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At that time will they be destroyed? Or can they come back? Or will we simply cut the laser power? And if we can make the size of the craft big enough so that a person can be on-board (without reducing speed), will he be come to the Earth?

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  • $\begingroup$ H Harry and Welcome to Space! Can you add some more information to your question? Where does "after 20 years" come from? I think you may have read something or saw something that you are asking about, can you mention what it is? Right now it may be hard to answer your question without some more background information. Thanks! $\endgroup$ – uhoh Dec 11 '18 at 9:15
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    $\begingroup$ The "20 years" mean it will take to reach Alpha Centauri (because it is 4 light years away from us, and the speed of light sail is 1/5 of speed of light. So, it will take 20 years to reach the star system. And I've read that Hawking wrote, "even if a nanocraft could be scaled up to a crewed vessel. It would be unable to stop." $\endgroup$ – Harry Dec 11 '18 at 9:19
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    $\begingroup$ So, I just would like to know why the nanocraft drive by light sail cannot stop. After its duty, what will happen? $\endgroup$ – Harry Dec 11 '18 at 9:21
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Welcome to Space Exploration Harry. I'm guessing you are talking about the Breakthrough Starshot proposals? The original idea was for a flyby of the target system because it was assumed that there would be no way to slow down the spacecraft once it had accelerated to 20% of the speed of light. As we know from Newton's First Law:

An object at rest will remain at rest unless acted on by an external force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an external force

so unless the probe has a similar force applied to it in the opposite direction, it will continue its journey into interstellar space. It will never return to Earth because we've pushed it away from us and nothing is pulling it back.

Some calculations have been carried out which suggests that it may be possible to decelerate the probe using gravity assists, light pressure from the target star, and a lower velocity (so a longer travel time), so that the probe could stay within the Alpha Centauri system.

All this is based on vast amounts of energy being transferred to a sail that can carry a microchip-sized payload. The larger the spacecraft, the more energy you'd need to carry out the mission. A spacecraft built to carry a person, with years of food, water, air and other supplies would probably need a solar sail thousands of miles across and more energy than we can imagine generating from our current technology.

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