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.