anything about technological advances with regard to black holes. Get your story straight if you are going to troll.

#### Helio

Einstein never said anything about technological advances with regard to black holes. Get your story straight if you are going to troll.
That seems a little unfair. Einstein said that they don't exist after Schwarzschild's solution for GR suggested they might. Einstein likely thought there wouldn't be events that would produce them. I think he later was more open to this idea.

TacovilleMC

#### iconoclast

BANNED
That seems a little unfair. Einstein said that they don't exist after Schwarzschild's solution for GR suggested they might. Einstein likely thought there wouldn't be events that would produce them. I think he later was more open to this idea.