Communication Options With Future Deep Space Probes?

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  • #1
Devin-M
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I was wondering if humans send spacecraft far enough into space in the future (for example Alpha Centauri or other star systems further away), is there any particular distance where it might make more sense to communicate between star systems by making a star blink with some sort of shutter array rather than trying to build a powerful enough radio transmitter? In other words, above a certain distance would it take less energetic effort to make a star blink with some sort of blocking mechanism rather than trying to build a big enough radio transmitter and power source?
 

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  • #2
Orodruin
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No. Any such mechanism would have to be enormous.
 
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  • #3
Baluncore
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In other words, above a certain distance would it take less energetic effort to make a star blink with some sort of blocking mechanism rather than trying to build a big enough radio transmitter and power source?
Planets orbiting stars show up as a short term cyclic reductions in brightness. To achieve that with a screen, would require the screen be the size of a planet.

Alternatively, if you modulate a star-light powered laser aimed accurately towards the Earth, you should be able to detect the AC component at the wavelength of the laser, against the background.
 
  • #4
Ibix
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I would think if you could do engineering on that scale a mirror would be a better bet. You could at least direct all the light falling on it towards Earth.
 
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  • #5
sophiecentaur
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I would think if you could do engineering on that scale a mirror would be a better bet. You could at least direct all the light falling on it towards Earth.

Turning the light from a star with a shutter would involve massive structures but how about tilting parts of the mirror (like a heliograph)? The mirror could have a 'reasonable' diameter of a few km (or tens or hundreds of km) and be positioned in an orbit large enough to allow it to be resolved against its parent star. The power density would be poor, compared with a laser.
I can see problems with signalling rate, though, if the modulation were done mechanically. kB/s only.
 
  • #6
Baluncore
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I can see problems with signalling rate, though, if the modulation were done mechanically. kB/s only.
Lower rigidity is needed if the mirror array is replaced with a PV array to generate the power needed for a laser source. The data rate could then be greatly increased.

The laser would be switched on-off at a fixed rate, with the switching phase modulated with data. The regularly switched optical carrier could be detected positively against the star background.

For small radiator antenna arrays, light has a gain advantage over the longer radio waves. Only for VLBI radio astronomy does radio produce greater resolution than optical.
 
  • #7
Vanadium 50
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Let me understand this - the premise is that we can build a device that travels fast enough to cross interstellar space, anfd build a shutter that can cover an entire star, but we can't build a working radio?
 
  • #8
Baluncore
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..., anfd build a shutter that can cover an entire star, ...
Not an entire star, but the size of a planet.
 
  • #9
Devin-M
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If the star were a neutron star could a hypothetical shutter mechanism for interstellar communication be smaller — around ~12 miles across?
 
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  • #10
Ibix
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Let me understand this - the premise is that we can build a device that travels fast enough to cross interstellar space, anfd build a shutter that can cover an entire star, but we can't build a working radio?
I think the question is, is it easier/cheaper/"better" to build a radio with the necessary power or to modulate the light from a nearby star. I suspect the answer is yes, because a gigantic modulating device requires gigantic power sources to manipulate its transmittance/reflectance. So it's probably easier to use that gigantic power source to run a big laser (or a lot of small ones).
 
  • #11
Vanadium 50
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gigantic modulating device requires gigantic power sources
And it will have terrible bandwidth.

This isn't even a solution looking for a problem. It's a not-even-a-solution looking for a problem.

We can make big radio transmitters today. We can't make planetary-scale machines today.
 
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  • #12
Devin-M
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Would a 200 meter mirror at a distance of 2 light years from Earth be able block all the light from neutron star RX J1856.5−3754 from reaching Earth?

That neutron star is around 40km in diameter and is at a distance of 400 light years from Earth.

https://en.wikipedia.org/wiki/RX_J1856.5−3754
 
  • #13
Orodruin
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Would a 200 meter mirror at a distance of 2 light years from Earth be able block all the light from neutron star RX J1856.5−3754 from reaching Earth?

That neutron star is around 40km in diameter and is at a distance of 400 light years from Earth.

https://en.wikipedia.org/wiki/RX_J1856.5−3754
No.
 
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  • #14
Orodruin
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As long as it was able to manoeuvre to stay between us and the star, and was actually a bit bigger than 200m to account for the finite diameter of the Earth. But why would you want to do that? What's 2ly away from us to want to signal about?
With a 20 km diameter source and a 200 m = 0.2 km diameter blocker, the umbra would reach 4 light years from the blocker. However, the umbra at 2 light years distance would be 100 m in diameter. Most of the Earth is still getting hit by at least some of the radiation from the neutron star.
 
  • #15
Ibix
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Would a 200 meter mirror at a distance of 2 light years from Earth be able block all the light from neutron star RX J1856.5−3754 from reaching Earth?
It would block it completely at a point, yes. But not anywhere away from that point (and the Earth is 6400km across with an orbit 300 million km across). So you'd need it to manoeuvre to be able to keep between the star and a fixed point on the rotating and orbiting Earth. And what is there at 2ly to write home about anyway?
 
  • #16
Ibix
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With a 20 km diameter source and a 200 m = 0.2 km diameter blocker, the umbra would reach 4 light years from the blocker. However, the umbra at 2 light years distance would be 100 m in diameter. Most of the Earth is still getting hit by at least some of the radiation from the neutron star.
Yeah, I already deleted that post before your reply when that (slightly belatedly) occurred to me.
 
  • #17
sophiecentaur
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Lower rigidity is needed if the mirror array is replaced with a PV array to generate the power needed for a laser source. The data rate could then be greatly increased.

I think we're being wildly speculative to come out with any useful figures to compare various methods. We'd have to compare two much better specified systems. The efficiency of PV and Laser systems and the actual power capability would go together to set the possible signalling bandwidth over several LY. Would the data rate be actually better than what you could get with a beam deflector and mirror? Beam width and pointing capability are also relevant.
So you'd need it to manoeuvre to be able to keep between the star and a fixed point on the rotating and orbiting Earth.
Station keeping would be slightly easier if the 'screen' were well away from 'orbital' problems.
And what is there at 2ly to write home about anyway?
I see what you did there!
 
  • #18
Baluncore
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Would a 200 meter mirror at a distance of 2 light years from Earth be able block all the light from neutron star RX J1856.5−3754 from reaching Earth?
Does our Moon stop all light from our Sun during an eclipse ?
How big is the corona of a neutron star ?
What will the diffraction angle be at the edges of the mirror or mask ?
Will it matter where the observer is on or near the Earth ?
 
  • #19
sophiecentaur
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Does our Moon stop all light from our Sun during an eclipse ?
I keep muttering "signal to noise ratio" to myself. No chance of estimating that until the geometry of a 'real case' has been worked out but masking a star reliably would require a much oversized disc.

It may be worth bearing in mind the problems of detecting exoplanets (impossible until thirty years ago), compared with spotting very high magnitude stars against a dark background. This is why I like the idea of a mirror.
 
  • #20
Filip Larsen
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Knowing DSN can pick up the 22 W transmitter of Voyager 1 at 1.8 light-days, means that everything else being equal you would need around 16 MW of transmitter power at 4.25 light-year for DSN to also pick up that up such signal. And that is just using RF technology; I would expect using an accurately pointed laser to transmit to a space telescope near earth, or something similar, would require far less power still.
 
  • #21
Devin-M
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Knowing DSN can pick up the 22 W transmitter of Voyager 1 at 1.8 light-days, means that everything else being equal you would need around 16 MW of transmitter power at 4.25 light-year for DSN to also pick up that up such signal. And that is just using RF technology; I would expect using an accurately pointed laser to transmit to a space telescope near earth, or something similar, would require far less power still.
For comparison I found Arecibo's transmitter (305m diameter) was roughly 1MW, and the most powerful radar on Earth (Eglin AFB Site C-6) is currently around 32MW....

https://en.wikipedia.org/wiki/Eglin_AFB_Site_C-6

https://www.hou.usra.edu/meetings/apophis2020/pdf/2013.pdf

2008_Eglin_AFB_Site_C-6_phased_array_building.jpg
 
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  • #22
Devin-M
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The furthest pulsar yet observed (NGC 5907 X-1) is 50 million light years away, and assuming it's 50km in diameter, it seems to imply that a chance alignment with a 1/2 meter object 500 light years away could block most of its light at least momentarily. It seems the biggest impediment for this to be a practical means of communcation would be maintaining the necessary precise alignment between the observer/receiver, the distant pulsar, and the intervening shutter mechanism that is attempting to broadcast a message.
 
  • #23
PeroK
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Is the idea to communicate in Morse code?
 
  • #24
Baluncore
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Is the idea to communicate in Morse code?
No.
Morse code would be the worst possible modulation scheme to use over a noisy channel without the ability to QSR this year.
 
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  • #25
PeroK
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No.
Morse code would be the worst possible modulation scheme to use over a noisy channel without the ability to QSR this year.
How do you communicate otherwise by blocking our a pulsar?
 
  • #27
Ibix
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@Devin-M was Morse code what you had in mind?
I assume you'd use binary with some error-correcting code scheme.
 
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  • #28
Devin-M
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Yes extremely low bandwidth morse code is what I had in mind. The only uses I could think of would be something like engine telemetry of an outgoing spacecraft, or if the shutter was close enough to the star and large enough, it could broadcast the presence of intelligence. For example if the pulses weren't regular but rather a sequence of prime numbers, it might seem odd, if anyone was watching.
 
  • #29
PeroK
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I assume you'd use binary with some error-correcting code scheme.
This giant shutter would have to open and close quite quickly.
 
  • #30
sophiecentaur
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Yes extremely low bandwidth morse code is what I had in mind.
But what use would such a low data rate be? A message like "Arrived at star system. Locals friendly" could take months. Not very good return on trillions of quid of investment.
 
  • #31
Vanadium 50
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And that is just using RF technology
And a much more omnidirectional antenna.

I am amused that a giant planet sized iris shutter is sensible, but a radio? Now that's just crazy talk!
 
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  • #32
Baluncore
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A planet sized mirror or metallic reflector, will not work near a pulsar because the eddy currents induced in the mirror by the pulsar rotation, will distort the mirror.
 
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  • #33
sophiecentaur
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A planet sized mirror or metallic reflector, will not work near a pulsar because the eddy currents induced in the mirror by the pulsar rotation, will distort the mirror.
The cold light of reality. It would involve crude amplitude modulation of a very dirty signal.

But, if we're actually interested in getting some useful data across, we would probably be better with a lower power and some clever modulation and coding of a coherent source. I suspect there may be some mileage in using a high power microwave source - a compromise between beam width and possible noise / bandwidth performance.
 
  • #35
Baluncore
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"Each eye's glass contains a liquid crystal layer which has the property of becoming opaque when voltage is applied, being otherwise transparent."
https://en.wikipedia.org/wiki/Active_shutter_3D_system
A back-lighted LC (liquid crystal) shutter does not need to remain accurately aligned, but it does need a sandwich of five layers made up of two polarizers, two transparent sheet electrodes, and a liquid crystal filling. The transparent sheet electrodes cannot be superconducting because then they would become mirrors.

To minimise weight, the LC shutter will have a minimum volume of liquid, so will have a high capacitance and require a high drive current to change the control voltage. So it can be rolled or folded, the outer layers must not bond together.

Maybe you can use an LC shutter as a solar sail to accelerate, then to slow down on arrival.
 

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