Latest Progress Towards Laser Pushed Solar Sails

Philip Lubin has led a team at UCSB to create laser arrays for propulsion and asteroid deflection. He has received NASA and Starshot funding. There have 100 students in his programs. They have created hardware components and performed various experiments.

The DE-STAR (Directed Energy System for Targeting of Asteroids and ExploRation) has several sizes. DE-STAR-1 is 10 meters on a side, DE-STAR-2 is 100 meters on a side, DE-STAR-3 is 1000 meters on a side and DE-STAR-4 is 10,000 meters on a side.

A full-scale DE-STAR 4 (100GW) will propel:

* a wafer-scale spacecraft with a 1 meter laser sail to about 26% of the speed of light in about 10 minutes (20kgoaccel)
* a wafer craft could reach Mars (1AU) in 30 minutes
* a wafer craft pass Voyager I in less than 3 days
* a wafer craft could pass 1,000 AU in 12 days and
* a wafer craft reach Alpha Centauri in about 20 years.

After a few minutes of laser illumination the wafers are launched. This allows hundreds of launches per day or 100,000 missions per year.

The same directed energy driver (DE-STAR 4) can also propel
* a 10 kg payload to 2.5% of light speed
* a 100 kg payload to about 1% of light speed and
a 10,000 kg payload to more than 1,000 km/s.

Systems in Earth orbit, Mars and the moon could accelerate and decelerate solar sail craft.

There are very large economies of scale in such a system in addition to exponential pho-tonics growth. The system has no expendables, is completely solid-state and can run continuously for years. Industrial fiber lasers and amplifiers have a mean time between failures (MTBF) in excess of 50,000 hrs. The revolution in solid-state lighting, including up-coming laser lighting, will only further increase the performance and lower the cost.

They have already achieved 43% wall-plug efficiency in our lab with efficiency limited by the pump diode efficiency. New diode designs promise ever higher efficiencies and allow full system amplifier efficiencies greater than 50% in the near future. The same basic system can also be used as a phased array telescope for the receive side for laser communications, as well as for future kilometer-scale telescopes for specialized applications such as spectroscopy of exoplanet atmospheres and high redshift cosmology studies.

Reducing the reflector thickness from 1 micron to 0.35 nm (single layer graphene) would increase the speed by (1000/0.35) 1/4 ∼7.4 in the non-relativistic limit. Such a reduction in reflector thickness would also allow the product of photon driver power and size (P0d) to be reduced by a factor of (1000/0.35)1/2 ∼53.

Mass Ejection Thrusters

Miniature ion engines are emerging, which may allow a near term option for even wafer-scale systems. For example, single tip electrospray thrusters are compatible with wafer-scale fabrication and could be effectively used for maneuvering during the cruise phase. With Isp in the 2000-4000 s range these would allow significantly more attitude and transverse maneuver controls than the baseline photon thrusters for the same power level. The amount of ejection mass needed is small compared to the system mass, and thus they are an attractive alternative, as they are vastly more energy-efficient for the same momentum transfer (ratio of Isp).

There is a NASA NIAC looking at 50,000 ISP lithium-ion propulsion.

There is a 2020 Path to Interstellar Flight article in the ESA Acta Futura.

There was a recent presentation at the TVIW (Tennessee Valley Interstellar Workshop).

40 thoughts on “Latest Progress Towards Laser Pushed Solar Sails”

  1. Not so far from what I was thinking, except that I was figuring on much, much less massive sails. Remember, to get them up to 0.2 C in a reasonable distance, they need to accelerate at multiple gs, which requires a very thin, (But NOT 0.3nm!) sail. So a massive sail is also a wide sail.

    But the very thin sail is also very weak, which means you have to minimize longitudinal forces within the sail. The easiest way to do that is make it small.

    So, say the sail is 10nm thick by 10cm by 10cm; It’s not even going to weigh a milligram. But there will be a LOT of them coming in, effectively a continuous stream of them.

    If you catch them in a magnetically confined plasma, you can use the magnetic field to even out the impacts, (While generating some power inductively.) and the plasma is composed of the previously captured sails, no onboard consumables, which means you get away from the rocket equation. And a 10nm thick sail can certainly be absorbed by a dense plasma.

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  2. I’m seeing the main craft as the return of Project Orion.
    A big old blast pusher plate with a shock absorption system.
    Only instead of carrying a set of nukes onboard, you have a set of simple lumps of matter. Chunks of ice or spheres of nickel-iron asteroid or something. Say 1 kg each.
    Then we have a 100 g solar sail incoming at 0.2C, heading towards the centre of the pusher plate aiming at the homing beacon on the craft and using MEMS ion drive steering built into the mass produced sail.
    At t-30 seconds the ship ejects a 1 kg iceball directly into the path of the sail. The 0.2C collision vaporises both 100 g sailcraft and 1kg iceball and the resulting 0.02C plasma cloud hits the pusher plate.., and pushes it.
    0.1 kg at 0.2C is ~43 kilotons of TNT, so it’s like dealing with a medium nuke, hence the Orion design.

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  3. Even when little kids play with guns and accidentally shoot grandma,

    Actually, my grandma did that when she was a little kid.

    She and her sister (my great aunt) were in the house one day, and found Great Grandpa’s rifle*
    One girl picks up the rifle. The other picks up the cat. “Shoot the cat, shoot the cat!”
    BANG
    Bits of cat all over the deafened and terrified girls.

    *or shotgun, I don’t know, it was a long time ago and relayed to me by people who didn’t know or care about such details.

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  4. I kind of picture the “scooter” as a fairly compact craft, with maybe a catcher consisting of a dense plasma in a magnetic bottle. The sails home on the bottle, and end up added to the plasma while transferring their momentum to the ship.

    Then you could even use the hot plasma for an additional thrust, in effect “reflecting” the sails.

    The small sails are already set up for using light propulsion, and the launch laser will be providing light levels sufficient for course correction long after the acceleration is no longer impressive. And the influx of high delta V sails will allow the probe a good energy budget for sending out a homing beacon. Think of the homing beacon as a light “funnel” that the incoming sails navigate to stay within.

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  5. Mmm… I see. 

    Maybe could have little micro-squibs of something stable-but-explosive-enough on board, to do the jettisoning. Or maybe the sailcloth wraps back up onto a kind-of-wad, to present less frontal surface to the IPM solar wind. Or, mayhaps that is the steering mechanism?  itsy-bitsy-tethers could pull in part of the sailcloth, giving the retarding force of the IPM a moment vector off-axis from the direction ‘thrown’.  Its not like it’d have to react very fast!  Days to weeks.

    I would kind of think tho’, that the receding scooter ship would want to ‘see an impact’ over the larger, sail-out surface. If the Δv is large enough, perhaps the impact would vaporize the sail completely (enough) to not change the mass of the pod. Some would stick, some would be vaporized. All of it would contribute to heat and momentum transfer to some degree.

    Hmmm… interesting graduate student research.  Doable on Planet Dirt in high vacuum chambers.  

    ⋅-=≡ GoatGuy ✓ ≡=-⋅

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  6. The big threat is that people are as crazy as they can possibly be. Only understanding repression and its symptoms, cure and prevention will allow much further time of this tech empowerment. Janov published 50 years ago, so time is ripe!

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  7. That’s why I suggest the sails in the mass beam be capable of terminal guidance. Long before you’re done with the acceleration phase of the mission, you’ll be absurdly too far from the ship to be actively tracking it from the sail launcher. All you can do is launch them in the right “general” direction, and let them find it themselves.

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  8. Yah.
    I see what you mean. 

    Throw sails “of mass” at the receding spacecraft. Since they are obviously sub-c, the ‘transmitter gals’ can figure out their final trajectory with probably remarkable precision; that, transmitted to ‘scooter’ tells scooter how to jig its position to capture the next, and next, and next sailcloth package. 

    It remains (to me) an interesting problem of deciding what Δv to arrange to deliver the sail-bits. Too much and ablative capture loss kind of ruins the mission before its day. Too little, and scooter isn’t scooting fast enough, either. Very interesting problem, since once several AU out, scooter is many light hours away from momma bear, who will have, months before, shot sail-bits towards S, hoping for an intercept at the right speed, and at the right place.  

    Almost feels like a different kind of show-stopper.  Scooter needs to get out of the glide path for one reason or another, and misses a sail-bit intercept. Then another. The calculations groundside would be maddening to compensate.  

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

    (NET though: I LIKE IT)

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  9. Posting limits cut me off, just wanted to state that I *am* aware that such a combined beam is going to leak around the edges. That actually permits beam cooling, but it would limit the range that the self focusing could be maintained.

    I honestly think the best bet for interstellar propulsion is a mass beam consisting of small sails accelerated in a fairly short (astronomical units, not light years) distance, but capable of their own course corrections to home on a beacon emitted by the craft they’re propelling. And then you need some separate propulsion system for slowing down at the other end, because who’s going to trust the home system to turn the beam back on after a century?

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  10. “Mere thermal expansion with varying reflected earthlight (!), just millidegrees K, will cause hundreds-of-nanometers of expansion, across kilometer arrays made from near-magic zero-tempco materials. ”

    It’s kind of silly to propose the array as a solid object held in alignment by the rigidity of the structure. On those sizes and time scales, solid objects are just little bits flying in formation, objects are only rigid on a time scale longer than the time sound takes to transit them anyway.

    You would need an external phase reference, possibly more than one, very distant, and probably operating at a much shorter wavelength than the array was transmitting, in order to maintain adequate phase control. Then you could apply a systematic correction for the actual beam aiming. And, of course, the emitters would have to be isolated from any vibrations at a higher frequency than the aiming system response time.

    The best proposal I’ve seen to maintain the necessary targeting stability involved a combined particle beam and EM beam, where the particle beam would act as a very low density optical fiber, containing the EM beam, while the gradient in the EM beam would focus the particle beam by dielectric forces.

    The fact that the EM radiation and the particle beam travel at different speeds actually stabilizes things. Each averages instabilities in the other.

    https://www.aau.edu/research-scholarship/featured-research-topics/combining-laser-and-particle-beams-interstellar

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  11. If the rogue is something the size of a ship and can be easily detected. If the rogue is the contents of someone’s tool box or assorted broken loose debris that drifts away from the inbound ship, that could be a deal breaker. The occasional rogue relativistic ball bearing could also ruin your whole day. 
    You might want to setup your highway so it does not end at your front door, but to one side and parallel to your solar system. Probably best to adopt a relativistic speed limit within the solar system, all violators will get the business end of a terawatt laser instead of a speed ticket.

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  12. But… it is like a gun. 

    There are precious few ways to “death by gun”, without the agency of “a person” to shoot it. Even when little kids play with guns and accidentally shoot grandma, there is the agency of a person. Same for accidental shootings when cleaning firearms. Or when fiddling with them.  But they don’t grow legs, hop out of holsters, run about shooting people on their own volition. 

    Same for these laser array proposals.  
    It takes the agency of a nefarious agent.
    To wreak havoc.

    ⋅-=≡ GoatGuy ✓ ≡=-⋅

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  13. Yah, and follow the dots a bit further: except for proving it can be done, what is the point of a freeflying sail?… mmm… well, next to nil.

    This however wouldn’t preclude sending dozens of freeflying sails about the Solar System for all sorts of perfectly plausible science purpose. 

    • How do they degrade as they waft thru the IPM (interplanetary medium)
    • How do they slow down… measuring displaced solar wind
    • How much reflectance is maintained. How badly degraded in acceleration phase.

    Since one might as well ‘impact’ Mars or Ceres or Io, Europa, Titan or flacking Chiron, what goes, there?

    In fact keeping my (tiring!) skepticism at bay, there appears to be a LOT of science-and-technology that could be done with a free-flying sail and accelerator setup.  

    Like … now to fabricate sailcloth in orbit.
    Like … how hard, really, to solve the ‘milliarcsecond pointing problem’?
    Like … how effecient can lasers get?
    Like … How stable are the transmitting platforms?
    Like … (all the auxilliary support shît, worked out)

    So, yah.
    Sailcloth to Pluto.

    Bring it on.
    At 2 gees. 

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

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  14. In rocketry ‘theoretical Isp’ proposes that minus radiation-of-heat losses that a hot gas can accelerate to many times the speed of sound supported by the hot fluid. (In physics, all gasses are ‘compressible fluids’.)

    For instance, in (2H₂ + O₂) → (2H₂O + 226,000 J/mol), there is a substantial ultraviolet radiation component and the infrared hot-gas radiation as well, which saps perhaps 15% of the 226 kJ/mol away. Still, that leaves 192 kJ/mol. 10.7 MJ/kg (of water), or so. 

    10.7 MJ of kinetic energy, remembering:

    Ek = ½mv²; … so
    v² = 2Ek/m; … and
    v = √(2E/m);

    v = √(2×10,700,000 J ÷ 1 kg);
    v = 4,620 m/s;

    Isp = v/G₁;
    Isp = 4,620 m/s ÷ 9.80655 m/s²;
    Isp = 471;

    See that? It was relatively easy to find a number very close to the published reaction thrust of the veneralbe Space Shuttle’s main engines. Close!

    If we follow that same logic for depositing a particular amount of energy into the H₂ plasma… let’s say what, 1 GJ/kg? we get:

    v = √(2E/m);
    v = √(2×1,000,000,000 ÷ 1);
    v = 44,700 m/s … ÷ 9.80665:
    Isp = 4,560;

    Wow.
    Spectacular!

    But it still follows Tsiolkovsky’s Rocket Equation:

    ΔV = Isp⋅G₁⋅log( Mi / Mo ); … where
    Mi, Mo are initial and final weight of the spacecraft. Say 3:1?
    ΔV = 44,700 log( ³⁄₁ ); … log is base-e logarithm.
    ΔV = 49,000 m/s

    26 days to Mars, if ¾ AU average separation.

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

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  15. Yah, that IS the problem … underneath this entire discussion.  It ALWAYS resorts to ‘magic’ in the end, but magic that is repackaged to sound perfectly plausible to those who are essentially innumerate. I mean, between beer buddies, what’s the difference between 1 µm and ⅓ of a nanometer, anyway!  Lets not wave-physics get in the way of a perfectly silly extension of an equation.  Right?

    Actually, there is not a 600 lb. gorilla in the room, but a 6,000 lb. elephant: pointing.  

    See, its one thing to blithely wax eloquent (as do the authors) about putting together phase-coherent billion (!!!) emitter fiber lasers and pods of beam-forming modules, to be harnessed by the square-kilometer (or tens) to emit a 100 gigawatt (!!!) beam that thru its exquisite phase coherence and time synchronization, just happens to have nano-radian targeting chops, and is able to focus the preponderance of that astounding beam onto a fleeing magic-material sail, whizzing off at many AU, a wee dot in the firmament.  … and hit it… square and centered.  

    Like … right. No. WRONG.

    Each and every element of the synthetic aperture laser needs to be aligned to nanometer precision. A billion emitters. Mere thermal expansion with varying reflected earthlight (!), just millidegrees K, will cause hundreds-of-nanometers of expansion, across kilometer arrays made from near-magic zero-tempco materials.  

    Moonquakes, displace micrometers, in waves.  

    Just saying… pointing.
    ⋅-=≡ GoatGuy ✓ ≡=-⋅

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  16. The difference being that you started to substitute in real numbers for the algebraic variables to see what sort of values come out the other end. Hence the

    about 9 millinewtons of force per square meter, with incident power of 1.6 megawatts.

    Though we really should take it one step further.
    The only information I can see on the OP about sail material is that they are thinking of 500 nm thick polymer film.
    Taking the density of UHMWPE (about 1000 kg/m^3) as an example of high strength, advanced polymer, that gives us a mass per square metre of 500 mg which we can combine with 9 mN to get 18 m/s/s. Which is almost 2G.
    Not hanging around. Not the months and months to get anywhere of existing solar sails. But it’s not being fired out of a cannon either.

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  17. “Reducing the reflector thickness from 1 micron to 0.35 nm (single layer graphene) would increase the speed by (1000/0.35) 1/4 ∼7.4 in the non-relativistic limit. Such a reduction in reflector thickness would also allow the product of photon driver power and size (P0d) to be reduced by a factor of (1000/0.35)1/2 ∼53.

    Little bitty problem here: Graphene is actually pretty transparent at anything above 300 nanometers. You’d have to use microwaves or at least far infrared to avoid 80-90% of the incident light going right through it.

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  18. All that laser power can be used to power large rockets by heating different types of fluids. It could heat hydrogen gas until it atomizes giving ISP in the 5,000 range.

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  19. Thank you, makes sense even to a layman. My main reaction as a layman is “let’s do it, what are we waiting for!”. Seems like we’re going to need enough space commercialization for private companies to build this, since our government procurement process is FUBAR.

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  20. Note that there is a “sweet spot” between pure-photonic drivers and beamed-energy-and-reaction-mass systems.  

    Like how over the last 70 years the turboFAN engine was found to be way-way-way more fuel efficient at comporting a large airliner tens of thousands of kilometers, compared to a turbo-jet. Why?

    Using the turboJET’s powerful thrust to spin up a much larger ‘bypass fan’ to push incoming atmosphere out the back makes a lot of straight physics sense:

    F = ma — and of course
    a = Δv/Δt — but setting Δt to ‘1 second’ means
    F = mΔv — for most intents in napkin calculations. Rearranging:
    m = F/Δv, used below;

    Also,

    E = ½mv² — in its pure form, but
    P = ½mΔv² — as far a jet engines are concerned, on 1 second Δt timescale

    Then

    P = ½ F/Δv • Δv², which algebraically reduces to
    P = ½ FΔv

    This ‘says’ that for a given FORCE, (accelerating some mass) to a speed (½ as much (using twice the accelerated mass)) still results in saving ½ the power.  

    Excellent!
    By any measure!
    This is what the high bypass turbo-fan engine does. 

    So, the same goes for BEAMING power instead of REFLECTING it.  
    Turning a pure photonic drive into a beamed-power reaction mass drive …
    Substantially increases its energy-per-thrust efficiency.

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

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  21. Yah, mostly.
    There are a few of us … still …
    That know some physics, and use of Scientific Notation.
    A few.

    ⋅-=≡ GoatGuy ✓ ≡=-⋅

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  22. I took a stab at it … before reading your comment.
    Oh well.
    Late to the party.
    Again.

    See replies to Jean Baptiste and R. Kimhi

    ⋅-=≡ GoatGuy ✓ ≡=-⋅

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  23. Here’s a nice relationship wirth remembering:

      D⋅d = 1.22 λ B … where
      D = diameter (meters) of the spacecraft ‘receiver’ film
      d = diameter 9meters) of the laser source, incl. synthetic-aperture business
      λ = wavelength of laser beam
      B = baseline (meters) of distance between ‘d’ and ‘D’

    Its kind of fun because with algebraic rearrangement, a calculating fiend can find the maximum baseline distance achievable with a pair of d⋅D apertures, or, for a given design-rule baseline, how big the d⋅D pair need to be to support receiving ‘most’ of the power of the laser to useful propulsive purpose. 

    With 1550 nm IR laser light (per this article), and for a 100 m transmitter, and a 100 m light sail, 

      100 × 100 = 1.22 × 1550×10⁻⁹ • B
      B = 10⁴ / (1.22 × 1.55×10⁻⁶ )
      B = 5.3×10⁹ m … or 0.035 AU

    Of course, if it is ‘OK’ to receive a lower-and-lower fraction of the transmitter’s power, or (and?), if the transmitter can increase power say 10× beyond the nominal ‘close-maximum’, then B can be 10× to maybe 25× higher. Perhaps up to 1 AU or so.  For this dual 100 meter system.

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

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  24. At first I wasn’t going to reply — yet you make a salient point: the power levels involved with interplanetary photon-repulsion transport are … in a word … gargantuan.  Ginormous to the point that were they to be pointed at say, Boston or Singapore or Lagos or Johannesburg, well the lased beams could easily fry the entire cities.  In a matter of a few hours, maybe.  Beamed nuclear weapons!

    The notion that the fusioning ‘ships’ could also serve as kinetic destroyers has been floating around the SciFi story-line literature for at least 50 if not 60+ years. Certainly 50. 

    Thing is, we want it all — speedy interplanetary scale transport, significant payloads, tolerable safety measures, and obviously, low transit costs.  

    Throwing that 4th factor in there, really puts the qabash on realistic plans. 
    And the beamed nuclear weapons prospect.  

    I guess a simple-enough response is, “if you have 1 rogue, you can focus the other non-rogue terawatt lasers against it, and fry its mechanism in … seconds. Maybe. Depends on how fast the arrays can be repositioned.”

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

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  25. It is. A problem.

    Basically there are 5 components that mutually compete to define the limit of incident power.  

     № 1 – Rr = reflectivity
     № 2 – Ra = absorptivity
     № 3 – P = Incident power
     № 4 – T = Maximum allowable film heating
     № 5 – η = degree of ‘black-body-ness’ of the film in isotropic IR emission

    So, for instance, let us say (for a competent, tough film:

     [1] Rr = 85%
     [2] Ra = 0.25%
     [3] P = the ‘x’ unknown
     [4] T = 350° K
     [5] η = 70%

    OK, the ‘heat equation’ is the defining part. Remembering that (P ≈ σT⁴), the (σ ≈ 5.67×10⁻⁸, the Stefan-Boltzman constant), then

     P = ( 5.67×10⁻⁸ • (T = 350°K)⁴ ) • η
     P = 600 W/m²

    That P however, is P absorbed and thermally re-radiated. So…

     Pi = P / ( (1 – Rr) • Ra )
     Pi = 600 / ( 1 – 0.85 ) × 0.0025 )
     Pi = 1,600,000 W/m²

    That’s the quantitative answer to your point. Yet, we can ‘do it’ further:

     F = (2 ( Pi • Rr ) + 1 ( Pi • (1-Rr) • Ra )) / c
     F = (2 ( 1.6×10⁶ × 0.85 ) + 1 ( 1.6×10⁶ × 0.15 × 0.0025 )) ÷ 299,792,458 
     F = 2,721,000 ÷ 299,792,458
     F = 0.009075 N/m²

    This particular “pretty darn good film” with limit-heating gives about 9 millinewtons of force per square meter, with incident power of 1.6 megawatts. 5.67×10⁻⁹ N/W.  OR, if you like the more productive inverse,

    175 MW per newton.

    Not much force for all that energy! But no reaction mass lost, either!!!

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

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  26. I think such papers are probably of more interest to the audience here

    You would think, but I’ve never gotten that impression. Always struck me as being more of a “popsci” crowd.

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  27. If you go to the linked paper he dives into many of the areas that he skips over in his talk to make space for “jokes” and weird analogies.

    However he still skips over two critical subjects, the sail itself and interstellar communication.

    The sail physics, and how they could deal with GW of radiation, are analysed in terms of mathematical relationships between reflectance, absorption, radiation, and resulting temperature. But comparing these results to any actual materials is ignored. Let alone actual experiments to see how it works in real life.

    Communication between a thin film electronic circuit and Earth many AU , or even lightyears, away is just glossed over completely.

    Now these seem like serious guys. Probably there are other papers somewhere that DO address these issues.
    I think such papers are probably of more interest to the audience here, for whom the basic idea is already established.

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  28. this is the key to rapid interplanetary travel, i expect most traffic in the colonised solar system will employ a laser highway network and fusion drives will be strictly regulated

    nobody will care that every space colonist has a laser pistol because the authorities will be too busy trying to manage the number of potential kinetic planetbusters flying around

    ships travelling on the laser highway simply arent as much of a threat as a tramp freighter with enough fusion fuel to speed up to a fraction of a percentage of lightspeed lol

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  29. How do they avoid vaporization of the laser sail wafer/spaceship?

    Putting all that power in a small place looks like a problem.

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  30. I think that the key here is fabricating in space sails as big and thin as possible, highly reflective, that can withstand high temperatures. Them laser beams will probably have to start with lower intensity and focus and increase as the object gets further away to never go beyond the intensity limit that the sail can withstand. A bigger sail will be able to collect the beam light further away as it get diffused. Placing several laser arrays along the object acceleration route will allow to push the objects for many times longer period even with lower intensity lasers. With a big sail the object can continue accelerating using light from space.

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