No, you have to zap them early. Lasers still spread out (see "beam divergence"), so you have to impart most of the momentum while they're still close enough for your lasers to hit with enough power.

Sadly, that also means you have to accelerate them hard if you want to get to a decent fraction of c before they're effectively out of range. Which means your solar sail has to be really, really tough while being really, really light.

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pavel_lishin 2 days ago | parent [-]

I haven't done the math, but if we had significantly powerful lasers, shooting from the vacuum, isn't it very possible that we could actually deliver more acceleration to a far-away probe in 20 years than we could from Earth-based (or even orbit-based) lasers right now?

I have no idea what the best-case scenario for laser acceleration is.

	▲	sfink 11 hours ago \| parent [-]
		No. Let's say our initial boost got it up to 0.1c. After 20 years, it's gone 2 light years. If we make our space-based laser aperture really big, let's say 1km, then the light reaching our probe is something like a 25000km wide radius. That's not going to power anything. If you slow down the initial 0.1c, then pretty quickly you're better off not sending it out at all and getting a bigger boost from your giant space laser being close for the initial acceleration. The diffraction limit is annoying.