# QuestionSolar sail acceleration

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#### Moon

I don't think that hundreds of large solar reflectors that can then focus sunlight on a solar sail spacecraft will work because the metal will burn up really fast if that's what they would be used for. And plus it wouldn't work because we don't have metal that works for us and the sun is too powerful.

#### Dratonia

I don't think that hundreds of large solar reflectors that can then focus sunlight on a solar sail spacecraft will work because the metal will burn up really fast if that's what they would be used for. And plus it wouldn't work because we don't have metal that works for us and the sun is too powerful.
While this would be a problem right now, by the time we are sending probes out of the solar system, we will probably have far stronger and lighter alloys, and the only real problem will be light refraction, which would be solved with lazers.

#### suneritz

May be the sun's gravity could be used as well by swinging probe around it to build up speed. Later firing small thrusters to free itself this orbit in a specific direction. Just look at how fast Parker probe is currently flying.

#### billslugg

May be the sun's gravity could be used as well by swinging probe around it to build up speed. Later firing small thrusters to free itself this orbit in a specific direction. Just look at how fast Parker probe is currently flying.
The Parker probe is going fast only by virtue of how far down the Sun's gravity well it has fallen. It will lose the same amount of speed as it exits the well, with no net gain.
A probe can use any of the planets orbiting the Sun to gain speed and thus exit the Solar System, but not the Sun itself. A probe interacting with the Sun is a "two body" problem. No two body interaction can eject one of the two. A "three body " problem is required such as interacting with a planet that is going around the Sun.

suneritz

#### JLivingstonSG

---
Would it be interesting to boost a solar sail spacecraft ,
so that it accelerate more,
by using mega-large mirrors in space
to reflect more sunlight to it?

MagI
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#### zzroom

Reflecting material / more = more weight = more weight = more heat
more heat means more degradation = failure Z
\\

#### billslugg

Any space mirror designed to accelerate an insterstallar vehicle does not need to be made of anything better than Kapton. In solar orbit, kapon lasts probably thousands of years. The boost we need to give to the space ship puts it going so incredibly fast that it is very soon out of sight. There is a proposal in the funding system called Breakthrough Starshot that propels with a bunch of ground based lasers an amount of light roughly equal to current world production capacity boosted by capacitors. The star ships weigh in at one gram and have a large solar sail. Millions would be launched hoping at least one will make it to the next star, and radio us back here at home. When the initial laser blast goes off and starts pushing them, they are out of laser range in ten seconds. The amount a laser spreads its signal is helped by going to shorter wavelengths and wider sources and they will take every advantage of all of that and still, after ten seconds, the ship is pretty much gone. Simple solar reflectors can't even do that well.
The proposed acceleration is 10,000 g for ten seconds. This would put the velocity, after one second, to be 1,000 kilometers per second and its distance 5,000 kilometers, by the end of the first day it is half way to the Sun.

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#### Dratonia

Any space mirror designed to accelerate an insterstallar vehicle does not need to be made of anything better than Kapton. In solar orbit, kapon lasts probably thousands of years. The boost we need to give to the space ship puts it going so incredibly fast that it is very soon out of sight. There is a proposal in the funding system called Breathrough Starshot that propels with a bunch of ground based lasers an amount of light roughly equal to current world production capacity boosted by capacitors. The star ships weigh in at one gram and have a large solar sail. Millions would be launched hoping at least one will make it to the next star, and radio us back here at home. When the initial laser blast goes off and starts pushing them, they are out of laser range in ten seconds. The amount a laser spreads its signal is helped by going to shorter wavelengths and wider sources and they will take every advantage of all of that and still, after ten seconds, the ship is pretty much gone. Simple solar reflectors can't even do that well.
The proposed acceleration is 10,000 g for ten seconds. This would put the velocity, after one second, to be 1,000 kilometers per second and its distance 5,000 kilometers, by the end of the first day, it is half way to the Sun.
*Eurobeat starts playing in background*

#### iconoclast

BANNED
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.

You don't have to worry about relativistic effects, you will NEVER get ANYWHERE near enough velocity. 1/r^2 baby, an initially trivial acceleration dies away quickly as you move a bit away from your star. For our sun, a milli g acceleration at Earth's orbit becomes less than a micro g out near Uranus, and continues to drop. You won't even catch up to a Voyager.

#### JLivingstonSG

Reflecting material / more = more weight = more weight = more heat
more heat means more degradation = failure Z
\\

I am not talking about haveing larger solar sails on the space craft.
I am talking about placing reflectors around the sun
to reflect more sunlight to the solar sail ship.
What if it was possible to build a dyson sphere
and use half of it to focus half the suns energy onto a solar sail space craft.

Could it work?

Regards from Sweden

#### Catastrophe

##### "Science begets knowledge, opinion ignorance.
A sail would have to be large, very thin and fragile. You then have to worry about too much energy which could burn them up and destroy them. You are caught in the '22' between fragility of sail and power which falls off with the inverse square law. That means you would have to increase power as lightsail gets further and further away.

Don't forget this:

There is a very good article in Astronomy May 2021, entitled Breakthrough Starshot: A Voyage to the Stars. Essential reading for anyone who thinks that this is a practical means of space travel.

"A necessary requirement for the Breakthrough Starshot mission is keeping the mass of each sail-equipped Starchip to just a few tenths of an ounce (a few grams). . . . . . . likewise the solar sail itself, which is expected to span up to around 13 feet (4 metres), will need to weigh in at less than 0.035 oz (1 gram). It will also need to be extremely thin, as otherwise the sail would absorb far too much heat and be vaporized by the barrage of laser light . . . . . . " the barrage of laser light to drive a few grams??
"Mass is the bane of accelerating objects to great speeds. To significantly increase the velocity of a heavy object takes a tremendous amount of energy. So, if the goal is to reach a distant star in a reasonable amount of time, say within a generation, a spacecraft must be extremely tiny and, therefore, robotic. Plus, it still requires an insanely energetic boost to get up to speed."
" . . . propelling a lightsail-equipped nanocraft, or Starchip, would require hundreds of individual lasers ,spanning roughly 200 acres (1 square kilometer). The array would also need access to enough energy to fire a coherent 100 gigawatt laser beam for several minutes during each . . . launch. That's roughly the amount of power generated by all the nuclear power plants in the U.S. in a given year."

Another problem occurred to me, which is not mentioned. 200 acres of individual lasers surely would not be very manoeuvrable. Would it be able to follow said Starchip, even for a few minutes (as the Earth turns)? Also, would it not be usable only at a fixed latitude - that at which it was built?

Not to mention stopping when it gets there!

My advice - forget about solarsails and get on with your life.

Cat

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Helio

#### Helio

I am not talking about haveing larger solar sails on the space craft.
I am talking about placing reflectors around the sun
to reflect more sunlight to the solar sail ship.
That seems very plausible to me.

At 1 AU, there are about 7.8E-6 Newtons per square meter of photon force upon a sail, per Wiki. [I assume this is both the inbound force and the exit photon force combined.]

So, say we had 1000 one-sq. meter mirrors placed at 1AU and, hopefully, have adaptive optics on one of the mirrors, assuming each is comprised of two or three mirrors, to adjust the focal length to the position of our little spacecraft -- the first mirror reflects to one or more mirrors so that the sunlight is passed forward. This would also need to include special high-precision gyro motors to maintain alignment with the spacecraft.

So, let's try a 10 gram spacecraft and give it only 10 sq. meters of sail area. If everything above is perfect (ha!), then the acceleration would reach about 30%c in about 2 months, and would be a little over 2 AU from the Sun. So, allowing for inefficiencies and alignment challenges, etc, perhaps technology may be capable of something around 20% c, which puts it at the Centauri system in about 21 years.

But, if this is plausible, why not more or larger mirrors?

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#### Helio

Correction,

If we had 1000 mirror each with an area of 1000 sq. meters, then it would reach close to 2 AU in 24 hours, traveling close to 20%c.

Ouch, I tried to build a spreadsheet while doing two other tasks. My setup was much worse than I thought.

Let's try again....

The problem is that to reach a very fast speed, say 20%c, the object would have to undergo enormous acceleration since it would be too far away after just a short period of time, as billslug notes.

So, if my spreadsheet is now correct (ug), a 10 gram craft with 10 sq. meters of sail that receives the sunlight from 100 mirrors, each with 1 km per side, will have an 800g acceleration. But, even after 8 days, it will only be traveling less than 0.1%c but will already be 21.7 AU away, so way too far for anything I can imagine in maintaining focus.

But I'll double check these numbers this weekend, perhaps. (yesh!)

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#### Catastrophe

##### "Science begets knowledge, opinion ignorance.
Travelling at the speed of light? | Space.com Forums

QUOTE
"I hope this will help, but if you are thinking about manned trips using lightsails - forget it - unless you want to completely ignore the science." Catastrophe

Actually, you can forget about any method of human interstellar transportation.
No method can work. (Assumptions, humans not in some sort of hibernation, a round trip within their lifetimes. Can't leave Earth until age 18)
Light sail is a one way trip.
Neither chemical, fission nor fusion can provide enough energy for a round trip.
No way to make or store enough antimatter to do the job.
QUOTE Billslugg

Cat

#### billslugg

Yes, the numbers just can't work out. I looked at it every which way I could.
For a one way trip, powered by Earth bound lasers, the problem becomes beam divergence. Formula for that is wavelength / (pi * beam diameter.) I don't know for sure but probably that assumes a coherent beam. A one meter diameter laser beam at 600 nm (green) would diverge with an angle of 6e-7 meters / (3.14 * 1) or 2e-7 radians. This is 6e-8 degrees. Tangent is 1e-9. So at a billion meters it would be two meters wide and power would be one fourth. At one AU, beam width would be 150 meters wide and power would be down to .4% of original beam strength. It goes downhill from there.

#### Catastrophe

##### "Science begets knowledge, opinion ignorance.
Billslugg, thank you for the calculations. It just seems one big catch 22, that the size needed for the sail, when weight is so critical, means that the sail can stand up to the power needed to propel it without failing.

Cat

#### Helio

Yes, the numbers just can't work out. I looked at it every which way I could.
For a one way trip, powered by Earth bound lasers, the problem becomes beam divergence. Formula for that is wavelength / (pi * beam diameter.) I don't know for sure but probably that assumes a coherent beam. A one meter diameter laser beam at 600 nm (green) would diverge with an angle of 6e-7 meters / (3.14 * 1) or 2e-7 radians. This is 6e-8 degrees. Tangent is 1e-9. So at a billion meters it would be two meters wide and power would be one fourth. At one AU, beam width would be 150 meters wide and power would be down to .4% of original beam strength. It goes downhill from there.
Yes. Couldn’t the beam become more focused, nevertheless? Couldn’t the mirrors use adaptive optical servers to improve focus?

Divergence, even for lasers is more than many think. I measured my ”1 watt” blue laser divergence and determined the beam width would be ~ 1 km when viewed by Don Pettit on the space station (@400 miles alt. for that particular night), thus unharmful to him.

#### Helio

I’m curious if several light impelling cycles would help. So, if the craft is sent to Jupiter, could it be turned completely around to later receive another cycle of thrust? [I think there are insertion limitations that don’t work as well as we can draw them. ]. Could the intense mag field of Jupiter also be exploited, though shielding complicates things, no doubt?

#### billslugg

The divergence of a beam of light is a function of diffraction. This cannot be cured at the sending end regardless of focussing mechanism. You would have to put a large lens or mirror at the far end to capture the expanded beam and refocus it. A space ship might carry a series of Fresnel lenses and deploy them behind it every so often.

Catastrophe

#### Catastrophe

##### "Science begets knowledge, opinion ignorance.
May I please remind you?

"A necessary requirement for the Breakthrough Starshot mission is keeping the mass of each sail-equipped Starchip to just a few tenths of an ounce (a few grams). . . . . . . likewise the solar sail itself, which is expected to span up to around 13 feet (4 metres), will need to weigh in at less than 0.035 oz (1 gram). It will also need to be extremely thin, as otherwise the sail would absorb far too much heat and be vaporized by the barrage of laser light . . . . . . " the barrage of laser light to drive a few grams??

billslugg

#### billslugg

Yes, of course, the weight requirement puts any sort of deployable lenses out of the question.

Catastrophe

#### billslugg

Could the intense mag field of Jupiter also be exploited, though shielding complicates things, no doubt?
There are several basic rules of magnetic fields. Among them:

- No arrangement of permanent magnets alone can levitate an object.
(I know, it's hard to believe but it's true. There must be at least one physical constraint to prevent the magnet from flipping over and slamming into the magnet below. Levitated trains use variable fields to maintain the levitation.)

Another one is:
- Travel in space using magnetic fields can only take one parallel to the field lines. In other words you could travel along the field lines towards either of the magnetic poles of Jupiter but then you could only return and it would be along the same path you went there.
Given some method of moving across field lines one could then choose a path towards a pole of Jupiter, coast for awhile, then accelerate towards Saturn along another field line. At some distance you would have to turn off your magnet and glide as eventually it would return you to Jupiter's opposite pole.

As for using the magnetic field of Jupiter to bend light, that would not work. EM waves do not respond to magnetic fields. This is a great advantage of gamma ray observatories, they know where the gamma ray came from. This is not so with charged particle detectors, they have no idea of the point of origin.

View: https://imgur.com/gallery/dWJRllL

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Helio

#### Catastrophe

##### "Science begets knowledge, opinion ignorance.
At last someone heard "And you’ll likely fry the solar sail"

. . . . . . . . . . . . . . . . . . and the rest.

Lightsails are dead duck. EOS.

Cat

#### billslugg

I was surprised to find aluminum only 90% reflective in visible wavelengths. At UV and also infrared wavelengths it is up around 98%-99% reflective. Consider one or two percent of a gigawatt per square meter and you're talking some serious heat loading. Yes, deep space will draw off a lot of that heat but it doesn't sound good.

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