Centauri Dreams
Imagining and Planning Interstellar Exploration
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.
Hayabusa2: Multiple Paths for Analyzing an Asteroid
Ryugu is classified as a carbonaceous, or C-type asteroid, a class of objects thought to incorporate water-bearing minerals and organic compounds. Carbonaceous chondrites, the dark carbon-bearing meteorites found on Earth, are thought to originate in such asteroids, but it has been difficult if not impossible to determine the source of most individual meteorites.
Hence the significance of the Hayabusa2 mission. JAXA’s successful foray to Ryugu represents the first time we’ve been able to examine a sample of a C-type asteroid through direct collection at the site. Ralph Milliken is a planetary scientist at Brown University, where NASA maintains its Reflectance Experiment Laboratory (RELAB). The laboratory expects samples collected at Ryugu to arrive in short order. Milliken is interested in the history of water in the object:
“One of the things we’re trying to understand is the distribution of water in the early solar system, and how that water may have been delivered to Earth. Water-bearing asteroids are thought to have played a role in that, so by studying Ryugu up close and returning samples from it, we can better understand the abundance and history of water-bearing minerals on these kinds of asteroids.”
Milliken is also one of many co-authors on a new paper in Nature Astronomy looking at the thermal history of subsurface materials exposed on Ryugu. The paper examines data the spacecraft collected during its operations there, which can now be compared to the sample collection. We learn that the asteroid may not be as water-rich as originally thought, leading to various scenarios about how it might have lost its water. High on the list of possibilities is that this ‘rubble pile’ asteroid — essentially loose rock maintaining its shape because of gravity — dried after a collision or other disruption and subsequent reformation.
Image: Japan’s Hayabusa2 spacecraft snapped pictures of the asteroid Ryugu while flying alongside it two years ago. The spacecraft later returned rock samples from the asteroid to Earth. Credit: JAXA.
Bear in mind how Hayabusa2 proceeded with its sampling at Ryugu. During the 2019 rendezvous, the spacecraft fired a projectile into the asteroid’s surface that exposed the subsurface rock examined here. A near-infrared spectrometer was used to compare the water content of the surface with the material below, showing the two to be similar in water content. The authors see that as a clue that Ryugu’s parent body dried out, rather than the surface of Ryugu being dried out by the Sun, perhaps in a close solar pass earlier in its history.
In other words, heating by the Sun in one or more close solar passes would be likely to occur at the surface, without penetrating deep into the asteroid. What Hayabusa2’s spectrometer shows is that surface and sub-surface are both comparatively dry, which is an indication that it was the parent body of Ryugu, rather than an event happening to Ryugu itself, that produced this result.
Ahead for the Ryugu analysis is the need to study the size of the particles excavated from below the surface, which could play a role in how the spectrometer measurements are interpreted.
“The excavated material may have had a smaller grain size than what’s on the surface,” says Takahiro Hiroi, a senior research associate at Brown and another study co-author. “That grain size effect could make it appear darker and redder than its coarser counterpart on the surface. It’s hard to rule out that grain-size effect with remote sensing.”
The beauty of the successful sample return is that hypotheses about Ryugu’s past can now be evaluated through comparison of the remote sensing data and actual laboratory work. These are exciting times indeed for the scientists studying the extensive collection of asteroid debris Hayabusa 2 brought back. We can expect significant papers on all this in 2021.
The paper is Kitazato et al., “Thermally altered subsurface material of asteroid (162173) Ryugu,” Nature Astronomy 4 January 2021 (abstract).
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.
A Statite ‘Slingshot’ for Catching Interstellar Objects
I see that a white paper on Richard Linares’ interesting ‘statite’ concept became available just before Christmas, and I want to call your attention to it (and thanks to Antonio Tavani for the pointer). Back in April, the idea received funding as a Phase 1 study in the NASA Innovative Advanced Concepts (NIAC) Program, renewing attention on the matter of interstellar objects (ISO) like ‘Oumuamua. The notion is to deliver a payload to an object discovered entering our Solar System so that, unlike the two we’ve found thus far (‘Oumuamua and 2I/Borisov), we can examine them up close before they depart. Statites are to be the enabling technology.
So let’s circle back to that concept, for statites are getting more interest these days given the plans for Solar Cruiser, NASA’s solar sail mission that may experiment with maneuvers that allow it to act in non-Keplerian ways. The idea is that a solar sail can achieve ‘station-keeping’ — hovering in place — by using light pressure from the Sun to offset gravitational forces. That would allow what Robert Forward termed ‘displaced orbits.’ Forward envisioned what he called ‘polesitters,’ satellites that could, if positioned at high Earthly latitudes, offer telecommunications services through geosynchronous coverage of the polar regions.
Solar Cruiser could use these methods in its experiments to observe the Sun’s polar regions, though how much the statite idea would be explored during the mission is not yet determined. What Linares has in mind is the fact that it’s extraordinarily difficult to imagine a mission to catch an incoming interstellar asteroid or comet. Such objects show up suddenly and with high heliocentric velocities. A constellation of statites, however, located stationary in terms of the Sun, would make it possible to move quickly into the appropriate flyby or rendezvous trajectory. The payload can be quickly placed on a trajectory toward the Sun, using the sail and, perhaps, electric propulsion along the way for course adjustment.
Here’s how Linares and team describe the idea:
The proposed mission concept is to have a constellation of statites where: A) each statite enters into a stationary, “low-energy” state, which it can hold indefinitely, awaiting a potential ISO; B) once an ISO is detected, a flyby or rendezvous trajectory is calculated with an expected 4-16 months of lead time; C) a single statite releases a CubeSat which enters into a freefall trajectory with respect to the Sun or uses attitude control to orient the solar sail and adjust the solar radiation pressure force to accomplish a rendezvous; D) the CubeSat, which may use propulsion or the solar sail, then adjusts its trajectory for a flyby or rendezvous with the ISO; and E) onboard sensors are used to make critical scientific measurements.
Image: To ensure the best coverage of the Solar System, MIT’s Richard Linares envisions a constellation of “statites” that communicate and work together, activating the statite in the optimum position to fly by or rendezvous with an interstellar object. Other statites in the constellation can continue to wait for the next ISO to appear. Credit: Linares et al.
So we use solar photons and the momentum they impart to cancel the Sun’s gravitational force, enabling a constellation of statites that can hover in place indefinitely. The effort takes shape under the umbrella of what Linares describes as a “dynamic orbital slingshot for rendezvous with interstellar objects.” We learn from the white paper that the NIAC work will take in the design of a statite constellation and its configuration, as well as its capability for achieving rapid rendezvous. The potential of added solar electric propulsion will be considered as a way of improving performance, while scientific instruments and operational factors will be analyzed.
As statites must cancel out the gravitational acceleration caused by the Sun, the area-to-mass ratio of the sail is critical, for both the gravitational force and the propulsive force of the sail vary as 1/R2. Any statite can thus cancel the gravitational force as long as it has a suitable area-to-mass ratio. Linares and team assume the aluminized, temperature-resistant CP-1 — an ultra-thin reflective polymer — as the sail material.
And when an incoming interstellar object is detected? The levitating sail has an inertial velocity of zero, and when released from ‘hover,’ it enters a Keplerian orbit. In the example given in the paper, a statite at 1 AU has a free-fall trajectory of 64 days. The choice of the term ‘slingshot effect’ is dictated by the fact that the sail is essentially being used to store energy that is released when needed, with the free-fall path to solar flyby adjusted along the way. Linares’ simulations show effective rendezvous with our two previous interstellar visitors:
…a DeltaV of 104 km/s and 145 km/s are achieved for the ‘Oumuamua and Borisov rendezvous trajectories, respectively. These high Delta-V values are possible due to the fact that the statite can cancel the solar gravity completely, independent of the distance to the Sun. Moreover, from these initial proof-of-concept results, we can see that the rendezvous times are relatively short for both of these notional missions. For ‘Oumuamua, the rendezvous time is approximately 5 months and the rendezvous occurred at 0.76 AU. For Borisov, the rendezvous time was approximately 8 years and the rendezvous occurred at 30.6 AU. Although the Borisov trajectory had a long time of flight, it was still within typical mission timescales, while requiring neither large propulsion systems nor long lead times.
The paper argues that a statite free-falling toward the Sun from an initial position at 1 AU and then deploying its sail away from the Sun at perihelion can achieve speeds of up to 25 AU/year, making it possible to deliver payloads to the outer Solar System. Voyager 1 has reached 3.6 AU per year by comparison, making the statite concept attractive beyond its value as a station-keeper for quick response missions to interstellar comets/asteroids.
The paper is Linares et al., “Rendezvous Mission for Interstellar Objects Using a Solar Sail-based Statite Concept,” now available as a preprint. For more on Solar Cruiser, see Heliophysics with Interstellar Implications. See also Les Johnson’s analysis “The Solar Cruiser Mission Concept — Enabling New Vistas for Heliophysics,” Bulletin of the American Astronomical Society, Vol. 52, No. 3 (June, 2020). Abstract.
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].
Conclusion
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.
References
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 Beamed Sail to the Sun’s Gravity Focus
Our recent discussions about Claudio Maccone’s FOCAL mission to the Sun’s gravitational focus, and the ongoing work at the Jet Propulsion Laboratory for NASA’s Innovative Advanced Concepts office, have had Alex Tolley thinking about alternative scenarios. Yes, a spacecraft moving along the focal line extending from the solar gravitational lens (SGL) would be capable of extraordinary imaging, and could serve as a communications relay for interstellar probes, but that tricky Sundiver maneuver suggested by Slava Turyshev and team in their ‘string of pearls’ concept puts huge demands on sail materials. Moreover, we’d ideally like to be able to slow the craft as it moves along the focus, to allow maximum time for observations. To achieve both fast transit and maneuverability at the gravitational focus, Alex advocates beamed propulsion, a method whose advantages and consequences are discussed below. Synergies with the ongoing Breakthrough Starshot effort are apparent.
by Alex Tolley
The huge increase in discovered exoplanets, many in the habitable zones (HZ) of their stars, has increased the push to determine if life exists on any of these worlds. With even our best telescopes, light from these worlds is just a single pixel in extent, which allows spectrographic analysis for biosignatures and techno-signatures. However, a 2D image of an exoplanet would answer many more questions about these worlds. The image makes a lot of difference both to scientists and the public. The astronomy books of my youth showed the moons of the gas giants as points of light, and Pluto was just a star-like object. As late as 2010, the Hubble telescope could only manage a crude blurry image of Pluto showing some light and dark regions. The New Horizon probe flyby with high-resolution images changed our view of the Pluto-Charon binary.
Image: Hubble 1200×1200 pixel image of Pluto in February 2010.
To acquire a megapixel image of even a nearby exoplanet would require a telescope of tens of kilometers in diameter to collect the light of the planet while masking out the far more intense light of its star. A possible solution was proposed by Claudio Maccone, with his proposed FOCAL mission to use our sun’s gravity to focus distant light rays on a telescope [5]. While there are technical issues still to be resolved in how to form a low noise 2D image of 10,000 (100 x 100) to 1 million (1000 x 1000) pixels, the big question is:
“How do we get there in a short enough time for a project , and how do we best collect the data to make the image?”
The Sundiver Problem
The solar gravitational focus (SGF) is a line extending from about 550 AU to infinity. The focus starts out in interstellar space. For comparison, Voyager 1 launched over 40 years ago and has just passed through the heliopause of our sun and into interstellar space. It is only ¼ of the way to the SGF. It will not reach the focus for another 120 years. Clearly, we need a much faster way to reach the focus so that scientists and engineers can reasonably engage in a realistic mission, rather than taking the generations that were required to build a cathedral.
At this point, we can rule out most propulsion technologies, even using gravity assists. The most promising is a solar sail, and as I will argue, a sail augmented with a beam.
The authors of the recent NIAC II report [1, 6] opt for a pure solar sail. They propose a mission that could reach the SGF within a reasonable 25 years using a very advanced solar sail and leveraging a ‘sundiver’ trajectory. This sail could achieve a velocity of 25 AU/yr (about 125 km/s), of which most of that velocity is achieved quickly as the sail leaves perihelion. When I say ‘advanced,’ the mission needs a sail that has an areal density of less than 10 g/m2, and possibly 1 g/m2. Note that this density is not just the sail material, but must include the probe structure, payload, and any auxiliary equipment, such as communications, maneuvering thrusters, etc.
To put this in perspective, the Planetary Society’s CubeSat LightSail has an areal density of about 143 g/m2, a similar density as the upcoming NEA Scout probe. The target areal density of the Breakthrough Starshot beamed sail is 1.4 g/m2, achieved by having just a chip-sized payload on the sail of just 1 m2 [4].
Without such low areal densities, 2 to 4 orders of magnitude lower than currently achieved, there is no hope of reaching the needed velocities. The sail craft must also do a sundiver maneuver to gain maximum thrust at perihelion. Maximum velocity is achieved by the orbit of Saturn. How close the sail can approach the sun depends on the sail materials. To get both the very low sail mass to achieve the needed areal density, and approach the sun to within 0.1 AU (15 million km) requires a strong, low-density material with a high melting point, such as a ceramic.
If such a solar sail is achievable, then the craft with its telescope payload will continue on into interstellar space along the focal line.
While the focal line continues to infinity, is there an optimum “sweet spot” to image the target? Maccone states that although complex, the best position might be fairly close to the 550 AU start of the SGF [1]. If that is correct, it is suboptimal to allow the telescope to continue traveling rapidly away from that position.
The NIAC authors solve this rather cleverly: launch a series of sails, a year apart, forming a “string of pearls” separated by 25 AU, so that each probe stays within 25 AU of the start of the SGF at 550 AU before handing off the data collection to the next probe, or even contributing to the collection of other data as it journeys on into interstellar space.
This solution has several disadvantages, not the least of which is the need to ensure that each sail can align independently and correctly with the target.
Therefore, ideally, one would want the sailcraft and its telescope to effectively stop at the best distance just beyond 550 AU. This has several advantages:
- The focus remains unchanged – reducing issues with image deconvolution
- The required coronagraph to isolate the Einstein ring of light is fixed in dimension and position.
- The tracking of both the target star’s position and the exoplanet in its orbit is simplified as the distance to track relative motion increases as the distance from the sun increases.
- Only one imaging telescope is needed, as well as the auxiliary equipment. The scale economies of building and flying multiple sails can be used to image multiple targets instead of just one.
- Data collection can be as long as desired, not fixed for the number of sails sent, allowing better images to be produced, as well as longer-term observations of the target for other purposes. If the payload includes a receiver for a communication bridge to a probe orbiting an exoplanet, the communication link can be established for as long as needed, perhaps many decades.
Using sail material that is off-the-shelf, how could we achieve such a mission?
The Benefits of Beamed Propulsion
My proposed solution is to use a beamed sail. The beam would most likely be a phased laser array located on Earth with the capability of pushing a sail in all but the highest latitudes. This is the type of power source that is being studied by the Breakthrough StarShot team. Because the sail need not achieve the fractional c velocities needed for interstellar flight, the size of the sail, payload, and beam intensity on the sail can be matched to the materials and payload requirements. Ideally, the sail velocity would exceed that of 25 AU/yr to shorten the flight time, but no longer restricted to the high-performance sundiver mission needed to reach that velocity. There is also no need to tolerate high temperatures at perihelion, although this will also depend on the laser power and duration.
Moreover, there is a way to stop a beamed sail, which was suggested by Robert Forward [7]. Figure 1 below shows a schematic of the concept for this deceleration maneuver.
The sail would separate into 2 parts with the larger part focussing the beam on the smaller sail to decelerate it. This was proposed by Forward as a way to decelerate a sail and its payload on arriving at its desired destination. The same approach could be used to decelerate a beamed sail when it reaches the 550 AU minimum focus position. When “stopped”, the sail would deploy its imager/receiver.
Image: Schematic of the round-trip interstellar lightsail concept proposed by Forward (not to scale), shown during the deceleration phase. Credit: Geoffrey Landis [2].
The relative sizes of the two sails will depend on the achievable laser strength on the sail over the deceleration distance that ends around 550 AU, and on how well the reflected beam can be targeted to the sail with the payload. The aim is to slow the smaller sail with the telescope payload down towards a dead stop, although any low velocity, like that of Voyager 1, is adequate to ensure a long data collection period and minimal changes to the required maneuvering to track the star and planet. Pointing the laser at the sail will require relatively simple position prediction up to about 160 hours in the future based on the most recent time-stamped location received from the craft. [160 hours is 2x the light travel time to 550 AU; signal sent from the craft that reaches earth 80 hours later and the next 80 hours for the beam to reach the sail.]
The slow movement of the stars relative to the sun will still require some traversal across the focus to track this motion and ensure the craft stays within the best position to receive the communication or visual image. For example, Proxima moves about 3.85 arcseconds per year across the heavens. At 550 AU, this implies that to keep the sun in line with Proxima, the craft must travel a modest 50 m/s to maintain position.
One issue not so far mentioned is the position of the stellar target in the sky. Unlike most of our planetary missions to date, where the planets fairly closely aligned with the ecliptic, the target stars for an SGL mission are spread out over the celestial sphere. For example, Alpha Centauri is in the southern sky with a declination of over -62 degrees. The TRAPPIST 1 planetary system, about 40 ly away, with some possibly very interesting exoplanets in close proximity for imaging, is about -5 degrees of declination when observed from Earth. The sail craft must travel in the opposite direction so that the target is directly behind the sun, so that for Proxima the telescope must be positioned with a declination of over +62 degrees and the appropriate 180 degrees offset to right ascension.
Each sail craft can only image one target star at a time, although if that star has several planets, all these planets may be imaged over time. The stars are far too separated over the celestial sphere for any reasonable time to navigate between them. For the 30 pc (100 ly) volume, the 15,500+ FGK stars would be separated by about 60 AU on average. Therefore each sailcraft could only image one star system. This would therefore require considerable care in selection. However, given the laser infrastructure and the scale economy of manufacturing the sail craft, it might well make sense to send many craft out to their SGF positions.
The benefit of a FOCAL mission to acquire relatively high resolution images of exoplanets far beyond any single telescope we can envision today is offset by the demands of reaching the gravitational focal line starting at 550 AU. Once there, ideally, the craft should not continue its outbound journey. To achieve this, a beamed sail that can decelerate its payload is proposed.
References
1. Gilster, P. (2020) JPL Work on a Gravitational Lensing Mission, https://www.centauri-dreams.org/2020/12/16/jpl-work-on-a-gravitational-lensing-mission/
2. Landis, G (1989). Optics and Materials Considerations for a Laser-propelled Lightsail” http://www.geoffreylandis.com/lightsail/Lightsail89.html
3. Vulpetti, G., Johnson, L., & Matloff, G. L. (2008). Solar sails: A novel approach to interplanetary travel. New York: Copernicus Books.
4. Montgomery, E, Johnson, L, (2017) Solar Sail Propulsion: A Roadmap from Today’s Technology to Interstellar Sailships. Presentation to Foundations of Interstellar Studies Workshop, New York City College of Technology, Brooklyn, NY
5. Maccone, C. (2009) Deep Space Flight And Communications: Exploiting The Sun As A Gravitational Lens (Springer Praxis Books / Astronautical Engineering)
6. Turyshev et al. (2020), “Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravity Lens Mission.” Final Report. NASA Innovative Advanced Concepts Phase II
7. Forward, R (1984) “Roundtrip interstellar travel using laser-pushed lightsails,” American Institute of Aeronautics and Astronautics, v21 No.2, pp 187-195
Follow with RSS or E-Mail
