Lasers on earth visible from Mars

[andrewbee]

Hi,

I read something interesting about some amateur astronomers lasing the ISS with a blue-light laser, and the astronauts were able to spot it.

How powerful would a laser have to be to be seen by the human eye from Mars? It would be shone from the dark side of Earth of course. Are we talking one (or several) guide star lasers from the big observatories? Metal-cutting beams (c. 50w)? Naval anti-missile experimental lasers (c. 100 kw)?

Any input appreciated.

Thanks

 

[mfb]

The human eye cannot resolve the disk of Earth from Mars, so you always have much more sunlight than laser light.
Even if we neglect this and just take the minimal distance (~55 million km), it depends on the focus of the laser beam: If you use a telescope (in the "wrong" direction of light) to focus the beam, the power can be reduced significantly compared to a regular laser.

Edit: For a laser pointer of 1mW, I calculated 4000km as lower bound on the visibility range a while ago. 10000 times this distance would need 100 million times the power, or 100kW, assuming the focussing remains the same.

 

[andrewbee]

That's really helpful,thanks. Did you make any assumptions about whether it was red, green or blue light?

 

[mfb]

700nm -> red
A shorter wavelength could improve the visibility limit a bit, but it also gives a lower number of photons per power, and it is harder to create.

 

[KenJackson]

For a laser pointer of 1mW, I calculated 4000km as lower bound on the visibility range a while ago. 10000 times this distance would need 100 million times the power, or 100kW, assuming the focussing remains the same.

That's surprising.

One of my favorite novels is https://www.amazon.com/dp/0974776963/?tag=pfamazon01-20, in which they propel a spaceship to Earth by the power of a laser shined from Alpha Centauri. If it takes 100kW just to be able to see it on Mars, it must take petawatts to propel a ship to the nearest star.

https://www.amazon.com/dp/0671741926/?tag=pfamazon01-20 is another novel that did a similar thing.

 

[mfb]

Well, such a system would have a much better focus than a laser pointer. If you use something like the E-ELT (40m-mirror, currently under construction), a strong laser pointer could be sufficient to be visible from Mars. The laser spot would have a diameter of a few kilometers there.

Direct propulsion with light pressure requires insane amounts of power - about 1 gigawatt (or 1 big powerplant) per 6N (enough to hold a mass 600g on earth), even if the whole laser hits the spacecraft .