Monday, July 27, 2015

Interstellar Communication – Microwave

Earthlings continue to push their technological capabilities for space communication. The Voyager I probe, now Earth’s farthest object, is about 130 AU out, or better, 36 light hours from Earth. It broadcasts back to Earth every six months, using the power from its radioisotope generator, which has about 250 watts at this time. It has a 3.7 m antenna and broadcasts at X-band. This is picked up by earthbound antennas, ranging up to 70 m in diameter.

For fun, let’s extrapolate this to 10 light years, or about 2500 times as far. Since received power drops by distance squared, somehow the system has to be upgraded a factor of about 6 million. For starters, assume there is a good reactor powering it, not just a plutonium RTG. Give it 1 million watts, which is a factor of 4000 already. Now we need a factor of 1500. Assume the antenna is 10 times the size, and since beamwidth goes as the square of the ratio of the frequency and the aperture, this is an improvement of 100. Now we need a factor of 15.

The existing receiving antenna is 70 m, but let’s assume we build a big, big antenna in space, 700 m in diameter. That’s another factor of 100 and we only needed 15. We now have built an interstellar communications channel. The Voyager I doesn’t talk frequently, or with a high baud rate, but the super-sized system we speculate about here could do plenty more that just get some minimal data back to Earth.

It would be possible to go into some interesting engineering calculations on how big a package this all would be, and how much propulsion power it would take to get it up to some nice fast interstellar speed like 10% of light speed. A lot would be needed, of course, but instead let’s talk about what it means for alien civilizations. Any alien civilization that has been around for millennia and has progressed to asymptotic technology would look at this design and see many ways to do it better, simpler, with lower weight, and so on. That is not the point. The point is that the galaxy can be wired for communications.

If an alien civilization sets up a colony 10 light years away, it can be in contact with it, given the 10 year time lag in receiving a message. Obviously a larger system could transmit more bandwidth. The point is that it is possible. Not only can an alien civilization set up colonies, it can set up a communications network so that information can be passed on it. An order for a thousand tons of silicon can be sent back. A report on the failure of a landing probe can be transmitted. Health status on the re-awakened crew can go home.

Recall that since alien society would have long passed into a stable state, we noted that traveling for aliens over long periods would not be the same as Earthlings would experience, in that when we got home from a hundred year journey, the whole face of the planet might be different. When aliens returned at year 174433, it looks pretty much like it did at year 174333 when they left. It is the same for communications. Ten years to get the message out and ten years to get the message back? No problem, the same master computer will get the message as sent it, the right citizens, robots or intellos will hear it and do what needs to be done, even if they are not the ones connected to the launch. This is abbreviated by saying that aliens in an aeons-old civilization are fungible. They are all able to do what is necessary, and who does it is likely unimportant.

In the previous post on interstellar communications, it was noted that there are two big reasons for using it. One was connected with colonization, and the possibility of communications means that a one-way trip is sufficient. Nobody needs to return home with the video. However, communication means a big reactor has to be loaded onto the probe, but surprisingly enough, a 1 megawatt reactor can be made compact, and shadow shielding used so the weight of all that lead around the reactor will not be needed. Shadow shielding means the payload is shielded by a wall, and that’s all. The rest of the neutrons can go spraying out to space with no worries at all. This is under the assumption that small fusion reactors are a no-go, but Earth has no clue about this at this time. If they are possible, a lighter weight probe may be possible.

A big antenna is also needed, but unless the probe is going to go flying through a dense cloud of interstellar gas, not much strength is needed. These clouds can be avoided in most cases.

There is still a question of deceleration in the destination solar system, in case the originating home planet wants to get a longer look at the planets there, perhaps do some close-ups. Time is not of the essence in a civilization whose age might be very many millennia. It might not be millions of years old, as it is important, if an alien civilization is going to do colonization, to get there before anyone else does, but that still leaves a very long time, except in some vanishingly small probability cases. Deceleration will raise the mass, but the reactor can be designed to have power for a long time, and running around the solar system at a slow speed, comparable with orbital speeds not light speeds, can be done to explore wherever the originating planet decides. It is very unlikely that the originating planet will have to decide anything, however, as by the time the probe is launched, artificial intelligence will certainly be able to make these decisions locally, using general guidelines set up by the launch team. One way communications will be what is important.

The second big reason is deterrence and surveillance. In the situation where an alien civilization sees itself on the colonization path of another alien civilization, one that has been around longer and has developed a sphere of colony worlds, having some surveillance that one of the colonies on the closest part of the outer envelope is not going to be sending something nasty their way would be a good thing. Having the ability to deter any such ventures with their own weapons, connected with a reliable communication channel, would be another good thing. Whether such deterrence is possible remains to be explored, but the communications part of it is not a show-stopper. There may be others, like the need to operate covertly, but that will require a bit more thought to get even the first glimpse of the options involved.

So, now that we know ET can call home, as long as he has a megawatt reactor in orbit, it is time to think about whether we can intercept any of these communications. Beamwidths on the supersized system are about a degree, so there are photons spewing all over the place near the beamline of one of these systems. However, IR or visible light may be a better choice; we need to discuss these options as well. Aliens may be using any part of the electromagnetic spectrum as far as we know, and it would be interesting to try and get a clue as to which part they might prefer.

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