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Solar Gravitational Lens: Sailcraft and In-Flight Assembly

The last time we looked at the Jet Propulsion Laboratory’s ongoing efforts toward designing a mission to the Sun’s gravitational lens region beyond 550 AU, I focused on how such a mission would construct the image of a distant exoplanet. Gravitational lensing takes advantage of the Sun’s mass, which as Einstein told us distorts spacetime. A spacecraft placed on the other side of the Sun from the target exoplanetary system would take advantage of this, constructing a high resolution image of unprecedented detail. It’s hard to think of anything short of a true interstellar mission that could produce more data about a nearby exoplanet.

In that earlier post, I focused on one part of the JPL work, as the team under the direction of Slava Turyshev had produced a paper updating the modeling of the solar corona. The new numerical simulations led to a powerful result. Remember that the corona is an issue because the light we are studying is being bent around the Sun, and we are in danger of losing information if we can’t untangle the signal from coronal distortions. And it turned out that because the image we are trying to recover would be huge – almost 60 kilometers wide at 1200 AU from the Sun if the target were at Proxima Centauri distance – the individual pixels are as much as 60 meters apart.

Image: JPL’s Slava Turyshev, who is leading the team developing a solar gravitational lens mission concept that pushes current technology trends in striking new directions. Credit: JPL/S. Turyshev.

The distance between pixels turns out to help; it actually reduces the integration time needed to pull all the data together to produce the image. The integration time (the time it takes to gather all the data that will result in the final image) is in fact reduced when pixels are not adjacent at a rate proportional to the inverse square of the pixel spacing. I’ve more or less quoted the earlier paper there to make the point that according to the JPL work thus far, exoplanet imaging at high resolution using these methods is ‘manifestly feasible,’ another quotation from the earlier work.

We now have a new paper from the JPL team, looking further at this ongoing engineering study of a mission that would operate in the range of 550 to 900 AU, performing multipixel imaging of an exoplanet up to 100 light years away. The telescope is meter-class, the images producing a surface resolution measured in tens of kilometers. Again I will focus on a specific topic within the paper, the configuration of the architecture that would reach these distances. Those looking for the mission overview beyond this should consult the paper, the preprint of which is cited below.

Bear in mind that the SGL (solar gravitational lens) region is, helpfully, not a focal ‘point’ but rather a cylinder, which means that a spacecraft stays within the focus as it moves further from the Sun. This movement also causes the signal to noise ratio to improve, and means we can hope to study effects like planetary rotation, seasonal variations and weather patterns over integration times that may amount to months or years.

Image: From Geoffrey Landis’ presentation at the 2021 IRG/TVIW symposium in Tucson, a slide showing the nature of the gravitational lens focus. Credit: Geoffrey Landis.

Considering that Voyager 1, our farthest spacecraft to date, is now at a ‘mere’ 156 AU, a journey that has taken 44 years, we have to find a way to move faster. The JPL team talks of reaching the focal region in less than 25 years, which implies a hyperbolic escape velocity of more than 25 AU per year. Chemical methods fail, giving us no more than 3 to 4 AU per year, while solar thermal and even nuclear thermal move us into a still unsatisfactory 10-12 AU per year in the best case scenario. The JPL team chooses solar sails in combination with a close perihelion pass of the Sun. The paper examines perihelion possibilities at 15 as well as 10 solar radii but notes that the design of the sailcraft and its material properties define what is going to be possible.

Remember that we have also been looking at the ongoing work at the Johns Hopkins Applied Physics Laboratory involving a mission called Interstellar Probe, which likewise is in need of high velocity to reach the distances needed to study the heliosphere from the outside (a putative goal of 1000 AU in 50 years has been suggested). Because the JHU/APL effort has just released a new paper of its own, I’ll also be referring to it in the near future, because thus far the researchers working under Ralph McNutt on the problem have not found a close perihelion pass, coupled with a propulsive burn but without a sail, to be sufficient for their purposes. But more on that later. Keep it in mind in relation to this, from the JPL paper:

…the stresses on the sailcraft structure can be well understood. For the sailcraft, we considered among other known solar sail designs, one with articulated vanes (i.e., SunVane). While currently at a low technology readiness level (TRL), the SunVane does permit precision trajectory insertion during the autonomous passage through solar perigee. In addition, the technology permits trimming of the trajectory injection errors while still close to the Sun. This enables the precision placement of the SGL spacecraft on its path towards the image cylinder which is 1.3 km in diameter and some 600+ AU distant.

Is the SunVane concept the game-changer here? I looked at it 18 months ago (see JPL Work on a Gravitational Lensing Mission), where I used the image below to illustrate the concept. The sail is constructed of square panels aligned along a truss. In the Phase II study for NIAC that preceded the current papers, a sail based on SunVane design could achieve 25 AU per year – that would be arrival at 600 AU in 26 years in conjunction with a close solar pass – using a craft with total sail area of 45,000 square meters (that’s equivalent to a roughly 200 X 200 square meter single sail).

Image: The SunVane concept. Credit: Darren D. Garber (Xplore, Inc).

With sail area distributed along the truss rather than confined to the sail’s center of gravity, this is a highly maneuverable design that continues to be of great interest. Maneuverability is a key factor as we look at injecting spacecraft into perihelion trajectory, where errors can be trimmed out while still in close proximity to the Sun.

But current thinking goes beyond flying a single spacecraft. What the JPL work has developed through the three NIAC phases and beyond is a mission built around a constellation of smaller spacecraft. The idea is chosen, the authors say, to enhance redundancy, enable the needed precision of navigation, remove the contamination of background light during SGL operations, and optimize the return of data. What intrigues me particularly is the use of in-flight assembly, with the major spacecraft modules placed on separate sailcraft. This will demand that the sailcraft fly in formation in order to effect the needed rendezvous for assembly.

Let’s home in on this concept, pausing briefly on the sail, for this mission will demand an attitude control system to manage the thrust vector and sail attitude once we have reached perihelion with our multiple craft, each making a perihelion pass followed by rendezvous with the other craft. I turn to the paper for more:

Position and velocity requirements for the incoming trajectory prior to perihelion are < 1 km and ∼1 cm/sec. Timing through perihelion passage is days to weeks with errors in entry-time compensated in the egress phase. As an example, if there is a large position and/or velocity error upon perihelion passage that translated to an angular offset of 100” from the nominal trajectory, there is time to correct this translational offset with the solar sail during the egress phase all the way out to the orbit of Jupiter. The sail’s lateral acceleration is capable of maneuvering the sailcraft back to the desired nominal state on the order of days depending on distance from the Sun. This maneuvering capability relaxes the perihelion targeting constraints and is well within current orbit determination knowledge threshold for the inner solar system which drive the ∼1 km and ∼1 cm/sec requirements.

Why the need to go modular and essentially put the craft together during the cruise phase? The paper points out that the 1-meter telescope that will be necessary cannot currently be produced in the mass and volume range needed to fit a CubeSat. The mission demands something on the order of a 100 kg spacecraft, which in turn would demand solar sails of extreme size as needed to reach the target velocity of 20 AU per year or higher. Such sails will be commonplace one day (I assume), but with the current state of the art, in-flight robotic assembly leverages our growing experience with miniaturization and small satellites and allows for a mission within a decade.

If in-flight assembly is used, because of the difficulties in producing very large sails, the spacecraft modules…are placed on separate sailcraft. After in-flight assembly, the optical telescope and if necessary, the thermal radiators are deployed. Analysis shows that if the vehicle carries a tiled RPS [radioisotope power system]…where the excess heat is used for maintaining spacecraft thermal balance, then there is no need for thermal radiators. The MCs [the assembled spacecraft] use electric propulsion (EP) to make all the necessary maneuvers for the cruise (∼25 years) and science phase of the mission. The propulsion requirements for the science phase are a driver since the SGL spacecraft must follow a non inertial motion for the 10-year science mission phase.

According to the authors, numerous advantages accrue from using a modular approach with in-space assembly, including the ability to use rideshare services; i.e., we can launch modules as secondary payloads, with related economies in money and time. Moreover, such a use means that we can use conventional propulsion rather than sails as an option for carrying the cluster of sailcraft inbound toward perihelion in formation. In any case, at some point the sailcraft deploy their sails and establish the needed trajectory for the chosen solar perihelion point. After perihelion, the sails — whose propulsive qualities diminish with distance from the Sun — are ejected, perhaps nearing Earth orbit, as the sailcraft prepare for assembly.

Flying in formation, the sailcraft reduce their relative distance outbound and begin the in-space assembly phase while passing near Earth orbit. The mission demands that each of the 10-20 kg mass spacecraft be a fully functional nanosatellite that will use onboard thrusters for docking. Autonomous docking in space has already been demonstrated, essentially doing what the SGL mission will have to do, assembling larger craft from smaller ones. It’s worth noting, as the authors do, that NASA’s space technology mission directorate has already begun a project called On-Orbit Autonomous Assembly from Nanosatellites-OAAN along with a CubeSat Proximity Operations Demonstration (CPOD) mission, so we see these ideas being refined.

What demands attention going forward is the needed development of proximity operation technologies, which range from sensor design to approach algorithms, all to be examined as study of the SGL mission continues. There was a time when I would have found this kind of self-assembly en-route to deep space fanciful, but there was also a time when I would have said landing a rocket booster on its tail for re-use was fanciful, and it’s clear that self-assembly in in the SGL context is plausible. The recent deployment of the James Webb Space Telescope reinforces the same point.

The JPL team has been working with simulation tools based on concurrent engineering methodology (CEM), modifying current software to explore how such ‘fractionated’ spacecraft can be assembled. Note this:

Two types of distributed functionality were explored. A fractionated spacecraft system that operates as an “organism” of free-flying units that distribute function (i.e., virtual vehicle) or a configuration that requires reassembly of the apportioned masses. Given that the science phase is the strong driver for power and propellant mass, the trade study also explored both a 7.5 year (to ∼800 AU) and 12.5 year (to ∼900 AU) science phase using a 20 AU/yr xit velocity as the baseline. The distributed functionality approach that produced the lowest functional mass unit is a cluster of free-flying nanosatellites…each propelled by a solar sail but then assembled to form a MC [mission capable] spacecraft.

Image: Various approaches will emerge about the kind of spacecraft that might fly a mission to the gravitational focus of the Sun. In this image (not taken from the Turyshev et al. paper), swarms of small sailcraft capable of self-assembly into a larger spacecraft are depicted that could fly to a spot where our Sun’s gravity distorts and magnifies the light from a nearby star system, allowing us to capture a sharp image of an Earth-like exoplanet. Credit: NASA/The Aerospace Corporation.

The current paper goes deeply into the attributes of the kind of nanosatellite that can assemble the final design, and I’ll send you to it for further details. Each of the component craft has the capability of a 6U CubeSat/nanosat and each carries components of the final craft, from optical communications to primary telescope mirror. Current thinking is that the design is in the shape of a round disk about 1 meter in diameter and 10 cm thick, with a carbon fiber composite scaffolding. The idea is to assemble the final craft as a stack of these units, producing the final round cylinder.

What a fascinating, gutsy mission concept, and one with the possibility of returning extraordinary data on a nearby exoplanet. The modular approach can be used to enhance redundancy, the authors note, as well as allowing for reconfiguration to reduce the risk of mission failure. Self-assembly leverages current advances in miniaturization, composite materials, and computing as reflected in the proliferation of CubeSat and nanosat technologies. What this engineering study is pointing to is a mission to the solar gravity lens that seems feasible with near-term technologies.

The paper is Helvajian et al., “A mission architecture to reach and operate at the focal region of the solar gravitational lens,” now available as a preprint. The earlier report on the study’s progress is “Resolved imaging of exoplanets with the solar gravitational lens,” (preprint). The Phase II NIAC report on this work is Turyshev & Toth, “Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravity Lens Mission,” Final Report NASA Innovative Advanced Concepts Phase II (2020). Full text.

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{ 18 comments… add one }
  • Martin Alfredsson July 22, 2022, 11:16

    What is the avantage of combining the separate cubesats. Is it easier to direct the telescope mirrors compared to freeflying mirrors?
    If the pixel get further from each others would it not be easier to increase the distance between freeflying mirrors or is this widening only apparent on large distances?

    Would not freeflying mirrors be able to cover a much larger area creating a super telescope like we have radio scopes that cover thousands of km on earth?

    • Alex Tolley July 22, 2022, 12:42

      We cannot do real-time optical interferometry on Earth let alone with free flying. Mirrors in space.

      Maybe it would be possible to send the data for each mirror to Earth for integration, if the mirror alignment could be made very accurately. IDK.

      But clearly this would add a whole new level of complexity to the mission, which is already complex enough.

  • James Early July 22, 2022, 12:39

    Most discussions of solar gravitational lens concept emphasis observations of extra-solar planets. High resolution observations of the star should also be valuable. Helioseismology observations may be possible. Accurate measurement of the rate and plane of rotation should be possible and would narrow the area to search for planets. Studies of sunspot activity might also be possible.

  • Alex Tolley July 22, 2022, 14:31

    AIUI, there will be a platoon (an imprecise term for a differing number of units that could range from 10 – up to 200) of spacecraft with 1 m telescopes with some form of occulter to block the unwanted light. Each telescope images a pixel based on its position to image the Einstein ring, and this data is sent to Earth for integration into a Megapixel{?} image. Each telescope must be positioned within 0.1 m at the SGL.

    Multiple crafts make sense not just to do the imaging, but to allow redundancy, although it appears that each craft only handles part of the total kit needed to measure the data and return it to Earth.

    When I think of the extremely accurate targeting of the direct descent Mars rover craft, this SGL accuracy must blow that away. Both the solar sails and the micro thrusters must ensure that the platoon of 6U CubeSat craft can assemble in a tight formation at 650 AU after decades. Almost all the technology is at low TRL, with the solar sails needing another order of magnitude reduction in aerial density to achieve the needed velocity, as well as surviving a sun dive perihelion nearly 1/2 that currently possible).

    Ambitious is an understatement. Almost makes the Breakthrough Starship proposal seem a doddle by comparison. I wonder what is a realistic timeframe for the technology to reach the levels needed to ensure a high probability of success?

    [I had an online argument with Dwayne Day over the value of CubeSats for interplanetary missions. He was, to say the least, very skeptical of their capabilities. This mission using multiple CubeSats, on a long-duration, extremely deep-space mission, would give even an enthusiast like myself, some reservations. But technology and capabilities march on…]

    • Leo July 25, 2022, 18:49

      Maybe I´m wrong, or it’s undefined yet, but I don´t have the impression that each and every microsatellite will be a 1m telescope. Some will be specialized in communication, others in data processing, and so on. BTW, how can a monolithic 1m-diameter telescope fit into a 6-units cubesat? what am I missing?

    • Brett Bellmore July 25, 2022, 20:46

      Each telescope is going to require its own occultation disk; The telescope has to be entirely within the disk’s umbra to not have the Einstein ring completely washed out by a tiny bit of visible Sun, and making the occultation disk any larger than necessary for the umbra to completely cover the scope requires the Einstein ring to be that much further out, delaying observations.

      Further, the occultation disk has to be fairly distant in order for the entire telescope to see the entire Einstein ring, requiring a disk substantially larger than the telescope size. Fortunately, the disk could just be a spin stabilized solar sail, it needn’t be massive.

      But both the telescope and the disk will have to have lateral propulsion, and remain aligned to high precision as they are scanned across the target. I’d suggest actively controlling the occultation disk’s location to do the scanning, and just have the telescope sense the edges of the umbra and track it.

  • Michael July 23, 2022, 5:14

    Even if we fail to accurately scan an exoplanet the deep field view using the technique would be of great interest with the possibly of seeing the first stars ever in the early Universe.

  • wdk July 23, 2022, 9:00

    To a certain extent, whether at a gravitational lens site or nearer to the sun, we are speaking of observatories based on phased arrays. Should near future heavy lift launchers achieve significant diameter increases, it will be much easier to deploy JWST scale telescopes. Consequently, near space (e.g. Earth-sun libration point) vs. gravity lens point locations as a trade study: There is time to IOC, quality of data obtained from target and slewing flexibility. Call me short sighted, but I would bet on the libration point. But in addition to the considerations described thus far, setting up at the gravitation lens implies that you know something about, say Proxima or Alpha Centauri to whet the study appetite. If the HZ planet turns out to be as devoid of development as the moon, for the trip to 550 AUs: What’s the backup plan? Beside transit time, other liabilities are the field of view and inability to slew – in any reasonable period of time.

    • Deanna Shaw July 26, 2022, 5:49

      That’s a really excellent point…. we are much closer to “routine” heavy lift capability….. Starship…. than we are to a 550 AU trek with a reasonable velocity. And entirely flexible compared to the gravity lens mission. Spot the promising candidates first, and then maybe in a generation, target it with a 550 AU, specific.

      • Brett Bellmore July 27, 2022, 6:02

        You actually need a very good ‘conventional’ scope to use the gravity focus; Because the Sun is in the way of any direct view, the Einstein ring is your only way of seeing the target, and you can’t reduce the inherent magnification of that. So there’s no way to have a spotter scope to tell you if you’re a little off. And you will be off, you’ll miss the target entirely if you’re only a few km off axis.

        We’re going to need to image the region around the target at the highest available conventional resolution, so that the focal mission scope can do an initial coarse scan, and be able to recognize where it is relative to the focal line, so that it can navigate to the correct spot. Otherwise if you miss even slightly, you’re totally lost.

  • David Jernigan July 23, 2022, 10:24

    Two thoughts about the early part of the mission:

    – could jettisoning the solar vanes near Earth orbit present an option for thier reuse? They would represent exotic materials manufactured and assembled to exacting requirements. This could also reduce future launch costs, opening up a multi-mission approach, important as a criticism of SGL missions is they each have one viewing target

    – could the vane-driven trajectory adjustments be so complete as to aim the various components at a close-formation rendezvous at the assembly point? This might even be required to achieve feasibility

    • Alex Tolley July 23, 2022, 14:19

      could jettisoning the solar vanes near Earth orbit present an option for thier reuse?

      I don’t see how. They are traveling at over 25 AU/yr. With their hyperbolic orbits, there is no obvious way to reduce their velocity as they cannot reverse the trust of their sails. [A caveat that if the sails are made of metamaterials such that they might be able to do this via difraction, then there is a small possibility of slowing down, although it seems unlikely to me. Carrying another form of propulsion, e.g. a magsail to use as a “parachute” under some clever trajectory changes seems to defeat to advantage of using a sail.]

      I think that having some sort of production economies of scale to reduce costs is perhaps a better approach. Each sail could carry a message like Pioneer and Voyager for future discovery.

      My concern with such long-term missions is that even with auxilliary science that can be done on the journey, if the mission fails to achieve its imaging goals, that is a long time to reach failure, especially for some scientists and engineers, they will have committed most of their working lives to the project. The telescope will be using technology decades old and there may even be the issue of a later mission overtaking this mission using better propulsion and getting results earlier. That is one extra risk for the project team.

  • Mike Serfas July 23, 2022, 11:41

    Given the linear focus of the Einstein rings, is it feasible to launch a probe to align with a target behind one of the Centauri suns, or some other close star? Just looking at an image ( https://en.wikipedia.org/wiki/File:Alpha,_Beta_and_Proxima_Centauri_(1).jpg ), the 13000 AU between Proxima and Alpha (not counting inclination) seems jam-packed with fairly bright stars, a few within a couple of hundred AU of being aligned with one of them. Are any of these interesting enough to be worth pursuing with another star’s lens, and is that feasible?

  • charlie July 25, 2022, 3:39

    “Position and velocity requirements for the incoming trajectory prior to perihelion are < 1 km and ∼1 cm/sec. Timing through perihelion passage is days to weeks with errors in entry-time compensated in the egress phase. As an example, if there is a large position and/or velocity error upon perihelion passage that translated to an angular offset of 100” from the nominal trajectory, there is time to correct this translational offset with the solar sail during the egress phase all the way out to the orbit of Jupiter. The sail’s lateral acceleration is capable of maneuvering the sailcraft back to the desired nominal state on the order of days depending on distance from the Sun. This maneuvering capability relaxes the perihelion targeting constraints and is well within current orbit determination knowledge threshold for the inner solar system which drive the ∼1 km and ∼1 cm/sec requirements."

    The recent Pluto mission had position errors at target of 60 miles; with accur. tracking. This can be done ?

    • Ron S. July 25, 2022, 15:15

      Dealing with a close dive past the Sun is different. The speed, density and composition of the corona/solar wind at any particular time is unpredictable, and therefore so is the drag and its effect on the trajectory. The spacecraft needs sufficient propulsive force to compensate for the range of predicted trajectory deviation (direction & speed) that it is likely to encounter.

  • Nat July 25, 2022, 21:17

    Future space telescopes surpassing JWST in size will have to be modular (due to payload-size constraints and with benefit of not hinging a whole mission on one launch). That is to say, some form of “in-flight” assembly will be required. Hopefully that will happen before a solar gravitational lens mission gets developed, in order to gain some experience with in-flight assembly.

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