The solar sail experiment

El_Machinae

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I first read about this idea in Larry Niven's A mote in God's eye, so it's a pretty old idea (as such things go). There's a donation-funded organisation, the Planetary Society, that tried to do a similar experiment nearly ten years ago, but the rocket failed on launch. Now they've finally scraped enough cash to try a variant on the experiment.

I'm kind of excited, but I also realize we could have had results from this experiment nearly ten years ago. I like the idea of the Planetary Society, where our private efforts actually help propel us along towards the future.

Planetary Society to launch solar sail test flight in May

Solar sails are designed to capture the momentum from solar energy photons using large, mirrored surfaces. The small, continuous acceleration allows a spacecraft propelled by solar sails to reach high speeds over time.
 
I remember this idea from Niven's novels. It's called a Bussard ramjet.


A solar sail and a ramjet are 2 different concepts for a spaceship drive. The ramjet idea is to scoop up matter and superheat it or ionize it to shoot it out as a reaction drive propellant. A solar sail has no propellant, and works by being pushed by stellar emissions.

Based on articles I read years ago, neither idea is likely to actually work as an interstellar drive. Neither the photon density nor the particle density is great enough to provide enough acceleration for travel to even nearby stars in acceptable time frames.
 
The advantage of the sail is more that the acceleration it gives you access to is fueled by stores that don't have to go onto the ship. A portion of the fuel used by the ship is being used to accelerate the fuel that will be used later. The solar sail is a fixed weight, but the photons pushing it come from elsewhere. It's the deceleration fuel that is more expensive, so the sail gives you deceleration potential.

The home system can add more and more lasers over time, so the amount of acceleration you get isn't fixed at the time of launch. And the total fuel required to slow the ship is lower.

It's also really useful in slow-and-steady scenarios, asteroid deflection and the like.
 
I think Arthur C. Clarke's short story The Wind From The Sun is be the first story on this concept strictly using the Sun. IIRC in NIven's books, the sails were driven by lasers instead of the Sun.
 
The home system can add more and more lasers over time, so the amount of acceleration you get isn't fixed at the time of launch. And the total fuel required to slow the ship is lower.

Unfortunately you would need really big laser beams to cover interstellar distances - about 100km in diameter. Even if we would be able to build such a thing at some point in the future, I doubt there would be space to build many of such lasers.
 
If you're expecting the destination star to cover the deceleration, then you don't need to push all the way to the destination star. The point where you turn off the lasers gets closer based on how much laser oomph you use. (in other words, the KE of acceleration needs to match the KE of decel).

These are still experiments that needed to be done!
 
If you're expecting the destination star to cover the deceleration, then you don't need to push all the way to the destination star. The point where you turn off the lasers gets closer based on how much laser oomph you use. (in other words, the KE of acceleration needs to match the KE of decel).

You would still need a kilometer wide laser beam to keep pushing within the solar system. With current technology it would be hard to keep pushing between Earth and Mars. I think it is a much better idea to just let the sun push. Lasers are hardly going to contribute anything over a a meaningful distance.
 
You would still need a kilometer wide laser beam to keep pushing within the solar system. With current technology it would be hard to keep pushing between Earth and Mars. I think it is a much better idea to just let the sun push. Lasers are hardly going to contribute anything over a a meaningful distance.


The concept Niven and Pournell were envisioning was clusters of lasers giving the solar sail a stronger initial acceleration, and then eventually cutting out and leaving the ship to accelerate the rest of the way, and then decelerate, with strictly solar. If those lasers were in orbit and solar powered, then perhaps justified. But it's still a cryogenic sleep for the ship's crew to a close by star.
 
Or a generational ship. I suspect any departure from the solar system is going to require us to accept that traveling be a way of life for the participants, not an adventure.

Question: does a laser generate a reactive force? It seems like it must, else if you have this solar sail dragging a laser it becomes free acceleration for as long as you can power the laser...but I can't think of any mechanism for a laser to have 'recoil'. I could easily be missing something though.
 
The concept Niven and Pournell were envisioning was clusters of lasers giving the solar sail a stronger initial acceleration, and then eventually cutting out and leaving the ship to accelerate the rest of the way, and then decelerate, with strictly solar. If those lasers were in orbit and solar powered, then perhaps justified. But it's still a cryogenic sleep for the ship's crew to a close by star.

I get the concept, but given the very limited range of reasonably sized lasers I am wondering what a solar sail could achieve that could not be achieved with conventional propulsion and gravity slingshots.

Question: does a laser generate a reactive force? It seems like it must, else if you have this solar sail dragging a laser it becomes free acceleration for as long as you can power the laser...but I can't think of any mechanism for a laser to have 'recoil'. I could easily be missing something though.

Yes. Usually it does.

The mechanism depends a bit in the type of laser, but for a laser with a two-mirror resonator it is like this: The light travelling around the laser resonator creates a force on both mirrors. You could say, that every time a photon bounces from a mirror it gives that mirror a kick equal to twice its momentum. This is exactly the same principle the solar sail uses. Usually, one of the mirrors is slightly less reflective to get the light out on one side. So you have less bounces and thus less force on one of the mirrors, so the whole resonator feels a net force backwards.

But actually, if you are okay with the beam coming out at both sides, there is no need to make one of the mirrors less reflective. In that case the beam going forward and the one going backward have the same momentum. That means that the total momentum of the system is zero and you could reflect one of the photons to get 'free' acceleration. At that point you should wonder why to bother with all this trouble with a sail, and indeed: Just use a regular laser on your craft and point it in the opposite direction of where you want to go (might want to turn away from earth first, but hey, you are never going to see them again, anyway).

The point why the acceleration is not free is a bit more subtle. To power the laser you need to generate energy. And to do that you have to use up stored energy, i.e. mass. That means despite not directly ejecting mass, your craft would still lose mass as long as you accelerate.
 
The mechanism depends a bit in the type of laser, but for a laser with a two-mirror resonator it is like this: The light travelling around the laser resonator creates a force on both mirrors. You could say, that every time a photon bounces from a mirror it gives that mirror a kick equal to twice its momentum. This is exactly the same principle the solar sail uses. Usually, one of the mirrors is slightly less reflective to get the light out on one side. So you have less bounces and thus less force on one of the mirrors, so the whole resonator feels a net force backwards.

Thanks. That explains that.

But actually, if you are okay with the beam coming out at both sides, there is no need to make one of the mirrors less reflective. In that case the beam going forward and the one going backward have the same momentum. That means that the total momentum of the system is zero and you could reflect one of the photons to get 'free' acceleration. At that point you should wonder why to bother with all this trouble with a sail, and indeed: Just use a regular laser on your craft and point it in the opposite direction of where you want to go (might want to turn away from earth first, but hey, you are never going to see them again, anyway).

Hey, if they don't want to come along they have to take whatever they get as a parting gift and like it, right?

The point why the acceleration is not free is a bit more subtle. To power the laser you need to generate energy. And to do that you have to use up stored energy, i.e. mass. That means despite not directly ejecting mass, your craft would still lose mass as long as you accelerate.

This goes to what I said...it only works for as long as you can power the laser. I think though that you can use mass much more efficiently for powering a laser than you can use it as ejection mass [insert nuclear reactor here]. Of course that may balance out depending on the difference in acceleration that you get.

As long as it at least breaks even on that though the laser propulsion seems like a good deal. Considering that with one system you arrive at your destination with a big empty reaction mass container, and with the other you arrive with a big freakin' laser...and we know those always come in handy on a trip to foreign territory.
 
Hey, if they don't want to come along they have to take whatever they get as a parting gift and like it, right?

They should be grateful to get free energy delivered. It is their own fault if they are not ready to harvest it.


This goes to what I said...it only works for as long as you can power the laser. I think though that you can use mass much more efficiently for powering a laser than you can use it as ejection mass [insert nuclear reactor here]. Of course that may balance out depending on the difference in acceleration that you get.

As long as it at least breaks even on that though the laser propulsion seems like a good deal. Considering that with one system you arrive at your destination with a big empty reaction mass container, and with the other you arrive with a big freakin' laser...and we know those always come in handy on a trip to foreign territory.

If you can freely convert mass to energy, emitting photons is the best you can do (without invoking questionable physics). That means theoretically, laser propulsion is very efficient. But even with nuclear fuel only a tiny fraction of its mass is converted into energy. You end up with a lot of mass that cannot be converted to energy any further and much less energy. In that situation it is probably a better idea to use the energy to accelerate the waste and hurl it out backwards.

But having a laser in case we need to 'liberate' the target system does sound attractive.
 
A solar sail and a ramjet are 2 different concepts for a spaceship drive. The ramjet idea is to scoop up matter and superheat it or ionize it to shoot it out as a reaction drive propellant. A solar sail has no propellant, and works by being pushed by stellar emissions.

Based on articles I read years ago, neither idea is likely to actually work as an interstellar drive. Neither the photon density nor the particle density is great enough to provide enough acceleration for travel to even nearby stars in acceptable time frames.
There was also some research by Robert Zubrin (I know, I know) and some other guy that showed that a ramjet would actually decelerate a ship and could never provide enough thrust to offer net forward thrust.

They then went on to show that this might actually be useful for an interstellar mission that used another form of propulsion for the acceleration phase (like fusion) and then turned on the ramscoop to slow down for free with a bonus of free hydrogen for further propulsive maneuvers on arrival.

I don't put much stock in Zubrin in general anymore. While I haven't seen proof of his academic idiocy, he is in general a kook with a bad reputation amongst academics and has a spot-focused, narrowed-minded, one-track mind.
 
Bill Nye just did an ep of Neil Degrasse Tyson's radio show on this. I haven't listened to it yet, but I'm intrigued.
 
Success

The glint from the spacecraft's solar sails is expected to be visible from the ground. The Planetary Society has a webpage that allows people to track the spacecraft relative to their location.

Link

Apparently they're sending up a second one to a higher altitude next year. Using entirely crowdfunding. B612 will probably get my $50 this year, but this could very likely get it next year.

Don't believe some of the hype, apparently the Mercury satellite Messenger used solar sailing to save fuel.
 
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