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# Physics/Effective range of laser weapons in space?

Question
Hi! What do you believe is the effective range of laser weapons in space given that the wikipedia says that the most powerful lasers are around 1 petawatt? How far would these laser be able to travel with still being energetic enough to damage the armor of a ship? Here is a summary of abilities of a petawatt laser in Texas:
Summary of Texas Petawatt Laser Capabilities
The Texas Petawatt Laser produces 170 fs pulses at a central wavelength of 1057 nm.
The pulses can be amplified up to 190 J in an f/40 target chamber, limited by the damage threshhold of the laser system.
The pulses can be amplified to 60 J in an f/3 target chamber, limited by the damage threshhold of the final focusing parabolic mirror. With the installation of a dielectric high reflective coated final focusing mirror in December 2011, the maximum pulse energy will be increased to 190 J.
At full power, the repetition rate is one shot per hour.
The Texas Petawatt Laser can be operated at a higher repetition rate with lower levels of amplification: four shots per hour at 7 J, or 2.5 Hz at ~100 mJ.
Pulse focus is near twice the diffraction limit.
Experimental runs are typically three weeks in duration. Could this laser have an effective range of let's say 3 million kilometers? How far would the beam travel to be not dangerous to an armor of a spaceship?

Armor of a ship?  So you're assuming armored ships in space.  And one shot per hour, with only 190 J of energy, they wouldn't significantly damage the in the slightest, even with pretty good focus.  You should focus on a highly-focused lasers which are lower-power but far far higher repetition rate.  3 million km is a large distance to shine a laser and not expect it to diffract significantly, but I know nothing about the aperture diameter.  You have to account for diffraction effects, but the parameters of the laser "weapon" you're looking at are ridiculously not-dangerous for "spaceship armor."

Physics

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#### Dr. Stephen O. Nelson

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I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

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