# Is this concept for very low energy space travel plausible?

Humanity is on its last lap. A once great empire has been reduced to a single Dyson sphere around the dying sun, and most of the matter of the solar system has already been incorporated into this last bastion of survival. Any activity which leaks energy out of the sphere is suspect, and only very rarely do humans venture outside anymore, on occasional expeditions to salvage stray asteroids or other rubble.

In this scenario, the only way space travel could be considered acceptable at all is if it can be performed at an extremely low energy cost, so that the energy gain of towing in an old shuttle or piece of rock surpasses the expenditure of making the trip in the first place.

My thought is that a type of travel that could be conceivable in this world would be using laser sails to travel outwards, and then falling back towards the home sphere through natural gravity. Heat inefficiencies from the lasers themselves could be absorbed back into the sphere, and the kinetic energy of the ships could be reclaimed on return.

Is this feasible? The ships can be assumed to be almost fully contained and self-sufficient and able to sustain a small crew for months or years. Some energy/heat leakage will be unavoidable, but is it possible that you can still go energy-positive through such a scheme, or is it just utterly ridiculous? Speed and comfort can be sacrificed, since we're talking about squeezing just a few more years of sustainable living from a solar system which is already at the limit.

If not reasonable based on what I laid out here, are there other options which could work better?

• Why would they use crewed ships for this? Even with 21st century tech, a computer capable of the job will use a fraction of one percent of the energy needed just to recycle water for a human crew of two or three over a similar period. Not to mention, best check your entropy; waste heat is waste because it's not hot enough to do anything cost-effective with it. Jun 24 at 12:55
• "Any activity which leaks energy out of the sphere is suspect:" Very obviously the Dyson sphere radiates out exactly as much energy as it captures from the sun; because otherwise every little rock inside would have the same temperature as the sun, which most people would consider rather uncomfortable. Energy and free energy are not the same thing. Jun 24 at 13:16
• @AlexP Worse than that. The cosmological signature of a Dyson sphere is longwave IR black body emission, the energy of a G or M star translated to brown dwarf temperature. Keep the star's heat from radiating, and the interior would be come hotter than the star's core over a cosmologically short time, leading to an explosion potentially mistaken for a hypernova. Hmmm. Jun 24 at 13:26
• What is a "dying sun" exactly? Stars get brighter as they age. And, how distant a future we are talking about here? Sun has billions of years of "lifetime" left. In that time, humans will be either extinct, or develop to incomprehensibly godlike level. Either way, they would not bother with lasers and Dyson spheres. Jun 24 at 19:07
• Regarding your setting, the only thing I can think of that sounds like your 'dying sun' is the end of a very-very-very long lived white dwarf, the creation of which probably shattered the planetary bodies around it - which is actually a great place to put a Dyson sphere, assuming you have the opportunity. Jun 25 at 5:59

The biggest issue with energy loss is stopping at the destination. A fuel burned to produce thrust will be pointed directly away from the Dyson sphere. Since mass is energy, the loss of any mass is the loss of energy. Barring that, there are a few options.

One method of reducing energy loss is for each departing ship to have a giant mirror on its rear. More speed is imparted by reflecting light than by absorbing it. In a perfect world, light could be generated by a high-energy laser, bounce against the mirror to impart speed, and reflect back to energy collectors on the surface of the sphere with zero loss. Even with some loss, it would result in minimal loss for maximum gain, but this is slow. The loss would increase over distance as lasers are not perfect, but they are decent enough. There would still need to be a method of stopping at the destination, but something like a solar sail might be a viable option, as the light from the approaching star would increase as the ship approached, providing more deceleration the closer it got.

You could also accelerate the ships along a magnetic track built across the outer surface of the sphere. This would allow for a constantly increasing acceleration until extreme speeds are reached. When the ship achieved its target velocity, it could simply release its magnetic clamps and be flung outward.

A fusion drive could potentially provide energy to the sphere, but it would still be at an overall loss. The drive plume from the ship could be concentrated into a laser type emission and directed at energy collectors. This would allow the energy from the drive to be collected. As with the laser reflector, there is bound to be some loss. There is also the loss of the energy mass of the fusion fuel needed to stop. The energy from burning the fuel to accelerate could be captured, but the ship will be pointed the wrong way when it wants to slow down. You could throw rocks at the ship. Well, maybe not rocks, but metal slugs, so... bullets? The ship could capture them in a magnetic field, stealing the velocity and taking it as its own, and then fire the slugs back towards the sphere, providing additional thrust. The ship would need to carry enough slugs to slow down in the end, but the thrust generated would be higher due to the higher mass of the slugs. Light has very little mass. Robotic factories could be sent ahead of the ship to start building mass drivers and slugs for when the ship approached, eliminating the need to carry additional slugs for slowing down.

Honestly though, your people should just fix their star. Star-lift material from its surface, extract everything except the hydrogen, and cool it down a bit. Heck, even keep some of the extra hydrogen and convert the star into a long-lived red dwarf. Every few billion years they could toss back in some hydrogen and keep it going for a while. Big stars burn fast, while small stars burn very slowly. If they have overcome the technical issues which plague a Dyson sphere (which was never meant to be a solid structure, but a swarm of smaller satellites) then they should be able to mend their dying sun. Although, now that I think about it, perhaps they already star-lifted the extra material from their sun and used that for their sphere. If they are running low on hydrogen, they could resort to electrolyzing their oceans and pouring that hydrogen back into the sun. A Dyson sphere's worth of water is a ton of hydrogen. Heck, just do away with the sun altogether and install a few thousand fusion candles to provide the light and heat you need. More energy efficient in the long run to change the light bulbs than to keep using a star for something so simple.

If you were to reduce the size of your scope to a planet-sized sphere with everyone living on the inner surface and held down primarily by spin gravity, the problem could be the lack of hydrogen for the fusion candle used to light the sphere. The "sun" is dying from a lack of fuel, and you have a ticking clock to race against. Such an object could potentially be traveling through the depths of space (maybe inter-galactic), far from an easy source of hydrogen for refueling. This eliminates the technical issues with Dyson spheres and could still provide a similar backdrop to tell your story.

• I'm marking this as the answer, but all of the contributions here have been very helpful! I can see I have plenty of aspects to consider, my main question being only one of them. This thread has given me many great ideas, so thank you very much. :) I actually failed to mention that I had been thinking already of a situation where most of the energy from the sphere feeds right back into suspending its natural life cycle, which is relevant for some of the answers. I'm going to process this one for a bit. Again, thanks for all the ideas! Jun 25 at 19:18
• Your description of laser sail plus mirror looks like there is a gain in acceleration for (in theory) zero energy cost, which contradicts the law of perseveration of energy and would be a perpetuum mobile. Jun 26 at 6:04
• @quarague It's not zero cost, it's zero loss (assuming perfect recovery of the reflected light). The light reflected from the receding spacecraft is red-shifted and carries less energy, which accounts for the energy imparted to the spacecraft. Jun 26 at 15:11

# In space, there is no free lunch

If you're thinking that waiting for gravity to bring you back is reasonable, then a solar sail option is inefficient and unreliable. You don't have an atmosphere to worry about, so just use a mass driver. If the Dyson sphere is fully contained, your worlds tech must have a way of redirecting the solar winds into vented plumes, so that would work.

After that, everything falls apart. You'd need a method of matching speeds with the target once you get there, or you'd crash into your target at high velocity. If you're thinking you'll do something to cushion the impact, you have to account for around 50kps, compared to a typical bullet's speed of .5kps.

Let's say you have super-strong materials, and you're talking about a rock smaller than your ship. You could grab it with a net on a tether, at which point you and the object would be orbiting each other like a thrown bola. If you draw the object in, the speed of rotation increases, eventually splattering the inside of the cabin with the human occupants.

You can use gyroscopes to slow down the rotation, but then you're using the same amount of energy as if you'd matched speeds with it. Maybe a little less because you're making the rock match speeds with you instead of the other way around. If it's a really small rock, you'd be ok. If it were something bigger than you, it wouldn't spin when the tether pulls on it and you'd swing into it and hit it even harder, like with tether-ball.

So you've matched velocity and grabbed/docked/landed on your target. Now both you and the other object are moving at the average of your momentum. Since you're adding "moving away" energy to the combined mass, odds are you aren't on a trajectory to intercept your home sphere, so you need thrust to change your trajectory.

So, from a modern human perspective, there is no way to make this a low-energy process. You'd have to apply whatever magic is used to keep the Dyson sphere from collapsing.

## Dying sun

On this point, I have to ask what you think a dying sun looks like. As it burns through the hydrogen, the helium core increases the density of the remaining star, increasing gravitational collapse, making the star burn faster and brighter. Our Sun has been increasing its luminosity by about 10% per billion years.

When the Helium starts burning, things get even worse. The outer portion of the star will expand into a red giant. You can imagine what that would mean for a Dyson sphere.

Maybe you're thinking of really, really far into the future when the Sun has become a white dwarf? Why would mankind return to huddle around the ashes of its old star if it's already been to so many others? Mankind would have had to go elsewhere to survive the red giant phase.

• As this answer starts to explain, our sun won't die by dimming then snuffing out, like a match. The sun will die by brightening and growing, in which case humanity's problem is too much energy, not too little energy. I think original question needs to be reconsidered. Jun 24 at 22:24

# No

• If natural gravity could pull the ship back to the Dyson Sphere, it would already be pulling the asteroids the ship was sent out to get.

• This process would be insanely slow. What value could anything inside the solar system have that would justify years of travel and the energy expense to move the ship?

• You've harnessed the energy output of a sun. The amount of energy needed to move a ship effectively is beyond the measure of irrelevant compared to that scale. If humanity's energy consumption actually brought the issue to bear, it's no longer relevant that the ship can fly because nothing it can reach or recover can solve the problem.

• If you're so nervous about energy consumption that you'd consider using laser sails to move a ship, you'd be insane to try to move any object more massive than the ship out of its trajectory for a useful purpose. In reality, said object can't be anywhere near as massive as the ship because the force of the trajectory is the problem. F=mA.

# But, should you care what I think?

No! I apologize for being a bit brutal, but you're basically asking us for permission to use an idea. Plausibility, reality, feasibility, etc. are all nonsense. How many people among your audience have the background to read your story and say, "wait a minute...." And of that handful of human beings, how many would be mean enough to stop enjoying the story to bring it to your attention?

OK, it has happened before. When MIT students stormed the venue at the 1971 World Science Fiction Convention chanting, "The Ringworld is Unstable!" Well, they did it.

But what a badge of honor!

People cared enough about your story to take the time to test it! Jeez, they're a bunch of losers compared to your creative genius! Therefore, please remember that this Stack's purpose is not to give you permission to use an idea, but to help you solve a problem that you cannot solve yourself. You've already solved this problem — you're just unsure about your solution. It's great! I love it! It would make the basis of an interesting story about subsistence living at a time that should be post-scarcity!

So, throw all of us the proverbial vulgar hand gesture, go write your story, and enjoy the wonder of your world!

<Bah-Humbug Mode Off>

Since this is a very high tech society, and presumably the bureaucracy only cares about the net energy or mass that is returned to the sphere.

Perhaps they temperature regulate the sphere by controlling and regulating the electromagnetic radiation by controlling the emissivity of the external surface in sophisticated ways including being able to direct the radiation to the spacecraft with the sails. They might even up-convert the radiation to visible or UV wavelengths, and make it coherent (Laser) to improve the sails efficiency.

To return, perhaps the sails have controllable diffraction gratings which might help steer the spacecraft, but more importantly since they are collecting mass, perhaps they have a fancy drive that converts the collected mass to energy.

Since the bureaucrats only care about the net return on the energy and mass balance they are allow to use that mass to energy conversion for an ion drive, and return home. Even small amounts of mass would suffice since $$E=mc^2$$ could potentially return a lot of energy.

It might be that some elements are easier to convert than others, hydrogen in ice easier than iron.

You would have you choice of handwaving technologies from it being fusion as we know it today, or perhaps some other as yet undiscovered technology converts atoms or protons etc to energy.

I'm afraid you're out of luck on this one.

The "sails" must be completely absorptive, since if they reflect the laser energy you'll get stray reflections running around the interior of the sphere until they hit something.

For an absorbing sail, in order to provide one pound of force you need (in rough numbers) about 2 billion watts of power. So, for instance, if you want to boost a one-ton payload to a speed of 96 feet per second (about 60 mph), and you want to do it in 10 seconds, you'll need a laser with a power of about 1.2 trillion watts. Since you've stated that high-energy or high-power is off the table, I don't see how it's going to work.

• "if they reflect the laser energy you'll get stray reflections running around the interior of the sphere until they hit something": since the interior is designed to absorb light and convert it to useful power, this is exactly what you want. Absorbing sails would also have a much lower power density limit before they melt. The more reflective you make the sails, the more efficient you make the whole system. Jun 27 at 20:37

As others have mentioned, stars don't die in the way you seem to be describing. The stellar objects that do slowly cool are already quite "dead"...that's why they're cooling. It's also hard to reconcile the possession of a Dyson sphere with an energy shortage...a more typical problem is finding enough matter to usefully do stuff with the energy.

Aside from that, photon propulsion of any kind is essentially the limit of how far you can go with increasing exhaust velocity to reduce propellant requirements at the expense of higher energy requirements. Laser sails are thus one of the least energy-efficient propulsion options you could consider, the only thing worse being to put the laser on the spacecraft as a photon rocket. The energy consumption will be vastly more than anything you could recover from kinetic energy of the returning spacecraft.

At least, this will be the case for sails outside the sphere. Inside the sphere, the reflected and slightly red-shifted light will just be absorbed by the inside of the sphere. If you are truly energy-limited, you could do the majority of the launch inside the sphere and recover most of the energy used as scattered and slightly red-shifted laser light.

The spacecraft could then use low value materials as propellant to do the relatively undemanding rendezvous maneuvers in the outer system and send objects back home, obtaining more reaction mass from them along the way.

Angular Momentum is Not Your Friend

The method to go in and out sounds good. Subject to the efficiency rating of your solar sails of course.

The problem is you need more energy to get that asteroid back home. Asteroids orbit the Sun just like everything else. They are moving very quickly in a circle. To get them into the same circle as your Dyson sphere you need to speed them up or slow them down. That takes energy.

By the way how does your Dyson sphere not fall into the Sun? A Dyson circle could just orbit. But the same thing doesn't work in three dimensions.

Perhaps you can plant solar sails in the asteroid and then sail it home like a sailboat.

• Solar sails won't work -- outside the sphere, the radiation is such long wavelength it hasn't got much pressure. Laser sails need as much energy to come down as they did to go up. Jun 24 at 13:30
• @ZeissIkon Just let some radiation out. Jun 24 at 13:37
• But the down-conversion of stellar radiation (yellow-white) to waste heat (thermal infrared) is the primary source of operating energy. That's exactly what they're trying to avoid, by ensuring that the energy recovered from scavenging exceeds that spent. I'm not convinced that's possible at all... Jun 24 at 14:28
• @Zeiss Ikon - unless it disperses waste heat in an exotic manner (neutrinos, dark matter...) the presence of a Dyson sphere would have zero effect on the total light pressure out from the Sun. Energy cannot build up in the sphere or it would fry! More, lower energy photons leave from a larger area than the surface of the sun, but they add up to the same energy/relativistic mass. Jun 24 at 21:31
• A photon sail close to the sphere will produce less thrust because of the geometry, though. The limiting case will be half the sky glowing in infrared with the surface brightness of the dyson sphere (which will depend on how big the sphere is), with the light coming in from a wide range of angles and only a relatively small patch of the sphere visible. Jun 25 at 18:01

If you're trying to predict the future, that might be scientifically feasible but roughly when do you hope suitable technology will be developed?

If this is fiction, it sounds far less unfeasible than the average "Fire up the FTL drive and let's get out of here…"

If this is fiction, the real limit to "feasibility" is the writer's ability to make a case.

For a vaguely similar example, take whichever episode of Star Trek: Deep Space 9 has the Siscos making a Kon-Tiki-like expedition in a hand-built reconstruction of a primitive, light-sailed space-ship. As I recall, that wasn't just inter-planetary, but capable of travelling between systems!

The major difference there is between laser- and sunlight and who's going to argue that might almost as well take up pin dancing with the angels.

I suggest DS9 got away with it purely because the quality of writing and production lured the audience into suspending their disbelief in the half-baked "science."

I see you've already accepted an answer, however I wanted to share some additional information. Per this answer on the physics stackexchange, you can approximate the time it will take any craft to fall back to the Dyson sphere (assuming the Dyson sphere is not too much larger than the Sun itself) with the following equation:

where t is time (in second), r is the distance from the sphere (in meters), G is the gravitational constant 6.6743E-11 and M is the mass of the star you are falling towards (in kg). In the case of our Sun, that gives you a simpler formula:

Note that as r is raised to the 3/2th power, return times will become very long if you want to go far out. Here are some examples of times it would take to fall to the Sun from different distances:

• Mercury: 2 weeks
• Earth: 2 months
• Asteroid Belt: 7-12 months
• Jupiter: 2 years
• Saturn: 5.5 years
• Kuiper Belt: 30-60 years

In other words, returning to the Sun using only gravity is not a solution that will work in all cases.

Also, note that you cannot get too close to anything larger than a small asteroid without getting pulled into their gravity well. So planets, moons, and larger asteroids are off-limits unless you have fuel and an engine.

# The sail ships are pushed home by exterior lasers

If the the timing of their system-wide mass sweep is planned well, they could intentionally leave several dwarf planets or other large masses remaining in the distant reaches of the solar system outside the sphere, and place laser sources (perhaps just more mirrors) on them to push sail ships back home. Any objects interior to these remaining laser emplacements are accessible by sail. Timing is crucial, because the dwarf planets should be in decaying orbits to counteract the transfer of momentum and keep from being ejected in the long term. They should still fall sunward faster than they push in reverse, so that in the end they are the final mass collected in the system. Thus the sail ships start collecting mass as far out as possible, moving inward as the dwarf planets fall.

In other words, a gravity-powered return trip is made feasible by using large external masses as intermediaries.