<p>A partial answer to your question emerged today on arXiv. <a href="https://arxiv.org/abs/1706.03795" rel="nofollow noreferrer">Hippke (2017)</a> analyses this exact problem - dealing with power requirements for the transmitter, arrangements for detection, the information transfer rate, extinction etc. etc.</p>

<p>Their conclusion is that there is no problem in principle with getting something to work at about 1 bit/s per Watt of power that the probe can obtain from photovoltaic power and <em>if</em> a dedicated 39-m telescope is observing the probe using a coronagraphic imager. They also conclude that the best wavelength to work at is 400nm for a ground-based receiving telescope and 300 nm if a 10-m space based telescope is available.</p>

<p>The basic transmission device is a $\sim 1$m diameter telescope, firing a coherent laser beam back towards Earth. The data rate scales as the area of the transmitting telescope. They do not discuss how such a thing could be built or sent to $\alpha$ Cen. A promised follow-up paper is to discuss the use of gravitational lensing to improve signal-to-noise.</p>

CC BY-SA 3.0
2017-12-03T14:17:37.230Z
@AlexanderCska I don't know what you saw, but that isn't correct. Anyway, we are not discussing radio transmissions here.
ProfRob
CC BY-SA 3.0
2017-06-14T13:57:04.510Z
2017-06-14T13:57:04.510Z
ProfRob
<p>It's possible to transmit an intelligible signal to a target roughly 4.3 light-years away using 0.1 milliwatts. The trick for that particular level of power is that both the sender and receiver have to be carefully positioned several hundred AU from a star, with those stars lined up between them, in order to take advantage of a gravitational lensing effect. If only one of the two radios can be positioned at a star's gravitational focus, then the necessary power rises, but can be under 40 watts, according to one of the diagrams reprinted at <a href="https://www.centauri-dreams.org/?p=10123" rel="nofollow noreferrer">https://www.centauri-dreams.org/?p=10123</a> .</p>

CC BY-SA 3.0
2017-06-07T10:59:44.017Z
The problem is, these small ships would be laser accelerated and have no means of slowing down.    Personally I think the very tiny laser accelerated ships are a bad idea cause they'd just fly through any solar system they are sent to in a few months.   Fly past a planet if they were lucky enough to get close to one in less than an hour and have great difficulty sending what little information they did collect back.   I'd rather send larger ships slower.  100 years, not 20 but much better.   https://www.scientificamerican.com/article/100-million-plan-will-send-probes-to-the-nearest-star1/
userLTK
CC BY-SA 3.0
2017-06-07T23:50:56.247Z
The plan outlined by the Breakthrough Starshot people (Stephen Hawking and Freeman Dyson are on board) is carefully planned to be theoretically accomplishable within a generation. They say that there are no "deal breakers" even though there are barriers to overcome. I would like to see the accelerating laser built in space, but was shot down by cost arguments. I'm sure that if anybody has better ideas, they are very open to suggestion. Personally, I would rather see a "fast flyby" mission than none at all. We probably want to swing by Proxima on the way, though.
Jack R. Woods
CC BY-SA 3.0
2017-06-07T09:54:13.833Z
2017-06-07T09:54:13.833Z
DataPacRat
<p>The StarShot project from <a href="https://breakthroughinitiatives.org/Initiative/3" rel="nofollow noreferrer">Breakthrough Initiatives</a> aims to send a paper light spacecraft to alpha centauri using pressure from a laser located on earth at 20,000kms. </p>

<p>What are the options for a transmitter that can be received from 1-4 lyrs away, if the transmitter weighs the same as a few jellybeans (&lt;10 grams)? What wavelength and power range can they work with for that distance using that weight of equipment?</p>

CC BY-SA 3.0
2017-06-04T08:32:36.693Z
Might be better for space exploration than here, but what I know about it is - right now they can't.   They might set up some relays, er, maybe.      The wavelength is easy - somewhere around the hydrogen-line, is ideal for transmission but the ability to communicate is still very hard at those distances.   Maybe they'll build a Texas sized receiving dish into the moon.    It's not just power of the signal, it's also size of the receiver and how pinpoint the transmission.   I think those tiny ships are a long ways off personally.   Lots to figure out.
userLTK
CC BY-SA 3.0
2017-06-04T09:12:02.477Z
@userLTK ditto and ditto.
uhoh
CC BY-SA 3.0
2017-06-06T09:47:02.137Z
Good point indeed. Even reflecting a laser from a nearby star would probably not be possible with today's laser photon parallelism errors. And the gravity effects near the other solar system would also make 2 way light communications difficult. and the object couldn't orbit and return accuratesly at 20,000km/h and 10 grams. It sounds like the 100 million project should spend 10 million first to see if the 2 way communication is impossible, it sounds like the most difficult component of the mission.
bandybabboon
<star><communication>
Power requirement for transmitting communications further than 1 ly?
bandybabboon
# Power requirement for transmitting communications further than 1 ly?

(Asked by: username_0 on 2017-06-03)

The StarShot project from [Breakthrough Initiatives](https://breakthroughinitiatives.org/Initiative/3) aims to send a paper light spacecraft to alpha centauri using pressure from a laser located on earth at 20,000kms.

What are the options for a transmitter that can be received from 1-4 lyrs away, if the transmitter weighs the same as a few jellybeans (<10 grams)? What wavelength and power range can they work with for that distance using that weight of equipment?

**Tags:** star, communication
> **Comments:**
> **username_1**: Might be better for space exploration than here, but what I know about it is - right now they can't.   They might set up some relays, er, maybe.      The wavelength is easy - somewhere around the hydrogen-line, is ideal for transmission but the ability to communicate is still very hard at those distances.   Maybe they'll build a Texas sized receiving dish into the moon.    It's not just power of the signal, it's also size of the receiver and how pinpoint the transmission.   I think those tiny ships are a long ways off personally.   Lots to figure out.  
> **username_2**: @userLTK ditto and ditto.  
> **username_0**: Good point indeed. Even reflecting a laser from a nearby star would probably not be possible with today's laser photon parallelism errors. And the gravity effects near the other solar system would also make 2 way light communications difficult. and the object couldn't orbit and return accuratesly at 20,000km/h and 10 grams. It sounds like the 100 million project should spend 10 million first to see if the 2 way communication is impossible, it sounds like the most difficult component of the mission.  

---

## 2 answers

### Answer by username_3 on 2017-06-14. Score: 2 | Accepted answer

A partial answer to your question emerged today on arXiv. [Hippke (2017)](https://arxiv.org/abs/1706.03795) analyses this exact problem - dealing with power requirements for the transmitter, arrangements for detection, the information transfer rate, extinction etc. etc.

Their conclusion is that there is no problem in principle with getting something to work at about 1 bit/s per Watt of power that the probe can obtain from photovoltaic power and *if* a dedicated 39-m telescope is observing the probe using a coronagraphic imager. They also conclude that the best wavelength to work at is 400nm for a ground-based receiving telescope and 300 nm if a 10-m space based telescope is available.

The basic transmission device is a $\sim 1$m diameter telescope, firing a coherent laser beam back towards Earth. The data rate scales as the area of the transmitting telescope. They do not discuss how such a thing could be built or sent to $\alpha$ Cen. A promised follow-up paper is to discuss the use of gravitational lensing to improve signal-to-noise.

> **Comments:**
> **username_3**: @AlexanderCska I don't know what you saw, but that isn't correct. Anyway, we are not discussing radio transmissions here.  

---

### Answer by username_4 on 2017-06-07. Score: 2

It's possible to transmit an intelligible signal to a target roughly 4.3 light-years away using 0.1 milliwatts. The trick for that particular level of power is that both the sender and receiver have to be carefully positioned several hundred AU from a star, with those stars lined up between them, in order to take advantage of a gravitational lensing effect. If only one of the two radios can be positioned at a star's gravitational focus, then the necessary power rises, but can be under 40 watts, according to one of the diagrams reprinted at <https://www.centauri-dreams.org/?p=10123> .

> **Comments:**
> **username_1**: The problem is, these small ships would be laser accelerated and have no means of slowing down.    Personally I think the very tiny laser accelerated ships are a bad idea cause they'd just fly through any solar system they are sent to in a few months.   Fly past a planet if they were lucky enough to get close to one in less than an hour and have great difficulty sending what little information they did collect back.   I'd rather send larger ships slower.  100 years, not 20 but much better.   https://www.scientificamerican.com/article/100-million-plan-will-send-probes-to-the-nearest-star1/  
> **username_5**: The plan outlined by the Breakthrough Starshot people (Stephen Hawking and Freeman Dyson are on board) is carefully planned to be theoretically accomplishable within a generation. They say that there are no "deal breakers" even though there are barriers to overcome. I would like to see the accelerating laser built in space, but was shot down by cost arguments. I'm sure that if anybody has better ideas, they are very open to suggestion. Personally, I would rather see a "fast flyby" mission than none at all. We probably want to swing by Proxima on the way, though.  

---

