Sail + laser propulsion? Tell me more.

[Laie]

So, someone wants to send probes to Proxima Centauri. The plan is to have a "probe" that weighs only grams, a solar sail, and a powerful laser on Earth. That laser is then supposed to accelerate the probe to 20% lightspeed in a matter of minutes.

https://breakthroughinitiatives.org/News/4

There's a lot of probing questions I could ask about that proposal, but here and now I'm interested in laser propulsion. Would anyone on this board happen to know a little about it?

Assuming a 10g probe, what kind of laser would be necessary, what power, and how large would the sail need to be? Let's assume a weightless sail.

Basically, I'd like to know how far-fetched this really is.

[Signo]

Based on my old readings on this (I was not aware of the "initiative") the laser should be on "Tera" scale to attain such a speed and the sail should be more or less the same size of the projected beam to avoid wasting precious energy. 

As far as I know nobody built or tested anything similar before so I must assume we still miss the tech to build such a big Earth based laser array (but we are actually close) and we do not have any proof of concept for the sail.

Moreover, I think the laser should be based in orbit to avoid beam depletion due to the atmosphere.

What would you think about the Muricans putting a huge laser cannon in GEO?

[Tex_NL]

13 minutes ago, Laie said:

...

Basically, I'd like to know how far-fetched this really is.

A probe that small can only carry the most basic equipment, will be very exposed to radiation as it won't have any shielding and won't be able to transmit its findings back home.
It's not only far-fetched, it's also completely useless.

[Gaarst]

Photons carry momentum: p = h / λ.

Photons hit the sail and are reflected, part of their momentum is therefore transmitted to the sail. (Or you can say that they are absorbed and re-emitted through BB radiation, but that pretty much adds up to the same thing).

Because I don't want to bother with complicated physics and maths, let's just assume 100% of the photons momentum is transmitted to the probe.

Say you need to accelerate a 10g probe to 0.2c in 5mins. Ignoring relativistic effects, that is a 600 000 kg.m/s change in momentum.
They plan to use infrared photons for their lasers, so take the wavelength of a photon to be 1µm, its momentum is then 6.63e-28 kg.m/s and energy E = cp = 1.24eV.
To accelerate your probe you then need to use 9e32 photons, or a total energy of 1.8e14 J. In 5mins that makes the required power 600 GW (or a thrust of 2kN like that ion thing you never use because it is so weak, interesting isn't it?) which represents a hundred big nuclear plants or 30 Three Gorges Dam (or one Ninety Gorges Dam).

For the sail size, it's just a matter of how accurately you can aim your laser, or how much diffraction likes to mess up with everything you're trying to do.

Now this is overly simplified and ignores a whole bunch of things that should not be ignored.

Also your probe will be on fire. Putting a lot of lasers at one place is a very good way to vaporise things or trigger nuclear fusion.

[kerbiloid]

Quote

Generating and storing a few gigawatt hours of energy per launch

And a fog above the laser means up to several kilotons of waste energy right above the laser array.

[pincushionman]

1 hour ago, Gaarst said:

Also your probe will be on fire.

Quoted for truth. Especially if you have only 10 g, any of it thin metal or plastic.

[Rakaydos]

 

I'm not sure where the "in 5 minutes" bit is coming from. My understanding was that even a fraction of a g of acceleration, applied until the probe is out of range, would get the probe up to an appreciable fraction of lightspeed.

Simplifying the flight path to an eliptic escape trajectory, and assuming you wanted to keep the earth "behind" the probe(s) for the entire boost period...

Launch on the far side of the sun from alpha centauri, on a trajectory for a venus slingshot flyby at periapse. Combine launch laser with solar sail propulsion for continuous acceleration. As the sail gets furthur from earth (and sun), build more lasers to maintain acceleration.

Off the eliptic, same idea, but plan the Venus slingshot to throw you into the proper inclination.

 

Once it's out of range, stop building lasers, staart building a bigger telescope, so we can read the probe's signals when it gets there.

Edited August 30, 2016 by Rakaydos

[Laie]

3 hours ago, Gaarst said:

Now this is overly simplified and ignores a whole bunch of things that should not be ignored.

Never mind, it seems as if give or take an order of magnitude will make no real difference.

This was just the kind of answer I was looking for, thank you very much. Too bad I can like it only once.
Now, where are we supposed to get 90 gorges?

2 hours ago, Rakaydos said:

My understanding was that even a fraction of a g of acceleration, applied until the probe is out of range, would get the probe up to an appreciable fraction of lightspeed.

Yes but. If it takes you a week to bring it up to speed, you'd need to keep up the pressure well into the Oort cloud. That's not a problem in itself... if you can keep your laser focused.

Edited August 30, 2016 by Laie

[Rakaydos]

6 minutes ago, Laie said:

Yes but. If it takes you a week to bring it up to speed, you'd need to keep up the pressure well into the Oort cloud. That's not a problem in itself... if you can keep your laser focused.

Probably even longer, but the flipside is that instead of efficntly focusing the laser through ort cloud and beyond, you can build more lasers as the probe gets furthur away and brute force it to maintain acceleration.

[wumpus]

First, I can't imagine a 10g "probe" sending back signals across a few light years.  You might want to redefine it as "message in a bottle" or "message in a bottle flying past the subject at a significant fraction of c".

Second, why a laser?  You have an unlimited source of light (at levels significantly above "brightest Earth Sunlight") in nearly all space.  Just make a bigger sail and fly that way.  Better suggestion, fly toward Jupiter then sling around as close to the Sun as your vessel can handle and spread your sail there.

Of course, you could always use the laser as well, but expect enough heat issues already with just the solar sail.

[Gaarst]

2 hours ago, wumpus said:

Second, why a laser?  You have an unlimited source of light (at levels significantly above "brightest Earth Sunlight") in nearly all space.  Just make a bigger sail and fly that way.  Better suggestion, fly toward Jupiter then sling around as close to the Sun as your vessel can handle and spread your sail there.

Of course, you could always use the laser as well, but expect enough heat issues already with just the solar sail.

Solar luminosity flux at Earth is about 1300 W/m². To get your 600 GW of power (from my calculation above), you'd need a 20x20km sail still weighing less than 10g. The Sun is really good at radiating all over the place but that's pretty much it.

Using a more reasonable 10m² sail (order of magnitude), you'd need to get within 22 000km of the Sun. Even if your probe is built to resist the temperatures there, it will not survive a long time before the magnetic field screws up everything. That, and the fact that going through a solar flare at 0.2c is not going to do you any good.

[Rakaydos]

2 hours ago, wumpus said:

First, I can't imagine a 10g "probe" sending back signals across a few light years.  You might want to redefine it as "message in a bottle" or "message in a bottle flying past the subject at a significant fraction of c".

Second, why a laser?  You have an unlimited source of light (at levels significantly above "brightest Earth Sunlight") in nearly all space.  Just make a bigger sail and fly that way.  Better suggestion, fly toward Jupiter then sling around as close to the Sun as your vessel can handle and spread your sail there.

Of course, you could always use the laser as well, but expect enough heat issues already with just the solar sail.

It only has to be  signal detectable above the noise. in 40 years we can build a big enough telescope to detect the probe we already sent.

[1of6Billion]

What kind of forces are we talking about here? How does the "push" of a light sail compare to drag of solar wind, micrometeorites, last bits of atmosphere, etc?

[cfds]

47 minutes ago, 1of6Billion said:

How does the "push" of a light sail compare to drag of solar wind, micrometeorites, last bits of atmosphere, etc?

On the earth's orbit, the photon pressure of the sun light is already stronger than the solar wind. And if you are above LEO, the atmosphere will not be a problem either. Micrometeorites will transfer not a whole lot of momentum but they will perforate the sail...

Edited August 31, 2016 by cfds

[magnemoe]

1 hour ago, 1of6Billion said:

What kind of forces are we talking about here? How does the "push" of a light sail compare to drag of solar wind, micrometeorites, last bits of atmosphere, etc?

Solar wind can also be used to push an spacecraft, benefit is that you only need an magnetic field not an sail membrane so you could make this far larger. 

[Laie]

7 hours ago, 1of6Billion said:

What kind of forces are we talking about here? How does the "push" of a light sail compare to drag of solar wind, micrometeorites, last bits of atmosphere, etc?

See Gaarst's first reply a few posts up.

In this particular case, we're talking about a substantial amount of light. The amount that hits the sail has equals the full output of ninety gorges. That is a lot of light. Also a lot of water. The occasional particle just can't stand up to it.

I'd expect intersting things (TM) to happen when such a probe encounters the occasional particle, however.

They plan to launch thousands, they say. Maybe the first thousand will clear the way or something. I don't know.

Edited August 31, 2016 by Laie

[Aethon]

 

I almost linked this the other day but he took his previous video down.

[cubinator]

Giant orbital laser? I think the US already denied a petition to build one...

Would make for an interesting (albeit incredibly expensive) concept for sending tiny, inert bits of metal to nearby star systems fast. 

[Bill Phil]

Wouldn't it be a better idea, at least in the short term, to use laser sails to move around interplanetary space as well?

[cubinator]

5 minutes ago, Bill Phil said:

Wouldn't it be a better idea, at least in the short term, to use laser sails to move around interplanetary space as well?

The problems I think of at first glance are:

• You still need a really gigantic laser to move anything useful around. Why send postage stamps to other planets in a few weeks when we can send car-sized robots in a few years?
• Even if you manage to get up to speed, you'll need something to slow the craft down at the destination. If it's travelling at a fraction of c that doesn't require scientific notation, that thing is probably another gigantic laser, which would be even harder to build away from Earth.

I think fusion and even fission drives are easier than this for large ships.

[Bill Phil]

9 minutes ago, cubinator said:

The problems I think of at first glance are:

• You still need a really gigantic laser to move anything useful around. Why send postage stamps to other planets in a few weeks when we can send car-sized robots in a few years?
• Even if you manage to get up to speed, you'll need something to slow the craft down at the destination. If it's travelling at a fraction of c that doesn't require scientific notation, that thing is probably another gigantic laser, which would be even harder to build away from Earth.

I think fusion and even fission drives are easier than this for large ships.

It's like trains/roads, you'll need a lot of infrastructure, but once it's there, you can get more economical transportation.

We'll make them big enough to send vehicles larger than cars. Sure, chemical rockets and other types would let us get even more, but they're limited. This would be high speed travel between planets, and by keeping the lasers at the destinations/"stationary" you get away with higher payload fractions.

Yeah you need a gigantic laser, but the question is: how fast do you want to go, and how much acceleration do you want?

Who ever said it would be a decent fraction of c? A few hundred km/s would due nicely. And even then, that's a very long term goal. Using a laser that can get you about 10km/s of Dv would be pretty useful.

Edited September 8, 2016 by Bill Phil

[todofwar]

I've wondered about a launch system using this. Basically, you get a cannon pointed straight up, and fire your payload with some reasonable velocity (would probably be more along the lines of a rail gun than a powder cannon) and you use lasers to give it enough propulsion to counteract gravity just enough for it to reach geostationary. Basically, a space elevator made of light instead of probably impossible materials. The laser wouldn't have to completely nullify gravity, just enough to make sure the craft reaches 0 velocity at geostationary.