Alternatively, scrap the laser and you have a perfectly serviceable photon   
   sail to use with your local star/a distant laser station.   
   Not precisely reactionless, but fuel-less (the fuel is consumed by the   
   star/laser station to generate light - and in the case of the star, it is   
   always on anyway, so using it will make no difference).   
      
   Remember that light is just matter that felt like being photons/matter is when   
   light decides to all have a party in one place.   
   Throwing your reaction mass as photons is the most fuel-efficient way possible   
   (neglecting the system's inefficiencies of course), but it is no different   
   from throwing your reaction mass as the fiery plume of a chemical rocket.   
   Putting a photon sail before would be like putting a brick wall in front of a   
   rocket engine. Sure, it will push the other way, but you'd be better off   
   turning the rocket around and get rid of the wall   
      
   So if you scrap the sail and keep the laser, remember that a photon rocket is   
   an obscenely powerful floodlight. We're talking 300 MW per Newton. TO get the   
   same thrust as a Saturn V, that is, about 35000 kN, you need 10500 TW (minus   
   inefficiencies). So    
   you are throwing the energy of a 2.5 Mt nuke every second.   
      
   So I strongly suggest _not_ using a laser, and instead use a conical   
   non-coherent floodlight instead (with a 10% angle, you won't loose that much   
   efficiency).   
   That way, while your engine will still *burn everything that dares to exist   
   through its exhaust until there is nothing left but subatomic particles*, at   
   least it won't do that from quite as far. You still don't want to point it at   
   anything too close (so    
   no Earth orbit for you), but at least your engine exhaust isn't an   
   interplanetary WMD anymore. Still, try not to point it at nearby planets,   
   though.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)