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The FOCAL Radio Bridge

Getting a probe to another star is a big enough problem, but woven inextricably through it is the issue of communications. Adding payload steepens the propulsion curve in dramatic fashion, which is why recent thinking has dwelled so firmly on miniaturizing the spacecraft. Thus Breakthrough Starshot, which envisions payloads roughly on the order of a computer chip. No wonder, with spacecraft of that size, getting data back to Earth is such a daunting challenge.

Can gravitational lensing help? We’ve seen that the Sun’s mass shapes spacetime around it, bending light from targets on the other side so that electromagnetic waves come to a focal point about 550 AU out. The implications for imaging are under intense study at the Jet Propulsion Laboratory, where Slava Turyshev’s team, working with a Phase III NIAC grant, is exploring “Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravitational Lens Mission,” taking two prior studies, a Phase I and II at NIAC, forward in terms of imaging.

But if we can fight off solar corona effects and achieve a signal-to-noise ratio high enough — Turyshev believes this is possible — we might do more than reconstructing an exoplanet image with 25-kilometer scale surface resolution. Such an image would help us understand the target for future interstellar probes, but Claudio Maccone has been writing about using the same lensing effect to boost communications in dramatic fashion. And if Starshot’s payloads might have little more than a cellphone’s available power, such levels might prove workable.

The Gravitational Lens as Relay

How is this possible? There are a lot of issues here, but we can start with Bit Error Rate, which has to do with the quality of a signal. Specifically, BER is the number of erroneous bits received divided by the total number of bits transmitted. We can compare the Bit Error Rate across interstellar distances with and without the gravitational lens effect, as Maccone first did in a 2011 paper. I’m going to use that paper and his recent presentation to examine the results and what they mean for current thinking about missions.

Before we start miniaturizing, let’s assume a probe at Alpha Centauri communicating to Earth using a 12-meter antenna, a fairly conventional spacecraft working with a Deep Space Network-class antenna (a 70-meter dish) on the other end, trying to communicate over the Ka band (32 GHz), with a bit rate of 32 kpbs. These assumptions come from recent missions: The Ka band is the highest frequency used by Cassini, while the 32 kbps bit rate equals that of ESA’s Rosetta spacecraft. Maccone then goes on to assume transmitting power of 40 W.

The paper (citation below) is available in full text, so there is no need to go through the chain of reasoning in detail here. The point is that the signal is shown to be unusable. But matters change if we deploy a FOCAL mission to the Sun’s gravity lens. According to the paper’s equations, the same equipment now allows received power and acceptable Bit Error Rate at Alpha Centauri. Thus the figure below, which Maccone used in his online talk.

If we follow the equations through, we don’t begin to introduce significant errors until we’re fully 9 light years out, not far off the distance to Epsilon Eridani. By the time we get to a probe at a distance of 100 light years, we reach a level of Bit Error Rate high enough to swamp the dataflow with this equipment. A spacecraft using the Sun’s gravitational lens, then, can communicate effectively assuming present-day power levels and infrastructure with a probe at Alpha Centauri, making the target of 550 AU an enabler for data return from small payloads.

If Breakthrough Starshot had a gravitational lens relay of this kind, the daunting communications issue would be cast in a new light. Just how complicated communications matters are without a gravity lens relay is something I’ll soon explore with a look at the just released Request for Proposals from Breakthrough that deals explicitly with the question.

Enter the Radio Bridge

What is it that S.R. Hadden says in the film Contact? “The first rule of government spending: why build one, when you can build two, at twice the price?” This is where Claudio Maccone is suggesting a forward path for interstellar probes that looks at what we do next after our initial interstellar probes fly past their target. The Starshot model, for example, assumes a fleet of sail-driven craft flung past a nearby star like Proxima Centauri to collect and return an image of the planets there. This is a flyby concept. A few hours in-system and gone.

Flybys are first steps and, as New Horizons has recently shown so dramatically (not to mention a host of other craft, like Voyager in the past), we can collect invaluable data from them. But once we’ve achieved flybys around nearby stars, we’ll want to continue our explorations by establishing a robotic (or one day human, perhaps) presence there. Now we would like not just the relatively slow communications methods enabled by the gravitational lens of the Sun but a much faster capability. We achieve this by using a second lens, the one near our target star.

If we’re talking about a mission to Alpha Centauri, then we go to the gravitational lens of Centauri A because it has the highest mass of the three stars, and thus offers the highest gain. Here the FOCAL spacecraft would be placed at a minimum distance of 750 AU for communications and data retrieval from targets around any one of the three stars. We have a radio bridge with communications capabilities that can be achieved by no other means. Here is the image Maccone uses to illustrate the point, along with his own caption from the paper.

If we establish the radio bridge, our capabilities at error-free communications between the Sun and Alpha Centauri, assuming two 12-meter FOCAL antennae, become striking: The minimum transmitted power falls to 10-4 watts. One-tenth of a milliwatt does the job, creating error-free communications between spacecraft and Earth, meaning we have created a workable channel for future activities in the Alpha Centauri system, whether human or robotic.

To make it happen, an initial FOCAL mission becomes our communications relay, while a second FOCAL mission creates the radio bridge. Multiple human generations may separate the two projects; this is not a task for the impatient. But should we see it through, huge, power hungry communications systems aboard the spacecraft are not necessary, while communications with our growing infrastructure around nearby stars become robust and all but routine.

Building radio bridges is a huge challenge, assuming not only the capability of reaching and exploiting the Solar gravity lens, but eventually reaching another star not just with a flyby but with a craft capable of decelerating into the target system and positioning itself into the focal lens of the star there. The sequence may be straightforward: Use our Sun’s gravity lens to communicate with the first interstellar probes, then expand capability by setting up the second communications relay. But the implementation, like everything interstellar, pushes all our limits well beyond anything we have yet achieved. Which is why I love writing about all this.

Technologies both suggest and enable new uses that push beyond the currently possible. If there is one thing my continuing dreams of Alpha Centauri have taught me, it’s that thinking beyond our own personal limitations builds the groundwork for future generations.

The idea that we should think only of what we as individuals will benefit from misses the point of intellectual inquiry, and blunts the blade of discovery. So I applaud the efforts of those currently working the hard equations of reaching 550 AU, even though many of them will probably not see such a mission arrive. Being a part of the effort, emplacing cornerstones, is what counts.

The paper is Maccone, “Interstellar Radio Links Enhanced by Exploiting the Sun as a Gravitational Lens,” Acta Astronautica Vol. 68, Issues 1-2 (January-February 2011), pp. 76-84 (abstract/full text).


Comments on this entry are closed.

  • Alex Tolley December 14, 2020, 16:50

    Arthur C Clarke once made the analogy concerning SETI that we might be in the position of stone age islanders using drums for communication assuming they were alone because they could not hear huge drums beating beyond the horizon, even as radio wave communications were passing invisibly by. That has often been used to suggest that there are some other forms of communication that do not use em radiation that we have not been able to harness, or even discover yet.

    The radio bridge communication method is effectively just such an “invisible” communication mode but using perfectly ordinary technology. The only difficulty preventing our civilization from using it a century ago is the ability to reach our sun’s gravitational focus and positioning the receiver/transmitter appropriately.

    If galactic civilizations exist and are using this almost costless technique to communicate over interstellar distances, the low power transmissions would both be extremely low and too directional for us to detect. The “galactic club” could be alive and well, communicating across the immensity of space, while we can only detect silence.

    How would a new civilization break into that club? It might require some cruder, high power transmissions to announce oneself, and a similar one from a club member to indicate position before setting up a transmitter/receiver at the correct point in the sphere with a radius of 550 AU. Or perhaps the onus is on the new member to be able to search the sphere surface for a low power signal already being beamed at our sun for us to receive. If so, some future SETI kit needs to be placed on that sphere to search for signals being beamed at us. If we can detect them, that is perhaps proof of our worthiness to join the galactic club. A single receiver orbiting at 550 AU with the galactic disc behind the sun could take a very long time to complete a search. Therefore a lot of cheap receivers need to be sent to improve the search time, and as with current SETI, they must be able to scan the full radio spectrum suitable for the likely data rates. This could keep SETI occupied for millennia!

    In the meantime, the communication bridge lends itself to swarm probes to target stars. Each probe should perform a gravitational slingshot maneuver that allows both a data collection of the target exoplanet and then a course that sends it behind the star as seen from Earth before it transmits its data over the bridge. Conceivably a chain of such probes could make many observations over time. In our solar system, a similar chain of the craft could be sent to the required focus to receive the exoplanet transmissions. This would obviate any requirement to loiter allowing high velocities to reach the focal position and each subsequent probe taking over the receiving as the prior one leaves the optimum receiving position. As 550 AU is about 3 light days away, a receiver traveling at 0.2c takes just about 15 days to reach that position. Send a probe every few days could create the requirements of reception continuity needed to receive the transmissions from the target star.

  • Harold Shaw December 14, 2020, 23:32

    The last paragraphs resonated with me. There is something about solar focal missions that inspires. They exemplify the premise of astronomical missions as cathedral building. Imagine a mission that, moving from target to target, delivers the same repeated anticipation of a long term comet.

    So people could be whispering past us using pennies to send messages. Certainly puts the FERMI paradox in perspective. A communication network would also likely be a surveillance network. We’ve been talking about solar focuses as telescopes. This network would conceivably include black holes and other ideal nodes. There would likely be more than one hardware network. For a K2 people, a network wouldn’t be that expensive to build and factions would likely appear.

    Give us 10,000 years and we will be ready to start a galactic network ourselves. All we need is mature biotech paired with industrial material technology. A people like us wouldn’t need long to build a galaxy spanning surveillance network. A people capable of navigating Deep Time, hopefully us, would have to benefit from a network. I don’t agree with assumptions that a people will spread though a galaxy, thoroughly eliminating the possibility of other people. A people will explore though. I don’t see how a people like us doesn’t build this network.

  • Laura December 15, 2020, 1:05

    Similar to Arthur C Clarke’s analogy of prehistoric people using drums to send messages to probe for far-away others mentioned by Alex Tolley, I’ve often used the analogy of sending smoke signals and watching for responding smoke signals on the Moon. Using radio signals to search for ETI may be as fruitless (though I support it).

    Radio signals have the obvious problem of being so terribly slow given the scale of the galaxy and the likely distances between any technological civilizations. Quantum entanglement teases us with idea of instantaneous communication, as does Stephan Wolfram’s idea about space-time being discrete rather than continuous, with individual space-time points having no location coordinates; only relations to other points. All quite speculative, but who knows what we’ve yet to discover.

    In the meantime, the idea of radio relays using gravitational lenses is an intriguing potential solution to the problems of communication with Starshot sized micro-probes. With the notable advantage of using known physics!

  • Afterthought December 15, 2020, 11:50

    We need far better astronomy of alphacen the we have now. With a better map of the destination we can answer many questions and plan. Also, best to build the transmitter upon arrival.

    For that one has to slow down. I favor sending extra mass that you fire forward near speed of light to slow a ship going say 1% speed of light. Isnt parker solar probe 1/17th that speed? In 100 years should be able reach 1% c.

  • Michael December 15, 2020, 12:14

    If we had many mirrors traveling along the light paths and a central processing receiver we could start to get images straight away with great resolution and data but low magnification. As time goes on and the mirrors converge we will get the final high magnification images.

  • Mike Serfas December 15, 2020, 15:06

    If this is a cathedral project, the motivation might be painted with a semi-religious character. After all, one of the Fermi Paradox solutions frequently suggested is that a Galactic Federation has a presence on Earth waiting, watching for humanity to cross some vital bureaucratic threshold before initiating contact. What if, instead of the construction of a warp drive, it happened to be the initiation of a *plan* to bring living organisms to the nearest star? So long as all the ducks are in a row for the report – the method of propulsion, the means of communication, the microbes to be sent – and some “material support” had been given, that might be sufficient for them to unveil themselves, with all the wonders and horrors that entails. Presumably their standards are more strict than for modern terrorism prosecutions, or humanity would have been contacted eons ago, but perhaps centuries of construction work do not have to be put in before contact is initiated. (Admittedly, this can of worms is probably best left shut, as it might start with good intentions and end with cultists rocking back and forth chanting prayers to the aliens while they wait for their dose of Kool-Aid, but I plead not guilty by reason of comedy!)

  • Adam December 15, 2020, 17:23

    Hi Paul
    Claudio’s ideas are fascinating and provocative. Thinking of a Galactic Network, another general relativistic effect to utilize is precession. Roger X Lenard proposed a cosmological mapping mission to Sirius B, to use the rapid precession experienced to scan the sky via a gravity lens scope in a highly elliptical orbit. Or (my proposal) scanning the sky for scheduled data packets in a delay tolerant network that links the Galaxy. Al Jackson has also proposed using white dwarfs as lenses for a neutrino communication network, though the neutrino beams ‘seen’ by the receivers would be pencil thin and require very accurate coordination by source and receiver.

    Using A-star-white dwarf binaries, like Sirius, also has the advantage that star-sails can access them at ~0.1-0.15 c for minimal additional propulsion. This would allow a Network to be installed unobtrusively, which might feed into the Fermi Paradox – we don’t see Them because They don’t want us to.

    • Michael December 16, 2020, 0:15

      More likely than neutrino comms is x-ray and gamma ray comms via free electron laser systems. The dispersion is vanishingly small at those wavelengths and should allow very high rates of comms by using effectively the whole area around the sun.

  • Alexander December 16, 2020, 6:31

    So if I understand correctly, for this to work the receiver needs to be in a particular region of space at 550 AU on the side of the sun opposite to alpha centauri.

    The trouble is, unless your using massive amounts of fuel, the receiver wouldn’t stay static. It would orbit the sun, and leave the gravitational lens! For a distance that far away, its orbit would be impractically long. For example comet Hale-Bopp’s aphelion is 370.8 au, its parahelion is a bit less than 1 AU and its period is around 2500 years.

    For this to be practical we would need numerous receivers calibrated to have orbits that put them at 550 AU at different times in the right place relative to alpha centauri so there is always an object there…

  • Michael December 16, 2020, 8:34

    Its a focal line, as long as we keep going we are ok, going into orbit is not needed.

    • Harold Shaw December 16, 2020, 15:25

      The sun and any target will be moving, a telescope would have to orbit our Sun to keep the target in view. If we are discussing the possibility of a vehicle moving from one target to another, then it sounds like keeping pace with one target is possible.

  • Project Studio December 16, 2020, 11:01

    Wondering if this discovery could be exploited by a focal mission?

    • Adam December 17, 2020, 5:24

      The super highways are somewhat slower than a quick trip to the SGL needs, but they minimize the total energy needed to make trips between the planets – if you’re patient.

  • Michael December 23, 2020, 16:03

    Perhaps we could have large relays between the stars to redirected signals by having them sit at the gravity stability points proportional to the stars masses, a sort of huge galatic exchange.