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One solution to interstellar space travel that I have heard popularised is to construct a ship with a solar sail and then use a laser to push it away. The advantages are obvious; you don't need to carry any fuel with you, massively reducing the amount of energy needed to thrust.

However, I heard that in the 1960's America wasted billions of dollars trying to develop a system that can intercept an ICBM with a laser. (It just wouldn't work)

Suppose you had colonised the sol system, and had a vast supply of fusion power from readily available gasses. You have a largely roboticised colony in a nearby star system, and you want to transport more human colonists with a continuous thrust of 1 g. (reversing at the half way mark.)

Is there any general law about intercepting projectiles with lasers over vast distances?

Is it viable?

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    $\begingroup$ There are two different questions here: your title asks about using a solar sail with a laser and your question body asks about intercepting projectiles with lasers. Which is it? $\endgroup$
    – Secespitus
    Commented Aug 14, 2017 at 13:54
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    $\begingroup$ On top of what has been said already: Viable to do what? Of course you can do those things theoretically, but viable economically, to reach a certain speed, with our current tech, for a specific application, to do something specifically? Do you want your ship to ever decelerate? What is it build for? Also you should specify what you mean by vast. Over continents or a couple of million light years? $\endgroup$
    – Raditz_35
    Commented Aug 14, 2017 at 13:58
  • $\begingroup$ Have you actually noticed the scale of the conceptual designs (Robert Forward uses one of them is Rocheworld)? You need to be able to engineer a Fresnel lens thousands of kilometers in diameter sited among the giant planet's orbits, and you still have a decades long mission to the nearest stars even making generous assumptions about the available power. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Aug 14, 2017 at 14:27
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    $\begingroup$ Vocabulary nit: this isn't about "cosmic" scale, we're only talking the distances between stars rather than the distances comparable to the inverse Hubble constant times the speed of light. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Aug 14, 2017 at 14:31
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    $\begingroup$ I think you're mixing up how solar sails work and how missile intercept would work. It seems to me that missile intercept wouldn't push the missile away, but instead aim to damage the projectile. On the other hand, solar sails (ideally) wouldn't be damaged by the laser pushing them along. $\endgroup$
    – bendl
    Commented Aug 14, 2017 at 16:41

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There isn't really a law per se, but laser attenuation (due to interstellar dust/gas) and decollimation are notable over interstellar distances.

The acceleration applied by the sun at 1 AU is 9.08 mm/s/s divided by the mass of the vehicle (in grams) divided by the area of the sail (in m^2). So to get a 1G acceleration for a person (approx 60kg), assuming zero weight for both the sail and the vehicle, you would need a (weightless) sail 256m on a side (for 66000 square meters), and provide the same light intensity as the noonday sun for the duration of the acceleration/deceleration.

Broadly, that kind of acceleration is not generally associated with solar sails over interstellar distances, because projecting that much power over that kind of distance is expensive and impractical. Projecting less power and using that to vaporize/accelerate propellant is probably the better approach.

Targeting, however, is not a problem, since NASA is already planning something using laser propulsion over interstellar distances.

Interstellar travel with humans in a hard science world is generally non-viable, unfortunately. Stross's writing takes a fairly realistic view of how humans get to leave the solar system (not in a corporeal fashion), much though it upsets fans.

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  • $\begingroup$ I wouldn't mind uploading my brain to a spaceship. I'd prefer skipping the "now you get killed so that you only exist once" part, though. $\endgroup$
    – John Dvorak
    Commented Aug 14, 2017 at 21:42
  • $\begingroup$ @JohnDvorak - I imagine once brain uploading is actually practical, there will be all sorts of legal and ethical entanglements. $\endgroup$
    – jdunlop
    Commented Aug 14, 2017 at 21:50

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