NIAC’s award of a Phase I grant to study a ‘swarm’ mission to Proxima Centauri naturally ties to Breakthrough Starshot, which continues its interstellar labors, though largely out of the public eye. The award adds a further research channel for Breakthrough’s ideas, and a helpful one at that, for the NASA Innovative Advanced Concepts program supports early stage technologies through three levels of funding, so there is a path for taking these swarm ideas further. An initial paper on swarm strategies was indeed funded by Breakthrough and developed through Space Initiatives and the UK-based Initiative for Interstellar Studies.
Centauri Dreams readers are by now familiar with my enthusiasm for swarm concepts, and not just for interstellar purposes. Indeed, as we develop the technologies to send tiny spacecraft in their thousands to remote targets, we’ll be testing the idea out first through computer simulation but then through missions within our own Solar System. Marshall Eubanks, the chief scientist for Space Initiatives, a Florida-based startup focused on 50-gram femtosatellites and their uses near Earth, talks about swarm spacecraft covering cislunar space or analyzing a planetary magnetosphere. Eubanks is lead author of the aforementioned paper.
But the go-for-broke target is another star, and that star is naturally Proxima Centauri, given Breakthrough’s clear interest in the habitable zone planet orbiting there. The NIAC announcement sums up the effort, but I turn to the paper for discussion of communications with such swarm spacecraft. As Starshot has continued to analyze missions at this scale, it explores probes with launch mass on the scale of grams and onboard power restricted to milliwatts. The communications challenge is daunting indeed given the distances and power available.
If we want to reach a nearby star in this century, so the thinking goes, we should build the kind of powerful laser beamer (on the order of 100 GW) that can push our lightsails and their tiny payloads to speeds that are an appreciable fraction of the speed of light. Moving at 20 percent of c, we reach Proxima space within 20 years, to begin the long process of returning data acquired from the flybys of our probes. Eubanks and colleagues estimate we’ll need thousands of these, because we need to create an optical signal strong enough to reach Earth, one coordinated through a network that is functionally autonomous. We’re way too far from home to control it from Earth.
Image: Artist’s impression of swarm passing by Proxima Centauri and Proxima b. The swarm’s extent is ∼10 larger than the planet’s, yet the ∼5000-km spacing is such that one or more probes will come close to or even impact the planet (flare on limb). It should be possible to do transmission spectroscopy with such swarms. Green 432/539-nm beams are coms to Earth; red 12,000-nm laser beacons are for intra-swarm probe-to-probe coms. Conceptual artwork courtesy of Michel Lamontagne.
The engineering study that has grown out of this vision describes the spacecraft as being ‘operationally coherent,’ meaning they will be synchronized in ways that allow data return. The techniques here are fascinating. Adjusting the initial velocity of each probe (this would be done through the launch laser itself) allows the string of probes to cohere. The laser also allows clock synchronization, so that we wind up with what had been a string of probes traveling together through the twenty year journey. In effect, the tail of the string catches up with the head. What emerges is a network.
As the NIAC announcement puts it:
Exploiting drag imparted by the interstellar medium (“velocity on target”) over the 20-year cruise keeps the group together once assembled. An initial string 100s to 1000s of AU long dynamically coalesces itself over time into a lens-shaped mesh network 100,000 km across, sufficient to account for ephemeris errors at Proxima, ensuring at least some probes pass close to the target.
The ingenuity of the communications method emerges from the capability of tiny spacecraft to travel with their clocks in synchrony, with the ability to map the spatial positions of each member of the swarm. This is ‘operational coherence,’ which means that while each probe returns the same data, the transmission time is related to its position within the swarm. The result; The data pulses arrive at the same time on Earth, so that while the signal from any one probe would be undetectable, the combined laser pulse from all of them can become bright enough to detect over 4.2 light years.
The paper cites a ‘time-on-target’ technique to allow the formation of effective swarm topologies, while a finer-grained ‘velocity-on-target’ method is what copes with the drag imparted by the interstellar medium. This one stopped me short, but digging into it I learned that the authors talk about adjusting the attitude of individual probes as needed to keep the swarm in coherent formation. The question of spacecraft attitude also applies to the radiation and erosion concerns of traveling at these speeds, and I think I’m right in remembering that Breakthrough Starshot has always contemplated the individual probes traveling edge-on during cruise with no roll axis rotation.
Image; This is Figure 2a from the paper. Caption: A flotilla (sub-fleet) of probes (far left), individually fired at the maximum tempo of once per 9 minutes, departs Earth (blue) daily. The planets pass in rapid succession. Launched with the primary ToT technique, the individual probes draw closer to one another inside the flotilla, while the flotilla itself catches up with previously-launched flotillas exiting the outer Solar system (middle) ∼100 AU. For the animation go to https://www.youtube.com/watch?v=jMgfVMNxNQs (Hibberd 2022).
Figure 2b takes the probe ensemble into the Oort Cloud.
Image: Figure 2b caption: Time sped up by a scale factor of 30. The last flotilla launched draws closer to the earlier flotillas; the full fleet begins to coalesce (middle), now under both the primary ToT and secondary VoT techniques, beyond the Kuiper-Edgeworth Belt and entry into the Oort Cloud ∼1000–10,000 AU.
When we talk about using collisions with the interstellar medium to create velocities transverse to the direction of travel, we’re describing a method that again demands autonomy, or what the paper describes as a ‘hive mind,’ a familiar science fiction trope. The hive mind will be busy indeed, for its operations must include not just cruise control over the swarm’s shape but interactions during the data return phase. From the paper;
With virtually no mass allowance for shielding, attitude adjustment is the only practical means to minimize the extreme radiation damage induced by traveling through the ISM at 0.2c. Moreover, lacking the mass budget for mechanical gimbals or other means to point instruments, then controlling attitude and rate changes of the entire craft in pitch, yaw, roll, is the only practical way [to] aim onboard sensors for intra-swarm communications, interstellar comms with Earth and imagery acquisition / distributed processing at encounter.
I gather that other techniques for interacting with the interstellar medium will come into play in the NIAC work, for the paper speaks of using onboard ‘magnetorquers,’ an attitude adjustment mechanism currently in use in low-mass Cubesats in low Earth orbit. It’s an awkward coinage, but a magnetorquer refers to magnetic torquers or torque rods that have been developed for attitude control in a given inertial frame. The method works through interaction between a magnetic field and the ambient magnetic field (in current cases, of the Earth). Are magnetic fields in the interstellar medium sufficient to support this method? The paper explores the need for assessment.
A solid state probe has no moving parts, but it’s also clear that further simulations will explore the use of what the paper calls MEMS (micro-electromechanical systems) trim tabs that could be spaced symmetrically to provide dynamic control by producing an asymmetric torque. This sounds like a kludge, though one that needs exploring given the complexities of adjusting attitudes throughout a swarm. We’ll see where the idea goes as it matures in the NIAC phase. All this will be critical if we are to connect interswarm to create the signaling array that will bring the Proxima data home.
Interestingly, the kind of probes the paper describes may vary in some features:
We note for the record that although all probes are assumed to be identical, implicitly in the community and explicitly in the baseline study, there is in fact no necessity for them to be “cookie cutter” copies, since the launch laser must be exquisitely tunable in the first place, capable of providing a boost tailored to every individual probe. At minimum, probes can be configured and assigned for different operations while remaining dynamically identical, or they can be made truly heterogeneous wherein each probe could be rather different in form and function, if not overall mass and size.
There is so much going on in this paper, particularly the issue of the orbital position of Proxima b, which you would think would be known well enough by now (but guess again). The question of carrying enough stored energy for the two decade mission is a telling one. But the overwhelming need is to get information back to Earth. How data would be received from these distances has always bedeviled the Starshot idea, and having followed the conversation on this for some time now, I find the methods proposed here seriously intriguing. We’ll dig into these issues in the next post.
The paper is Eubanks et al., “Swarming Proxima Centauri: Optical Communication Over Interstellar Distances,” submitted to the Breakthrough Starshot Challenge Communications Group Final Report and available online.