# Accelerate sunsail with sun reflectors

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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

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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 a million square meters. And you’ll likely fry the solar sail, because you don’t have perfect reflection.

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There is also the factor of what percentage of light that your reflector sends can actually hit the solar sail. The further it goes, the harder that the target is to hit. You might start off with nearly 100% of the light hitting the sail, but you would probably end up closer to 0%.

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I bet it is possible to develope this technology to make this work in 100 to 1000 years with artificial intelligens that have an IQ around 200 000 .

We can not today even imagin what technology we will have in some hundred years.

Because this is probably a strong candidate to get to other stars because we need speeds close to speed of light

to slow down time to get to other stars in some years.

The astronaut will be able to calculate his speed to be 10 to 20 times faster than the speed of light because of the

time dilation in the spacecraft.

MagI

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2 hours ago, jlivingstonsg said:

The astronaut will be able to calculate his speed to be 10 to 20 times faster than the speed of light because of the

time dilation in the spacecraft.

Can you please expand on this? I'm not sure what you mean.

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If you travel close to the speed of light,  then the time slow down in the spacecraft.

So lets say you travel 100 light years to another star and our sun is beaming a photon beam to push it,

so that the sun sail spacecraft accelerate at 1 g half the way.

In this webpage it calculate a deacceleration the en half of the trip at the same 1 g.

(This can be done if we first send many solar sails to the stars that can deaccelerate our ship.

These large reflectors can maybe be built to stand the heat and deaccelerate close to the star or something..... )

Set up the page like this and then click "Calculate"

Then you get these numbers.

You can see that from earth, the time to go to the star 100 lightyears away will in the earth timeframe take about 101,9 years to go to the star

and from earth they will see that the ship move close to the speed of light around max  0,9998 % of the speed of light.

On the ship the time slow down and you can see that in the timeframe for the astronauts,  the trip will take  about  9 years.

So the astronauts can calculate their average speed is about 10 times faster than the speed of light in their timeframe.

(They never moved faster than the speed of light but when you move so close to the speed of light, then the time slow down.)

Regards from Sweden

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I am not very knowledgeable of the details so please excuse any foolish questions, but don't the astronauts calculate their distance to the distant star to be much shorter than 100 light years, thus making their calculation of average speed to be something less than c?

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That is if they calculate the distance and the time when they travel close to speed of light.

The shorter distance/travel time will be a speed slower that the speed of light and that is correct.

But we are doing the calculation when they have arrived and  an hour is the same as on earth.

If they now just calculate the distance that is about 102 light years/9 years they get a travel speed about 10 times faster than the speed of light.

Regards from Sweden

MagI

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Edited by jlivingstonsg
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So you are calculating speed by using the time as measured in one frame and the distance as measured in a different frame?

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When I am on the other planet 100 light years from earth I can calculate what speed I had.

You calculate distance divided by time to get speed.

The distance between earth and the star system I am on is 100 light years.

The time it took to travel was 9 years.

100/9 = about 10 light years per year

Regards MagI

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Thanks for all of this, including the calculators!

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