Now it's 2085, and human civilization has started to reach the planets. Using the most advanced fission rockets, we can travel from Earth to Mercury in 8 months and from Earth to Jupiter in 1 year. The asteroid belt is now the high frontier, but we are still far from interstellar travel.

Laser or maser pushed lightsail is currently the only propulsion system we can feasibly build to reach a considerable fraction of the speed of light (say 0.25c). However, such a propulsion system emits thousands of terawatts of laser (equivalent to detonating one thermonuclear warhead per second) from its base station, so it can easily be a weapon of mass destruction too, and there will be opposition if someone were to build one.

So the question is, how can a laser-pushed propulsion system be built successfully by a small group, despite the political opposition it may incur? Assume there are no serious technical problems in constructing such a system, and the difficulties are mainly political. To answer this question, you may re-design the propulsion system in any way you see fit, as long as it meets the requirements below. You can also decide who this small group of people is that built the propulsion system.

Background information:

  • Technology has been progressing, but humans have not discovered any new law of physics. So by 2085, all new technologies are still based on today's science.

  • Many nations and corporates are involved in the competition for space, and none can monopolize space yet. No country can assert absolute hegemony on the Earth either. That said, you can't simply eliminate all who oppose you.

  • The economy in space relies on mining asteroids and collecting solar power. Many powers, primarily chartered companies, focus on scrambling for the asteroid belt. There are some permanent settlements on the moon too, and the moon is mostly under the control of nation-states on the Earth. The development and exploitation of Mercury have just become cost-effective due to the advancements in rocketry, and the competition for Mercury has just started.

  • The laser-pushed propulsion system will locate in the orbit of Mercury and operate on the abundant solar power there. It also needs a giant Fresnel zone plate located a few astronomical units away from the sun to focus its laser beam. Advanced AIs can mine Mercury and execute the construction project mostly automatically, with little human labour cost.

  • A thousand-terawatt laser system around mercury will threaten the settlements on the surface of the Moon and cities on the earth (if it can aim and focus its beam on them at all). It can also easily scorch the mining asteroids in the asteroid belt and destroy the surface outposts and the space stations all over the solar system. Therefore, no party will feel comfortable if they know or suspect such a system is built with any military capability, unless it is under their control, or so they think.

Requirements for the answer:

  • The laser-pushed propulsion system must be able to focus thousands of terawatts of laser in the distance of thousands of astronomical units. It doesn't need to have any ability to serve as a weapon, but such an ability is good to have.

  • The reason for constructing the propulsion system must be an initiative of a small group of people (and they have the resource to do it). It must not be because of international cooperation or some situation/disaster forcing humanity to do it.

  • The small group of people who built the propulsion system must not be a secret society, and they must not hold too much power like the fictional Illuminati.

  • No matter who nominally owns or supervises the propulsion system, the small group of people who proposed to build it must have substantial control over it and can use it for their purposes. (Again, they don't plan to use it as a weapon.) They should also have the power to decide who can use the system.

Optional requirements for the answer: (Try to satisfy them if feasible.)

  • The construction of the propulsion system should not risk any hostile sabotage or military action to stop it from being built.

  • The propulsion system should be built openly (you really can't hide something that big) and in a non-threatening manner. It should be advertised as built for exploration and scientific purposes only.

  • The small group of people who constructed the propulsion system should be private (non-governmental, and not international organizations). But they don't have to be a profit organization.

  • The construction of the propulsion system should improve the public relation of the small group which controls it, or at least does not hurt their public relations.

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    $\begingroup$ What's the nature of the opposition, why are they against it? The rest of the requirements strike me as excessive and to do with the way you go about writing your story and the complexities of the politics - which amounts to story-based answers which would be off-topic. $\endgroup$ Apr 18, 2022 at 19:33
  • $\begingroup$ @EveninginGethsemane the potential to repurpose a tera-watt range laser as a WMD. It's stated in the first paragraph of the Q. $\endgroup$
    – GOATNine
    Apr 18, 2022 at 19:34
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    $\begingroup$ But your optional requirements state that this is one of the things it should be able to be re-purposed for. Again, that's getting into the complexities of solar-system politics which we know nothing about. $\endgroup$ Apr 18, 2022 at 19:37
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    $\begingroup$ It looks like you're asking for us to write a story about how a small group of individuals can build laser propulsion in the face of undefined political opposition. Firstly we're not here to write your story for you. Secondly you'd need to describe your political system, and the opposition specifically for this question to be answerable (if the core ask was on topic in the first place). $\endgroup$
    – sphennings
    Apr 18, 2022 at 19:41
  • $\begingroup$ Are you thinking of some sort of oversight committee? It's tough to know how this could work (or if it could) given unknown powers and factions at play. $\endgroup$ Apr 18, 2022 at 19:42

6 Answers 6


Kare Sailbeam

Jordin Kare developed a significant improvement over the classical beam sails. One just accelerates small sails to near the speed of light and uses nuclear pulsed propulsion concepts (either Orion or Mag-Orion) to push the spacecraft. The advantage is that you can focus the energy you want over lightyears onto a few square meters with this. Admittedly, the sails need some steering but that can be done.

The advantages of this are that the light sails you need for propulsion are poor weapon systems for planetary bombardment. Even space colonies can shoot them down with their own lasers (pick a frequency that is absorbed, this is the strategy the Mag-Orion concept uses to get its plasma). Additionally, you don't need big lenses to focus the laser. The paper suggests that the sails can go form 0 to c in under a second with much less laser power.

As an added bonus, this is also great for propelling vessels in the solar system (any nuclear pulse propulsion system would need little in the way of adjustment to use Sailbeam. Alternatively you can trade energy with colonies in the outer solar system. They just need a magnetohydrodynamic generator you can target.

To sum it up, the smaller scale, the lack of the frescl lense, the saver energy transfer and the economic usefulness of the system make it easier to sell to others. Furthermore, the economic aspects could sell it as a prototype project whose final version could be duplicated and sold to others.

PS: Check out Fusion Highways as another cheap interstellar option.

  • $\begingroup$ I did some simple calculations using relativistic momentum. Assume the sails move at 0.97c and completely inelastic collisions between the sails and the vessel, it takes 159 kilometre-squares of the Kare sail to accelerate the 1000-ton vessel to 0.25c. If the average acceleration of the vessel is 0.3g, it requires the base station to launch ~1.56 square meters of sail per second. The power required is 177TW, which is much more manageable but still huge. If using heavier and slower sails, the power required would decrease further. $\endgroup$ Apr 19, 2022 at 17:38
  • $\begingroup$ Fusion highway is an attractive option in real life, but it does not fit in my story. In my story, the launching system must be quickly ready to use and there is no time to wait for the fuel to be spread out onto the highway (which is probably hundreds to thousands of astronomical units long). $\endgroup$ Apr 19, 2022 at 18:23
  • $\begingroup$ @VegetableNewMan If my math is correct that means a 3.3 year acceleration phase over 0.164 lightyears. Those numbers make me worry about sail drift (not to mention the inevitable loss rate) it might be worth launching antimatter, fusion or fission powered guidence stations along the future path of the sails to give them readjusted pushes with lasers along the way. $\endgroup$ Apr 19, 2022 at 21:23
  • $\begingroup$ In my calculation, the acceleration only takes 0.8 years and 4000 astronomical units. But this distance is still huge. One possible alternative to guidance stations is to make diffraction gratings on the sails to control their bearing with lasers. The "guidance" laser can probably also clear some dust on the path of the sails. $\endgroup$ Apr 19, 2022 at 21:57

They will not own the energy generator.

The machine is so big that everybody can see where it is pointing to. the builders will only build the machine, not the power generators, they will need so much energy that they will have to buy it from different providers when they use the machine.

If the providers see that the machine is pointed in the wrong direction they will cut the energy.



(Collectively Assured Destruction)

As you note, MAD is not super applicable here because of the first-strike advantage.

This is pretty simply solved by the fact that every party that does not control the laser has great opportunity to destroy it, at considerably less cost. This statite is an enormous, and enormously costly, civil engineering project that can have its day ruined by one ship with a mass driver (or one ship’s mass, driven at it)

The moment this device is turned against any living target, the sole immediate priority of every other living thing in the solar system is going to be destroying it. Unless this small group’s goal is to devastate a planet or two before their investment is liquidated permanently, they’re going to be strongly inclined to toe the line of propriety.

Everyone else will know this too. This makes the device less a WMD and more a nuclear power plant. It has the potential to be misused by malicious actors to cause limited devestation, but as long as no one gets too gung ho about refining uranium hexafluride, that danger is limited.

The Wife of Caesar

A gun is always loaded, so pointing one at someone is an active death threat. That’s true here as well.

This means that there is going to be a very strong incentive for the group to be above suspicion, to openly and clearly communicate where the barrel is pointed, communicate any traversal with exceeding clarity, and likely permit or encourage (even offer to fund) nearby observation posts for major powers in the solar system. The outposts will have the data and positioning of their relevent home resources, and if they see the laser starting to point at any of them, will retaliate immediately and preemptively.

Having representatives of those powers nearby will have the bonus of providing de facto protection against smaller bad actors who may have intentions to sabotage the device.

In effect, you have a cafeteria lady with a revolver locked in a room with thirty hungry preteens armed with steak knives. As long as no one gets stabby or shooty, everyone’s safe, the food slop gets delivered, and life goes on. The kids will hold each other back from getting stabby because they want to keep getting food, and the cafeteria lady doesn’t get shooty because she knows that as soon as she murks a kid, the rest will swarm her and stab her to death, and she’s only got so many bullets.


Sharing! (Well kinda)

Every group wants the laser to be under their control, now this would be made of numerous arrays of lasers to have constant coverage. So simply have each party controlling some lasers. Since each party has equal capabilities, this introduces MAD (mutually assured destruction) meaning no group will attempt to use them maliciously, because it will cause them to be attacked.

This is basically the same reason that humanity is not extinct. So many people have nukes that nobody can use nukes. If people had giant lasers left and right, nobody would use giant lasers (to kill).

Of course, the politics that would be required for this would be complex. Perhaps other groups would get word of a group making one and independently decide to make their own to defend themselves, who knows. the main small group could just be privately funded by (rich) people wanting to explore. Of course, international cooperation rather than peaceful holding of WMDs could also be a good choice, but you said no to that. All the groups coming together under one group to propel humans together would definitely be good in the eyes of the public.

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    $\begingroup$ I have considered MAD, but it does not quite work for lasers because lasers move at the speed of light. If your opponent decides to launch a surprise attack on your laser system, the intelligence of the attack will only reach you together with the laser, so you will have no chance to return fire with your laser. $\endgroup$ Apr 18, 2022 at 20:45
  • $\begingroup$ @VegetableNewMan that could be avoided by placing all the lasers on the same planet, such as mercury, which also has the advantage of easy solar power. Since it’s on the same planet, any laser can’t reach another laser. Additionally lasers would need time to aim, meaning some of small warning would be possible. $\endgroup$
    – Topcode
    Apr 18, 2022 at 22:10

Maybe an option to to have the laser be very low-powered and focused over a large area. This way a large enough sail can still take advantage of it, but it would be very difficult to use it as a weapon due to the actual energy density per square meter. However, this would be a much more gradual acceleration, over a protracted period of time, so you would need many of them to push any large amount of ships.


You split them up and make them dual use- its just a skyborn green house illumination system, no beam laser here- everyone in this part of the oort has one. Mc Millers next door have one to drill come ice from asteroids.

And if needed, they can all be brought online, and aligned on the same spot to accelerate a ship. But if you do and get caught, you know the jig is up and the authorities of this world will try to get them all.


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