Stopping when you are going really fast is hard, stopping or slowing down while going really fast while flying through the vacuum of space is really hard, slowing down while going really fast in space without using fuel wicked hard to do.
Breakthrough Starshot, the joint initiative between Stephen Hawking and Russian Billionaire Yuri Milner, discussed in the post Additioanl Uses for the Hawking Milner Interstellar Probes (sorry I didn't know the name of the initiative, if I had studied journalism or writing I would feel far more guilty about the mistake), is currently outlined as a fly-by mission, similar to how NASA's New Horizon spacecraft whipped past Pluto. As stated before, the faster you're going in space the harder it is to slow down. For an initiative like Breakthrough Starshot slowing down is probably right out off the table for the first generation of probes, the reason is simple, any fuel the probe would carry with it, would make the mission that much more difficult to get to a useful speed, and the amount of fuel you would need to slow down from .2C, well we won't be doing the math today, but it would be a poop ton (possibly quite literally).
If mission designer's at Breakthrough Starshot can't use fuel how could they slow down their probes enough to achieve orbit?
Quick answer, using probes to focus the star light of Alpha Centauri (this will not be as easy as it sounds)
The long answer: The initial generations of Starshot probes will most likely be relatively simple in design, a large surface area solar sail attached to the actual sensor and communication suite, at least at launch, it seems (to this rather uninformed nerd) unlikely the space craft will try to remain attached to the solar sail once outside of the range of the laser propulsion system. Why would the probe keep carrying something that can increase the likely hood of colliding with a piece of material that could throw everything off course? As technology gets better it may become possible to create a solar sail material that can be manipulated in ways similar to how blinds can be manipulated to alter how much light can enter your home. The ability to create a solar sail that can dynamically alter its shape after deployment would open up a world of possibilities, now the probe can protect their sails across the interstellar void, patiently waiting until the probe gets close enough to its target star and start to put on the breaks.
(note to self add some drawings 6/1/16, dear readers please remind me to make some artwork)
Alas, as noted before, putting on the breaks when you are going at such extreme speeds would be difficult under even the best of circumstances. In out solar system sunlight doesn't have sufficient density to provide any real pressure much past the orbit of Mars*. With Mars having an average distance to the sun of about 1.5 AU, for sunlight to reach Earth at 1 AU it takes 8 min 20 seconds, or 12 min 30 seconds for light to reach Mars' orbit, this means if you are going at 20% the speed of light, that means you have just over an hour to use sunlight to slow yourself down, this makes it unlikely that your probe will sufficiently slow down before burning up.
But we aren't thinking about the individual probes, Starshot probes are like ants in a colony individually cool but not super effective, but when they work together they can achieve wonderful things. The probes that are able to control their sails will work together. A simplified mission profile would look something like this. Wave A is sent out, they are definitely all going to die horribly, when approaching their target stars, they reconfigure their sails to act as Fresnel lenses, focusing starlight towards those who will come behind them this focused light will increase the amount of breaking force supplied to the probes. The waves will continue, some waves will be launched with lower initial velocities, the slower you are going when you leave the more time you will have to break on the other side, but it also means you are waiting that much longer to get results. Eventually you will have a steady string of probes focusing the light of a far off star further and further out, providing a breaking zone for larger probes who come down the line. Many years after the first wave of probes either burn up in the fires of Alpha Centauri or are left to wander the void beyond, a probe massing a whopping 10 grams slows enough to enter orbit. This probe left Earth going a mere 5% the speed of light, but has now slowed to a much more manageable 22 km/s to begin its orbit around Alpha Centauri A.
The advantage of this approach is that all of these probes could be made and launched incredibly cheaply, and if an individual probe is lost, no harm no fowl.
The negative, and it is potentially a big one, it will take at least 80 years for the suggested 10 gram probe to get all of the way to its target destination. If we don't figure out how to make a space ship that can carry enough fuel to park itself in orbit or some truly sci-fi sounding propulsion method no big deal, humans weren't going to really be heading that way in the near term anyway. On the other hand, if it turns out we can make some kind of FTL drive, or a fusion powered starship, we suddenly have a rather dangerous traffic jam of dangerous bullets on a potentially intercept course with your ships. Who knows?
Another version of this idea, will be written about later, which will instead be optimized for helping really really big spaceships get into planetary orbit.
Any suggestions, questions, please leave a comment.
*citation will be added, as of June 1 2016 I am going off of recollection of some research I did.
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