Progress to a 20% of Light Speed Sail

The Limitless Space Institute is reviewing the work of Interstellar grant recipients.

It is feasible but very difficult to make a laser pushed sail reach 20% of the speed of light.

The general parameters are for an array of lasers with 100 Gigawatts of power to push a 1 gram solar sail that has 4 meter width by 4 meters length.

This means about 200 gigawatts of power that is converted into laser power with 50% efficiency. This would be about double the installed US nuclear power. It would also be about 11 times the power of China’s Three Gorges Dam.

The 1 gram sail would be 40% of the weight of a penny.

A few years ago photonic laser crystals were made that had dimensions of 350 microns by 350 microns. This means the width and the length would need to be increased by 10,000 times in each dimension.

Recently, the University of Delft has made photonic crystals that are 4.5 centimeters by 4.5 centimeters. We need increase the size by 900 times on each side.

How are the photonic crystals made. They use Silicon Nitride and they use electron beams to etch away unneeded material. They leave very thin supports holding each corner. The strand supporting each corner were made as thin as 50 nanometers. They etch many holes into the material to reduce weight and so optical interactions enable the material to become a mirror. They have made it 99.6% reflective.

The current size of the photonic crystal is limited by the size of a chip wafer.

Special purpose fabrication facilities will need to be made to expose a 4 meter by 4 meter ultrathin membrane with electron beams.

Written By Brian Wang,

25 thoughts on “Progress to a 20% of Light Speed Sail”

  1. For added bonus points, don't use a nuclear explosion, just use the energy released when your 0.2C probe slams into a (hopefully uninhabited) moon or something.

    The Tunguska event was actually just an alien space probe sending it's report home.

    When they read the report that clearly indicated not just life but an industrialising civilization, the response on Lalande 3 was a quiet, embarrassed "oops…."*

    And THAT's why they have kept out of contact since then.

    *Translated of course. The actual communication consisted of a flashed pattern of differently polarised light from the skin polarochromophores of the space scientists gathered around the parchment screen.

  2. Spent the weekend indulging in fiction where "I can't think right now of a reason that's not theoretically possible when given the most generous possible interpretation" is the accepted standard, so in that spirit:

    Nuclear pumped laser gives off a frequency determined by the crystal structure and exact elemental makeup of the laser crystal. The laser crystal is a use-once design where the crystal is vaporised from one end and the beam is generated in the crystal as it is being vaporised by a nuclear blast. (In theory, I think that the couple of attempts to do this didn't work.)

    So, in SF level theory, if you have a series of different crystals arranged end-to-end to give a total x-ray rod, then this will produce a beam that varies in frequency as the rod is consumed.

    Build your rod to encode the message and aim it back to the radio telescopes around Sol. You might get a few thousand bits.

    For added bonus points, don't use a nuclear explosion, just use the energy released when your 0.2C probe slams into a (hopefully uninhabited) moon or something.

  3. I calculate that the diminution of a 100 GW beam, nominally able to reach out to about 0.1 AU (expanding beyond the 250 m diameter catcher), at 4.1 LY is oh, about 0.23 µW/m² = 230 nW/m²

    Sounds tiny?

    Well, put it in perspective!  At 1 AU (149.5×10⁹ m), Earth receives about 1,363 W/m² or so of inSOLation. Back calculating the surface of a sphere of that radius, (A = 4π ²), we get 2.81×10²³ m², so Sol's total output omnidirectional is 3.83×10²⁶ W.  

    α-Cen is what, 4.1 LY off, or about 3.88×10¹⁶ m.  that radius (turned into a sphere) has an area of 1.9×10³⁴ m².  Divide total Sol output by that, and you get 20×10⁻⁹ W/m² (20 nW/m²)

    Mind you, α-Cen and Sol are very similar stars, from an absolute output point of view. So, it is fair to say on one of α-Cen's planets, Sol would twinkle with about the same intensity as α-Cen does in our night sky.  

    Therefore, if Earth's 100 GW laser were in some optically observable band to the Centaurians, it would look like Sol brightened by a factor of 10 or so!  

    In that perspective, at least communicating TO α-Cen could be done at many megabits per second! The reflected whiff though … might still be nearly completely invisible back here on Planet Dirt.

    ⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
    ⋅-=≡ GoatGuy ✓ ≡=-⋅

  4. Hmmmm. Let's see. The claim is that they can get acceleration to 0.2C in 3 minutes. I assume they'd go for more performance if they could, so this suggests that after 3 minutes the beam is diffuse enough to not accelerate the sail usefully any more.
    We can take this as the point at which our beam is not intense enough to burn through the Martian colony Pon Farr stadium.

    So, assuming constant acceleration (not true) then acceleration distance = v/2 * t

    v= 0.2 C = 60 000 000 m/s.
    t = 3 minutes = 180s
    distance = 5.4 billion km = 0.036 AU

    The lasers will mess up the moon colonies, but even Mars will be safe.

    (This is assuming constant acceleration, in practice the acceleration will die away with distance giving a slightly longer range. But even if the ENTIRE journey was at max speed that still only gives you a factor of 2.)

  5. Yeah, the precision they'd need to pull that off in the first place, then for us to detect it… I'm having a hard time thinking how they could do that even with some serious handwavium. How much transmitting power can you stuff into a 2.2-gram payload package, anyway?

    I'm thinking even a nuclear-pumped laser like they were looking at in the '80s would have a hard time being detected. (And seriously, how would you modulate THAT beam to have any chance to transmit data?)

  6. Forget the lasers, make the sails! Looks like an ideal product for In Space Mfg, esp the large vacuum volumes avail, needed for electron beams. Sun and less intense lasers should work for Solar System sailing needs.

  7. Indeed, even at 99.6% reflectivity (U of Delft photonic crystal) that's 400MW of absorbed power to deal with. 25MW per square metre for the 4×4 metre example. Instantaneous destruction seems the more likely outcome, either by incineration or shattering due to thermal shock.

  8. At that range they might be able to detect it using good telescopes.

    It won't be sinking their favourite cruise ship or burning down their olympic stadium. The beam will have spread out far too much by then.

  9. agreed. we are designing a vehicle without knowing the terrain. One of the Voyagers must have some info on 'beyond the heliopshere'?

  10. Not convinced that we are considering the cosmic oceans for which we intend to sail.
    The heliosphere has some research background, but the Local Interstellar Blob (my nomenclature) which encompasses much of that for many, many AUs out, is understood to have a fair and varying density of hot plasma which would seem to be a navigational nuisance if not complete drag. Though, A lovely General Interstellar Medium, beyond and beyond that, of likely less than one atom per 10,000 cubic inches' density may be smoother run outbound (if we are moving away from the galactic centre) – but are we then still on course and at some useful dV?

  11. The 100 GW laser array is doable. Its going to cost you money. Getting the price per MW down would be something to focus on over then next couple of decades.

    You can power the laser with gas turbine engines. They are cheap per MW. Just expensive to run. You can also buy power during the night cheaply.

    My suggestion is to spend money developing the technology over the next few decades that way you can see whether or not it is feasible. Start small.

  12. If it turns out that there is a civilisation on Alpha Centauri, would they not be offended at being hit by our 50GW death ray and consider it an act of war?

  13. They're going to somehow shape the sail into an optically perfect transmitting aperture, and transmit back once they've reached the gravitational focal line on the other side of the destination star.

    I mean, that would about work, if the universe was otherwise black. I don't see it getting above the noise floor in the universe we actually live in.

  14. Have they ever addressed the "We made it here, now we've got to report back what we found" problem? Or is that still 'to be invented' tech?

  15. I have a monumental headache today, so please pardon me if my replies are unusually obtuse.

     × 100 GW ≡ pure fiction, straight up.

    100,000,000,000 watts, with the potential to be a kind of weapon against critical infrastructure of almost everyplace on the planet.

    Not good.

    Apart from that, there's the cost. HUNDREDS of millions of kilowatt solid state lasers.
    Colossal PV arrays, one believes — in space — to generate the juice.

    • ALL THIS for a big fat penny payload?

    Really? If one were to have claimed, 'a megawatt of laser for a sub-kilogram payload', well, that's much more rational. Don't need millions of lasers. Sub-kg payloads offer interesting instrumentation, computing, communications and so on. Art thee crazed, m'Lord?

    • PHOTONIC crystals, a zillionth of a meter thick, and hadnling TeraPascals of tensile force.

    As Brett et alia said, on what planet?
    On p.Dirt, we have diamond — but now flawless sheets larger than an automobile?

    Thing is, it takes a budget of about three bags of popcorn and a box of rubber duckies to keep a few feverishly self-motivated science monkeys banging away at their spreadsheets to keep this idea alive. Everyone gets paid, and there isn't either a theoretically possible, or hinted-to fictional account of a paradigm changing breakthrough that'd bring the 10 to 12 orders-of-magnitude disconnect down to Urth.

    And that's what bothers me.
    As it remains.

    ⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
    ⋅-=≡ GoatGuy ✓ ≡=-⋅

  16. The reason they're talking about such absurd accelerations and power levels, is that they want to minimize the necessary aperture. Reduce the acceleration 100 fold, you increase the distance 100 fold, and thus the required aperture. And 100 times the width means 10,000 times the area you have to maintain phase control over.

    Pretty soon you're in Robert Forward territory. Cost scales as the inverse square of the acceleration.

  17. I also see as a problem putting several gigawatts of laser energy into an area of 4×4 meters.

    Not sure how reflective the sail will be, but even a small percent of thermal absorption would be very bad for the sail.

  18. The biggest issue I see, is that a sail that thin and large, at that acceleration, is less a single object, than a bunch of pieces flying in formation. The forces applied to the sail would vastly exceed the sail's own internal structural strength.

    At an acceleration of 10,000G, a 1 gram sail would be under 980 Newtons of force.

    Let's say the sail is constructed of flawless diamond. (The photonic crystal won't be remotely that strong.) The total force necessary to tear it in half would be less than a twentieth of that. And, of course, the sail has no rigidity at all, practically speaking.

    I suppose if you spun it up to keep it from just folding up, (Like that sheet I tried using as a parachute as a child!) and subjected it to an extremely uniform light field, it might survive. But you'd need the loading to be distributed, not a point load.

    Possibly you might work some meta-material magic, cause reflections off axis to neutralize localized stresses.

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