Feasibility of Interstellar Probe Communication

Question

So, I understand that to transmit information across interstellar distances, you need to use a lot of power (or less bandwidth?), something for example a small space probe wouldn't exactly have.
If we knew exactly what signal to look for, could we detect and receive information from a probe we sent to, let's say, the nearest star system, or are the distances just too great? If so, how far could we possibly take it?

Answer 1

Overview

Having an interstellar probe brings up several major design issues. Two of the biggest are power and communication systems. Additional problems such as thermals, propulsion/trajectory are relatively trivial compared to the systems above.

---

Major Problematic Subsystems

Power: As solar intensity drops off at 1/r^2 anything beyond about 8 AU isn’t really feasible, thus solar panels are not a viable power generation option. Using RTG’s (radioisotope thermal generators) or similar power generation systems would be required. RTG’s (higher temperature gradient allows for much better efficiency and power generation) in deep space are very beneficial as you use deep space to increase the temperature gradient to help increase efficiencies. Depending on the RTG selected the half-life will change and can increase the overall mission duration. Ideally, multiple RTG’s or a duel string system would be required.

Communication: This is a tricky one, the further you travel the more power you need to transmit power and larger transmitter dish and ground station dish. Optical communication would probably not be feasible due to the vast distance and distortion of space dust/particles between you and the receiving station. To put things in perspective, the nearest star (assuming we would travel there) is 268,700 AU (Alpha Centauri). The satellite communication system would have to operate in the Milimeter band with frequencies of around 40-300 GHz (most spacecraft’s operate around 300 MHz to 40 GHz). With a higher antenna gain, a higher antenna diameter for ground station and transmitter (spacecraft side) is required. This would be easily be on the order of 100+ meters in diameter. A deployable antenna dish or assembling the dish in space might be a better option since current rockets would not be able to fit the payload in the launch vehicles fairing. All this is excluding the mass of the antenna, which is another issue of its own. There are other options such as setting up communication system relay to reduce size of a single antenna by increasing the number of hops the single takes.

Trajectory: Using a combination of electric propulsion systems and gravitation assists from the planets and the sun you could achieve a higher delta-V and delta-au per year.

System Architecture: Assuming you can generate around 100 au per year using a combination of electric propulsion and gravity assists maneuvers. It would take approximately 2,687 years to travel there. Let’s review a potential probe/spacecraft architecture. We will base our set values on the Decadal Ice Giants Study for a preliminary architecture.

Subsystems

• Attitude Control System (Values from Ice Giants Spacecraft Report)
  • Power Active: 50 W
  • Power Standby: 25 W
• Command and Data System (Values from Ice Giants Spacecraft Report)
  • Power Active: 60 W
  • Power Standby: 30 W
• Communication System
  • Discussion: Voyager uses approximately 22.6 Watts to transmit data at a range of 139.3 AU. Assuming similar transmitting power is required at 139 AU for the spacecraft with current ground stations, we would require 19.29x more power (436 Watts).
  • Power Active: 436 W
  • Power Standby: 0 W
• Mechanical System
• Power System (Values from Ice Giants Spacecraft Report)
  • Power Active: 24 W (Always active)
• Propulsion System
  • Power Active: 0 W (Assume off - propellant depleted)
• Thermal System (Values from Ice Giants Spacecraft Report)
  • Power Active: 25 W (Always active - a heater to keep components within operational temperature range)

Certain subsystems like propulsion can be turned off, other subsystems such as command and data handling, communication, and attitude control systems can go into a standby or low power mode periodically to preserve power.

1. Mode 1 – Active – Power Required: 159
2. Mode 2 – Transmitting – Power Required: 595 W
3. Mode 3 – Standby – Power Required: 104 W

So the minimal power required in 2,687 years is 104 W, however to transmit any sort of data back would require 595 W. Looking at Voyager’s three RTG’s, combined they generated 470 W at being of life. Our spacecraft would need 4 RTG’s, allowing us to generate 626 W. Factoring in RTG’s half-life, which has a half-life of 87 years makes things even more complicated and hits limitation of what is considered feasible. Such a mission would easily require 50+ RTG’s and a large supply of plutonium, which is a very limited resource. Factoring the overall mission duration, power required, and destination, most hardware systems would likely fail, power would be a major concern, and radiation would be another issue.

Conclusion

Based on the very preliminary overview, it is safe to say that such a system is not feasible with current technology. Hopefully in the future as new technologies are discovered and implemented into space systems such missions will become feasible.

---

Sources

Ice Giants Report - https://www.lpi.usra.edu/icegiants/mission_study/Full-Report.pdf

Alpha Centauri - https://en.wikipedia.org/wiki/Alpha_Centauri

Satellite Communications - http://space.au.af.mil/au-18-2009/au-18_chap14.pdf

Space Communication Calculations - http://www.spaceacademy.net.au/spacelink/spcomcalc.htm

RTG - https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator

Answer 2

This 2012 blog post reports progress from one study into this question. In summary, using a 1MW transmitter with a 40m dish on the interstellar probe, and an array of telescopes spread over a 10km or so circle on Earth, they estimated a data rate of a few hundred kilobits per second from a nearby star. They also suggest some more exotic solutions, such as using the sun's gravity to focus the incoming signal (which requires positioning the receiver very carefully about 550 AU from the sun in the opposite direction).