by Paul Gilster | Apr 20, 2024 | Astrobiology and SETI |
Radio and optical SETI look for evidence of extraterrestrial civilizations even though we have no evidence that such exist. The search is eminently worthwhile and opens up the ancillary question: How would a transmitting civilization produce a signal strong enough for us to detect it at interstellar distances? Beacons of various kinds have been considered and search strategies honed to find them. But we’ve also begun to consider new approaches to SETI, such as detecting technosignatures in our astronomical data (Dyson spheres, etc.). To this mix we can now add a consideration of gravitational lensing, and the magnifications possible when electromagnetic radiation is focused by a star’s mass. For a star like our Sun, this focal effect becomes useful at distances beginning around 550 AU.
Theoretical work and actual mission design for using this phenomenon began in the 1990s and continues, although most work has centered on observing exoplanets. Here the possibilities are remarkable, including seeing oceans, continents, weather patterns, even surface vegetation on a world circling another star. But it’s interesting to consider how another civilization might see gravitational lensing as a way of signaling to us. Indeed, doing so could conceivably open up a communications channel if the alien civilization is close enough, for if we detect lensing being used in this way, we would be wise to consider using our own lens to reply.
Or maybe not, considering what happens in The Three Body Problem. But let’s leave METI for another day. A new paper from Slava Turyshev (Jet Propulsion Laboratory) makes the case that we should be considering not just optical SETI, but a gravitationally lensed SETI signal. The chances of finding one might seem remote, but then, we don’t know what the chances of any SETI detection are, and we proceed in hopes of learning more. Turyshev argues that with the level of technology available to us today, a lensed signal could be detected with the right strategy.
Image: Slava Turyshev (Jet Propulsion Laboratory). Credit: Asteroid Foundation.
“Search for Gravitationally Lensed Interstellar Transmissions,” now available on the arXiv site, posits a configuration involving a transmitter, receiver and gravitational lens in alignment, something we cannot currently manage. But recall that the effort to design a solar gravity lens (SGL) mission has been in progress for some years now at JPL. As we push into the physics involved, we learn not only about possible future space missions but also better strategies for using gravitational lensing in SETI itself. We are now in the realm of advanced photonics and optical engineering, where we define and put to work the theoretical tools to describe how light propagates in a gravity field.
And while we lack the technologies to transmit using these methods ourselves (at least for now), we do have the ability to detect extraterrestrial signals using gravitational lensing. In an email yesterday, Dr. Turyshev offered an overview of what his analysis showed:
Many factors influence the effectiveness of interstellar power transmission. Our analysis, based on realistic assumptions about the transmitter, shows that substantial laser power can be effectively transmitted over vast distances. Gravitational lensing plays a crucial role in this process, amplifying and broadening these signals, thereby increasing their brightness and making them more distinguishable from background noise. We have also demonstrated that modern space- and ground-based telescopes are well-equipped to detect lensed laser signals from nearby stars. Although individual telescopes cannot yet resolve the Einstein rings formed around many of these stars, a coordinated network can effectively monitor the evolving morphology of these rings as it traces the beam’s path through the solar system. This network, equipped with advanced photometric and spectroscopic capabilities, would enable not only the detection but also continuous monitoring and detailed analysis of these signals.
We’re imagining, then, an extraterrestrial civilization placing a transmitter in the region of its own star’s gravitational lens, on the side of its star opposite to the direction of our Solar System. The physics involved – and the mathematics here is quite complex, as you can imagine – determine what happens when light from an optical transmitter is sent to the star so that when it encounters the warped spacetime induced by the star’s mass, the diffracted rays converge and create what scientists call a ‘caustic,’ a pattern created by the bending of the light rays and their resulting focused patterns.
In the case of a targeted signal, the lensing effect emerges in a so-called ‘Einstein ring’ around the distant star as seen from Earth. The signal is brightened by its passage through warped spacetime, and if targeted with exquisite precision, could be detected and untangled by Earth’s technologies. Turyshev asks in this paper how the generated signal appears over interstellar distances.
The answer should help us understand how to search for transmissions that use gravitational lensing, developing the best strategies for detection. We’ve pondered possible interstellar networks of communication in these pages, using the lensing properties of participating stellar systems. Such signals would be far more powerful than the faint and transient signals detectable through conventional optical SETI.
Laser transmissions are inherently directional, unlike radio waves, the beams being narrow and tightly focused. An interstellar laser signal would have to be aimed precisely towards us, an alignment that in and of itself does not resolve all the issues involved. We can take into account the brightness of the transmitting location, working out the parameters for each nearby star and factoring in optical background noise, but we would have no knowledge of the power, aperture and pointing characteristics of a transmitted signal in advance. But if we’re searching for a signal boosted by gravitational lensing, we have a much brighter beam that will have been enhanced for best reception.
Image: Communications across interstellar distances could take advantage of a star’s ability to focus and magnify communication signals through gravitational lensing. A signal from—or passing through—a relay probe would bend due to gravity as it passes by the star. The warped space around the object acts somewhat like a lens of a telescope, focusing and magnifying the light. Pictured here is a message from our Sun to another stellar system. Possible signals from other stars using these methods could become SETI targets. Image credit: Dani Zemba / Penn State. CC BY-NC-ND 4.0 DEED.
Mathematics at this level is something I admire and find beautiful much in the same way I appreciate Bach, or a stunning Charlie Parker solo. I have nowhere near the skill to untangle it, but take it in almost as a form of art. So I send those more mathematically literate than I am to the paper, while relying on Turyshev’s explanation of the import of these calculations, which seek to determine the shape and dimensions of the lensed caustic, using the results to demonstrate the beam propagation affected by the lens geometry, and the changes to the density of the EM field received.
It’s interesting to speculate on the requirements of any effort to reach another star with a lensed signal. Not only does the civilization in question have to be able to operate within the focal region of its stellar lens, but it has to provide propulsion for its transmitter, given the relative motion between the lens and the target star (our own). In other words, it would need advanced propulsion just to point toward a target, and obviously navigational strategies of the highest order within the transmitter itself. As you can imagine, the same issues emerge within the context of exoplanet imaging. From the paper:
…we find that in optical communications utilizing gravitational lenses, precise aiming of the signal transmissions is also crucial. There could be multiple strategies for initiating transmission. For instance, in one scenario, the transmission could be so precisely directed that Earth passes through the targeted spot. Consequently, it’s reasonable to assume that the transmitter would have the capability to track Earth’s movement. Given this precision, one might question whether a deliberately wider beam, capable of encompassing the entire Earth, would be employed instead. This is just [a] few of many scenarios that merit thorough exploration.
Detecting a lensed signal would demand a telescope network optimized to search for transients involving nearby stars. Such a network would be capable of a broad spectrum of measurements which could be analyzed to monitor the event and study its properties as it develops. Current and near-future instruments from the James Webb Space Telescope and Nancy Grace Roman Space Telescope to the Vera C. Rubin Observatory’s LSST, the Thirty Meter Telescope and the Extremely Large Telescope could be complemented by a constellation of small instruments.
Because the lens parameters are known for each target star, a search can be constructed using a combination of possible transmitter parameters. A search space emerges that relies on current technology for each specific laser wavelength. According to Turyshev’s calculations, a signal targeting a specific spot 1 AU from the Sun would be detectable with such a network with the current generation of optical instruments. Again from the paper:
Once the signal is detected, the spatial distribution of receivers is invaluable, as each will capture a distinct dataset by traveling through the signal along a different path… Correlating the photometric and spectral data from each path enables the reconstruction of the beam’s full profile as it [is] projected onto the solar system. Integrating this information with spectral data from multiple channels reveals the transmitter’s specific features encoded in the beam, such as its power, shape, design, and propulsion capabilities. Additionally, if the optical signal contains encoded information, transmitted via a set of particular patterns, this information will become accessible as well.
While microlensing events created by a signal transmitted through another star’s gravitational lens would be inherently transient, they would also be strikingly bright and should, according to these calculations, be detectable with the current generation of instruments making photometric and spectroscopic observations. Using what Turyshev calls “a spatially dispersed network of collaborative astronomical facilities,” it may be possible not only to detect such a signal but to learn if message data are within. The structure of the point spread function (PSF) of the transmitting lens could be determined through coordinated ground- and space-based telescope observations.
We are within decades of being able to travel to the focal region of the Sun’s gravitational lens to conduct high-resolution imaging of exoplanets around nearby stars, assuming we commit the needed resources to the effort. Turyshev advocates a SETI survey along the lines described to find out whether gravitationally lensed signals exist around these stars, pointing out that such a discovery would open up the possibility of studying an exoplanet’s surface as well as initiating a dialogue. “[W]e have demonstrated the feasibility of establishing interstellar power transmission links via gravitational lensing, while also confirming our technological readiness to receive such signals. It’s time to develop and launch a search campaign.“
The paper is Turyshev, “Search for gravitationally lensed interstellar transmissions,” now available as a preprint. You might also be interested in another recent take on detecting technosignatures using gravitational lensing. It’s Tusay et al., “A Search for Radio Technosignatures at the Solar Gravitational Lens Targeting Alpha Centauri,” Astronomical Journal Vol. 164, No. 3 (31 August 2022), 116 (full text), which led to a Penn State press release from which the image I used above was taken.
by Paul Gilster | Feb 21, 2024 | Missions |
Having just discussed whether humans – as opposed to their machines – will one day make interstellar journeys, it’s a good time to ask where we could get today with near-term technologies. In other words, assuming reasonable progress in the next few decades, what would be the most likely outcome of a sustained effort to push our instruments into deep space? My assumption is that fusion engines will one day be available for spacecraft, but probably not soon, and antimatter, that quixotic ultimate power source for interstellar flight, is a long way from being harnessed for propulsion.
We’re left with conventional rocket propulsion with gravity assists, and sail technologies, which not coincidentally describes the two large interstellar missions currently being considered for the heliophysics decadal study. Both JHU/APL’s Interstellar Probe mission and JPL’s SGLF (Solar Gravity Lens Focal) mission aim at reaching well beyond our current distance holders, the now struggling Voyagers. The decadal choice will weigh the same question I ask above. What could we do in the near term to reach hundreds of AU from the Sun and get there in relatively timely fashion?
A paper from the JPL effort in Experimental Astronomy draws my attention because it pulls together where the SGLF concept is now, and the range of factors that are evolving to make it possible. I won’t go into detail on the overall design here because we’ve discussed it in the recent past (see for example Building Smallsat Capabilities for the Outer System and Self-Assembly: Reshaping Mission Design for starters). Instead, I want to dig into the new paper looking for points of interest for a mission that would move outward from the Sun’s gravitational lens and, beyond about 650 AU, begin imaging an exoplanet with a factor of 1011 amplification.
Image: This is Figure 1 from the paper. Caption: The geometry of the solar gravity lens used to form an image of a distant object in the Einstein ring. Credit: Friedman et al.
Carrying a telescope in the meter-class, the spacecraft would reach its target distance after a cruise of about 25 years, which means moving at a speed well beyond anything humans have yet attained moving outward from the Sun. While Voyager 1 reached over 17 kilometers per second, we’re asking here for at least 90 km/sec. Remember that the focal line extends outward from close to 550 AU, and becomes usable for imaging around 650 AU. Our spacecraft can take advantage of it well beyond, perhaps out to 1500 AU.
So let’s clear up a common misconception. The idea is not to reach a specific distance from the Sun and maintain it. Rather, the SGLF would continue to move outward and maneuver within what can be considered an ‘image cylinder’ that extends from the focal region outward. This is a huge image. Working the math, the authors calculate that at 650 AU from the Sun, the light (seen as an ‘Einstein ring’ around the Sun) from an exoplanet 100 light years from our system would be compressed to a cylinder 1.3 kilometers in diameter. Remember, we have a meter-class telescope to work with.
Thus the idea is to position the spacecraft within the image cylinder, continuing to move along the focal line, but also moving within this huge image itself, collecting data pixel by pixel. This is not exactly a snapshot we’re trying to take. The SGLF craft must take brightness readings over a period that will last for years. Noise from the Sun’s corona is reduced as the spacecraft moves further and further from the Sun, but this is a lengthy process in terms of distance and time, with onboard propulsion necessary to make the necessary adjustments to collect the needed pixel data within the cylinder.
So we’re in continual motion within the image cylinder, and this gets further complicated by the range of motions of the objects we are studying. From the paper:
Even with the relatively small size of the image produced by the SGL, the spacecraft and telescope must be maneuvered over the distance of tens of kilometers to collect pixel-by-pixel all the data necessary to construct the image… This is needed as the image moves because of the multiple motions [that] are present, namely 1) the planet orbits its parent star, 2) the star moves with the respect to the Sun, and 3) the Sun itself is not static, but moves with respect to the solar system barycentric coordinates. To compensate for these motions, the spacecraft will need micro-thrusters and electric propulsion, the solar sail obviously being useless for propulsion so far from the Sun.
Bear in mind that, as the spacecraft continues to move outward from 650 AU, the diameter of the image becomes larger. We wind up with a blurring problem that has to be tackled by image processing algorithms. Get enough data, though, and the image can be deconvolved, allowing a sharp image of the exoplanet’s surface to emerge. As you would imagine, a coronagraph must be available to block out the Sun’s light.
What to do with the sail used to reach these distances? The mission plan is a close solar pass and sail deployment timed to produce maximum acceleration for the long cruise to destination. Solar sails are dead weight the further we get from the Sun, so you would assume the sail would be jettisoned, although it’s interesting to see that the team is working on ways to convert it into an antenna, or perhaps even a reflector for laser communications. As to power sources for electric propulsion within the image cylinder, the paper envisions using radioisotope thermoelectric generators, which are what will power up the craft’s communications, instruments and computing capabilities.
Image: This is Figure 4 from the paper. Caption: Trajectory of the mission design concept for a solar sailcraft to exit the solar system. Credit: Friedman et al./JPL.
Let’s clear up another misconception. If we deploy a sail at perihelion, we are relying on the solar photons delivering momentum to the sail (photons have no mass, but they do carry momentum). This is not the solar wind, which is a stream of particles moving at high velocity out from the Sun, and interesting in its own right in terms of various mission concepts that have been advanced in the literature. The problem with the solar wind, though, is that it is three orders of magnitude smaller than what we can collect from solar photons. What we need, then, is a photon sail of maximum size, and a payload of minimum mass, which is why the SGLF mission focuses on microsats. These may be networked or even undergo self-assembly during cruise to the gravity focus.
The size of a sail is always an interesting concept to play with. Ponder this: The sail mission to Halley’s Comet that Friedman worked on back in the mid-1970s would have demanded a sail that was 15 kilometers in diameter, in the form of a so-called heliogyro, whose blades would have been equivalent to a square sail half a mile to the side. That was a case of starting at the top, and as the paper makes clear, issues of packaging and deployment alone were enough to make the notion a non-starter.
Still, it was an audacious concept and it put solar sails directly into NASA’s sights for future development. The authors believe that based on our current experience with using sails in space, a sail of 100 X 100 square meters is about as large as we are able to work with, and it might require various methods of stiffening its structural booms. The beauty of the new SunVane concept is that it uses multiple sails, making it easier to package and more controllable in flight. This is the ‘Lightcraft’ design out of Xplore Inc., which may well represent the next step in sail evolution. If it functions as planned, this design could open up the outer system to microsat missions of all kinds.
Image: This is Figure 5 from the paper. Caption: Xplore’s Lightcraft TM advanced solar sail for rapid exploration of the solar system. Credit; Friedman et al./JPL.
Pushing out interstellar boundaries also means pushing materials science hard. After all, we’re contemplating getting as close to the Sun as we can with a sail that may be as thin as one micron, with a density less than 1 gram per square meter. The kind of sail contemplated here would weigh about 10 kg, with 40 kg for the spacecraft. The payload has to be protected from a solar flux that at 0.1 AU is 100 times what we receive on Earth, so the calculations play the need for shielding against the need to keep the craft as light as possible. An aluminized polymer film like Kapton doesn’t survive this close to the Sun, which is why so much interest has surfaced in materials that can withstand higher temperatures; we’ve looked at some of this work in these pages.
But the longer-term look is this:
Advanced technology may permit sails the size of a football field and spacecraft the size of modern CubeSats, and coming close to the Sun with exotic materials of high reflectivity and able to withstand the very high temperatures. That might permit going twice as fast, 40 AU/year or higher. If we can do that it will be worth waiting for. With long mission times, and with likely exoplanets in several different star systems being important targets of exploration we may want to develop a low cost, highly repeatable and flexible spacecraft architecture – one that might permit a series of small missions rather than one with a traditional large, complex spacecraft. The velocity might also be boosted with a hybrid approach, adding an electric propulsion to the solar sail.
It’s worth mentioning that we need electric propulsion on this craft anyway as the craft maneuvers to collect data near the gravitational focus. Testing all this out charts a developmental path through a technology demonstrator whose funding through a public-private partnership is currently being explored. This craft would make the solar flyby and develop the velocity needed for a fast exit out of the Solar System. A series of precursor missions could then test the needed technologies for deployment at the SGL We can envision Kuiper Belt exploration and, as the authors do, even a mission to a future interstellar object entering our system using these propulsion methods.
I recommend this new paper to anyone interested in keeping up with the JPL design for reaching the solar gravitational focus. As we’ve recently discussed, a vision emerges in which we combine solar sails with microsats that weigh in the range of 50 kilograms, with extensive networking capabilities and perhaps the ability to perform self-assembly during cruise. For the cost of a single space telescope, we could be sending multiple spacecraft to observe a number of different exoplanets before the end of this century, each with the capability to resolve features on the surface of these worlds. Resolution would be to the level of a few kilometers. We’re talking about continents, oceans, vegetation and, who knows, perhaps even signs of technology. And that would be on not one but thousands of potential targets within a ten light year radius from Earth.
The paper is Friedman et al., “A mission to nature’s telescope for high-resolution imaging of an exoplanet,” Experimental Astronomy 57 (2024), 1 (abstract).
by Paul Gilster | Dec 15, 2023 | Sail Concepts |
What an interesting object Methone is. Discovered by the Cassini imaging team in 2004 along with the nearby Pallene, this moon of Saturn is a scant 1.6 kilometers in radius, orbiting between Mimas and Enceladus. In fact, Methone, Pallene and another moon called Anthe all orbit at similar distances from Saturn and are dynamically jostled by Mimas. What stands out about Methone is first of all its shape and, perhaps even more strikingly, the smoothness of its surface. We’d like to know what produces this kind of object and would also like to retrieve imagery of both Pallene and Anthe. If something this strange has equally odd companions, is there something about its relationship with both nearby moons and Saturn’s rings that can produce this kind of surface?
Image: It’s difficult not to think of an egg when looking at Saturn’s moon Methone, seen here during a Cassini flyby of the small moon. The relatively smooth surface adds to the effect created by the oblong shape. NASA/JPL-Caltech/Space Science Institute.
Our path to interstellar missions will see us ramp up the velocities of our probes to objects in our own system, made more accessible by shorter mission times, sail technologies and miniaturization. There is no shortage of targets between high-interest moons like Europa, Titan and Enceladus and Kuiper Belt Objects like Arrokoth. For that matter, the interstellar interloper ‘Oumuamua may yet be within range of faster missions (and in fact we’ll be examining ‘Oumuamua prospects in at least one upcoming article). But the point is that intermediate steps to interstellar will enhance exploration of objects we’ve already visited and take us to numerous others.
One way to proceed is discussed by Greg Matloff and Les Johnson in a recent paper for the Journal of the British Interplanetary Society that grew out of a presentation at the 6th International Space Sailing Symposium this summer. Here the idea is to adjust the parameters of a solar sail so that a balance is achieved between the gravitational force of the Sun and the solar photon radiation impinging upon it. The parameters are clear enough: We need a sail of a specific thickness (areal density), and tightly constrained figures for its reflectance and absorbance. We want to cancel out the gravitational acceleration imposed by the Sun through the propulsive effects of solar photons, allowing us to effectively ‘hover’ in place.
Hovering isn’t traveling, but bear with me. We’ve looked at this kind of sail configuration before and discussed its development in the hands of Robert Forward. It was Forward who dubbed the configuration a ‘statite,’ implying that when the force on the sail from solar radiation exactly balances the gravitational force acting upon it, the spacecraft is effectively in what the paper calls a ‘force-free environment.’
This gets interesting in terms of fast probes because while the statite is normally considered to remain stationary (and it will do so when the sail is stationary relative to the Sun during sail deployment), something else happens when the craft is orbiting the Sun when the sail is deployed. The sail now moves in a straight line at its orbital velocity at the time of deployment. The authors style this ‘rectilinear sun-diving.’ As Matloff noted in an email the other day:
“To do this operationally, it is necessary to maintain the sail normal to the Sun – broadside facing the Sun – during the acceleration process. The sail moves off at its velocity relative to the Sun at sail deployment because radiation pressure force on the sail balances solar gravitational attraction. This is a consequence of Newton’s First Law.”
Using this method we can fling the sail and payload outward. What is known as the sail’s lightness factor is the ratio of solar radiation forces divided by the solar gravitational force, and in the case of the rectilinear trajectory described above, the lightness factor is 1. So consider a sail being deployed from a circular orbit of the Sun at 1 AU. The statite, free of other forces, now moves out on a rectilinear trajectory at 30 kilometers per second, which is the Earth’s orbital velocity. The number is noteworthy because it practically doubles the interstellar velocity of Voyager 1. Matloff and Johnson point out that at this velocity, the Sun’s gravitational focus at 550 AU is reachable in 87 years.
Moving at the same pace gets us to Saturn (and the interesting Methone) in 1.5 years. I’m going to run through the other two scenarios the scientists consider to show the range of possibilities. Assume an orbit that is not circular but rather one having a perihelion of 0.7 AU and aphelion at 1 AU. Deploying the sail at perihelion allows the spacecraft to reach 38 kilometers per second, getting to the inner gravitational focus in about 66 years. Finally, with an aphelion at 1 AU and perihelion at 0.3, our craft achieves a velocity after sail deployment of 66 km/sec, reaching the focus in 38 years.
As regards to ‘Oumuamua, the third scenario, with sail deployment at perihelion some 0.3 AU out from the Sun, achieves enough interstellar cruise velocity to catch the object roughly around 2045, when it will be some 220 AU from the Sun. To these times, of course, must be added the time needed to move the sail from aphelion to the sail deployment point at perihelion, but the numbers are still quite satisfactory.
This is especially true given that we are talking about relatively near-term technologies that are under active development. Matloff and Johnson calculate using an areal mass thickness of 1.46 X 10-3kg/m2 for the proposed missions. They show current state of the art solar sail film as 1.54 X 10-3kg/m2 (this does not include deployment mechanisms, structure, etc). The point is clear, however: Achieving 30 km/sec or more offers us fast passage to targets within the outer Solar System as we analyze options for missions beyond it, using technologies that are not far removed from present capability.
The authors note that we can’t assume a constant value for solar radiation; the solar constant actually varies by about 0.1% in response to the Sun’s activity cycle. Hence the need to explore options like adjusting the curvature of the sail or using reflective vanes for fine-tuning. Controlling the sail will obviously be critical. The paper continues:
Control of the sail depends upon the ability of the system to dynamically adjust the center of mass (CM) versus the center of (photon) pressure (CP). Any misalignment of the CM versus the CP will induce torques in the sail system that have to be actively managed lest the offset result in an eventual loss of control. The sail will encounter micrometeorites and interplanetary dust during flight that will create small holes in the fabric, changing its reflectivity asymmetrically and inducing unwanted torques. Depending upon how the sail is packaged and deployed, there may also be fold lines, wrinkles, and small tears that occur with similar end results.
Hence the need for a momentum management system, which could involve possibilities like reflective control devices for roll or diffractive sail materials that manipulate the exit direction of incoming photons as needed to counter these effects. The authors point out that the solar sail propulsion systems for this kind of mission are at TRL-6 despite recent failures such as the loss of the Near-Earth Asteroid Scout Cubesat mission, which carried an 86 square meter solar sail that was lost after launch in late November 2022. With solar sails under active development, however, the prospect for exploring rectilinear sundiver missions in the near term seems quite plausible.
The paper is Matloff & Johnson, “Breakthrough Sun Diving: The Rectilinear Option,” Journal of the British Interplanetary Society Vol. 76 (2023), 283-287.
by Paul Gilster | Oct 25, 2023 | Communications and Navigation |
I sometimes imagine Claudio Maccone having a particularly vivid dream, a bright star surrounded by a ring of fire that all but grazes its surface. And from this ring an image begins to form behind him, kilometers wide, dwarfing him and carrying in its pixels the view of a world no one has ever seen. The dream is half visual, half diagrammatic, but it’s all about curving Einsteinian spacetime, so that light flows along the gravity well to be bent into a focus that extends into linear infinity.
My slightly poetic vision of what happens beyond 550 AU or so doesn’t do justice to the intrinsic beauty of the mathematics, which Maccone learned to unlock decades ago as he explored the concept of an ‘Einstein ring’ as fine-tuned by Von Eshleman at Stanford. When I met him (at one of Ed Belbruno’s astrodynamics conferences at Princeton in 2006), we and Greg Matloff and wife C talked about lensing at breakfast one morning. Even then he was afire with the concept. He’d been probing it since the late 1980s, and had submitted a mission proposal to the European Space Agency. He had written a short text that would later be expanded into the seminal Deep Space Flight and Communications (Springer, 2009).
Maccone said in his presentation at the Interstellar Research Group’s Montreal symposium that he was delighted to see the Sun’s gravitational focus moving into the hands of the next generation, citing the 2020 NASA grant to Slava Turyshev’s team at JPL, where a Solar Gravitational Lens mission is being worked out at the highest level of detail as an entrant into the sweepstakes known as the Heliophysics 2024 Decadal Survey. To see how far the concept has gone, have a look at, for example, Self-Assembly: Reshaping Mission Design, or A Mission Architecture for the Solar Gravity Lens, among numerous entries I’ve written on the JPL work.
Image: A meter-class telescope with a coronagraph to block solar light, placed in the strong interference region of the solar gravitational lens (SGL), is capable of imaging an exoplanet at a distance of up to 30 parsecs with a few 10 km-scale resolution on its surface. The picture shows results of a simulation of the effects of the SGL on an Earth-like exoplanet image. Left: original RGB color image with (1024×1024) pixels; center: image blurred by the SGL, sampled at an SNR of ~103 per color channel, or overall SNR of 3×103; right: the result of image deconvolution. Credit: Turyshev et al., “Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravity Lens Mission,” Final Report NASA Innovative Advanced Concepts Phase II.
The astounding magnification we could achieve by using bent starlight was what drew me instantly to the concept when I first learned about it – how else to actually see not just pixels from an exoplanet around its star, but actual continents, weather patterns, oceans and, who knows, even vegetation on the surface? But at Montreal, after his praise for the JPL effort that could become our first attempt to exploit the gravitational lens if adopted by the Decadal survey, Maccone took a much more futuristic look at what humans might do with lensing, delving into the realm of communications. What about building a radio ‘bridge’?
The concept is even more audacious that reaching 650 AU with the payloads we’ll need to deconvolve imagery from another star. In fact, it’s downright science fictional. Suppose we achieve the technologies needed to send humans to Alpha Centauri. We have there in the form of Centauri A a G-class star much like the Sun (although we could also use the K-class star Centauri B). Both of these stars have their own distance from which gravitational lensing occurs, Align your spacecraft properly to look back towards the Earth from Centauri A and you can now connect to the ‘relay’ at the lensing distance from the Sun. You’ve drastically changed the communications picture by using lensing in both directions.
The consequences for contact and data transfer are enormous. Consider: If we want to talk to our crew now orbiting Centauri A and try to do so with one of the Deep Space Network’s 70-meter dish antennae using today’s standards for spacecraft communications, we’d have no usable signal to work with. Assume a transmitting power of 40 W and communications over the Ka band (32 GHz) at a rate of 32 kbps (these are the figures for the highest frequency used by the Cassini mission). The distances are too great; the power too weak. But if we factor in a receiver at the lensing point of Centauri A directly opposite to the Sun, we get the extraordinary gain shown in the diagram below.
This raises the eyebrows. Bit Error Rate expresses the quality of the signal, being the number of erroneous bits received divided by the total number of bits transmitted. Using a spacecraft at the solar gravitational lens distance from the Sun talking to one on the other side of Centauri A (alignment, of course, is critical here), we have a signal so strong that we have to go over 9 light years out before it begins to degrade. A radio bridge like this would allow communications with a colony at Alpha Centauri using power levels and infrastructure we have in place today.
Obviously, this is a multi-generational idea given travel times to and from Alpha Centauri. But it’s a step we may well need to take if we can solve all the problems involved in getting human crews to another star. Maccone told the audience at Montreal that in terms of channel capacity (as defined by Shannon information theory), the Sun used as a gravitational lens allows 190 gigabits per second in a radio bridge to Centauri A as opposed to the paltry 15.3 kilobits per second available without lensing.
Realizing that any star creates this possibility, Maccone has lately been working on the question of how a starfaring society of the future might use radio bridges to plot out expansion into nearby stars. He is in fact thinking about the best ‘trail of expansion’ humans might use to keep links being built and used between colonies at these stars. This turns out to be no easy task: The first goal must be to convert the list of nearby stars being studied (the number is arbitrary) into Cartesian coordinates centered on each star (their coordinates are currently given in terms of Right Ascension and Declination with respect to the Sun). Maccone calls this an exercise in spherical trigonometry, and it’s a thorny one.
A network of radio bridges between stars could evolve into a kind of ‘galactic internet,’ a term Maccone uses with an ironic smile as it plays to the journalist’s need to write dramatic copy. Be that as it may, the SETI component is intriguing, given that older civilizations may even now be exploiting gravitational lensing. It would be an interesting thing indeed if we were to discover a bridge relay somewhere at our Sun’s gravitational lensing distance, for its placement would allow us to calculate where the receiving civilization must be located. Using a gravitational lens for communications is, after all, extraordinarily directional. Might we one day discover at the lensing distance from the Sun an artifact that can open access to a networked conversation on the interstellar scale?
Human expansion to nearby stars would likely be a matter of millennia, but given the age of the galaxy, it would represent just a sliver of time. Whether humanity can survive for far shorter timeframes is an immediate question, but I think it’s refreshing indeed to look beyond the current work on reaching the solar gravitational lens to the implications that would follow from exploiting it. The radio bridge is great science fiction material – we might even call it the stuff of dreams – but solidly rooted in physics if we can find the tools to make it happen.
by Paul Gilster | May 12, 2023 | Sail Concepts |
The game changer for space exploration in coming decades will be self-assembly, enabling the growth of a new and invigorating paradigm in which multiple smallsat sailcraft launched as ‘rideshare’ payloads augment huge ‘flagship’ missions. Self-assembly allows formation-flying smallsats to emerge enroute as larger, fully capable craft carrying complex payloads to target. The case for this grows out of Slava Turyshev and team’s work at JPL as they refine the conceptual design for a mission to the solar gravitational lens at 550 AU and beyond. The advantages are patent, including lower cost, fast transit times and full capability at destination.
Aspects of this paradigm are beginning to be explored in the literature, as I’ve been reminded by Alex Tolley, who forwarded an interesting paper out of the University of Padua (Italy). Drawing on an international team, lead author Giovanni Santi explores the use of CubeSat-scale spacecraft driven by sail technologies, in this case ‘lightsails’ pushed by a laser array. Self-assembly does not figure into the discussion in this paper, but the focus on smallsats and sails fits nicely with the concept, and extends the discussion of how to maximize data return from distant targets in the Solar System.
The key to the Santi paper is swarm technologies, numerous small sailcraft placed into orbits that allow planetary exploration as well as observations of the heliosphere. We’re talking about payloads in the range of 1 kg each, and the intent of the paper is to explore onboard systems (telecommunications receives particular attention), the fabrication of the sail and its stability, and the applications such systems can offer. As you would imagine, the work draws for its laser concepts on the Starlight program pursued for NASA by Philip Lubin and the ongoing Breakthrough Starshot project.
Image: NASA’s Starling mission is one early step toward developing swarm capabilities. The mission will demonstrate technologies to enable multipoint science data collection by several small spacecraft flying in swarms. The six-month mission will use four CubeSats in low-Earth orbit to test four technologies that let spacecraft operate in a synchronized manner without resources from the ground. Credit: NASA Ames.
The authors argue that ground-based direct energy laser propulsion, with its benefits in terms of modularity and scalability, is the baseline technology needed to make small sailcraft exploration of the Solar System a reality. Thus there is a line of development which extends from early missions to targets like Mars, with accompanying reductions in the power needed (as opposed to interstellar missions like Breakthrough Starshot), and correspondingly, fewer demands on the laser array.
The paper specifically does not analyze close-pass perihelion maneuvers at the Sun of the sort examined by the JPL team, which assumes no need for a ground-based array. I think the ‘Sundiver’ maneuver is the missing piece in the puzzle, and will come back to it in a moment.
Breakthrough Starshot envisions a flyby of a planetary system like Proxima Centauri, but the missions contemplated here, much closer to home, must find a way to brake at destination in cases where extended planetary science is going to be performed. Thus we lose the benefit of purely sail-based propulsion (no propellant aboard) in favor of carrying enough propulsive mass to make the needed maneuvers at, say, Mars:
…the spacecraft could be ballistically captured in a highly irregular orbit, which requires at least an high thrust maneuver to stabilize the orbit itself and to reduce the eccentricity…The velocity budget has been estimated using GMAT suite to be ?v ? 900?1400 m s?1, depending on the desired final orbit eccentricity and altitude. A chemical thruster with about 3 N thrust would allow to perform a sufficiently fast maneuver. In this scenario, the mass of the nanosatellite is estimated to be increased by a wet mass of 5 kg; moreover, an increase of the mass of reaction wheels needs to be taken into account given the total mass increment.
Even so, swarms of nanosatellites allow a reduction of the payload mass of each individual spacecraft, with the added benefit of redundancy and the use of off-the-shelf components. The authors dwell on the lightsail itself, noting the basic requirement that it be thermally and mechanically stable during acceleration, no small matter when propelling a sail out of Earth orbit through a high-power laser beam. Although layered sails and sails using nanostructures, metamaterials that can optimize heat dissipation and promote stability, are an area of active research, this paper works with a thin film design that reduces complexity and offers lower costs.
We wind up with simulations involving a sail made of titanium dioxide with a radius of 1.8 m (i.e. a total area of 10 m2) and a thickness of 1 µm. The issue of turbulence in the atmosphere, a concern for Breakthrough Starshot’s ground-based laser array, is not considered in this paper, but the authors note the need to analyze the problem in the next iteration of their work along with close attention to laser alignment, which can cause problems of sail drifting and spinning or even destroy the sail.
But does the laser have to be on the Earth’s surface? We’ve had this discussion before, noting the political problem of a high-power laser installation in Earth orbit, but the paper notes a third possibility, the surface of the Moon. A long-term prospect, to be sure, but one having the advantage of lack of atmosphere, and perhaps placement on the Moon’s far side could one day offer a politically acceptable solution. It’s an intriguing thought, but if we’re thinking of the near term, the fastest solution seems to be the Breakthrough Starshot choice of a ground-based facility on Earth.
What we have here, then, could be described as a scaled-down laser concept, a kind of Breakthrough Starshot ‘lite’ that focuses on lower levels of laser power, larger payloads (even though still in the nanosatellite range), and targets as close as Mars, where swarms of sail-driven spacecraft might construct the communications network for a colony on the surface. A larger target would be exploration of the heliosphere:
…in this last mission scenario the nanosatellites would be radially propelled without the need of further orbital maneuvers. To date, the interplanetary environment, and in particular the heliospheric plasma, is only partially known due to the few existing opportunities for carrying out in-situ measurements, basically linked to scientific exploration missions [76]. The composition and characteristics of the heliospheric plasma remain defined mainly through theoretical models only partially verified. Therefore, there is an urgent need to perform a more detailed mapping of the heliospheric environment especially due to the growth of the human activities in space.
Image: An artist’s concept of ESA’s Swarm mission being deployed. This image was taken from a 2015 workshop on formation flying satellites held at Technische Universiteit Delft in the Netherlands. Extending the swarm paradigm to smallsats and nanosatellites is one step toward future robotic self-assembly. Credit: TU Delft.
Spacecraft operating in swarms optimized for the study of the heliosphere offer tantalizing possibilities in terms of data return. But I think the point that emerges here is flexibility, the notion that coupling a beamed propulsion system to smallsats and nanosats offers a less expensive, modular way to explore targets previously within reach only by expensive flagship missions. I’ll also argue that a large, ground-based laser array is aspirational but not essential to push this paradigm forward.
Issues of self-assembly and sail design are under active study, as is the question of thermal survival for operations close to the Sun. We should continue to explore close solar passes and ‘sundiver’ maneuvers to shorten transit times to targets both relatively near or as far away as the Kuiper Belt. We need test missions to firm up sail materials and operations, even as we experiment with self-assembly of smallsats into larger craft capable of complex operations at target. The result is a modular fleet that can make fast flybys of distant targets or assemble for orbital operations where needed.
The paper is Santi et al., “Swarm of lightsail nanosatellites for Solar System exploration,” available as a preprint.
