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It is the not-too-distant future in an alternate timeline, and humanity has tripled-down on solar power.

We built a network of solar focusing satellites that orbit Sol at close range (so they can capture more sunlight). "Close range" means somewhere between the orbit of Mercury and the solar corona. So, not as close as the Parker probe, but still pretty close. The details of how they focus the light without being incinerated are outside the scope of this question; assume handwavium.

These satellites focus light at a variety of targets distributed throughout the solar system to provide power, all of which have super-advanced solar-conversion tech aboard. The targets that get the most love are, in priority order:

  1. Electric-powered "solar trains" on their way to and from Earth's space facilities. Each must be powered continuously for the bulk of its climb and descent. We have five or six of these at present.
  2. Large permanent space stations near Jupiter, Ceres, and Saturn, whose batteries need to be recharged every month or so. (The colonies are primarily robotic, hence their outsized consumption.)
  3. A giant solar-collection station in orbit around Earth, which charges massive batteries that we then carefully land on Earth. It demands power continuously, and can handle the light from 2 satellites simultaneously. It does not provide all the power Earth needs, but we want it to do more; the limiting factor is the number of batteries we have, and we're building more (subject to material scarcity).

These satellites must be very hardy to survive so close to Sol, so building them is expensive and consumes scarce resources. Thus, we haven't built a bajillion of them; more in the neighborhood of two or three dozen. (Though that number is up for grabs.)

We obviously want each satellite to have line-of-sight on as much of the solar system as possible for as much time as possible. Each satellite has a "firing arc" of 50° from its Solar radius (apologies if I've expressed that weirdly).

So: what kinds of orbits are best-suited for these satellites?

Here are a couple of obvious ideas, which (for starters) I'd appreciate help evaluating:

  • one evenly-spaced ring around the sun, in the plane of Earth's orbit
  • evenly spaced around the entire surface area of the sun, with orbits calculated to avoid collisions

The biggest objection I can think of to the ring is that many targets will frequently be obscured by planets, leading to blackouts.

The biggest objection to the second pattern is that the limited firing arc means that many satellites would have line-of-sight on nothing for a long time, which means we'd need to build a gazillion satellites (which we haven't), which once again leads to power blackouts.

You might think that Earth can take a blackout because we have other sources of power, but the environment is near the breaking point, and the Terrarium Laws are absolute and totally merciless: anyone who tried to turn on a coal plant for even a few minutes would be summarily executed. If that means a hospital full of babies dies, that is what happens.

The stations and the space trains really can't take a blackout either: the few people on them will die without heat and recycled atmosphere, and if station-keeping goes untended for too long, the station itself could be lost.

If the solution requires a minimum number of satellites higher than what I've proposed, feel free to add them, but then I want some kind of demonstration showing that nothing lower than your number will suffice. Assume my number is 36.

If the firing arc is unworkable, you can change that, too, subject to similar requirements about proof.


Edit to add: there are no mirrors or lenses anywhere in this system. At least, nothing anybody would recognize as such. I said "handwavium," and that's all you're going to get (in this question). I think this would not evade the problem of etendue, but it should short-circuit a host of other concerns that answerers might get tripped-up by.

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    $\begingroup$ "Many targets will frequently be obscured by planets": No they won't. It will be a very rare event. The planets do not orbit in the same plane as the Earth. (And batteries are a real thing. You can easily bridge over the very brief very infrequent eclipses with suitable batteries.) $\endgroup$
    – AlexP
    Jul 5 at 8:23
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    $\begingroup$ There are issues regarding station-keeping with large mirrors close to the sun, will that comprise a separate question? $\endgroup$ Jul 5 at 14:49
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    $\begingroup$ ferrying batteries up and down the gravity well is an absurdly inefficient way to transport energy. why not beam the sunlight directly to the surface? $\endgroup$
    – ths
    Jul 5 at 17:38
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    $\begingroup$ if they have magick which breaks conservation of energy (they need to get up to orbital speed too!) they should use that to generate energy, not sunlight. $\endgroup$
    – ths
    Jul 5 at 17:59
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    $\begingroup$ Addressing the edit: you describe the system as focusing sunlight. The etendue issues are basically a matter of geometry and will exist for any system that handles sunlight. This is the main reason why no real proposed beam power system uses beams of sunlight, they convert it to microwave or laser beams. Lasers can be constructed to produce light as if it came from a point source at an infinite distance, microwaves are low enough in frequency that we can directly emit them with the characteristics of our choosing. Another benefit is better conversion efficiency at the receiver. $\endgroup$ Jul 5 at 21:47
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The approach is fundamentally flawed. Focusing optics, whether mirrors or lenses (realistically mirrors in applications like this), obey conservation of etendue. The incoming light comes spread across an angular area, and that spread still exists after passing through a focusing element.

It works out such that for a mirror to just equal the sun's intensity, it must appear as large as the sun from the viewpoint of the target it's focused on. Your arrangement with the focusing satellites as close to the sun as possible is the worst possible case, maximizing the required mirror sizes. Equaling the intensity of the sun with this setup would require a mirror the size of the sun. A mirror at the distance of the moon would "only" need to be as large as the moon (which coincidentally has about the same angular size as the sun as seen from Earth). One in geostationary orbit would be about 3500 times smaller than one near the sun, with about 12 million times less surface area.

Placing satellites near the sun only makes sense if the conversion is done at the satellite, and something else used to transport the power to its point of use. Microwave beams would likely be more efficient to generate and collect than lasers, but it's easier to get narrower beams with lasers. And even with these, longer distances mean more transmission losses and larger transmitters/reflectors, and it's unlikely that putting them as close to the sun as possible is the most effective use of resources. There's plenty of room in Earth orbit for big solar collector arrays, and you could easily arrange things so only a small fraction of the arrays could ever be in Earth's or the moon's shadow at any given time.

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  • $\begingroup$ This is the answer. +1 $\endgroup$ Jul 5 at 15:55
  • $\begingroup$ I believe Elon Musk has already attempted something similar already with much less reflected sunlight, and people still got mad $\endgroup$ Jul 5 at 16:44
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    $\begingroup$ I am a little skeptical that you've applied the conservation law correctly, or that it's actually a barrier. I'm definitely not an expert, but your statement makes it sound like there is literally no such thing as "focusing," which seems obviously false. I also think it's not a requirement for me to preserve 100% of the sun's intensity to make this worthwhile. I wonder if I could prevail upon you to walk me through this in some more detail? I have read the WP article several times, but the whole thing is incomprehensible, being 35% formulae in Greek. $\endgroup$
    – Tom
    Jul 5 at 17:47
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    $\begingroup$ @Tom So, with optics, the best you can do is make the surface of your lens appear as bright (per circular radian) as the thing you are focusing. So a magnifying glass right next to a piece of paper can (in theory) create the illusion of the sun covering a huge angle of the "sky". The same magnifying glass further away limits the "optical sun" cover less of the "sky". But the brightness of the image projected cannot be any brighter than the thing you are projecting; the optics can make it larger, but no larger than the optics are to the target. $\endgroup$
    – Yakk
    Jul 6 at 13:12
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    $\begingroup$ @Tom Lasers are not optics; well, at least, not only optics. A solar energy collector which then powers a laser is very fundamentally different thing than a set of lenses that focus the sun's light. You still have a bunch of entropy issues, but the math is very different. $\endgroup$
    – Yakk
    Jul 6 at 13:15
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The biggest obstacle your mirrors will have to go around is the Sun itself, meaning that you would prefer a polar orbit for them, rather than an orbit laying in the ecliptic.

When you are above the poles, you can sweep basically the entire ecliptic plane with your mirror as you please. If you are on the ecliptic, there will always be some portion of the sky blocked by the Sun itself.

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  • $\begingroup$ I think a polar orbit runs afoul of the narrow firing arc, though. $\endgroup$
    – Tom
    Jul 5 at 11:38
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Choice of orbit is an interesting topic, but..

How to target high energy interplanetary light beams ?

Precision of the beam will be the main challenge and I even think it's a show stopper.

Sending high energy concentrated light beams has been discussed in the past, when solar satellites in Earth's orbit were considered. Even from 100-400 miles altitude, a tiny direction fault of the beam would impose huge safety risks on the planet surface. Here you propose to send beams across the solar system.. such problems multiply. Even when the alignment error is extremely small, e.g. a beam gets 10e-11 degree misaligned, or your solar sat gets displaced 2 microns, because a dust particle impact. The beam will not hit the intended receiver, it can be miles off at the target.

Also at this scale, take into account any gravitational fields in the path of the light beam (planets, moons) will bend the light beam and result in angle deviations.

Solution

As mentioned above, the easiest orbit (equatorial) has also the disadvantage of not being effective for >50% of the time. imho there is no reflected beam solution at all.

Consider instead: storing the solar energy on the spot, by synthesizing some type of fuel in the process.. and then move that fuel out and distribute it, using robot spacecraft.

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Shooting a moving train full of people with concentrated light from a far-off satellite seems like asking for trouble. I am sorry. I am shy at parties, too. Maybe I am too risk averse?

Other ideas

1: Satellites can bounce light to each other. If you really like satellites beaming light, you can have one without a straight shot pass the light to one that does.

  1. Venus has lots of sunny real estate. Huge solar collectors orbiting Venus could collect a lot of energy and if that cools down Venus so much the better. Or Mercury - you could have your plant on giant circumplanetary train tracks to crawl along in pace with the long Mercury day.

  2. Turn solar energy into fungible, portable energy ASAP. You are charging batteries with it and that is good. Maybe charge those batteries on the satellite or on Venus? Or if you are really digging the cross-space energy beam, shoot it someplace no-one will get hurt when the beam gets frisky. Maybe Luna? The old beam scars of molten moonstuff will be interesting to see for new arrivals.

  3. Batteries shmatteries. Batteries are fine but kind of 1970s. I am sure you can come up with more energy-dense near future methods to save solar energy. Different species of antimatter? Something even cooler (hint: make it "quantum"! That means it is cool.) Or retro - the moon has lots of aluminum. Aluminum metal is a great way to store energy and it is very stable.

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  • $\begingroup$ You're not wrong about the danger, but I propose to solve the problem thus: the train's solar collector completely obscures the train from the satellite's point of view, the collector can autonomously reposition and reorient itself in real-time to best-catch the ray, and the ray is turned on gradually until tracking is solid (which could take a couple hours because of the round-trip delay). $\endgroup$
    – Tom
    Jul 5 at 17:30
  • $\begingroup$ How does aluminum store energy? And, can't "battery" refer to any technology that stores energy? Like, those giant spinning stone cylinders would be "batteries." I never said these were big Duracells. $\endgroup$
    – Tom
    Jul 5 at 17:31
  • $\begingroup$ I forgot to add, re: danger mitigation: "flight plans" are coordinated at an excruciating granularity by AIs. Space trains don't get to do any maneuvering. This is one of the two reasons I refer to them as "trains" rather than space ships. $\endgroup$
    – Tom
    Jul 5 at 17:40
  • $\begingroup$ @Tom - all good! Re aluminum - it burns. It likes to be the oxide and will again if you let it. That is what makes thermite hot. $\endgroup$
    – Willk
    Jul 5 at 17:47

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