Centauri Dreams
Imagining and Planning Interstellar Exploration
SETI and Signal Leakage: Where Do Our Transmissions Go?
The old trope about signals from Earth reaching other civilizations receives an interesting twist when you ponder just what those signals might be. In his novel Contact, Carl Sagan has researchers led by Ellie Arroway discover an encrypted TV signal showing images from the Berlin Olympics in 1936. Thus returned, the signal announces contact (in a rather uncomfortable way). More comfortable is the old reference to aliens watching “I Love Lucy” episodes in their expanding cone of flight that began in 1951. How such signals could be detected is another matter.
I’m reminded of a good friend whose passion for classical music has caused him to amass a collection of recordings that rival the holdings of a major archive. John likes to compare different versions of various pieces of music. How did Beecham handle Delius’ “A Walk in the Paradise Garden” as opposed to Leonard Slatkin? Collectors find fascination in such things. And one day John called me with a question. He was collecting the great radio broadcasts that Toscanini had made with the NBC Symphony Orchestra beginning in 1937. His question: Are they still out there somewhere?
Image: A screenshot of Arturo Toscanini from the World War II era film ‘Hymn of the Nations,’ December, 1943. Credit: US Office of War Information.
John’s collection involved broadcasts that had been preserved in recordings, of course, but he wanted to know if somewhere many light years away another civilization could be listening to these weekly broadcasts, which lasted (on Earth) until 1954. We mused on such things as the power levels of such signal leakage (not to mention the effect of the ionosphere on AM radio wavelengths!), and the fact that radio transmissions lose power with the square of distance, so that those cherished Toscanini broadcasts are now hopelessly scattered. At least John has the Earthly versions, having finally found the last missing broadcast, making a complete set for his collection.
Toscanini was a genius, and these recordings are priceless (John gave me the complete first year on a set of CDs – they’re received a lot of play at my house). But let’s play around with this a bit more, because a new paper from Reilly Derrick (UCLA) and Howard Isaacson (UC-Berkeley) tweaks my attention. The authors note that when it comes to the leakage of signals into space, a 5 MW UHF television picture has effective radiated power of 5 x 106 W and an effective isotropic radiated power (EIRP) of approximately 8 x 106 W. ERP tells us the strength of an actual signal in a specific direction, while EIRP describes an isotropic ideal antenna.
It’s interesting to see that a much more powerful signal than our TV broadcasts comes from the Deep Space Network as it communicates with our spacecraft. Derrick and Isaacson say that DSN transmissions at 20 kW power have an EIRP of 1010 W, making such signals (103 times higher than leakage, thus more likely to be detected. With this in mind, the authors come up with a new way to identify SETI targets; viz., find stars that are within the background of sky positions occupied by our spacecraft at such times as the transmissions to them from the DSN were active.
Ingenious. Remember that identifying interesting SETI targets has led us to study such things as the ‘Earth Transit Zone,’ which would identify stars so aligned as to be able to see transits of Earth across the Sun. In a similar way, we can study stars whose ecliptic planes align with our line of sight to intercept possible communications in those systems. It turns out that most of our outbound radio traffic to spacecraft occurs near the ecliptic, and the assumption would be that other civilizations might do the same.
Image: With the Pluto/Charon flyby, we have performed reconnaissance on every planet and dwarf planet in our solar system, with the help of the Deep Space Network. The DSN comprises three facilities separated by about 120 degrees around the Earth: Goldstone, California; near Madrid, Spain; and near Canberra, Australia. Above is the 70-meter Deep Space Station 14 (DSS-14), the largest Deep Space Network antenna at the Goldstone Deep Space Communications Complex near Barstow, California. Credit: NASA/JPL-Caltech.
So what Derrick and Isaacson are doing is an extension of this earlier work (see SETI: Knowing Where to Look and Seeing Earth as a Transiting World for some of the archival material I’ve written on these studies). The authors want to know where our DSN signals went after they reached our spacecraft, and that means building an ephemeris for our deep space missions that are leaving or have left the Solar System: Voyagers 1 and 2, Pioneer 10, Pioneer 11, and New Horizons. They then consult the positions of over 300,000 stars within 100 parsecs as drawn from the Gaia Catalogue of Nearby Stars, checking to ensure that the stars they identify will not leave the radius of the search in the time it takes for the cone of transmission to reach them.
The Voyagers were launched in 1977, with both of them now outside the heliosphere. As to the others, Pioneer 10 (launched in 1972) crossed the orbit of Neptune in 1983, while Pioneer 11 (launched in 1973) crossed Neptune’s orbit in 1990. New Horizons, launched in 2006, crossed Neptune’s orbit in 2014. All these craft received or are receiving DSN transmissions, though the Pioneers have long since gone silent. Their ephemerides thus end on the final day of communication, while the other missions are ongoing.
The universe is indeed prolific – the Gaia Catalogue includes 331,312 stars within 100 parsecs, and as the authors note, it is complete for stars brighter than M8 and contains 92 percent of the M9 dwarfs in this range. We learn that Pioneer 11 – I should say the signals sent to Pioneer 11 and thus beyond it – encounters the largest group of stars at 411, while New Horizons has the least, 112. The figures on Voyager 1, 2 and Pioneer 10 are 289, 325, and 241 stars, respectively. Transmissions to Voyager 2, Pioneer 10 and Pioneer 11 have already encountered at least one star, while Voyager 1 and New Horizons signals will encounter stars in the near future. From the paper:
Transmissions to Voyager have already encountered an M-dwarf, GJ 1154, and a brown dwarf, Gaia EDR3 6306068659857135232. Transmissions to Pioneer 10 have encountered a white dwarf, GJ 1276. Transmissions to Pioneer 11 have encountered a M-dwarf, GJ 359. We have shown that the radio transmissions using the DSN are stronger than typical leakage and are useful for identifying good technosignature targets. Just as the future trajectories of the Voyager and Pioneer spacecraft have been calculated and their future interactions with distant stars cataloged by Bailer-Jones & Farnocchia (2019), we now also consider the paths of DSN communications with those spacecraft to the stars beyond them.
The paper provides a table showing stars encountered by transmissions to the spacecraft sorted by the year we could expect a return transmission if a civilization noted them, along with data on the time spent by the star within the transmission beam. DSN transmissions are several orders of magnitude smaller in EIRP than the Arecibo planetary radar (1012 W), but it’s also true that the positions of the spacecraft, and hence background stars during transmissions, are better documented than background stars that would have encountered the Arecibo signals.
So what we have here is a small catalog of stars whose systems are in the background of DSN transmissions, and the dates when each star will encounter such signals. The goal is to offer a list of higher value targets for scarce SETI time and resources, especially concentrating on those stars nearest the Sun where civilizations may have noticed us. I don’t hold out high hopes for our receiving a signal from any of these stars, but find the process fascinating. Derrick and Isaacson offer a new way of considering our position in the galaxy in relation to the stars that surround us.
The paper is Derrick & Isaacson, “The Breakthrough Listen Search for Intelligent Life: Nearby Stars’ Close Encounters with the Brightest Earth Transmissions,” available as a preprint. Thanks to my friend Antonio Tavani for the pointer to this work. The Bailer-Jones & Farnocchia paper mentioned above is likewise interesting. It’s “Future Stellar Flybys of the Voyager and Pioneer Spacecraft,” Research Notes of the AAS Vol. 3, No. 4 (April 2019) 59 (full text).
Probing the Shifting Surface of Icy Moons
In celebration of the recent JUICE launch, a few thoughts on what we’re learning about Ganymede, with eight years to go before the spacecraft enters the system and eventually settles into orbit around the icy moon. Specifically, let’s consider a paper just published in Icarus that offers results applicable not just to Ganymede but also Europa and Enceladus, those fascinating and possibly life-bearing worlds. We learn that when we look at the surface of an icy moon, we’re seeing in part the result of quakes within its structure caused by the gravitational pull of the parent planet.
Image: ESA’s latest interplanetary mission, Juice, lifted off on an?Ariane 5 rocket?from?Europe’s Spaceport?in French 09:14 local time/08:14 EDT on 14 April 2023 to begin its eight-year journey to Jupiter, where it will study in detail the gas giant planet’s three large ocean-bearing moons: Ganymede, Callisto and Europa. Credit: ESA.
The Icarus paper homes in on the link between such quakes, long presumed to occur given our understanding of gravitational interactions, and the landslides observable on the surface of icy moons. It’s one thing to tag steep ridges surrounded by flat terrain as the result of ‘ice volcanoes’ spouting liquid, but we also find the same result on moons whose surface temperature makes this explanation unlikely.
Thus the new work, described by lead author Mackenzie Mills (University of Arizona), who analyzed the physical pummeling icy terrain takes during tidally induced moonquakes:
“We found the surface shaking from moonquakes would be enough to cause surface material to rush downhill in landslides. We’ve estimated the size of moonquakes and how big the landslides could be. This helps us understand how landslides might be shaping moon surfaces over time.”
Image: NASA’s Galileo spacecraft captured this image of the surface of Jupiter’s moon Ganymede. On Earth, similar features form when tectonic faulting breaks the crust. Scientists modeled how fault activity could trigger landslides and make relatively smooth areas on the surfaces of icy moons. Credit: NASA/JPL/Brown University.
This is ancient terrain indeed, located within Ganymede’s Nicholson Regio near the border with Harpagia Sulcus. Which leads to a quick digression: One of the pleasures of discovery is the growing familiarity with the names of surface features we are beginning to see close up. We’ve known enough about Ganymede thanks to craft in the system (the above image is from the Galileo probe) to have already named many features, but with New Horizons we were naming as we went, seeing surface detail for the first time. Ponder how familiar we will become with the surface features of Ganymede once JUICE settles into its multi-year orbit around the moon. We’ll be tossing off references to Nicholson Regio with ease.
As to the latter, the terrain is ancient indeed, heavily cratered, and as you can see, riddled with steep slopes and cliffs (scarps) causing crustal fracturing. We’re seeing frozen geological history here, a useful pointer to how moon and planet have interacted over the aeons, and information which may tell us about Ganymede’s interior structure when complemented by the data we can expect from JUICE. Here the scarps form a series of blocks that delineate the boundary between dark and light terrain.
The image in question covers approximately 16 by 15 kilometers, and was taken on May 20, 2000 at a range of just over 2000 kilometers. It’s been some time since I’ve written about Galileo imagery used for anything other than the study of Europa, but of course the craft gave us priceless data about the entire Jovian moon system despite its high-gain antenna problems. Here the resolution is 20 meters per pixel. Below is another Galileo snapshot, this one of Europa and likewise showing scarp formations.
Image: An image of Jupiter’s moon Europa captured in the 1990s by NASA’s Galileo shows possible fault scarps adjacent to smooth areas that may have been produced by landslides. Credit: NASA/JPL-Caltech.
We’re looking at what appear to be fault features, scarps adjacent to much smoother areas. Is this the result of material cascading out into surrounding terrain as the result of a landslide? Co-author Robert Pappalardo (JPL) notes the likelihood, even when we’re talking about much smaller celestial objects than Ganymede or Europa. Much studied Enceladus, in fact, has a mere 3% of the surface area of Europa:
“It was surprising to find out more about how powerful moonquakes could be and that it could be simple for them to move debris downslope. Because of that moon’s small gravity, quakes on tiny Enceladus could be large enough to fling icy debris right off the surface and into space like a wet dog shaking itself off.”
The paper underlines the point:
By measuring scarp dimensions, we aim to better understand the formation of faults and associated mass wasted deposits, given abundant evidence of past and/or recent tectonic and seismic environments on these icy worlds. For studied scarps, we estimate a moonquake moment magnitude range Mw = 4.0–7.9. On Earth, quakes of similar magnitude are the middle and upper end of the log-based magnitude scale and commonly cause significant destruction, including causing mass movements such as landslides. Occurrence of similarly large quakes on icy satellites, which have surface gravities much less than Earth, implies that such quakes could induce significant seismic effects.
You can imagine how much JUICE and Europa Clipper will help in the decoding of such surface features, with sharp improvements in the resolution of our imagery and the prospect of stereo imaging along with subsurface radar sounding deployed for Ganymede and Europa. Thus we build our library of information about the geological processes at work in such exotic venues, and also learn about whether or not their surfaces continue to be active. The nature of the ice shell on Europa is a prime science objective for Europa Clipper, providing further information about the ocean beneath.
The paper is Mills et al., “Moonquake-triggered mass wasting processes on icy satellites,” Icarus Vol. 399 (15 July 2023), 115534 (full text).
HIP 99770 b: Astrometry Bags a Directly Imaged Planet
It took a combination of astrometry and direct imaging to nail down exoplanet HIP 99770 b in Cygnus, and that’s a tale that transcends the addition of a new gas giant to our planetary catalogs. Astrometry measures the exact position and motion of stars on the sky, so that we are able to see the influence of an as yet unseen planet. In this work, astrometrical data from both the ESA Gaia mission and the earlier Hipparcos mission flag a world that is directly imaged by the Subaru Telescope extreme adaptive optics system, which enabled its near-infrared CHARIS spectrograph to see the target.
Supporting work at the Keck Observatory using its Near-Infrared Camera and Keck II adaptive optics system allowed in combination with the CHARIS spectrum the discovery of the presence of water and carbon monoxide in the atmosphere, while the temperature was shown to be about ten times hotter than that of Jupiter. The joint measurements revealed a planet some 14-16 times the mass of Jupiter, in a 16.9 AU orbit around a star with twice the Sun’s mass and 13.9 times its luminosity.
Thayne Currie (Subaru Telescope) is lead author of the study:
“Performing both direct imaging and astrometry allows us to gain a full understanding of an exoplanet for the first time: measure its atmosphere, weigh it, and track its orbit all at once. This new approach for finding planets prefigures the way we will someday identify and characterize an Earth-twin around a nearby star.”
Image: Infrared image of HIP 99770 taken by the Subaru Telescope. The bright host star at the position marked with * is masked. The dashed ellipse shows the size of Jupiter’s orbit around the Sun for scale. The arrow points to the discovered extrasolar planet HIP 99770 b. Credit: T. Currie/Subaru Telescope, UTSA.
We’ve retrieved direct images of gas giants before, including massive planets around HR8799, the first such worlds detected with the method, relying on advances in adaptive optics systems for ground-based telescopes. But without astrometrical data, astronomers selected targets based on properties like age and distance, producing a small harvest of exoplanets. In this work, astrometry is first used to reveal a likely planetary system by virtue of its star’s motion, allowing the follow-up discovery.
The direct imaging program at the Subaru Telescope (like Keck, at the summit of Mauna Kea in Hawai?i) relies on the extreme adaptive optics system called SCExAO, which filters out the turbulence caused by Earth’s atmosphere. Astrometrical data from Gaia identify stars with telltale gravitational influences that can then be imaged, an approach far more likely to produce exoplanet results than the kind of ‘blind surveys’ earlier in use. One day we may well be using observatories like Earth-based Extremely Large Telescopes or a space observatory like the projected Habitable Worlds Observatory to image an Earth-like world. Thus Motohide Tamura (University of Tokyo):
“The indirect detection method will point us to a star around which a rocky, terrestrial planet could be imaged. Once we know when to look, we hope to learn whether this planet has an atmosphere compatible with life as we know it on Earth.”
The discovery is reported in Currie et al., “Direct imaging and astrometric detection of a gas giant planet orbiting an accelerating star,” Science Vol. 380, Issue 6641 (13 April 2023), 198-293 (abstract).
Go JUICE
Take a look at our missions to Jupiter in context. The image below shows the history back to 1973, with the launch of Pioneer 10, and of course, the Voyager encounters. We also have the flybys by Ulysses, Cassini and New Horizons, each designed for other destinations, for Jupiter offers that highly useful gravitational assist to help us get places fast. JUICE (Jupiter Icy Moons Explorer) joins the orbiter side of the image tomorrow, with launch aboard an Ariane 5 from Kourou (French Guiana) scheduled for 1215 UTC (0815 EDT) on Thursday. You can follow the launch live here or here.
The first gravitational maneuver will be in August of next year with a Lunar-Earth flyby, followed by Venus in 2025 and then two more Earth flybys (2026 and 2029) before arrival at Jupiter in July of 2031. I’ve written a good deal about both Europa Clipper and JUICE in these pages and won’t go back to repeat the details, but we can expect 35 icy moon flybys past Europa, Ganymede and Callisto before insertion into orbit at Ganymede, making JUICE the first mission that will go into orbit around a satellite of another planet. Needless to say, we’ll track JUICE closely in these pages.
Image: Ariane 5 VA 260 with JUICE, start of rollout on Tuesday 11 April. Credit for this and the above infographic: ESA.
Uranus: Diamond Rain, Bright Rings
Thinking about the ice giants, as I have been doing recently in our look at fast mission concepts, reminds me of the ‘diamond rain’ notion that has grown out of research into experiments with the temperatures and pressures found inside worlds like Uranus and Neptune. The concept isn’t new, but I noted some months ago that scientists at the Department of Energy’s SLAC National Accelerator Laboratory had been studying diamond formation in such worlds in the presence of oxygen. Oxygen, it turns out, makes it more likely that diamonds form that may grow to extreme sizes.
So let me turn back the clock for a moment to last fall, when news emerged about this exotic precipitation indicating that it may be more common than we had thought. Using a material called PET (polyethylene terephthalate), the SLAC researchers created shock waves within the material and analyzed the result with X-ray pulses. The scientists used PET because of its balance between carbon, hydrogen and oxygen, components more closely mimicking the chemical composition of Neptune and Uranus.
While earlier experiments had used a plastic material made from hydrogen and carbon, the addition of oxygen made the formation of diamonds more likely, and apparently allowed them to grow at lower temperatures and pressures than previously thought possible. The team, led by Dominik Kraus (SLAC/University of Rostock), suggests that such diamonds under actual ice giant conditions might reach millions of carats in weight, forming a layer around the planetary core. Silvia Pandolfi, a SLAC scientist involved in this work, was quoted in a SLAC news release last September:
“We know that Earth’s core is predominantly made of iron, but many experiments are still investigating how the presence of lighter elements can change the conditions of melting and phase transitions. Our experiment demonstrates how these elements can change the conditions in which diamonds are forming on ice giants. If we want to accurately model planets, then we need to get as close as we can to the actual composition of the planetary interior.”
Image: Studying a material that even more closely resembles the composition of ice giants, researchers found that oxygen boosts the formation of diamond rain. The team also found evidence that, in combination with the diamonds, a recently discovered phase of water, often described as “hot, black ice” could form. Credit: Greg Stewart/SLAC National Accelerator Laboratory.
Diamond rain is a startling concept, hard to visualize, and given the possibility that ice giants may be one of the most common forms of planet, the phenomenon may be occurring throughout the galaxy. Something to ponder as we look at the new image from Uranus just in from the James Webb Space Telescope, which highlights the planet’s rings as never before, while also revealing features in its atmosphere. The rings themselves have only rarely been imaged, but have been seen through Voyager 2’s perspective and through the adaptive optics capabilities of the Keck Observatory.
The brightness of the rings is striking, the result of the telescope’s Near-Infrared Camera (NIRCam) working through filters at 1.4 and 3.0 microns, shown here in blue and orange. The Voyager 2 imagery was as featureless as Voyager 1’s image of Titan, showing a lovely blue-green orb in visible wavelengths, but the power of working in the infrared is clear with the JWST results. Note the brightening at the northern pole (Uranus famously lies on its side almost 90 degrees from the plane of its orbit). This ‘polar cap’ formation appears when it is summer at the pole and disappears in the fall.
Image: This zoomed-in image of Uranus, captured by Webb’s Near-Infrared Camera (NIRCam) Feb. 6, 2023, reveals stunning views of the planet’s rings. The planet displays a blue hue in this representative-color image, made by combining data from two filters (F140M, F300M) at 1.4 and 3.0 microns, which are shown here as blue and orange, respectively. Credit: NASA, ESA, CSA, STScI. Image processing: J. DePasquale (STScI).
A few other features emerge here beyond the edge of the cap, including a second bright cloud at the left limb of the planet that seems to be related to storm activity. As to Uranus’ 13 known rings, 11 of them appear in the image. The other two, quite faint, are more visible, according to JWST scientists, during ring-plane crossings, which is a time in the planetary orbit when we see the rings edge-on. The Hubble instrument first discovered them during the last crossing, in 2007; the next will occur in 2049.
We’re dealing here with a brief exposure (12 minutes), but even so, a number of the planet’s moons can be found in the wider view shown below. Looking at this oddball system, I have to wonder whether the idea of a giant impact knocking it onto its side holds water. Can we get this result from resonance effects and the gravitational influence of the gas giants through periods of migration? The fact that the question can even be asked highlights how little we know about this particular ice giant. And whatever the cause, imagine a world where the Sun disappears for 42 years, a world of water, methane and ammonia, a rocky core and perhaps a rain of diamonds.
Image: This wider view of the Uranian system with Webb’s NIRCam instrument features the planet Uranus as well as six of its 27 known moons (most of which are too small and faint to be seen in this short exposure). A handful of background objects, including many galaxies, are also seen. Credit: NASA, ESA, CSA, STScI. Image processing: J. DePasquale (STScI).
As I’ve mentioned before, a mission to Uranus including an orbiter has been identified as a priority in the 2023-2033 Planetary Science and Astrobiology decadal survey. A flagship mission of Cassini-class at Uranus would be a great boon to science, but the suspicion grows that before it can fly, we’ll have learned how to reach the ice giants faster, and with mission strategies far different from those used for Cassini.
The paper on diamond rain is Zhiyu He et al., “Diamond formation kinetics in shock-compressed C?H?O samples recorded by small-angle x-ray scattering and x-ray diffraction,” Science Advances Vol. 8, Issue 35 (2 September 2022). Full text.
Self-Assembly: Reshaping Mission Design
It’s interesting to contemplate the kind of missions we could fly if we develop lightweight smallsats coupled with solar sails, deploying them in Sundiver maneuvers to boost their acceleration. Getting past Voyager 1’s 17.1 kilometers per second would itself be a headline accomplishment, demonstrating the feasibility of this kind of maneuver for boosting delta-v as the spacecraft closes to perhaps 0.2 AU of the Sun before adjusting sail attitude to get maximum acceleration from solar photons.
The economic case for smallsats and sails is apparent. Consider The Planetary Society’s LightSail-2, a solar sail in low Earth orbit, which demonstrated its ability to operate and change its orbit in space for multiple years before reentering Earth’s atmosphere in November of 2022. Launched in 2018, LightSail-2 cost $7 million. NASA’s Solar Cruiser, a much larger design still in development despite budging hiccups, weighs in at $65 million. Slava Turyshev and team at the Jet Propulsion Laboratory independently verified a cost model, with the help of Aerospace Corporation, of $11 million for a one-year interplanetary flight based on their Technology Demonstrator design.
Those numbers go up with the complexity of the mission, but can be reduced if we take advantage of the fact that spacecraft like these can be repurposed. A string of smallsat sailcraft sent, for example, to Uranus to conduct flybys of the planet, its moons and rings, would benefit from economies of scale, with successive missions to other outer system targets costing less than the ones that preceded them. Here the contrast between dedicated flagship missions (think Cassini or the Decadal Suevey’s projected Uranus Orbiter) could not be greater. Instead of a separately developed spacecraft for each destination, the modular smallsat/sail model creates a base platform allowing fast, low-cost missions throughout the Solar System.
To the objection that we need orbiters at places like Uranus to get the best science, the answer can only be that we need both kinds of mission if we are not to bog down in high-stakes financial commitments that preclude targets for decades at a time. Of course we need orbiters. But in between, the list of targets for fast flybys is long, and let’s not forget the extraordinary range of data returned by New Horizons at Pluto/Charon and beyond. As the authors of the recent paper from the JPL team note, heliophysics can benefit from missions sent to various directions in the heliosphere:
The shape of the heliosphere and the extent of its tail are subject to debate and the new model of the heliosphere—roughly spherical with a radius of ?100 AU—needs confirmation. Of course, every mission out to >100 AU will test it, but a series of paired missions (nose and tail, and in perpendicular directions) would provide a substantial improvement in our understanding of ISM/solar wind interactions and dynamics. High-velocity, low-cost sailcraft could probe these questions related to the transition region from local to pristine ISM sooner and at lower cost than competing mission concepts. Since the exact trajectory is not that crucial, this would also provide excellent opportunities for ad hoc trans-Neptunian object flybys.
Image: This is Figure 5 from the paper. Caption: New paradigm – fast, low-cost, interplanetary sailcraft with trajectories unconstrained to the ecliptic plane. Note the capability development phases from TDM (at 5–6 AU/yr) to the mission to the focal region of the SGL (20–30 AU/yr). Credit: Turyshev et al.
What I see emerging, however, is a new model not just for flyby missions but for the kind of complicated mission we’ve gotten so much out of through spacecraft like Cassini. We are on the cusp of the era of robotic self-assembly, which means we can usefully combine these ideas. Ten fast smallsats capable of flying considerably faster than anything we’ve flown before can, in this vision, self-assemble into one or more larger craft enroute to a particular destination. The Solar Gravitational Lens mission as designed at JPL relies on self-assembly to achieve the needed payload mass and also draws on the ability of smallsats with sails to achieve the needed acceleration.
We can trace robotic self-assembly all the way back to John von Neumann’s self-replicating probes, but as far as I know, it was Robert Freitas who in 1980 first took the idea apart in terms of a serious engineering study. Freitas applied self-assembly to a highly modified probe based on the Project Daedalus craft. Freeman Dyson considered robotic methods using robot swarms to build large structures and also proposed his famous ‘Astrochicken,’ a 1 kg self-replicating automaton that was part biological and was conceived as a way of exploring the Solar System. Eric Drexler is well known for positing nanomachines that could build large structures in space.
So the idea has an interesting past, and now we can consider the Turyshev paper we’ve been looking at in these past few posts as the outline of an overall rethinking of the classic one-destination-per-mission concept, one that allows cheap flybys but also alternate ways of putting larger instrumented craft into the kind of orbits the 2022 Decadal has recommended for its putative Uranus mission. Modular smallsat design might incorporate self-assembly including propulsion modules for slowing the encounter speed of a mission to the outer planets. Here is what the paper says on the topic as it relates to a possible mission to search for life in the plumes of Enceladus:
Another mission type may rely on in-flight aggregation [8], which may be needed to allow for orbital capture. For that, after perihelion passage and while moving at 5 AU/yr (?25 km/s), the microsats would perform inflight aggregation to make a fully capable smallsat to satisfy conditions for in situ investigations. One such important capability may be enhanced on-board propulsion capable of providing the ?v needed to slow down the smallsat. In this case, before approaching Enceladus, the spacecraft reduces its velocity by 7.5 km/s using a combination of on-board propulsion and gravity assists. Moving in the same direction with Enceladus (which orbits Saturn at 12.6 km/s) it achieves the conditions for in situ biomaterial collection.
We might, then, consider the option of either multiple flybys of small probes or larger payloads in self-assembling smallsat craft of the ice giants and other targets in the outer reaches of the system. The paper names quite a few possibilities. Among them:
The so-called ‘interstellar ribbon,’ evidently determined by interactions between the heliosphere and the local interstellar magnetic field.
Indirect probing of sailcraft trajectory in search of information about the putative Planet 9 and its gravitational effects somewhere between 300 and 500 AU of the Sun (Breakthrough Starshot has also discussed this). And if Planet 9 is found, target missions to a world much too far away to study with chemical propulsion methods.
The Kuiper Belt and beyond: KBOs and dwarf planets like Haumea, Makemake, Eris, and Quaoar within roughly 100 AU of the Sun, or even Sedna, whose orbit takes it well beyond 100 AU.
Observations of Earth as exoplanet, observing its transits across the Sun and improving transit spectroscopy.
Missions to interstellar objects like 1/I ‘Oumuamua, which are believed to occur in substantial numbers and likely to be a rich field for future discovery.
Studies of the local interplanetary dust cloud responsible for the zodiacal light.
Exoplanet imaging through self-assembling smallsats, the JPL Solar Gravitational Lens mission.
Image: This is Figure 9 from the paper. Caption: IBEX ENA Ribbon. A closer look suggests that the numbers of ENAs are enhanced at the interstellar boundary. A Sundiver spacecraft will go through this boundary as it travels to the ISM. Credit: SwRI.
As examined in JPL’s Phase III study for the SGL mission (the term ‘microsat’ below refers to that category of smallsats massing less than 20 kilograms):
The in-flight (as opposed to Earth-orbiting or cislunar) autonomous assembly [8] allows us to build large spacecraft from modules, separately delivered in the form of microsats (<20 kg), where each microsat is placed on a fast solar system transit trajectory via solar sail propulsion to velocities of ?10 AU/yr. Such a modular approach of combining various microsats into one larger spacecraft for a deep space mission is innovative and will be matured as part of the TDM flights. This unexplored concept overcomes the size and mass limits of typical solar sail missions. Autonomous docking and in-flight assembly are done after a large ?v maneuver, i.e., after passing through perihelion. The concept also offers the compelling ability to assemble different types of instruments and components in a modular fashion, to accomplish many different mission types.
To say that robotic assembly is an ‘unexplored concept’ underlines how much would have to be resolved to make such a daring mission work. The paper goes into more details, of which I’ll mention the high accuracy demanded in terms of trajectory. Remember, we’re talking about flinging each microsat into the outer system after perihelion on its own, with the need for successful rendezvous and assembly not in Earth orbit but in outbound cruise. Docking technologies for structural, power and data connections would go far beyond those deployed on any missions flown to date.
Even so, I’m persuaded this concept is feasible. It’s also completely brilliant.
Autonomous in-space docking has been demonstrated, while proximity operation technologies specific to such missions can be developed with time. I’ve referred before in these pages to NASA’s On-Orbit Autonomous Assembly from Nanosatellites (OAAN) project, and note that the agency has followed with a CubeSat Proximity Operations Demonstration (CPOD) mission. Needless to say, we’ll keep an eye on these and other efforts. I’m reminded of the intricacies of JWST deployment and have to say that from this layman’s view, we are building the roadmap to make self-assembly happen.
Image: An early artist’s impression of OAAN. Credit: NASA.
Alex Tolley has been looking at self-assembly issues in the comments to the previous post. I highly recommend reading what he has to say. I noted this about redundancy, an issue I hadn’t considered. Quoting Alex:
“Normally, a swarm of independent probe sails would offer redundancy in case of failure. A swarm of flyby sail probes can afford the odd failure. However, this is not the case with probes that must be combined into a functioning whole. Now we have a weakest link problem. Any failure could jeopardize the mission if a failed probe has a crucial component needed for the final combined probes. That failure could be with the payload, or with the sail system itself. A sail may fail with a malfunctioning blade, which prevents being able to rendezvous with the rest of the swarm, or more subtly, be unable to manage fine maneuvering for docking.”
Self-assembly is complex indeed, making early missions that can demonstrate docking and assembly a priority. Success could re-shape how we conceive deep space missions.
For a more detailed look at how the JPL team views self-assembly in the context of the SGL mission, see Helvajian et al., “A mission architecture to reach and operate at the focal region of the solar gravitational lens” (abstract). The Turyshev et al. paper is “Science opportunities with solar sailing smallsats,” available as a preprint. I’ve also written about self-assembly in Solar Gravitational Lens: Sailcraft and In-Flight Assembly.
Follow with RSS or E-Mail
