Breakthrough Discuss Ongoing

There is a public YouTube channel for watching the Breakthrough Discuss meetings, which began today and extend through tomorrow. Click here to go to sessions on “The Alpha Centauri System: A Beckoning Neighbor.” I’ll have thoughts on some of these presentations in coming weeks.


Juno at Jupiter: Extended Mission Flybys of Galilean Moons

The news that NASA will extend the InSight mission on Mars for two years, taking it through December of 2022, is not surprising, given the data trove the mission team has collected through operation of the mission seismometer. A live asset on Mars also deepens our knowledge of the planet’s atmosphere and magnetic field, all reasons enough for pushing for another two years. But the extension of the Juno mission to Jupiter deserves more attention than it’s getting, given that Juno’s remit will be expanded deep into the Jovian system.

Image: NASA has extended both the Juno mission at Jupiter through September 2025 and the InSight mission at Mars through December 2022. Credit: NASA/JPL-Caltech.

For those of us fascinated with the outer system, this is good news indeed. I’m looking over two documents, the first being a presentation based on a report submitted to NASA’ Outer Planets Assessment Group (thanks to Ashley Baldwin for passing this along). The OPAG document was produced by Scott Bolton (Southwest Research Institute); it gives the overview of what a mission extension could look like. Also on my desk this morning is the text of the 2020 Planetary Missions Senior Review (PMSR), outlining a set of three mission scenarios. The context of both analyses is the success of the mission in studying Jupiter’s interior structure, magnetic field and magnetosphere, not to mention the examination of its atmospheric dynamics, seen in such roiling imagery as that depicted with stunning complexity in many of the JunoCam images.

Launched in 2011 and operational at Jupiter since 2016, Juno’s prime missions were to have ended in July of this year, with the spacecraft having completed 34 polar orbits, each of 53 day duration. The OPAG report refers to the subsequent extended mission as “a full Jovian system explorer with close flybys of satellites and rings.” The extended mission is to last through September, 2025, with observations of the planet’s ring system, its large moons, and a series of targeted observations and close flybys of Ganymede, Europa and Io.

That last clause really got my attention, as I hadn’t seen it coming. Juno is in an elliptical orbit with a 53-day period whose perijove migrates northward. This bit from the Senior Review reveals in depth the interactions between the various mission scenarios and satellite flybys. The three scenarios mentioned offer alternatives given varying science and budget considerations:

The proposed Juno extended mission (EM) would take advantage of the natural northward progression of the periapsis of the spacecraft’s orbit and the consequent lowering of spacecraft altitudes over Jupiter’s high northern latitudes. The EM would run until the end of the mission, with an expected duration of approximately four years. Under the High and Medium Scenarios, propulsive maneuvers would be utilized not only to target Jupiter-crossing longitude and perijove altitude, as during the prime mission, but also to target close flybys of Ganymede, Europa, and Io. The flyby maneuvers would act to shorten the spacecraft orbital period, yielding more close passes of Jupiter within a given time interval, and increase the rate of northward movement of spacecraft perijove. Under the Low scenario for EM operation, the satellite gravity assists and close satellite flybys would not be attempted.

So mission scientists have a number of options to work with. The extended mission investigates the northern hemisphere and probes the region above Jupiter’s polar cap aurora. The northern adjustment in Juno’s orbit is what makes the satellite flybys possible and enables as well close analysis of its ring structures. The Juno team can look forward to 3D mapping of Jupiter’s polar cyclones and studies of the planet’s unusual dilute core, the latter an earlier Juno discovery revealing a core consisting of both rocky material and ice as well as hydrogen and helium.

Both Europa Clipper and the European Space Agency’s JUICE mission (Jupiter Icy Moons Explorer) should benefit from Juno data on the radiation environment they will operate within. At Europa, Juno will continue the search for possible plume activity while examining the ice shell and mapping surface features, while studies of Io’s magma, polar volcanoes and interactions with Jupiter’s magnetosphere will be enabled by its encounters there. At Ganymede, magnetospheric interactions and surface composition data should be produced in abundance.

In the OPAG presentation, most of the Juno flybys will be at Io, with 11 possible between mid-2022 and 2025. Two encounters are planned for Ganymede (and recall that JUICE is scheduled to orbit the huge moon), and three encounters are feasible for Europa. The actual number of flybys will, according to the Senior Review, depend upon budget choices. In that document, I find this overview of Juno’s satellite flybys:

The orbit of Juno in the EM [extended mission] would take the spacecraft through the Io and Europa plasma tori and in close proximity to Io, Europa and Ganymede. Maps of Ganymede’s surface composition would allow studies to understand the importance of radiolytic processes in surface weathering, identify changes since Voyager and Galileo, and search for new craters. Juno’s Microwave Radiometer (MWR) is particularly sensitive to the upper 10 km of Europa’s ice shell. Studies at wavelengths complementing expected results from Europa Clipper’s radar would identify regions of thick and thin ice and search for regions where shallow subsurface liquid may exist. Juno’s visible and low-light cameras would search Europa for active plumes and changes in color/albedo that may reveal eruption regions since Galileo. The fields and particles experiments would look for evidence of recent activity. Finally, the Juno EM would include a flyby of Io and search for evidence of a magma ocean.

What an interesting development Juno’s extended mission turns out to be! Continuing science operations with existing equipment far undercuts the cost of new missions while extending long-duration datasets and, in the case of Juno, enabling a set of exciting new targets. We have the option here of a series of Galilean moon flybys that were never in Juno’s original mission, observations that could inform later choices made for Europa Clipper and JUICE. All told, Juno’s unanticipated extended mission is a heartening contribution to outer system science.


The Red Dwarf Habitable Zone Dilemma

Henry Cordova, whose recent critique of traditional SETI kicked off a lengthy discussion in these pages, has been mulling over issues of habitability in the galaxy’s vast population of red dwarf stars. While we’ve focused on the questions raised by stellar flare activity and the climate challenges of tidal lock, the narrow band of habitability among the fainter M-dwarfs poses its own problems. How big a factor is a narrow circumstellar habitable zone? Henry comes by his interest in these matters by way of US Navy training in both astronomy and mathematics. A retired geographer and map maker now living in southeastern Florida, he’s keeping up with exoplanetary issues as an active amateur astronomer and collector of star atlases.

by Henry Cordova

I am curious as to how the width of a star’s habitable zone varies with respect to its luminosity.

It would not be unreasonable to assume that the surface temperature of a planet is directly related to the radiant flux of its star. Furthermore, it seems reasonable that the range of surface temperatures in which water can be a liquid on at least part of a planet’s surface is directly related to the stellar flux at its orbital distance. There may be many other factors involved, such as the properties of the planet’s atmosphere, its rotational characteristics, orbital elements and the variability and spectrum of its parent star; but let us ignore them for the moment and simply consider the geometrical parameters involved.

All else being equal, the radiant flux received by the planet must then be directly proportional to the luminosity of the star, and inversely proportional to the square of the planet’s distance from it. In other words, if one star is a hundred times more luminous than another, a planet orbiting the fainter star must be ten times closer to its primary in order to receive the same flux. The same reasoning can be applied to both the inner and outer edges of the star’s habitable zone. Regardless of how we define the HZ, it will become much narrower as the luminosity of the primary decreases. And the narrower an HZ is, the less likely there is a planet there.

Consider our own Sun’s habitable zone. Although there is some controversy about its dimensions, let us for the purposes of this argument say that it is limited by the orbits of Mars and Venus.The two planets have semi-major orbital axes of roughly 1.5 and 0.7 AU, respectively. These two figures mark the limits of Sol’s HZ, and their difference gives an HZ width of 0.8 AU, plenty of room to squeeze Earth in.

If our Sun were a hundred times less luminous, the HZ boundaries would be at 0.15 and 0.07 AU, which translates to an HZ width of only 0.08 AU! Clearly, the HZs of faint stars can be very narrow. The chances of a planet forming there, or migrating in, are substantially reduced.

Astronomers have detected planets in the HZs of some red dwarfs, but I feel this is due primarily to selection effects. Many of our planet detection techniques are very sensitive to big planets orbiting small stars in close, highly elliptical orbits, circumstances which are not conducive to life and yielding statistics that may give us a distorted idea of how solar systems form. Because of these considerations, red dwarfs may not be good candidates for life, even if we disregard other problems such as flares and tidal locking. It is true that these stars are often old, stable for long periods of time and by far the most common type of star, but I think we’re better off looking at brighter main sequence stars, such as spectral classes K, G or even F.

Several recent papers have pointed out that albedo effects on the planets of red dwarf stars may significantly expand the sizes of their habitable zones. See Joshi and Haberle, “Suppression of the water ice and snow albedo feedback on planets orbiting red dwarf stars and the subsequent widening of the habitable zone,” Astrobiology Vol. 12, No. 1 (23 Jan 2012); here’s the abstract. For Centauri Dreams‘ discussion on this, see M-Dwarfs: A New and Wider Habitable Zone.

These monographs suggest that on worlds with significant snow and ice cover, the effective albedo is much lower than snow and ice on Earth because frozen water absorbs more radiant heat at the red and infrared wavelengths emitted by red dwarfs than it does in the visual part of the spectrum as in Sol’s case. This effect would certainly extend the size of the habitable zone, but that would be dwarfed (no pun intended) by the much greater inverse square law effect (several orders of magnitude) of the much lower M-dwarf stellar luminosity.

Of the 53 known systems (66 stars) within 5 parsecs (16.3 ly) of the Sun, there are 48 red dwarfs (spectral class M) ranging from absolute magnitude 8.09 to 16.20 (an enormous range of luminosities!), and only 2 of them brighter than absolute magnitude 10.0. Absolute magnitude is the intrinsic brightness, how bright the star would appear if it were exactly 10 pc distant.

Keep in mind that magnitudes are exponential: A 5th magnitude star is 100 times brighter than one of 10th magnitude. Or alternatively, each magnitude is 2.512 times brighter than the next. The Sun’s absolute magnitude is 4.84, Barnard’s star, 13.23. Proxima Centauri is absolute magnitude 15.56. Although our own Sun is considered to be in the mid-range of stellar luminosities, it is still much brighter than most other stars. Red dwarfs may be very numerous, but they are very, very faint. All together, they don’t provide much habitable space.


Beamer Technology for Reaching the Solar Gravity Focus Line

Alex Tolley’s essay on using beaming technology to reach the solar gravity focus (SGF) caught the eye of Jim Benford, who has been exploring the prospects for beamed sails for many years. Along with brother Greg, Jim did laboratory work at the Jet Propulsion Laboratory some 20 years ago to demonstrate the method, and in the years since has written extensively on the uses of beaming within the Solar System as well as on interstellar trajectories. But what kind of beam are we talking about? Benford, a plasma physicist and CEO of Microwave Sciences, has done recent work on a gravitational focus mission in connection with Breakthrough Starshot. He points to the maturity of microwave technology and the cost savings involved in using microwaves for a mission far faster than anything that has yet flown.

by James Benford

An intermediate destination for beamed energy interstellar probes, such as Starshot, is the Sun’s Inner Gravitational Focus (SGF). Alex Tolley suggests using Beamer technology for this mission. Gregory Matloff and I studied this approach in 2018 in work on the Starshot Project and published it [1]. This is a summary and update of that work.

The on-going Starshot technology development program will build a modular Beamer system that will incrementally achieve steadily higher launch speeds. As the Starshot technology develops, velocity regimes beyond anything available now will be attained. This will include flyby probes of the outer solar system planets and moons, exploration of the Kuiper belt objects and interstellar precursors to investigate beyond the heliopause. All these missions have the advantage of not requiring any deceleration as the objective is reached. Thus consideration of earlier missions and destinations nearer than the Centauri system is in order.

Here we consider a specific application of the basic Starshot concept, to fly a mission at 100 km/sec. We take sailcraft parameters from Parkin’s Starshot System Model, a thin-film circular photon sail with a mass of 4 grams, a payload of 1.5 grams, a diameter of 5 meters and a thickness of about 0.1 micron (0.2 g/m2, in the range of graphene) [2]. In order not to choose the system parameters arbitrarily, we use the Beamer cost optimization method developed by Benford [3], which minimizes the total system cost.

Why Cost Matters

The approach in our paper is to stipulate the key parameters; mass and velocity, then minimize the cost of the system. All other parameters, such as the sail diameter and, most importantly, the frequency of the Beamer are varied in order to minimize costs. Why does cost matter? These are very expensive systems: note that Starshot is designed/optimized to have a system cost below $10B. We showed SGF Beamer Systems can be in order of magnitude lower.

Economies of Scale

The costs include the decrease in unit cost of hardware with increasing production, economies of scale [3]. The components we’re modeling here, sources of microwave, mm-wave and laser beams, antennas and optics, must be produced in large quantities for the large scales of directed energy-driven sails. High-volume automated manufacturing would drive costs down.

Cost-Optimized Systems

Microwave Beamer cost is 580 M$. (Parameters are wavelength 0.03 m, frequency 10 GHz) parameters for 100 km/sec, 3 gram, 5-meter diameter sail, perfect reflectivity, 0.3m wavelength.) Microwave costs have reached true economies of scale and are now available in quantity at about 0.01 $/W and about 100 $/m2. Consequently, there is no need to extrapolate future microwave cost because present costs are low enough to use.

Millimeter-Wave Beamer cost is 2 B$. Thus far, millimeter-wave (wavelength 3mm, 100 GHz) devices at ~ 1 MW are available at $6/W and 10,000 $/m2. No large market has developed for millimeter-wave devices, so economies of scale have not been firmly established. We assume the learning curve of millimeter-wave tubes will be approximately that of similar tube devices, such as klystron, for which the learning curve is well established. At present the largest application for a megawatt-level millimeter-wave sources is the ITER fusion project, which requires hundreds of devices. An emerging near-term application for millimeter–wave technologies is for 5G Wi-Fi. Although the power levels will be low because of the short-range requirement, mass manufacture of millimeter-wave transmitters and apertures may enable substantial cost reductions to be realized in the next few decades.

Laser Beamer cost is >5.3 B$. Parkin estimates contemporary costs as at least $150/W and 1M $/m2. There are several options for the technology of the laser Beamer: from small mm-scale wafers at ~ 1 W power to larger ~500 W lasers with long coherence length (a key constraint in operating an array). Cost elements include emitters, optics and amplifiers. Lasers are being used for LIDAR in autonomous vehicles and at powers of 10-100 W, cost 100-$1000 $/W. At the higher figure, the Beamer would cost 23 B$!

The large number of sails needed to provide a useful image of an exoplanet means that we must take into consideration the cost of sails. Each sail will cost far less than the Beamer. We estimated the cost of such sails at ~1M$ each [1].

Technology Readiness and Feasibility

  • State-of-the Art. Several practical factors favor microwave and millimeter waves over lasers, because they have practical advantages: Microwave equipment such as sources, anechoic rooms, antennas and diagnostics are commonly available than the emerging technology of high power lasers. That’s because microwave and millimeter wave sources, waveguide and supporting equipment, such as power supplies, are a developed industry. That means it is cheaper and faster to build systems. Lasers are developing fast, but at present are still expensive, and are produced in small numbers at slow rates.
  • Efficiency. Microwaves are more efficient than lasers, typically 50-90%. Millimeter wave generation technologies now make it possible to generate wavelengths as short as 0.1 cm with relatively high efficiency (>40%). Laser efficiencies are ~40% now and have been slowly rising.
  • Phased Arrays. Microwave phased arrays of transmitters and apertures are relatively easily done and are widely used, while phased arrays of laser beams, although possible in principle, subject to the coherence length constraint related above, are thus far little developed in practice. Work to date on laser phased arrays has been limited to small numbers of sources and modest power levels.

Desorption-Assisted Sail Missions

A different method that the JPL group has apparently not noticed is to use the desorption of various materials from the sail, ‘paints’, as it passes perihelion near the sun. That multiplies the utility of the solar sail technique substantially.

Thermal desorption consists in atoms, embedded in a substrate, that are liberated by heating, thus providing an additional thrust. Desorption can attain high specific impulse if low mass molecules or atoms are blown out of a lattice of material at high temperature.

Desorption of materials from hot sails in flight was observed in 2000 in microwave beam-driven carbon sail experiments I was conducting [4]. We found out that photon pressure could account for 3–30% of the observed acceleration, while the remainder came from desorption of embedded molecules.

After we understood what we were observing, my brother Gregory suggested it be used as a means of propulsion for sails [5,6]. The extraordinary potential of this sort of propulsion mechanism: if properly used, desorption could enhance thrust by orders of magnitude, shorten mission times.

Roman Kezerashvili and his fellow researchers have conducted detailed studies using desorption for solar sail missions to obtain high velocities [7]. Kezerashvili recently published a review article about this [8].


Therefore if we are to send probes to the SGF in this era, my calculations show that the lowest cost Beamer will be a microwave system. This will enable a transportation system within the Solar System that could be realized far sooner than laser arrays.

A solar sail augmented by desorption propulsion may give better performance for solar sail missions to the Sun’s Gravitational Focus.

If exoplanet imaging from the SGF is to be done soon, microwave or millimeter-wave beam systems could be built with existing technology now. Developing the phased array laser Beamer and driving the cost down to where larger arrays can be afforded will take decades. Similarly, it will take decades to conduct the test demonstrations required to prove the solar sail approach in the inner solar system. Advocates of both approaches should acknowledge these necessary timescales.


1. James Benford & Gregory Matloff, “Intermediate Beamers for Starshot: Probes to the Sun’s Inner Gravity Focus”, JBIS 72, 51-55, 2019.

2. Kevin Parkin, “The Breakthrough Starshot System Model”, Acta Astronautica 152 370, 2018.

3. J. Benford, “Starship Sails Propelled by Cost-Optimized Directed Energy”, JBIS 66 85, 2013.

4. James Benford et al., Flight and Spin of Microwave-Driven Sails, Final Report, Contract Number NAS8-99135, 2000. See also short version: “Flight and Spin of Microwave-driven Sails: First Experiments”, James Benford, Proc. Pulsed Power Plasma Science 2001, IEEE 01CH37251, 548, 2001.

5. Gregory Benford & James Benford “Desorption Assisted SunDiver Missions”, AIP Conf. Proc. 608, 462–469, 2002.

6. Gregory Benford, & James Benford, “Acceleration of Sails by Thermal Desorption of Coatings”, Acta Astronautica 56, 593–599, 2005.

7. Elena Ancona, Roman Ya. Kezerashvili, & Gregory L. Matloff, “Exploring the Kuiper Belt with sun-diving solar sails”, Acta Astronautica 160, 601–605 2019.

8. Elena Ancona & Roman Ya. Kezerashvili, “Extrasolar Space Exploration by a Solar Sail Accelerated via Thermal Desorption Of Coating”, Advances in Space Research 63 2021–2034, 2019.


A Holiday Thought Looking Ahead

I want to send along best wishes for the season to all of you. Centauri Dreams started as a book and became a study guide for me as I tried to keep up with ongoing developments in deep space research. But turning the site into a community, which I did in 2005 by adding comments, has been what really made it go, as I’ve continued to learn from the discussions between readers, finding new resources and different insights I would never have achieved on my own. So thank all of you for this continuing gift, and may this holiday season be the prelude to great discoveries ahead.


JPL Work on a Gravitational Lensing Mission

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).