12 ms·

Well, if the ISS is around 420km up, and Mars is on average 225 million km away, and all other parameters of the laser are identical, then the laser would need

Well, if the ISS is around 420km up, and Mars is on average 225 million km away, and all other parameters of the laser are identical, then the laser would need to be around 700GW. However, you could probably get away with a slightly less powerful laser if you decrease the bit-rate a little, improve the sensitivity of the receiver, and tighten the beam (which wouldn't be a problem if the laser is bigger). I got the impression that the 2.5W laser was way more powerful than necessary for basic communication. Let's face it - Mars Explorer has a total power availability of around 450W, and it aims its dish at the earth with an accuracy of 0.04 degrees, and it manages to communicate. Replacing the radio with a laser may improve matters, but it may also require too much power.

throwaway_yy2Di 12y ago
Here's a paper from NASA's (budget-axed) Mars Laser Communications Demonstration. They estimate 10-100 Mbps bandwidth, on a 5 W laser system (300 W pulsed, but only drawing 5 W from the PSU). http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38254/1/04-0101.pdf http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38254/1/04... and also http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38024/1/04-0216.pdf http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38024/1/04... context: https://en.wikipedia.org/wiki/Mars_Telecommunications_Orbiter https://en.wikipedia.org/wiki/Mars_Telecommunications_Orbite... There's a much more interesting paper somewhere that designs a system for interstellar lasers.. edit: I can't find what I'm thinking of, but this optical SETI experiment is relevant. It looked at 577 nearby stars (<50 parsecs = 163 light years), and claims it could detect a 50 kW (!!) optical laser pointed from one of them (presumably at a negligible bitrate). http://arxiv.org/abs/astro-ph/0112479 http://arxiv.org/abs/astro-ph/0112479
- mnw21cam 12y ago
  That's cool, thanks. Basically, the ISS laser experiment was not stretching the bounds of the method at all. Put a proper telescope at the receiving end, and it changes the game completely. It's simply amazing how little light modern telescopes are capable of detecting and measuring. Note that your arxiv paper describes detecting a 50kW laser that is diffraction limited for a 10m aperture. That in itself would be a quite interesting piece of equipment to build. Edit: Interesting - the abstract seems to have been mis-copy&pasted, with 60kW instead of 50kW.
- deleted 12y ago
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- throwaway_yy2Di 12y ago
  I found what I was thinking of: "The Technical Case for Optical and Infrared SETI" http://seti.harvard.edu/oseti/tech.pdf http://seti.harvard.edu/oseti/tech.pdf http://seti.harvard.edu/oseti/ http://seti.harvard.edu/oseti/
saraid216 12y ago
> Well, if the ISS is around 420km up, and Mars is on average 225 million km away, and all other parameters of the laser are identical, then the laser would need to be around 700GW. What if we tried more power? http://what-if.xkcd.com/13/ http://what-if.xkcd.com/13/ (I am a bad person. I am sorry.)

throwaway_yy2Di 12y ago

Here's a paper from NASA's (budget-axed) Mars Laser Communications Demonstration. They estimate 10-100 Mbps bandwidth, on a 5 W laser system (300 W pulsed, but only drawing 5 W from the PSU). http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38254/1/04-0101.pdf http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38254/1/04... and also http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38024/1/04-0216.pdf http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38024/1/04... context: https://en.wikipedia.org/wiki/Mars_Telecommunications_Orbiter https://en.wikipedia.org/wiki/Mars_Telecommunications_Orbite... There's a much more interesting paper somewhere that designs a system for interstellar lasers.. edit: I can't find what I'm thinking of, but this optical SETI experiment is relevant. It looked at 577 nearby stars (<50 parsecs = 163 light years), and claims it could detect a 50 kW (!!) optical laser pointed from one of them (presumably at a negligible bitrate). http://arxiv.org/abs/astro-ph/0112479 http://arxiv.org/abs/astro-ph/0112479

mnw21cam 12y ago
That's cool, thanks. Basically, the ISS laser experiment was not stretching the bounds of the method at all. Put a proper telescope at the receiving end, and it changes the game completely. It's simply amazing how little light modern telescopes are capable of detecting and measuring. Note that your arxiv paper describes detecting a 50kW laser that is diffraction limited for a 10m aperture. That in itself would be a quite interesting piece of equipment to build. Edit: Interesting - the abstract seems to have been mis-copy&pasted, with 60kW instead of 50kW.
deleted 12y ago
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throwaway_yy2Di 12y ago
I found what I was thinking of: "The Technical Case for Optical and Infrared SETI" http://seti.harvard.edu/oseti/tech.pdf http://seti.harvard.edu/oseti/tech.pdf http://seti.harvard.edu/oseti/ http://seti.harvard.edu/oseti/

saraid216 12y ago

> Well, if the ISS is around 420km up, and Mars is on average 225 million km away, and all other parameters of the laser are identical, then the laser would need to be around 700GW. What if we tried more power? http://what-if.xkcd.com/13/ http://what-if.xkcd.com/13/ (I am a bad person. I am sorry.)