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QHow much watts would a laser beam from Mars have to be, in order to be detectable from Earth?

A typical laser pointer has an output of 5 mW . The most powerful weapon developed laser has an output of 100 kilowatts .

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#1DickAnswered at 2013-02-22 00:44:43
Hubble could detect a mico -watt laser ... Do you think the hubble
#2atifAnswered at 2013-07-18 08:58:42
"Detectable" is a funny thing. If a 1 watt laser (for example) could not be detected with 1 hour of staring, it might be detectable with 10 hours. We can already detect signals carried by just a few 100 photons per second (Lunar Laser Ranging). At the typical distance to Mars, a laser there would have to send about 2.6 x 10^9 photons per second. That's only about 30 nanowatts.

On the other hand, at 20 nanowatts, the information rate would be less than 1 bit per second. The noise from the vibration of the laser (from winds and Mars quakes) would blur the signal so much, the laser would not be detectable any more. Better go up to 20 microwatts, and get about 30 bits per second. Still very slow.

When we bounced radar signals from Earth to Mars and back, we had to stare at Mars for about 30 minutes to detect the return signal.
#3CarmianerloAnswered at 2013-09-18 11:48:05
Depending on the aperture and integration time it could be anywhere from milliwatts to megawatts.

It's always daytime on the side of Mars facing us, so you would have to choose your wavelength carefully to avoid interference from sunlight reflected off the surface of Mars.
#4ra nandyAnswered at 2013-10-12 23:28:26
Detectable how? A telescope with a few meters aperture and a high-quality narrow-band filter tuned to the laser frequency could easily detect a 1 watt laser from Mars.
#5NoeliaAnswered at 2013-11-15 10:49:31
They were going to send an orbiter with laser communication (Mars Telecommunications Orbiter) capability but it got canceled. It would have used a 5 watt laser. That was to send a gigabit per second of data. You could detect a much smaller one.
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