20

Between power limitations and aperture-limited diffraction I just can't see how it's possible.

uhoh
  • 148,791
  • 53
  • 476
  • 1,473
Schlusstein
  • 2,157
  • 9
  • 30
  • 1
    Besides the gigawatt laser array, I believe this is one of the problems they hope to be solved in the coming decades. – Phiteros Mar 06 '17 at 22:50
  • 2
    One "possible way" which I know they won't take, is a chain of relays. Launch new craft every year or so, forming a chain to relay radio signal back to Earth. Of course reliability problems are significant. – SF. Mar 06 '17 at 22:55
  • I've been wondering this for years. – Bear Mar 07 '17 at 13:19
  • @SF. Aren't they supposed to be solar powered? – Schlusstein Mar 07 '17 at 14:39
  • https://breakthroughinitiatives.org/Challenges/3 looks like they are wondering the same thing. This is one of those "fancy startup" things, where someone rich and famous proposes something ridiculous (getting to mars) then makes a bunch of money while doing something else (making electric cars) – Mohammad Athar Mar 07 '17 at 15:13
  • @Schlusstein: The current method assumes so - and simultaneously doesn't offer radio capability. They would definitely need to be solar-propelled, but they could carry own power to work in the interstellar space to provide power for the relay to work. – SF. Mar 07 '17 at 15:33
  • Personally, I believe it could work as a chemical battery. There are chemical processes that can be quite precisely timed, and say, lead to a break-up of a membrane separating two inert substances 40 years from now; upon mixing they start acting as a chemical battery, providing electricity. – SF. Mar 07 '17 at 15:36
  • As you note, aperture limited diffraction makes it impossible. To inspect nearby star systems, much better to spend the resources on a large space-based optical interferometer. – antlersoft Mar 07 '17 at 20:03
  • The chain of relays should be very dense, because every spacraft of the chain has only a very small antenna dish and very limited power for the transmitter. The effect of a large DSN antenna dish together with its very powerful transmitter on the maximum transmission distance is immense. If the failure of one spacecraft of the chain should be tolerated, the distance from craft to craft should be halved. Failure of more than one spacecraft could be tolerated if there is no failure of two adjacent crafts. – Uwe Mar 08 '17 at 15:30
  • @Uwe Given that they're solar powered, I wouldn't expect them to be able to generate enough power to communicate at all in interstellar space. I suppose RTGs could be made small enough in principle to fit on board, but I'm not convinced that they could be shielded well enough to not kill the electronics due to the extreme mass constraints. – Schlusstein Mar 08 '17 at 15:37
  • If the antenna dish is much smaller than usual due to mass constraints, the maximum distance for the chain of relays is even smaller. RTGs should be located outside for better radiative cooling of the RTG. – Uwe Mar 12 '17 at 15:29
  • And what would be the mass of such an RTG? – Schlusstein Mar 12 '17 at 16:37
  • 1
    @Schlusstein: RTGs can be a couple grams, heart pacemaker batteries are a couple grams and could be stripped further, to ~1-2 grams. Add another gram of a supercapacitor and you can get a ~50W burst for 10-20 seconds. Followed by a couple days of recharging... – SF. Mar 13 '17 at 12:18
  • That same GW laser array that launched them should be able to provide them with enough illumination to power solar cells for quite a distance. I'd always assumed that after the initial boost they would have a secondary guidance phase where they formed up into a monolithic structure which enabled a large phased RF array. Thinking outside the box, why not give them tunable surface polarization? They could blink low-bandwidth data to us by changing the polarity of reflected laser light. I'm full of speculation today. – Kengineer Sep 19 '17 at 21:40
  • Duplicate question: How could we hear back from Breakthrough Starshot?. Has a great answer, compliments the one here. – William R. Ebenezer May 25 '20 at 06:10
  • Can you explain how power limitations or aperture-limited diffraction might be problems, or should we just guess what you meant? – Robbie Goodwin Jan 11 '22 at 22:01

4 Answers4

9

In the video of the Keynote Speaker (and former NASA ARC director) Pete Worden (also here)'s talk Breakthrough Discuss 2016 – Breakthrough Starshot Challenges the challenges of using the deployed photon sail (for the initial laser-based acceleration) as a reflector "dish" for an optical transmission back to Earth is discussed a bit. The speaker felt that this might be one of the most difficult technological challenges in the short term, possibly with no current or near-future solution.

Getting a tiny spacecraft to somehow get the 300 angstrom thin membrane to form an accurate optical surface to collimate an optical transmission does indeed sound pretty difficult.

Let's use a burst of 100W and a 10 meter reflector and a distance of 5 light years (5E+16 meters).

800nm photons from a simple III-V solid state laser have an energy of about 2.5E-19 Joules each, so that's 4E+20 photons per second. An Airy disk half-width of 1.22 $\lambda$/d of 8E-08 radians means the footprint at Earth has a radius of 8E-08 x 5E+16 = 4E+09 meters. The radius of the Extremely Large Telescope's aperture is about 20 meters, so the ratio of the areas is 2.5E-17.

Since we're transmitting 4E+20 photons per second, that's about 10,000 photons per second at the receiver!

Your milage may vary, but in terms of energy, is quantitatively realistic.

However, from an engineering point of view, one would have to back off from this starting point. Forming a 10 meter parabola with $\lambda$/4 from a 300 angstrom film automatically using a chip-size satellite is quite a challenge - probably nobody really knows how it might be done at the moment.

Pointing is also important; what kind of star cameras could one include within a tiny chip that would provide 0.02 arcsec pointing accuracy is currently unknown.

So the real challenge will be building something even remotely close to this. In order to transmit meaningful data, millions of photons would have to be collected. Probably dedicated, lower-tech photon collectors in the microgravity of space would be better than expensive Earth-based telescopes.

The inaugural 2016 Breakthrough Discuss Workshop was held April 15-16, 2016 at Stanford University and sponsored by the Breakthrough Initiatives and the Stanford Physics Department. Breakthrough Discuss is a forum for scientists to present and discuss leading edge ideas in space exploration. The 2016 workshop focused on the following three areas:

  1. Optical SETI and the Detectability of Directed Energy Systems – Chaired by Jill Tarter

  2. Exo-planet Detection Programs Focused on Alpha Centauri – Chaired by Olivier Guyon

  3. The Sun as a Gravitational Lens – Chaired by Avi Loeb

Community
  • 1
uhoh
  • 148,791
  • 53
  • 476
  • 1,473
4

Power limitations are tricky, in particular choosing a power source whose mass isn't much more than that of the spacecraft. Nonrechargeable batteries have too short a shelf life. Internal combustion engines that small have been tried, and abandoned. A milligram-scale RTG + supercap, maybe with a Diamond battery, is the only way I know to sustain 40 Kbps to the terrestrial 30 m reflector mirror.

Details and references are here: https://space.stackexchange.com/a/17072/1235

Camille Goudeseune
  • 12,128
  • 1
  • 50
  • 83
  • You necessarily have a very large laser back on Earth which can point at the probes (since that's how you launched them). Surely that can power them. – Steve Linton Oct 26 '19 at 23:21
2

I think that the problem of communication could be done without needing an interstellar power source. What if the super powered telescopes of earth are used to retrieve signal from the ship, and the ships signals are how it reflects incoming beams from a high-powered emitter on earth. Or Since they are micro-ships, have a specific maneuver or pack pattern as a signal.

Java
  • 129
  • 10
1

Either we would need to launch waves of spacecraft to form a web of relay signals, or we would have to manually retrieve spacecraft sometime in the future. Power could easily be solved by using the same principle of 'torch ships' (that is, breaking down the materials of the ship into energy). However, any communication would be next to impossible.

Durin the Deathless
  • 89
  • 1
  • 5
  • 1
    It would be good to explain why "any communication would be next to impossible." What's the basis for the impossibility? If it's just that you can't think of a way to do it at the moment, that's probably not good enough to support this as a stackexchange answer. If there are some overwhelming technical challenges or laws of physics broken, you should state them explicitly. Otherwise this looks like a personal opinion rather than a proper answer. – uhoh Sep 19 '17 at 01:12
  • 2
    That is not what "torchship" means. – Nathan Tuggy Sep 21 '17 at 14:54
  • Spaceships as relays almost never works. The bigger dishes and better amplifiers you can have back on Earth almost always outweigh the advantages of any reasonable number of relays. – Steve Linton Oct 26 '19 at 23:20