Seeing oceans, continents and seasonal changes on an exoplanet pushes conventional optical instruments well beyond their limits, which is why NASA is exploring the Sun’s gravitational lens as a mission target in what is now the third phase of a study at NIAC (NASA Innovative Advanced Concepts). All of this builds upon the impressive achievements of Claudio Maccone that we’ve recently discussed. Led by Slava Turyshev, the NIAC effort takes advantage of light amplification of 1011 and angular resolutions that dwarf what the largest instruments in our catalog can deliver, showing what the right kind of space mission can do.

We’re going to track the Phase III work with great interest, but let’s look back at what the earlier studies have accomplished along the way. Specifically, I’m interested in mission architectures, even as the NASA effort at the Jet Propulsion Laboratory continues to consider the issues surrounding untangling an optical image from the Einstein ring around the Sun. Turyshev and team’s work thus far argues for the feasibility of such imaging, and as we begin Phase III, sees viewing an exoplanet image with a 25-kilometer surface resolution as a workable prospect.

But how to deliver a meter-class telescope to a staggeringly distant 550 AU? Consider that Voyager 1, launched in 1977, is now 152 AU out, with Voyager 2 at 126 AU. New Horizons is coming up on 50 AU from the Earth. We have to do better, and one way is to re-imagine how such a mission would be achieved through advances in key technologies and procedures.

Here we turn to mission enablers like solar sails, artificial intelligence and nano-satellites. We can even bring formation flying into a multi-spacecraft mix. A technology demonstration mission drawing on the NIAC work could fly within four years if we decide to fund it, pointing to a full-scale mission to the gravitational focus launched a decade later. Travel time is estimated at 20 years.

These are impressive numbers indeed, and I want to look at how Turyshev and team achieve them, but bear in mind that in parsing the Phase II report, we’re not studying a fixed mission proposal. This is a highly detailed research report that tackles every aspect of a gravitational lens mission, with multiple solutions examined from a variety of perspectives. One thing it emphatically brings home is how much research is needed in areas like sail materials and instrumentation for untangling lensed images. Directions for such research are sharply defined by the analysis, which will materially aid our progress moving into the Phase III effort.

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.

Modes of Propulsion

A mission to the Sun’s gravity lens need not be conceived as a single spacecraft. Turyshev relies on spacecraft of less than 100 kg (smallsats, in the report’s terminology) using solar sails, working together and produced in numbers that will enable the study of multiple targets.

The propulsive technique is a ‘Sundiver’ maneuver in which each smallsat spirals in toward perihelion in the range of 0.1 to 0.25 AU, achieving 15-25 AU per year exit velocity, which gets us to the gravity lensing region in less than 25 years. The sails are eventually ejected to reduce weight, and onboard propulsion (the study favors solar thermal) is available at cruise. The craft would enter the interstellar medium in 7 years as compared to Voyager’s 40, making the journey to the lens in a timeframe 2.5 times longer than what it took to get New Horizons to Pluto.

Image: Sailcraft example trajectory toward the Solar Gravity Lens. Credit: Turyshev et al.

The hybrid propulsion concept is necessary, and not just during cruise, because once in the focal lensing region, the spacecraft will need either chemical or electrical propulsion for navigation corrections and for operations and maintenance. Let’s pause on that point for a moment — Alex Tolley and I have been discussing this, and it shows up in the comments to the previous post. What Alex is interested in is whether there is in fact a ‘sweet spot’ where the problem of interference from the solar corona is maximally reduced compared to the loss of signal strength with distance. If there is, how do we maximize our stay in it?

Recall that while the focal line goes to infinity, the signal gain for FOCAL is proportional to the distance. A closer position gives you stronger signal intensity. Our craft will not only need to make course corrections as needed to keep on line with the target star, but may slow using onboard propulsion to remain in this maximally effective area longer. I ran this past Claudio Maccone, who responded that simulations on these matters are needed and will doubtless be part of the Phase II analysis. He has tackled the problem in some detail already:

“For instance: we do NOT have any reliable mathematical model of the Solar Corona, since the Corona keeps changing in an unpredictable way all the time.

“In my 2009 book I devoted the whole Chapters 8 and 9 to use THREE different Coronal Models just to find HOW MUCH the TRUE FOCUS is PUSHED beyond 550 AU because of the DIVERGENT LENS EFFECT created by the electrons in the lowest level of the Corona. For instance, if the frequency of the electromagnetic waves is the Peak Frequency of the Planckian CMB, then I found that the TRUE FOCUS is 763 AU Away from the Sun, rather than just 550 AU.

“My bottom-line suggestion is to let FOCAL observe HIGH Frequencies, like 160 GHz, that are NOT pushing the true focus too much beyond 550 AU.”

Where we make our best observations and how we keep our spacecraft in position are questions that highlight the need for the onboard propulsion assumed by the Phase II study.

Image: Our stellar neighborhood with notional targets. Credit: Turyshev et al.

For maximum velocity in the maneuver at the Sun, as close a perihelion as possible is demanded, which calls for a sailcraft design that can withstand the high levels of heat and radiation. That in turn points to the needed laboratory and flight testing of sail materials proposed for further study in the NIAC work. Let me quote from the report on this:

Interplanetary smallsats are still to be developed – the recent success of MarCO brings them perhaps to TRL 7. Solar sails have now flown – IKAROS and LightSail-2 already mentioned, and NASA is preparing to fly NEA-Scout. Scaling sails to be thinner and using materials to withstand higher temperatures near the Sun remains to be done. As mentioned above, we propose to do this in a technology test flight to the aforementioned 0.3 AU with an exit velocity ~6 AU/year. This would still be the fastest spacecraft ever flown.

The report goes on to analyze a technology demonstration mission that could be done within a few years at a cost less than $40 million, using a ‘rideshare’ launch to approximately GEO.

String of Pearls

The mission concept calls for an array of optical telescopes to be launched to the gravity lensing region. I’ll adopt the Turyshev acronym of SGL for this — Solar Gravity Lens. The thinking is that multiple small satellites can be launched in a ‘string of pearls’ architecture, where each ‘pearl’ is an ensemble of smallsats, with multiple such ensembles periodically launched. A series of these pearls, multiple smallsats operating interdependently using AI technologies, provides communications relays, observational redundancy and data management for the mission. From the report:

By launching these pearls on an approximately annual basis, we create the “string”, with pearls spaced along the string some 20-25 AU apart throughout the timeline of the mission. So that later pearls have the opportunity to incorporate the latest advancements in technology for improved capability, reliability, and/or reductions in size/weigh/power which could translate to further cost savings.

In other words, rather than being a one-off mission in which a single spacecraft studies a single target, the SGL study conceives of a flexible investigation of multiple exoplanetary systems, with ‘strings of pearls’ launched toward a variety of areas within the focus within which exoplanet targets can be observed. Whereas the Phase I NIAC study analyzed instrument and mission requirements and demonstrated the feasibility of imaging, the Phase II study refines the mission architecture and makes the case that a gravity lens mission, while challenging, is possible with technologies that are already available or have reached a high degree of maturity.

Notice the unusual solar sail design — called SunVane — that was originally developed at the space technology company L’Garde. Here we’re looking at a sail design based on square panels aligned along a truss to provide the needed sail area. In the Phase II study, the craft would achieve 25 AU/year, reaching 600 AU in ~26 years (allowing two years for inner system approach to the Sun). [Note: I’ve replaced the earlier SunVane image with this latest concept, as passed along by Xplore’s Darren Garber. Xplore contributed the design for the demonstration mission’s solar sail].

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

The report examines a sail area of 45,000 m2, equivalent to a ~212×212 m2 sail, with spacecraft components to be configured along the truss. Deployment issues are minimal with the SunVane design. The vanes are kept aligned edge-on to the Sun as the craft approaches perihelion, then directed face-on to promote maximum acceleration.

We have to learn how to adjust parameters for the sail to allow the highest possible velocity, with areal density A/m being critical — here A stands for the area in square meters of the sail, with m as the total mass of the sailcraft in kilograms (this includes spacecraft plus sail). Sail materials and their temperature properties will be crucial in determining the perihelion distance that can be achieved. This calls for laboratory and flight testing of sail material as part of the continuing research moving into the Phase III study and beyond. Sail size is a key issue:

The challenge for design of a solar sail is managing its size – large dimensions lead to unstable dynamics and difficult deployment. In this study we have consider[ed] a range of smallsat masses (<100 kg) and some of the tradeoffs of sail materials (defining perihelion distance) and sail area (defining the A/m and hence the exit velocity…). As an example, for the SGLF mission, consider perihelion distance of 0.1 AU (20Rsun) and A/m=900 m2/kg; the exit velocity would be 25 AU/year, reaching 600 AU in ~26 years (allowing 2 years for inner solar system approach to the Sun). The resulting sail area is 45,000 m2, equivalent to a ~212×212 m2 sail.

The size of that number provokes the decision to explore the SunVane concept, which distributes sail area in a way that allows spacecraft components to be placed along the truss instead of being confined to the sail’s center of gravity, and which has the added benefit of high maneuverability. A low-cost near-term test flight is proposed with testing of sail material and control, closing to a perihelion in the range of 0.3 AU, with an escape velocity from the Solar System of 6-7 AU per year. Several such spacecraft would enable a test of swarm architectures.

Thus the concept: Multiple spacecraft would be launched together as an ensemble — the ‘pearl’ — using solar sails deployed on each and navigating through the Deep Space Network, with the spacecraft maintaining a separation on the order of 15,000 km as they pass through perihelion. Such ensembles are periodically launched, acting interdependently in ways that would maximize flexibility while reducing risk from a single catastrophic failure and lowering mission cost. We wind up with a system that would enable investigations of multiple extrasolar systems.

I haven’t had time to get into such issues as communications and power for the individual smallsats, or data processing and AI, all matters that are covered in the report, nor have I looked in as much detail as I would have liked at the sail arrays, envisioned through SunVane as on the order of 16 vanes of 103 m2, allowing the area necessary in a configuration the report considers realistic. This is a lengthy, rich document, and I commend it to those wanting to dig further into all these matters.

The report is 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 (full text).