When you read the article there are lots of challenges. Interstellar dust may (and probably will) destroy the nano spacecrafts.
And corrections afterwards are obvously impossible. Even with New Horizons that is difficult.
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In case you’re wondering, some care will probably need to be taken not to hit airplanes or satellites with the laser beam, although the situation won’t be quite as dangerous as it might seem at first.
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And 100GW on one square km equals to 100kW on one sq meter which is 100 times the Sun's radiation and can really pose a harm to aircraft or satellites. Looks something like the 'heat rays' in HG Wells 'War of the Worlds'.
So this is not really proven technology.
And why 1/5th of light speed in 4 minutes, why not (almost, e.g. 90%) full light speed in, say, 6 minutes ?