# QuestionSolar sail acceleration

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#### Catastrophe

##### "There never was a good war, or a bad peace."
Paper thin sails burnt up by start power pressure.

#### Helio

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
I was thinking more about generating a loop current while passing through the intense mag field, then applying the right hand rule to get thrust direction.

#### billslugg

There is a problem in that the return side of the current loop would exactly oppose the desired thrust generated by the portion of the loop we want to use.

Any dipole we create within a planet's magnetic field can serve only two purposes, one I described earlier is to attract towards one or the other pole and provide thrust along the planet's field lines.

The other thing it can do is torque the spacecraft to be in line with the planet's field.

What might be done is to beam a current of electrons into space from the ship. In that case the electrons would follow the left hand rule* and they would simply spiral around the planet's magnetic field lines as they moved towards one or the other pole. If injected precisely at 90° to the field lines they would make closed circles.

*Positive charges obey the right hand rule. Assuming "north" to be that pole of a magnet which is attracted to Earth's north pole. The north pole of the Earth being a magnetic south pole. Yes, I know, its confusing. Electrical engineering is not for the weak of heart. I nearly bailed when the prof asked us to imagine the square root of negative one then told us the north pole of the Earth is a south magnetic pole. Is nothing sacred any more?

#### Catastrophe

##### "There never was a good war, or a bad peace."
If you require a beam to power the outward journey, how come you don't require one to return - apart from billslugg's excellent point that you would then be opposed by the outward power beam?

Cat

#### billslugg

I read somewhere of a proposal to take a large mirror along with you and put ahead of you to reflect the Earth beam back at you to slow you down. Seems like a huge weight penalty to me.
Perhaps the light from the star the craft is visiting might slow it down.

#### Catastrophe

##### "There never was a good war, or a bad peace."
Sounds to me like perpetual motion.

Cat

P. S. "Perhaps the light from the star the craft is visiting might slow it down."

Then why not use the Sun for the outward journey|
Why bother with the beam?

#### billslugg

The beam at our end greatly hastens things at our end. In something like ten minutes the craft gets up to .2c. Once approaching the far away star, the deceleration phase would depend on a comparatively weak source of light and might last several years.

Project Breakthrough Starshot envisions a 4mx4m light sail receiving 1TJ of light during the acceleration phase.

At 1 AU each meter of sail area collects 1300 J per second from a star the same power as the Sun. At this distance, a 16 m^2 sail would need 1.5 years to decelerate. But if it starts 1.5 years before getting there it is going to be .3 light years away. At that distance the intensity of the light is but a billionth the needed power. You're right - it won't work. Looks like a one way journey without a stop at the destination!

#### Catastrophe

##### "There never was a good war, or a bad peace."
billslugg, are you sure that, at quoted energy figures, the sails are not going to be damaged? What thickness and weight figures are you using for the sails compared with my post #37?

Cat

#### Helio

The high speed entry to the star system will produce more light force against it, and perhaps a larger sail area might be possible, but these will not be enough, apparently.

What if the sail material could be of a material that could serve as fuel? How much hydrogen could it scoop-in to assist with this? If the probe has some maneuvering power then could it attempt atmospheric braking. [ We want it close to the planet regardless. ]

#### Catastrophe

##### "There never was a good war, or a bad peace."
Helio, please look at my post #37. Light sails are dead duck, except for real tiny tiny ones with real tiny tiny cameras.

Cat

#### Helio

Helio, please look at my post #37. Light sails are dead duck, except for real tiny tiny ones with real tiny tiny cameras.

Cat
Where there's a will....

Multiple booster stages, multiple planetary boosts and multiple light pulses, even ones to support U-turns around Jupiter, multiple this and, without a doubt, multiple that! What could go wrong?

#### Pogo

In the early 1960s as a Boy Scout, Boys Life, the scouting magazine had a short story about several youths operating light sails in Earth orbit in a race taking several orbits to leave orbit and pass a finish line. They had to know how to manipulate the sail to face full sun during the acceleration phases and edge-on to the sun during the neutral phases, all the while keeping it from ripping or wrinkling. Large or small, sail ships are great stuff for sci-fi, but practical? I think not. Not enough light where you need it, suitable materials not available, etc.

#### Catastrophe

##### "There never was a good war, or a bad peace."
Pogo,
This is a prime area for ignoring contra effects to those alternatives one may wish to adopt. One example is: Don't confuse me with fact - my mind is made up. I know because, like most people, I have suffered from it myself. I may even be doing it now, though I hope not.

If one needs a booster at the outset, it suggests that relying on stars alone is not really an option. The figures in post #37 show that, and also prove that any beyond tiny sails are totally impractical. And how do you conveniently turn sails around to manoeuvre in space? Action and reaction. Or do you have the (impossible) choice of carrying extra oxygen (or other gas) to squirt out in order to navigate?

And, of course, no one seems to have taken account of all the stuff floating (or travelling very fast relatively) in space - even space dust - when you are relying on ultra thin, reflective, heat sensitive sail to get you where you want to go (and possibly return from). Spare sails? How many spares? There is a lot of space dust out there, varying in size from microns to Ceres size and possibly larger. Mostly smaller, I agree.

I also totally agree with your succinct assessment "but practical? I think not. Not enough light where you need it, suitable materials not available, etc. "

Cat

Pogo

#### billslugg

billslugg, are you sure that, at quoted energy figures, the sails are not going to be damaged? What thickness and weight figures are you using for the sails compared with my post #37?
Cat
I have no delusions a solar sail could permit interstellar travel by humans. In an article in The Economist* (mentioned in wiki article on Breakthrough Starshot) "at least a dozen off-the-shelf technologies will need to improve by several orders of magnitude."

The proposal is to put 100 GW onto a 5 meter diameter sail for about ten seconds, delivering a Terajoule of energy. An excellent mirror only absorbs 1% of the incident energy. This would be a gigawatt for ten seconds absorbed by a few grams of matter. No way.

* "A new plan to send spacecraft to the stars: replace rockets with lasers". The Economist. 12 April 2016. Retrieved 13 April 2016

Pogo

#### suneritz

Why can’t we lunch dozens of them separately and reconnect them at the drop zone like drones to form a single large swarming sail which could be useful for slowing down and flying around. Because if it can’t decelerate is there a point of even going? It’s like throwing jumping rocks into the pond hoping one will boomerang back.

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#### Catastrophe

##### "There never was a good war, or a bad peace."
"form a single large sail which could be useful for slowing down and flying around"

How?

Cat

A thought:

If you take the sail down, you will continue to move (Newton's Laws).
If you re-erect it, you will continue to accelerate (OK, very slowly).
The source (beam) will still be there. The star(s) will still be there.
What mechanism might you have for re-erecting a large, very flimsy sail at a different angle?
What extra mass will this entail (during earlier stage?)

And what about all those nasty bits of rock destroying your sail, and leaving you stranded in space for ever?

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#### suneritz

There is probably only one thing you could use in space to "tractor" the bots together into sail or sphere, it is electro magnetism. Although we may not be ready to go down this rabbit hole yet. If you have dozens you could loose some to various misfortunes and still have enough left to accomplish your mission.

#### Catastrophe

##### "There never was a good war, or a bad peace."
What components of electromagnetism / power sources are you planning on taking and what is their mass?

Cat

#### suneritz

The same superconducting components which were used in the article above. I don't know their dimensions or other details except it's at microscopic level which should work in our favor. Ok I have no clue, but that doesn't mean it cannot work

Catastrophe

#### Catastrophe

##### "There never was a good war, or a bad peace."
suneritz, by the same logic:
I have no clue that there are ten foot high green aliens with three feet and thirty ears, but that doesn't mean there are none. There may be, but the probability is very small that we will meet any.

Cat

suneritz

#### Catastrophe

##### "There never was a good war, or a bad peace."
suneritz, the point I am making is that any such additional ideas all require additional complexity which might entail greater lift off mass, acceleration, power requirements, mechanical components, et cetera.

#### billslugg

Remember the ten seconds worth of acceleration proposed by Breakthrough Starshot? The reason it is only ten seconds is the probe is so far away by then that the beam becomes useless. It spreads out unavoidably by refraction and no more useful work can be gotten from it. Any thought of using the beam to decelerate the craft at the far end is futile.

The tangent of the beam angle cannot be any less than 1e-9 at visible wavelengths. See post #41.
At 1AU the beam width is 150 meters and .4% of initial strength.
At 4.3 lightyears (266,000 AU) it is 4e7 meters wide and down to 1e-11the original power density.
The beam width at the nearest star is bigger than the Earth.

#### Catastrophe

##### "There never was a good war, or a bad peace."
Don't ignore this very important issue:

"And what about all those nasty bits of rock destroying your sail, and leaving you stranded in space for ever?"

It is, of course, very unlikely that a rock/asteroid would hit directly head on, so that it would be deflected off course, maybe only a little, but enough to put you off course. If very unlucky, it might hit the outer frame.

Cat

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#### Dratonia

When you consider how sparse deep space is, I really don't believe that it is likely that there would be enough damage for it to really be relavant

#### Helio

The proposal is to put 100 GW onto a 5 meter diameter sail for about ten seconds, delivering a Terajoule of energy. An excellent mirror only absorbs 1% of the incident energy. This would be a gigawatt for ten seconds absorbed by a few grams of matter. No way.
Is that the input wattage at the sail or the output light wattage of the "leyser" (light-geyser)? But either way, I agree, it is too much energy I think for any sail, that is, any "one" sail. Perhaps several layers of sails would be required so that the last one isn't vaporized.