One thing people might not be getting is that this space craft would be tiny. The light sail would be huge (it would use a bunch of ground based lasers for the initial acceleration) but the actual craft is small. It will basically have the ability to do take some readings, have a camera or two and the ability to beam data back. And that’s about it. We aren’t talking about something the size of the voyagers here or any of our other space probes, but something maybe the size of a shoe box or even a chip. So, no real ability, aside from the sail to course correct, and even then I’m not sure you could even reorient the thing to try and use solar wind from the target star to do much, if anything. It’s just going to be a drive by to show we could do it and maybe get some science out of it.
There is about 111 billion kilometers of stuff between us and the α-Cen system. Granted, it is not all that much stuff, but it is still a non-trivial amount. When the probe gets much closer to the target, it will be looking at it through that much less stuff, and the things it will be looking for will subtend a considerably greater angle of view. Given that it is automated and will have decades of approach time, it will find a lot of things-of-interest.
On the other hand, at a peak speed of 0.2c, it will still take close to a century to get there. We currently do not have a reliable power source that would be able to support its brains and eyes for an entire century. In theory, it might be possible to create something that could generate some amount of power from cosmic ray flux, the interstellar magnetic field and ambient EM radiation, but it would not be very much. Probably not enough to maintain the computer.
And to succeed, the craft would have to have massive redundancy to account for component failure. Self repair is very likely feasible up to a point, but running into a random ping pong ball at 0.2c would almost certainly end the mission.
Then, of course, there is the aforementioned cosmic ray flux. All the electronics would have to be heavily shielded, with orders of magnitude excess capacity and highly adaptable in order to deal the damage that cosmic rays would do to it. It would be hugely expensive for a design that we could not be sure would even survive beyond the Oort Cloud.
The fastest is Helios 2, over 4 times faster than Voyager 1. The Parker Solar Probe won’t reach its record-breaking top speed for another 5 years or so.
I hadn’t seen your link before I sent my post. Very intereresting. The wikipedia article you linked to and didn’t see anything about how these small ships with a mass of just a few grams are going to be able to send a signal across 4 lightyears that we would be able to pick up on earth. I assume that they have thought about this, and would welcome any information you have about it.
I would also note that the article included this line,
So we may not be quite there yet. 
Well, I’m pretty skeptical that we would need ‘orders of magnitude’ improvement on anything proposed. As the Economist article is behind a pay wall and looks like mainly a fluff piece from 2016 I can’t really say, but my WAG is the only thing lacking orders of magnitude is funding. 
In any event, this is one of the only concepts I’ve seen to do what the OP is suggesting, so I assume it’s what the OP was using as a basis (there are a few others using other propulsion systems, but they all have similar mission profiles). Assuming that’s the case, I don’t think most of the OP is really applicable wrt maneuvering the spacecraft. Like I said, the thing would be tiny even compared to most other probes…more like the very early satellites that were put up with respect to the size of the actual spacecraft.
This is a non-small hurdle, for any instellar spacecraft of any size. A one millimeter laser beam spreads to around a billion miles across over a light year, with the attendant diffusion of intensity. The worldwide radio telescope array might be able to resolve a signal from over four light years away, but it is not a certainty.
It’s not the stuff in between that makes it difficult to look at exoplanets. For the most part, interstellar space is so empty that we can see clear through it, except right through the middle of our galaxy. Even our naked eyes can see a galaxy 2.3 million light-years away (Andromeda).
It’s difficult to see exoplanets because they have small angular size (i.e. they look small because they are far away), and because they are very close to a much brighter object (the star).
An approaching probe will have the same problem up until the very last part of its trip. Let’s say it’ll fly to alpha centauri (4.37 light-years away) at 0.1c, i.e. in 44 years. The latest it can take a picture and send it to Earth in time to get updated instructions from Earth is about 8 years before arrival. At that point it’s only about 4/5 of the way there (i.e. 5 times closer). But the telescopes back in our solar system would be a whole lot more than 5 times better than any telescope you can put on a probe, you’re still getting better pictures from our own solar system.
The Voyager 1 Family Portrait (of which the “Pale Blue Dot” photo is part of) was taken 40.5 AU from the Sun. The 0.1c probe would cover that distance in about 50 hours.
What we need to do is aim it at a star that has a black hole further beyond it - fly through the target solar system on the way out, make observations about likely targets, then onward to use the black hole for a 180 degree turn, back through the target solar system (this time, flying closer to one of the previously observed targets, then back to our solar system so that the data can be transmitted to us at lower power on the (fast) trip through our own solar system.
None of that is simple, however, and I know black holes aren’t magical gravity machines, but I don’t think there’s any trajectory around a star that would enable a 180 degree turn at 0.25c, without passing through the star itself. I imagine getting close enough to a black hole to do that would be troublesome too if there is any kind of accretion disk
Compared to the technical challenges of building the probe, building a space based antenna should be trivial. We can already stuff an 86 square meter solar sail into the size of a shoebox. I can easily imagine a 100 square kilometer antenna floating in L4 or L5.
Consider that during the additional centuries or millenia of travel time to the closest black hole, you could gradually build better starship technology. And develop one with the capability to slow down.
Ship of Thebes. Not just redundancy, the ship must have a machine onboard capable of fabricating any part of the ship including this machine itself. Living cells can do this so it is technically possible. So the entire starship would be recently manufactured as it would be constantly tearing down and rebuilding itself.
This also would allow reuse: the engine technology used to slow down might be entirely different from the one used to leave Sol.
One idea would be to ride a smart pebble beam of tiny iron pellets, launched from a giant accelerator somewhere in our solar system. This gets the ship up to speed and avoids the rocket equation.
Then to slow down, antimatter. The antimatter engine uses magnets (to steer the pions used as propellant) and fuel handling lasers. And the materials in these would be mostly recycled from the linear mass driver engine that “catches” the pellets fired at it during the boost phase.
Or ramscoop, might be a lot safer than antimatter.
As for hitting a golf ball - that is mission ending. Most likely the civilization sending starships would need to send several or many per star since some of them will be destroyed by collisions during the voyage.
The sail may be able to do double-duty as an antenna…
I predict we may never, ever use sails. It’s a bad idea like using a gunpowder fired cannon to go into orbit, Jules Verne style. We’ll never do that, either, even though we eventually did find a way. (with a light gas gun you can use gunpowder to make the projectile go much faster than the explosion velocity of the gunpowder, and reach orbital velocities at sea level)
Why? Because a probe that can’t slow down is just not useful. As alluded to above, at reasonable transit speeds (5-10% the speed of light), the observation time when it finally flies through the destination star is just too short. Mere minutes, and the probe is so low mass it’s optics are crap, and it’s so low mass it had no self repair, so it’s instruments are probably trashed from decades in transit.
Plus, that one planet in the goldilocks zone that you actually care about? That might not be a lifeless rock like everything else in a typical star system? You get just a few seconds, a few blurred images, after waiting 80+ years.
Bear this in mind: both gravity and EM (solar radiation) follow the inverse-square law, which means that the pressure on the sail will drop off at a constant rate that concurs with the decrease in gravity as the probe moves away from the sun. To continue to accelerate toward a ridiculous fraction of c, one would need to use some other form of propulsion when the sail becomes ineffectual (probably no later than 200 AU out).
And, of course, upon approach to the target, the sail can be redeployed to stern for braking, its effect again following the inverse-square law, making it more effective the closer the probe gets to the system.
Right, and during the midflight period it’s useless.
With a fusion rocket (ISP ~100,000) the engine burn is 4 years long. Project Daedalus - Wikipedia
And the endpoint is 12% of the speed of light.
I don’t even need to pull out a calculator to say that over 4 years of steady acceleration, nearly all of the time spent burning the engine is outside the solar system and probably out of range of the light beam emitter.
The starship masses 10 million metric tons. (or just 50,000 tons if you intend to use a pebble beam to accelerate and a fusion engine to slow back down). And if you use a ramscoop to slow back down it might be lighter still.
A pebble beam, by the way, offers much higher accelerations. If you think about what such a beam is, it’s like constantly pushing an iron rod away from the spacecraft.
One potential problem with light sails is that it would be real difficult to undeploy and then redeploy them. Think about it. Light sails are going to be made of the flimsiest stuff you can find. How are you going to stuff that back into a box? Well, maybe you don’t do that but just leave it deployed for the whole trip. Just swing the payload around to the other side of the sail and now it’s pointing at the target star for deceleration.
Then I thought to make them out of the thinnest stuff we know of: graphene. It turns out I’m not the only one who’s thought of it: here and here for example. Anyway, graphene doesn’t seem to be as flimsy as one might expect from such a thin material, so maybe undeploying won’t such a major problem. Or maybe it makes it easier to leave it deployed the whole way.
Ah, so you are citing dead-end bullshit science from nearly half a century ago. Back when fusion power was only twenty years in the future. The inertial confinement concept they suggested was tried at the great big NIF not so long ago, with a massive laser array, and they never managed to achieve ignition. Try again.
Before you can find out whether a ramscoop would function (hint: no) you have to attain controlled fusion. It is not clear that fusion is at all doable short of an M-class-solar-mass reactor. Please be realistic.
First, update your own knowledge. Fusion has worked in plenty of cases, including NIF. It simply turns out to be harder to get high yields than originally planned. (you know, using incorrect/incomplete computer models, versus what the real equipment does).
Second, ramscoops do function for braking. Research it, I am not going to bother to link.
Finally, your last statement is not even worth addressing. We have performed fusion thousands of times, with absolutely enormous yields, and it doesn’t take anywhere near the energy you are claiming. It just is really hard on the device that does it.
The daedalus engine is pulsed fusion. As in, take a nuclear bomb and pull out just the fusion fuel cartridge. Create the effect of the fission device that normally sets off the fusion reaction with lasers or magnetic fields or both. This does work and has been successfully demonstrated to varying levels of success.
Saying it will never work because a few smaller building-sized government efforts didn’t perform as well as expected is ignorance and ignoring the forest for the trees.
The forest is that by attempting to crush a fusion fuel pellet with similar pressures found next to a detonating fission bomb, it’s obviously tough to do with a reusable apparatus, but clearly possible and will work.
The tree you are obsessed with is focusing on how a single large effort failed.
Yeah, it works, in Known Space. We do not live there. It is fantasy.
Regarding the OP’s original question, I can not think of a way to control something which is several light years away.
In regards to sending a probe to a nearby star, what information is the probe supposed to return to us? Remember that the probe will have to have a very powerful transmitter with a tightly focused beam to reach Earth, and that the rate of download is going to be slow.
One alternative is to use the gravity of the target sun to send the probe back towards Earth. Even if it did not return directly to Earth, getting it within a light year would help to improve reception.
Our technology needs to mature considerably before we consider sending something to another star. Even if we master fusion power, we still need reaction mass of some kind, unless we are able to develop some unforeseen method of propulsion. The energy requirements to reach another star are enormous, if we wish to do so in anything like 200 years.