LightSail 2 Meets the Photons

[hitmouse]

I read, many years ago, an article which considered the use a giant space-based fresnel lenses to maintain the sun's photon flux on receding solar sails. Might have been in New Scientist, 1980s.

 

[StilLearning]

Me too. From what I've read applying solar sailing tech to launch anything bigger than an Xbox sized probe to another star system is going to mean scaling the sail and everything associated with it up to giant size.

 

[StilLearning]

An interesting read on the subject: Planetary Society Deploys LightSail 2’s Solar Sail. What Does The Future Hold For Solar Sails? - Universe Today

 

[Vertigo]

One or two people have questioned sailing towards the sun. I suspect you are thinking incorrectly about how you navigate around a star system. Though please correct me if I'm wrong.

We tend to get this image of battlestar galactica type spaceships whizzing around all over the place. however when in a star system I believe you don't sail away from the sun by simply pointing in the direction you want to go and blasting away (unless you have truly awesomely powered thrusters). Everything in a star system is permanently falling towards the star; if in a stable orbit then it never hits the star and instead orbits like a planet, if it were 'stationary' ie. not orbiting the star (though not really possible) then it will simply fall directly towards the star either hitting it or just missing it and settling into a highly elliptical orbit. So if you want to move farther from the star you simply increase your orbital speed and you inevitably move to a higher orbit farther from the star. If you want to go closer you decelerate and you fall to a lower orbit. Looked at in those terms, so long as your sail is trimmed to be pulling slightly tangential to and against your direction of orbital travel then you will slow and fall to a lower orbit closer to the star. That should be perfectly possible with a light sail.

In other words you don't get closer to the star by actually sailing towards it, but rather slow down and let the star's gravity draw you closer spiralling in towards the star. And you don't fly out of a star system in a straight line towards your destination star (that would require an enormous thrust) but instead steadily increase your velocity and spiral out from the star. Along the way you might use some of the planets to give your speed an extra sling shot boost. I guess eventually, having achieved the star system escape velocity, you will then be able head direct for your destination but you wouldn't start off this way.

Or I've got it all hopelessly wrong!

 

[Venusian Broon]

One or two people have questioned sailing towards the sun. I suspect you are thinking incorrectly about how you navigate around a star system. Though please correct me if I'm wrong.

We tend to get this image of battlestar galactica type spaceships whizzing around all over the place. however when in a star system I believe you don't sail away from the sun by simply pointing in the direction you want to go and blasting away (unless you have truly awesomely powered thrusters). Everything in a star system is permanently falling towards the star; if in a stable orbit then it never hits the star and instead orbits like a planet, if it were 'stationary' ie. not orbiting the star (though not really possible) then it will simply fall directly towards the star either hitting it or just missing it and settling into a highly elliptical orbit. So if you want to move farther from the star you simply increase your orbital speed and you inevitably move to a higher orbit farther from the star. If you want to go closer you decelerate and you fall to a lower orbit. Looked at in those terms, so long as your sail is trimmed to be pulling slightly tangential to and against your direction of orbital travel then you will slow and fall to a lower orbit closer to the star. That should be perfectly possible with a light sail.

In other words you don't get closer to the star by actually sailing towards it, but rather slow down and let the star's gravity draw you closer spiralling in towards the star. And you don't fly out of a star system in a straight line towards your destination star (that would require an enormous thrust) but instead steadily increase your velocity and spiral out from the star. Along the way you might use some of the planets to give your speed an extra sling shot boost. I guess eventually, having achieved the star system escape velocity, you will then be able head direct for your destination but you wouldn't start off this way.

Or I've got it all hopelessly wrong!

Excellent post @Vertigo. I get where your coming from. I got a tad confused with 'slow down' and 'decelerate' (acceleration in physics is complicated by the fact that an object in a circular orbit at a constant linear velocity is still constantly accelerating! ) which threw me as I know that object's orbit closer to the sun is faster that what it would be if it was further out. But that increase in speed comes from the spacecraft falling towards the sun, converting graviational potential energy into kinetic energy.

The point about solar sail craft is that, pragmatically, they will always start in orbit around the earth. For it to change orbit to make it 'go higher' it will angle the solar sail so that it boosts the force on the craft only when it is in the direction of it's orbital motion. (When the craft comes round so that it's pointing the other way, you just pull the sails in so that the opposite force is not applied) This will put it into a larger elliptical orbit, which depending on it's mass and force applied to it, may get escape velocity (or it could do a number of passes, increasing the size of the ellipical till it does). It could then change it's orbit to take into account other planets and use them to accelerate or deceleate the craft using gravitational assists to alter the course onto one you want. (see for example Parker Solar Probe - Wikipedia for the orbital dynamics of the Parker Solar Probe that uses Venus to brake and bring it closer to the Sun in it's orbit. It's not a solar sail but the type of orbit it was given I found instructive)

Or you could use sails as well to decelerate by using them at the correct points of its orbit.

Yes I think it would be quicker to send solar sails out from Earth, as one could always organise the orbit of the solar sail to actually use the Sun as a focal point of the ships orbital ellipse, so that when you wanted to send it off to Jupiter/Neptune, or to a star, say, you start it with the correct sail set up, right next to the Sun where it gets maximum thrust. And returning to Earth from far away will be awkward. But it's a form of transport that doesn't require fuel and may be a useful technology if we don't have more efficient thrust engine. (And we haven't looked into boosting the thrust artifically by shooting a laser at it, say.)

 

[Vertigo]

I get where your coming from. I got a tad confused with 'slow down' and 'decelerate'

Yeah wasn't quite sure how that should've been worded. Possibly decreasing your orbital... speed, rather than velocity might be more correct as your velocity is constantly changing anyway.