My understanding is that one hypothetical way for a spaceship to travel to a nearby star system is a solar sail. A laser based on the earth or the moon would push a solar sail attached to a spaceship, causing it to accelerate and obtain a faster and faster velocity.
Halfway through the trip, the laser would then be used to decelerate the spaceship so it could stop at its destination.
So my question is, how do you decelerate a solar sail when the laser is based on the moon, and not physically based at your destination?
I don’t understand much about sailing in the sea, but I thought you needed a keel to help you sail against the wind. Something like that wouldn’t exist in interstellar space.
If the sail is accelerating then the mass of the craft would be the keel. It would be a weight being dragged by the sail.
If your destination had a sun, which is likely. Maybe turn off the laser and let the sun slow the craft?
No idea if that would work fast enough. But at that point your laser back home is probably much less force compared to the nearer sun. The sun at the destination might be too powerful at some point in the trip. Furl the sail and coast till you are close, then unfurl and use it to brake.
I suppose our sun would add to the force for the outgoing trip to some amount.
Bob Forward, scientist and author, solved this problem decades ago. I read of it in the book The Great Mambo Chicken And The Transhuman Condition. I don’t recall all of it.
It involved part of the solar sail being detached and ejected forward. The laser would then hit the ejected sail which would reflect the light back onto the remaining sail and slow the ship. A similar process would be use to get the ship moving back to earth.
From what I’ve read, we don’t have near the laser technology to solar-sail an actual “ship” as it’s commonly thought of.
Most of the talk about using solar sails now is concerned with sending payloads the size of a deck of cards or smaller.
I don’t believe there are plans to slow them down, just send them out to anther system, take pics and beam them (the pics) back. I doubt the craft would be of much use after that.
Solar sails work by the change in momentum of the light. If the sail is normal to the light the reflection is directly back along the diction the light came in, and the resulting force is along the direction the light was initially travelling. If you angle the sail, the light is reflected at an angle, and the resulting force is along the median of the incoming and reflected paths. So you gain a transverse component to the force. But never a reverse force. A transverse component means you may be able to do useful things like change orbital plane, or orbit. So it has been suggested as a cheap way of getting things into high orbital inclinations around the sun.
The idea of a keel doesn’t work in space. In a sailing boat the keel does two things, it provides righting moment, but critically provides lateral resistance. It is the lateral resistance that allows a sailing boat to travel at angles other than directly downwind. The energy driving a sailing boat comes from the relative motion of air relative to the water - not motion relative to the boat. Traction on the water is what allows the boat to extract energy from the interface. Small high performance, and insane very high performance, sailing boats don’t have a weighted keel at all. Large very high performance sailing boats don’t get any lateral resistance from their keels and depend on separate elements for that.
There is no medium in space, so nothing to gain lateral resistance to, and no keel like mechanism possible.
This is what a large very high performance sailing boat keel looks like. Perhaps a bit extreme, but they are canted out sideways in normal sailing. Usually not actually above the water. But if your principle sponsor makes suits and you want to make spectacular promotional video, this is what you do.
The detached sail concept looks like this.
This method only really works for very lightweight payloads, and is quite inefficient, especially at interstellar distances. But (until we can get a deceleration laser in orbit around the destination star) it is one of the better options.
Think of a sail as sort of like a parachute or kite - the “wind” is the light.
As you approach the target star, the sail uses the light from the destination to slow down, lke a parachute… Presumably the lack of a destination laser means the craft must approach much closer to that sun to slow sufficiently. (IIRC this is the concept of start of The Mote in God’s Eye, the craft is plunging close to the sun for maximum deceleration following a laser launch decades before ).
Naturally, not enough deceleration and the craft simply does the comet orbit or hyperbola, so the relative velocities for launch and arrival have to be calculated carefully.
Also, solar sails suffer from “It’s just engineering” handwaving like much of sceince fiction - how to create and deploy hundreds of square miles of super-thin lightweight durable reflective material harnessed to a ship of minimal weight, etc. etc.
This question is complicated by the fact that it’s conflating multiple things. A laser sail is not the same thing as a solar sail, and use of a solar sail within a solar system works differently than using it for interstellar travel.
We know how to use a solar sail within a solar system, and we’ve launched proof-of-concept missions. A solar sail can be used to go both inwards and outwards in a solar system, through a process that’s sometimes called tacking, but which is only superficially analogous. The key is that the sail is reflective, and the direction you’re reflecting the light to is just as significant as the direction the light is coming from. Angle your sail so that light is reflected to ahead of you in orbit, and you’ll slow down, which lowers your orbit. Angle your sail so that light is reflected to behind you in orbit, and you’ll speed up, which raises your orbit. Note that, even with the best material science we can come up with, the force of gravity is still much greater than the force from the light pressure, so these changes to the orbit happen very slowly.
For an interstellar sail, you’d need to have much better technology, such that the light pressure is greater than gravity. This might be done with better lightweight and strong materials, such that the light from the Sun is enough. If you can manage that, then slowing down at the other end is obvious: You just use light from the other star.
Alternately, instead of making the mass less, you could make the light pressure more, by using a brighter light source than the Sun (i.e., a laser). This is phenomenally inefficient, but the advantage is that all of the big expensive infrastructure can stay right here in our Solar System, and doesn’t need to be carried with the spacecraft. This is where the detached-sail retrothrust method comes in. Which is even less efficient, because you’re both slowing down the payload, and speeding up the forward sail even more. But it could in principle work.
Just a clarification for anyone not familiar with racing sailboats but what is shown in that video is actually what is called a dual keel boat; in contrast to most monohull boats that have a single keep that is aligned with the centerline of the boat, that boat has a pair of keels that are canted out sideways (usually around 35 degrees) so that it can heel over at significant angle and still have good lateral resistance at the expense of some small amount of additional drag when upright. Since normal monohull sailboats typically make their best speed on a close reach because the keel is angled up, having a dual keel allows the boat make more speed pointing near close hauled (see this diagram for pointing descriptions)
it is the difference between force vectors of the wind generating lift across the sail (in anything other than running downwind) and the lift produced by the keel that generates motive force for a sailboat and as @Francis_Vaughan notes there is no medium akin to water to gain lateral resistance, and the solar wind—the outward flow of charged particles—is too diffuse to produce any lift or act as a fluid medium on that scale (although they can be treated as an electrodynamic fluid on the scale of the solar system and in their interactions with planetary magnetic fields), and so the only propulsion comes from the transfer of momentum of the solar wind and photons impinging directly upon the solar sail. Although this cannot pull the spacecraft inward, the sail can be angled in a way that there is a component of the net vector which can slowly reduce the orbital speed of the spacecraft about the Sun so that it ‘spirals’ inward.
The diagram posted by @eburacum45 shows how the ‘accelerator stage’ of the lightsail detaches and reflects impinging light onto the ‘back’ of the spacecraft lightsail, directly decelerating it. That accelerator stage of the mirror continues to accelerate forward so it becomes less effective over time. Note that this system requires a Fresnel lens to collimate the laser light on the lightsail (because light emitted from a laser has a characteristic divergence angle and will disperse over distance) in order to concentrate the light upon the sail. It appears that this spacecraft is heading in the direction of ε Eridani, which is a start 10.5 lightyears distant from Earth, so depending upon the divergence of the beam the lens is probably somewhere on the order of 1010 km 1011 km to in diameter; by comparison, the orbit of the Earth around the Sun is about 3⋅109 km in diameter. All of this would support sending a spacecraft about the size of a golf ball across interstellar space over a period of roughly a couple of centuries or a bit more.
It should also be noted without the use of a laser as a source of thrust, solar sails are effectively limited to the inner Solar System because solar irradiance falls off by a square of the distance to the Sun, and is really only practical with CubeSat-sized payloads or smaller because of the amount of sail area and thrust structure that would be required for a heavier payload. So, they are useful for small scientific payloads or for orbital stationholding and very slow maneuvers for satellites, but not for larger probes, missions beyond the orbit of Mars and the inner asteroid belt, or any kind of crewed vehicle.
But gravity also falls off the same way. You get less thrust further out, but you also need less thrust further out. At least, if you’re patient enough.
It isn’t an issue of gravity but of inertial mass, which doesn’t change regardless of where you are and will be dominated by the minimum mass per unit area of the solar sail for any useful degree of impulse. At Mars orbit (between 1.38 AU and 1.67 AU) solar irradiance is about 1/2 to 1/3 of what it is at Earth orbit, and a corresponding reduction in impulse per unit of spacecraft mass (including the solar sail). At the orbit of Jupiter (4.95 AU to 5.46 AU) irradiance is about 0.35% to 0.41% of Earth orbit, and at Saturn (9.0 AU to 10.1 AU) you might as well just get out and push for all of the good that the solar wind will do for you. At some point dictated by how thin and lightweight you can make the solar sail (and all of its supporting structure) you just can’t add any useful amount of payload and get a rate of acceleration useful for a anything on the timeline of an interplanetary mission (i.e. a couple of decades or less) or to perform trajectory maneuvers to intercept a celestial body and for current and foreseeable sail materials that is well inside the orbit of Jupiter. All current and seriously proposed solar sail missions that I’m aware of are actually at or inside Earth orbit although there are somewhat fanciful proposals for an Earth to Mars mission within a month using enhanced microwave-powered projection.
Here is a thread from several years ago where I did some general calculations about using solar sails (specifically about going to Mercury but they apply more generally to any use of solar sails).
