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Comment by yeldarb | Hacker News Reader

Interesting. If a 2.5 watt laser was used for communication from the ISS to Earth what wattage would be needed to send a high-bandwidth message back from Mars (or Titan)? How does that compare to traditional radio-based communication?

This point-to-point communication is also interesting in relation to the Fermi Paradox. It seems likely that a sufficiently-advanced race would beam their messages directly to their recipients rather than wasting energy (and privacy) transmitting an omnidirectional signal.

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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.

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... and also

http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38024/1/04...

context: 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

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.

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/

> 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/

(I am a bad person. I am sorry.)

Well, presumably since laser light is coherent, you only need to worry about the Earth's atmosphere. In this case, it seems that a 2.5 watt laser would be sufficient, assuming you can point it at Earth precisely and ignoring Mars' atmosphere.

There is still a problem with regards to the Fermi paradox: sure you'd want to beam directly, but where do you point your lasers? You only know by observing the not-so-coherent light leaving Earth and arriving at your planet, which may be very far away. You haven't really solved anything because you have to know where to point your laser in the first place.

>you only need to worry about the Earth's atmosphere

I assume you're meaning this is the main source of attenuation (signal loss) if we assume the space between Earth and Mars is an empty vacuum. This (technically called scattering) is not the main source of loss for such a system.

Coherent just means the light is all in phase, it doesn't mean it doesn't spread out as it travels. If it didn't spread out at all, your receiver on earth would be just as wide as the laser transmitter (about the size of a coin), and you'd have to be locked on the entire time... not very practical. It looks like NASA has intentionally focussed their LASER into a wide cone for precisely this reason.

The distance from Mars would cause the beam (cone) to spread out too much, thereby reducing the power received on earth. You would have to increase your laser power on Mars to compensate for this or make the beam more directional with some focusing optics.

Larger than the power issue, it seems like latency would be a big challenge that would need to be solved to achieve Mars <=> Earth communication. There's already a very high latency (about a day) with this proof-of-concept while waiting for the ISS to pass over the same station in California again.

The latency would be significantly longer than a day for two single transceivers, one on Earth/ISS, and one on Mars, to align again once they've broken alignment—Earth-Mars oppositions happen about every two years.

A solution to both problems would be more transceivers: the Earth to ISS station problem could be addressed by setting up more transceivers on earth underneath the ISS's ground tracks, enabling more frequent transmissions. The Mars solution would probably require a lot more cost/effort/creativity, but a mesh of transceivers, all in concentric orbit around the Sun between the Earth and Mars, should be able to relay data between the two points with a best-case 4- to 30-minute latency (bounded by speed of light, distance varies between Mars and Earth on a two-year period)

This is mentioned in the comments in the TCP implementation of the Linux kernel:

  373  *                                    ... Note that 120 sec is
  374  * defined in the protocol as the maximum possible RTT.  I guess
  375  * we'll have to use something other than TCP to talk to the
  376  * University of Mars.

http://www.cs.fsu.edu/~baker/devices/lxr/http/source/linux/n...

Why would you need to wait for opposition? As long as you have direct line of sight to Mars, that is good enough. Granted, the latency is only going to increase when not in opposition, but they already have ways to deal with that, such as IPN. [1]

Conjunction, of course, would be a much tougher feat without some sort of relay system. I'm sure that wouldn't be insurmountable though, and I think they built that into IPN as well, just like how the internet can route around problem areas.

[1] http://en.wikipedia.org/wiki/Interplanetary_Internet

This is one of the things SETI struggles with. Their entire system is designed around the assumption that radio waves are going to be the way these advanced civilizations communicate. It might just be likely they use lasers or other methods and maybe treat radio as an antiquated backup.

>Their entire system is designed around the assumption that radio waves are going to be the way these advanced civilizations communicate.

Yes, but don't think of it like Sweet 98's greatest hits. SETI specifically looks in the hydrogen frequencies because they're less noisy. This goes with the assumption that if they're at least as smart as us, they'll have similar astrophysics knowledge, and therefore know that a radio signal there would be easier to pick out against background.

Using line of site communication such as laser would probably not be the best way for interstellar communications. Largly because as empty as the universe is. It's hard to shoot a straight line in any direction and not hit something, and also the high energy requirements.

Lasers are only being used in this case for bandwidth issues not latency issues. As both lasers, and radio travel at the speed of light.

SETI is using the most likely method of detecting communications as we don't know of any faster means to communicate than EMR.

Complete tangent of course, but I wonder if aliens using laser communication methods might be why we aren't being bathed in radio waves.

Radio waves go in multiple directions, lasers can only work in short range, unless you find a superb trick to keep the alignment perfect at great distances.

Simple, you just keep tapping it with a rubber hammer until you get alignment.

I don't know if you need a "trick"... Just several orders of magnitude more precision?

Yeah, and just like for telescopes, when you need several orders of magnitude more precision, you need something huge to do so. Unless we find a way to build huge spaceships in space, it's not coming anytime soon.

Actually, I think this is less of a problem than you think. Lasers have significant divergence, see for example the Marcy paper an astro-ph that was referenced elsewhere in this thread. Telescopes can already track targets at the diffraction limit, so it's no different to send a laser.

What is different is that you have to know the relative proper motion of the target, since you have to "lead" the target just like you do when skeet shooting. I don't know whether the lead angle is significant, but proper motions of nearby stars have been measured (e.g. by Hipparcos http://en.wikipedia.org/wiki/Hipparcos) and calculating the resulting lead angle isn't difficult.

That's one of the proposed solutions to the Fermi Paradox. Human TV signals are already dying out as they get supplanted by cable/fiber, and what's left like cell phones is constantly being engineered to use less power.

Maybe the falloff in output has convinced them our civilization is dying and not worth contacting.

Considering that our radio waves haven't traveled very far, I doubt this is true. Remember, a planet 60 light years away is just now hearing Elvis for the first time.

The ships hiding on the dark side of the moon have heard everything...

That seems unlikely to me. Why wouldn't at least a few alien civilisation keep broadcasting radio signals just to let others know they are there too ?

Not for very long. http://xkcd.com/1377/

Maybe there a much better ways of doing so that we discover in another couple decades? Who knows.

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