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Comment by RIMR | Hacker News Reader

So, how is a tiny smartphone-sized probe supposed to transmit data back to Earth from 4.37ly away?

That is a good question. The Voyager spacecraft send out detectable signals with a ~20 Watt signal. This is larger than the ~3 W signal that a smartphone sends, but by less than an order of magnitude.

Voyager is at ~130 AU. Alpha Centauri is at ~275,000 AU. With the 1/r^2 decrease in signal, that means the signal from earth will be smaller by a factor of ~3 x 10^6. Now, supposing we have ~1000 of these smartphone probes, this means that their combined signal will be weaker than the Voyager signal by a factor of ~3000. We might imagine that they are somewhat more optimized to send a stronger signal than an ordinary smartphone. Let's say they have a 30 W radio instead of 3 W. This means that they will have a weaker signal by a factor of ~300. This is a lot, but if they manage to build a detector that has a larger effective diameter than the Deep Space Network by a factor of ~15. Given the budget of the project, and the fact that they have 20 years of time to prepare after the launch, this could be feasible.

Looking at the project's website, though, it looks like they are taking the approach of using a laser onboard the probe instead of using a traditional radio transmitter [1]. This would increase the efficiency of the transmission by several orders of magnitude. It might even be possible to detect the signal from a single probe with this approach.

[1]: http://breakthroughinitiatives.org/index.php?controller=Foru...

Another problem to consider is that the interstellar medium is noisier and interferes with the signal to a greater extent. Voyager is only just reaching that region, but this signal would have to travel through that for several lightyears.

https://en.wikipedia.org/wiki/Interstellar_medium#Radiowave_...

Not to mention there is also additional cosmic radiation outside the heliosphere. Something the size of a chip is going to be extremely vulnerable. Slower rad-hard electronics are going to be the order of the day, but even with thousands of these things, it's going to take a toll.

it seems ridiculous to use anything other than a laser IMO.

Almost certainly. But using a laser requires much better aim than a radio transmitter.

"The technique uses cryo-cooled nanotubes. The current performance is of order 2 bits per photon."

How do you keep it cool with a 100GW laser pointed at it over 2 minutes?

> Alpha Centauri is at ~275,000 AU.

I wonder if it would be easier to string ~300 of these at ~1000 AU intervals, and use them for a relay? Or perhaps ~3000 at ~100 AU intervals.

And I wonder if that would be construed as mining a common hyperspace lane by the galactic council... ;-)

Use the same tech you used to send the first probe to send relay satellites to Alpha Centauri to create an Alpha Centauri-Sol bridge.

http://link.springer.com/content/pdf/10.1007%2F978-3-642-274...

Interestingly, with no way to stop those relays we'd have to keep firing them at regular intervals until we no longer need them.

Thinking out loud: You need an ingenious design that allows the laser sail to double as a directional antenna. And you pick a frequency which is not radiated by many other objects in space so it's easier to pick up the dim signal. And then you build a ginormous radio telescope bigger than any ever built before. Luckily you have a decade or two to build the telescope after the probes are launched.

And/or establish a chain of these spanning the distance between here and Alpha Centauri. They don't need to actually stop, but if enough are traveling at lower speeds or depart sufficiently late in the program then they'll be in position to act as repeaters. Possibly larger probes (which would travel slower) that have more power available for reception and retransmission.

Planar directional antennas have been done before, in EHF-type frequencies (most research has focused on 2.4 GHz and 5 GHz for obvious reasons). Lower-frequency signals would probably be harder. No idea what a nice clear frequency might be though, particularly out in the interstellar medium.

Depending on what maneuvering profile these things will have, it might be desirable to have a "semi-phased" design that can be steered to a limited degree without physically moving the spacecraft.

And, it also goes without saying that this antenna probably needs to be flexible or at least articulated in order to deploy.

NASA has previously done some really interesting antenna designs with genetic algorithms. You just need to figure out what the goodness function here should be.

https://ti.arc.nasa.gov/m/pub-archive/1244h/1244%20(Hornby)....

I would think you could modulate a retroreflector to send data directly back to Earth using a small piece of the same laser used for propulsion.

Similar to how the retroreflectors on the moon allow you to bounce a laser off of them and send it directly back to you.

Since there will be many probes sent, they will probably be able to relay the signal all along the way. It would still be difficult, but maybe manageable.

Slowly and very carefully.

Does having them spread out over a few AU make beamforming possible? They do have their light sails to do double duty for comms.

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