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).
The Arxiv preprint is also available.
I found the paper at https://arxiv.org/pdf/2107.11473.pdf
I get the impression that to keep the description simple, the phrase “pixel by pixel” glosses over a much more interesting imaging problem. I understand using the term if there are many microsats tiled in a regular pattern, but here one SGLF spacecraft moves within the image. Though I assume it can only run though one line of “pixels” at once, it should have a continuous stream of data to work with. Nor is it working with a mere point of light: the Einstein ring is stretched around the edge of a coronagraph. Adjacent features, though dimmer, will appear mostly on one side of the other. I would think some extra information could be wrung from observations at a single point by blocking different portions of the ring, such as with a rotating bar. The spacecraft is imagine a rotating, changing planet by making many passes through its ever-accelerating image in a way calculated to conserve fuel. The processing here goes well beyond deconvolution; it should actually be moving to predict which pass to make to confirm and expand on previous data. The craft deserves some very intelligent software, written by some very intelligent people, to make timely decisions to gather the most informative line of data during each pass.
Very good concept, and a spinning spacecraft could make it possible to keep the sails in position without adding to much mass to said booms.
If this could be as ‘affordable’ as suggested so that multiple spacecrafts could be sent to various positions to imagine different star systems – and explore the Kuiper belt as well, I’m all for the idea. While a visit to any of the major KBO’s might turn out to be tricky, there should be plenty of smaller ones,
I cannot but wonder if the range could be extended from the proposed 10 light years mentioned, as we do not currently have any true Earthlike candidate in that volume of space.
So to actually see vegetation, lets say moss or lichens, will require a look at planets further away. This not to say that some of those nearby world might have microbial life, in fact if my hypothesis is correct we will eventually find so many such worlds we will be quite tired of hearing the announcement that another such have been found.
And the spacecraft would indeed need propulsion on top, so that it would be able to refocus and imagine several targets in one system – the time span it will spend where proper focusing will be possible will be limited as it continues outward and leave the focus point. So in most cases there will not be time to wait until a planet move in it’s orbit to get into the right position. In fact the best observations might be when the planet is on the far side of the star, having a larger part of the surface area lit up by the central star.
In related news: Either the Kuiper belt is wider than thought, or there might be a second population according to this press release.
https://www.nasa.gov/missions/new-horizons/nasas-new-horizons-detects-dusty-hints-of-extended-kuiper-belt/
As someone who has been both interest in and struggling with the fine details of SGL mission, wish to say thanks for these added details and clarifications. I will continue to be both interested and never quite all the way there. But in addition there seem to have been unexpected discoveries along the way.
The first figure perhaps illustrates as well what many of us have debated about or struggled with. The sun as an object in the field of view at 590 AUs or so blots out a disk of about 1.75 arc seconds radius. For conventional geometric optics, that’s would be at a distance of 1 parsec (206,264 AUs), a radius of about 100 AUs. Depending on the type of star or planet, but say Earth and Sun, the action is within a radius of 1/100th of that. Ten parsecs, the FOV is ten times wider. Now since the diagram suggests, that a central field is blotted out like a coronagraph, I have to wonder how you maneuver the spacecraft to focus on a planet, if the ring of captured light is very narrow. Or is it?
Now when the May 1919 eclipse was observed, light from the star in question was bent by the sun by about the same deflection angle. It was still a point source, I believe, rather than a ring.
Descriptions of the experiment seem to focus on the bending, but I am not sure where behind sun the star was located. At its limb or near the center? Similarly, with many Einstein rings, we observe galactic images, likely distorted, but identifiable as such structures. If these were convoluted somehow, I hope that we will be as lucky with an SGL. In any case, how SGL images can be deconvoluted for a planet will certainly be an interesting follow up on this topic.
On the other hand, whatever the answers are to these issues, when it comes to allocation of resources, I still can’t help wondering if ever larger space telescopes based on larger launchers and better space access – would result in more effective exoplanet surveys. A phased array of such telescopes might do rather well on a particular target – and then move on to others which would be out of the FOV of an SGL. Since they would be hard or expensive to slew even for one target, angular shifts might be limited to stellar clusters. While habitable worlds are truly attractive targets, this technology might need to be matched with other investigations to which it might be well adapted.
I think antimatter propulsion should still be considered.
What happened with James Bickford’s proposals?
https://www.centauri-dreams.org/2016/08/03/antimatter-production-harvesting-in-space/
Good question, Frank. I need to update my previous post on Bickford. Will hope to get to that soon.
I think it is worth pointing out that Claudio Maccone wrote a book about a mission to the ‘Solar Gravitational Lens’ , Deep Space Flight and Communications: Exploiting the Sun as a Gravitational Lens, 2009. (Different spacecraft.)
As far as I know the first to point this out was Von R Eshleman , in 1979, in Science, Gravitational Lens of the Sun: Its Potential for Observations and Communications over Interstellar Distances.
Of course Einstein that made note of gravitational lensing in 1936.
(Einstein A. Lens-like action of a star by the deviation of light in the gravitational field. Science. 1936.)
Most times , technically speaking, ‘geometrical optics’ is used , fine for weak lensing, but then the amplification along the focal line is infinite.
The full up wave optics calculation gives amplification that goes like one over the wavelength. Which is were the 10 to the 11 amplification comes from. This is an enormous intensity. Worth exploiting.
Hi Al & Paul
Greg Benford was one of the first to use the concept in fiction in his “Across the Sea of Suns” – the “Lancer” drops an imaging package on its way to Lalande 21185 which then uses the Solar gravitational lens to view other star systems. Greg uses a bit of poetic license to make it capable of viewing more than one star system, for dramatic effect.
Had forgotten about that one! But then, I read Across the Sea of Suns when it first came out. A long time ago!
When this mission was considered on Centauri Dreams earlier, for illustration there was an image of Earth in full phase, suggesting that it would be possible to obtain such an image of Earth analog. Of course, what results will depend very much on the target, but it might be in order to continue to contemplate what problems could arise with the target planet. For starters, it is unlikely to be locked in full phase pointing toward us. At the very least after an orbital year there will be night and dark sides. But if the planet does not rotate synchronously, it will show different surface features while it is supposedly posed for a portrait. In addition, our Earth analog might display divisions between oceans and continents – if we are lucky. It might not have oceanic and continental divisions – and it might have an overlying clouds.
Some of these problems could be addressed by a range of sensors with bands more likely to distinguish these distinctions. But I would imagine that these issues will complicate the problem of picture puzzle approach to obtaining a high resolution image.
In addition, if it will take 25 years or so to reach the deployment station, there’a unique validation issue here: What kind of test program can be implemented to make sure the concept will work as described?
There was another paper (posted on CD?) where a rotating planet could have its entire surface mapped over time as the light was integrated over time. The changing cloud patterns would seem to make that difficult unless they can be accounted for in some way. When we use weather satellite images, we know the geography below the clouds that can obscure much of the surface, but this will be unknown for exoplanets, just as it was on Venus and Titan before radar was used.
The simulated image of Earth used before was created by what means – using a static image as the source and then reconstructing it? Whether this can be done by the SGL telescope may depend on how fast it can sample the focal area to do the deconvolving of the light. If fast, then a partially illuminated planet’s surface can be reconstructed, and over time all of the planet as it orbits its star. If slow, then it may prove very difficult to image the planet. But who knows what computational effort and algorithms can be applied to the data once it is collected and transmitted to Earth for image construction?
As for validation, even today we can build models to test this. A video image of the Earth could be used as the target, with a lens only allowing a narrow ring of light to pass, and a camera with a small aperture moved around the focal area, would be a start to test the data algorithms needed. This could be made more realistic (noisy) to determine the limits of the techniques to resolve the image of Earth over time.
Thanks, A.T.,
My personal picture of how this will work has changed as more details are obtained and clarified. Now if the image headed to the focal point is an annulus of about 1.3 kilometers in diameter, than the spacecraft would be obliged to hold position or track within the “walls” of the nominal cylinder as it recedes from the solar system. Now at 10s of kilometer per second velocity an asymptopic or hyperbolic path would not deflect very much at such distance, but the source of the beam would. The sun is shifting at significant velocity on a 500,000 kilometer orbital path created or perturbed by interaction with planets. If it is mainly a jovian cycle, that would be about 8 meters per second in Solar center of mass terms. But this also constitutes an angular rate.
My angular rate calculations extended out to 550 AUs is pretty substantial.
Oscillatory and bound, but it means that the velocity amplitude gets up to a thousand km/sec at peak.
Maybe looking at stars perpendicular to the ecliptic plane would be preferable.
Wondering if a sail with a diameter equal to the image cylinder’s (1.3 km) could be used as ‘film’ to capture all the pixels of a complete image at once. This would reduce the amount of lateral maneuvering the craft would need to make, somewhat simplifying the challenges of an otherwise ‘long exposure.’
If we use the sail or electric type propulsion we can use the structure to slow down along the SHL line by interacting with the solar wind which has practically stalled in interstellar space. This would give us extra viewing time although not needed and perhaps a source of energy and direction change abilities.
Hi Paul
The preprint version mentions the NIAC study which preceded the Journal submission:
Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravity Lens Mission
Which is available as a preprint too. There’s a Journal preview available on the ResearchGate version of the paper:
A mission to nature’s telescope for high-resolution imaging of an exoplanet