While trying to figure out a way that individuals could own spacecraft in my sci-fi setting without having access to things that could double as weapons of mass destruction, I settled on the widespread adoption of civilian solar sails, or more specifically, photon-particle sails.

It has quite a few advantages, like being mechanically simple and cheap to produce with bulk graphene production, along with having an omnipresent power source. The only problem is this omnipresent power source is unidirectional.

Even worse, there is a limit to how many lasers can be fired from X, Y, and Z bodies, to redirect these hundreds of thousands of crafts, along with the economic costs of firing all that energy and possible lack of infrastructure depending on where you're going.

• Food for thought: a perfectly reflective solar sail at 1AU will felt a radiation pressure of 9 micro Newton, assuming light from Sun to be monochrome. (Saturn V rocket lifting off is 35,000,000,000,000 micro Newton) – user6760 Sep 11 '17 at 5:14
• Food for thought 2: gravitational force between 1kg mass and Sun at 1AU is 5.9631298548245E-18 which is less than the weight of a bacteria on Earth! (I rounded mass of Sun up) – user6760 Sep 11 '17 at 5:52
• @user6760 Would that be 9 micro Newton per square meter? – Arthur Sep 11 '17 at 6:38
• @user6760 you could use $\text{µN}/\text{m}^2$ notation to help clarify — i.e. $\text{µN}/\text{m}^2$ MathJax, or simple µN/m^2 otherwise. – can-ned_food Sep 11 '17 at 11:49
• Did you try flipping the sail so it goes in the other direction? Hey, why is security coming over here? Put me down! – corsiKa Sep 11 '17 at 19:25

There are a number of ways to use solar sails to make round trips. For relatively short distances like going from the Earth to Mars, the sail can be sent on an elliptical orbit which "grazes" Mars. At that point the payload is released and aerobrakes into the Martian atmosphere, either to settle into a circular orbit around Mars or land on the Martian surface. The sail continues in a long, elliptical orbit and eventually returns to Earth about two and a half years later. (Unfortunately, I seem to have misplaced the orbital calculations where this was explained, so perhaps someone can find a link or post the actual calculations).

In a more general sense, sails can actually "tack" with the solar wind. This is more usable when dealing with trip to the outer planets, since a flypast orbit like the Martian example will be going so fast that the aerobraking will likely be unsuccessful, and the sail itself will be heading out to deep space, not returning for centuries. The genera idea for taking a solar sail is that if the sail is set so the thrust is aligned against the gravitational field of the sun, the sail will spiral outwards, while if the thrust is in line with the Sun's gravitational field, it will spiral inwards.

Tacking with a solar sail

By careful use of sunlight and a suitably lightweight sail, you can achieve very fine control of the sail and move throughout the Solar System with ease.

You can use the sail to deflect sideways the light and thus increase or decrease your orbital speed.

If you stop your orbital speed then all you have to do is to close the sail to plummet straight down (toward sun).

In practice Sun has two force sources:

• gravity: pulling
• solar wind: pushing

While the first is always pulling in a well defined direction you can use sail to maneuver, to a point.

In particular you can use it to increase or decrease your orbital speed amd let gravity do the rest.

• Can you alter the vector force imparted by a photon by altering the direction is which is it reflected? – Willk Sep 10 '17 at 23:40
• @Will: of course! the net impulse transferred from photon to mirror is equal to the inverse of difference between incoming and outgoing vectors. i.e.: if the photon changes its impulse vector from $\vec{b}$ to $\vec{a}$ its $\vec{\Delta i} = \vec{a} - \vec{b}$ and thus the mirror gets a $\vec{\Delta i}_{mirror} = - \vec{\Delta i}_{photon} = \vec{b} - \vec{a}$. You will get an impulse which is always orthogonal to mirror. Module of resulting impulse will decrease, though. – ZioByte Sep 10 '17 at 23:59
• I follow. Thank you! Now if you please, can you explain or link up some reading on module of impulse. My google turned up a lot of things that I am not sure are related. – Willk Sep 11 '17 at 0:51
• @Will: this is actually part of Physics, Kinematics, in particular. You can find the related concepts of Moment of inertia and Angular momentum of interest, but a complete understanding needs some study. The concepts and their application are a large part of "Analytical mechanics" which is a well known stumbling block for students in Physics and Engineering. – ZioByte Sep 11 '17 at 6:47
• Hmm. Disregard; i read incorrectly. – can-ned_food Sep 11 '17 at 13:17

The other posters have covered the main question of orbital adjustment by angling the solar sail. I can't improve on those answers. However...

...without having access to things that could double as weapons of mass destruction...

Given that a solar sail is a giant mirror, and not a completely rigid one at that, it should be reasonably simple to add a small amount of curve to the mirror to effectively focus the incoming light onto a target smaller than the sail area. Assuming that your sail hasn't been specifically designed for such a thing you probably won't get a great focus, but you can make up for that by sheer size. Ants under a magnifying glass would be a reasonable comparison.

If you specifically design the sail to be adjusted to any curve you specify (using sail spin and a tensioning system perhaps) then you could get several thousand square kilometers of reflector focusing to a point a few hundred meters across. It's going to drift as the sail accelerates away, but if you adjust focus as you move then you could probably get enough energy into the target zone to vaporise it and the rock beneath.

It would be interesting to see what happens to a society that suddenly discovers that the safe, cheap and slow method of allowing citizens to cruise around their solar system suddenly becomes a method of destruction greater - if much slower - than nuclear weapons. How would Earth react if all of the space-faring citizens ganged up to carve a great big molten X across the face of the moon before demanding that Earth stop ordering them around?

The moral of the story is: don't assume that just because it's big and slow that it's not going to eat you.

• Not to mention an object traveling at orbital velocity makes a great projectile. – QuyNguyen2013 Sep 11 '17 at 16:53

Solar sails must be paired with traditional gravity maneuvers to achieve practical mobility. The exception is things like permanent habitats where station-keeping is the real purpose of the sail.

This makes the course planning of such a vehicle extremely important and limits the useful tracks available within a planetary system, though it places few limitations on tracks among stars (as the target star can be used to gravity assist a return path on a grazing approach, which implies that the craft would have to be equipped to withstand a close approach).

From the storytelling angle...

If your problem is that you want civilians to have access to advanced technologies that cannot be used as weapons of mass destruction then your task is hopeless in the face of any serious investigation. I would recommend as a writer or world designer that you commit to solar sails not because other propulsion systems may be related to nuclear technology, but because you want to place a mechanical constraint on the actors in the system that can be usefully employed to drive the plot or game system. For example, because solar sails require gravity maneuvers the useful tracks within a planetary system are constrained. This also would have the effect of concentrating human habitation along the useful tracks -- which already gives a population and therefore political focus to your world (assuming you need such a thing).

It is much easier to justify the technology of solar sails by cost than "safety". In the somewhat near future it is very likely that nano manufacturing techniques will give rise to things profoundly more devastating than mere nuclear weaponry, and such things will probably be impossible to keep out of the hands of civilians. And that's not even sci-fi. (Incidentally, I smell another interesting hard sci-fi premise here...)

Gravity assisted maneuvers are an important part of stellar travel. Not only can you use a gravity well to change directions, but they can provide huge boosts in speed. This diagram shows that using a planet to reverse your direction adds your own initial velocity to twice the partial velocity of the planet in the direction you are heading. That can get things moving pretty quickly.

Assuming your solar sailors can steer their ships, not just directly away from the suns rays, but perhaps tangential to them, they can sail out to planets (or other objects orders of magnitude more massive than their ship) and use them to turn into the sun. From there, they lower their sails and coast into port.

Just make sure you don't sail past the furthest gravity turn-around. The Solar Coast Guard would have to rescue you with their expensive rockets.

The late Dr. Bob Forward wrote numerous papers on this, most famous among them is http://www.lunarsail.com/LightSail/rit-1.pdf

He also popularized the concept of laser-boosted concentric-ring sails in his Rocheworld series of SF books. By dropping the outermost ring and reversing the ship, the remaining mirror can be used to slow/stop/reverse the ship.

There are heaps of caveats to this approach, most notably the law of diminishing returns, as each concentric sail has less area with which to collect light.

• Hi, as links may go out of date, it's generally a good idea to summarise the important parts of the link in your answer – Mithrandir24601 Sep 11 '17 at 16:54

As others have explained, solar sails can be used to apply gradual delta-V to an orbit in either direction, to go in or out.

However... that means you can apply delta-V to any object, such as a large rock. It only takes a small amount of it to change a "near miss" into a "planet-destroying hit". We get a few decent near misses every year - see https://en.wikipedia.org/wiki/List_of_asteroid_close_approaches_to_Earth

You don't even need to use the sails to move them. You can deflect them in any number of ways, so long as you can get close to them at matched velocity, early enough - which a solar sail would let you do, if you knew the object's expected path.

So all planets and stations having a defense against such asteroids would be necessary to fulfull your "no weapons of mass destruction". Since... well, a near-earth-shatteringly large mass that's been weaponized is very literally a weapon of mass destruction.

In 2013, NASA estimated it had identified 95% of the potential "planet-killers" of 1km or more (about the same as the dinosaur-killer), and only 10% of those smaller than 300 meters.

Tracking rocks appears easy, but there are many rocks that are visible to earth only by obscuring other things, or on close approaches to the sun. Even among those that have been detected, they aren't constantly tracked; their orbital paths are calculated once they're detected, then they are filed away and forgotten about until that orbit is due to become interesting again.

That changes if there are things which might push them out of those predictable orbits.

Even if 100% of dangerous objects were identified and were continuously tracked... solar sails are essentially mirrors, which makes them black to anything outside a very narrow angle, so they are undetectable other than in their effect on the rock. If the solar sail obscures the rock, or if the rock has been covered in carbon-black, then the rock will not be visible to telescopes either.

So the whole system of falling-rock-detection currently on earth would need to be improved several thousandfold, to detect and track rocks far more constantly and aggressively than our current snapshot-based system. We could no longer rely on objects moving in predictable orbits, and no longer rely on passive image detection; a networked array of telescopes detecting stars being obscured, and using some form of active detection (very long distance radar?) for concealed rocks.

Deflecting a rapidly-moving rock requires a whole lot more force than aiming; dropping a thing on a planet is a lot harder than it oughtta be, because it is likely to just get caught in an orbit instead of spiralling inwards, but still, the person aiming has the significant advantage of lead time. The closer it gets, the more force must be applied to deflect it, and I think the increase scales with 1/distance^2.

There's also the shotgun problem: no matter how many rock-deflecting missiles you have in your armory, your foe only needs to send one more planet-destroying rock than that, in order to destroy your planet. Only one needs to get through.

Have one large visible rock followed by a stream of smaller blackened ones hidden behind it.

Have what appears to be one rock spread apart into many once the planet deploys deflection measures.

Scatter larger high-albedo and smaller low-albedo rocks together in the same "shotgun blast", so that the glare from the shiny ones masks the stealthy ones until it's too late to deflect them.

...and so on.

• To be fair, tracking all large bodies should be doable, and deflecting them is just as easy as aiming them. Solar sails are bright, so if they deflect something so that it takes a lot of effort to reaim it the solar sail has to be at it for a long time, which gives lots of time to respond. Ie, this looks like a policing problem more than anything (police deflecting of said rocks, move rocks that are at all close so they aren't at all close, etc). – Yakk Sep 11 '17 at 17:57
• Good arguments, which I'll edit to address. – Dewi Morgan Sep 14 '17 at 20:31

In sailing, you can tack, or zig zag, to head back upwind. You do this by aiming about 15-45 degrees away from the wind, or in this case, the sun. By pulling in your sail, it allow wind to move past the sail, but still exerts pressure on the sail. Using this, you could tack back towards a star. Given, wind doesn't act exactly like photons, but heading on a bearing and degrees farther out from the star. Also, depending on the shape of the sail, you could pull it in closer to the body of the ship, and use mirrors to reflect light, and therefore photons back into the sail, giving a powered descent down into the gravity well if you were in a hurry.

• The potential issue here is that there would be a momentum transfer to the mirror... – Mithrandir24601 Sep 10 '17 at 17:36
• Quick note: the only reason tacking is possible in a boat is because there is water to push against with the boats hull. You can't tack a solar sail in a similar way to a boat. Instead you use it to change orbital velocity. space.stackexchange.com/questions/370/… – sdfgeoff Sep 10 '17 at 17:36
• If this would work, you could also sail balloons (you can't). – Oscar Bravo Sep 11 '17 at 11:48