Centauri Dreams
Imagining and Planning Interstellar Exploration
The Transition from Rocky to Non-Rocky Planets
As I decompress from the Tennessee Valley Interstellar Workshop (and review my notes for next week’s report), I have the pleasure of bringing you Andrew LePage’s incisive essay into a key exoplanet question. Are some of the planets now considered potentially habitable actually unlikely to support life? Recent work gives us some hard numbers on just how large and massive a planet can be before it is more likely to be closer to Neptune than the Earth in composition. The transition from rocky to non-rocky planets is particularly important now, when our instruments are just becoming able to detect planets small enough to qualify as habitable. LePage, who writes the excellent Drew ex Machina, remains optimistic about habitable planets in the galaxy, but so far the case for many of those identified as such may be weaker than we had thought. A prolific writer, Drew is also a Senior Project Scientist at Visidyne, Inc., where he specializes in the processing and analysis of remote sensing data.
by Andrew LePage
For much of the modern era, astronomy has benefitted greatly from the efforts of amateur scientists. But while amateur astronomers equipped with telescopes have certainly filled many important niches left by the far less numerous professionals in the field, others interested in astronomy equipped with nothing more than a computer and an Internet connection are capable of making important contributions as well. One project taking advantage of this resource is Planet Hunters.
The Planet Hunters project was originally started four years ago by the Zooinverse citizen science program to enlist the public’s help in searching through the huge photometric database of NASA’s Kepler mission looking for transits caused by extrasolar planets. While automated systems have been able to uncover thousands of candidate planets, they are limited to finding only what their programmers designed them to find – multiple, well defined transits occurring at regular intervals. The much more adaptable human brain is able to spot patterns in the changes in the brightness of stars that a computer program might miss but could still indicate the presence of an extrasolar planet. Currently in Version 2.0, the Planet Hunters project has uncovered 60 planet candidates to date through the efforts of 300,000 volunteers worldwide.
A paper by a team of astronomers with Joseph Schmitt (Yale University) as the lead author was just published in The Astrophysical Journal which describes the latest find by Planet Hunters. The target of interest for this paper is a billion year old, Sun-like star called Kepler 289 located about 2,300 light years away. Automated searches of the Kepler data had earlier found two planets orbiting this distant star: a large super-Earth with a radius 2.2 times that of the Earth (or RE) in a 34.5-day orbit originally designated Kepler 289b (called PH3 b in the new paper) and a gas giant with a radius of 11.6 RE in 125.8-day orbit, Kepler 289c (now also known as PH3 d). The new planet, PH3 c, has a radius of 2.7 RE and a mean orbital period of 66.1 days. With a mean stellar flux about 11 times that of Earth, this planet is highly unlikely to be habitable but its properties have profound implications for assessing the potential habitability of other extrasolar planets.
The planet had been missed by earlier automated searches because its orbital period varies regularly by 10.5 hours over the course of ten orbits due to its strong interactions with the other two planets, especially PH3 d. Because of this strong dynamical interaction, it was possible for Schmitt et al. to use the Transit Timing Variations or TTVs observed in the Kepler data to compute the masses of these three planets much more precisely than could be done using precision radial velocity measurements. The mass of the outer planet, PH3 d, was found to be 132±17 times that of Earth (or ME) or approximately equivalent to that of Saturn. The mass of the inner planet, PH3 b, was poorly constrained with a value of 7.3±6.8 ME. The newest discovery, PH3 c, was found to have a mass of 4.0±0.9 ME which, when combined with the radius determined using Kepler data, yields a mean density of 1.2±0.3 g/cm3 or only about one-fifth that of the Earth. Models indicate that this density is consistent with PH3 c possessing a deep, hot atmosphere of hydrogen and helium making up about half of its radius or around 2% of its total mass.
PH3 c is yet another example of a growing list of known low-density planets with masses just a few times that of the Earth that are obviously not terrestrial or rocky in composition. Before the Kepler mission, such planets were thought to exist but their exact properties were unknown because none are present in our solar system. As a result, the position in parameter space of the transition from rocky to non-rocky planets and the characteristics of this transition were unknown. So when astronomers were developing size-related nomenclature to categorize the planets they expected to find using Kepler, they somewhat arbitrarily defined “super-Earth” to be any planet with a radius in the 1.25 to 2.0 RE range regardless of its actual composition. Planets in the 2.0 to 4.0 RE range were dubbed “Neptune-size”. This has generated some confusion over the term “super-Earth” and has led to claims about the potential habitability of these planets being made in the total absence of an understanding of the true nature of these planets. Now that Kepler has found planets in this size range, astronomers have started to examine the mass-radius relationship of super-Earths.
The first hints about the characteristics of this transition from rocky to non-rocky planets were discussed in a series of papers published earlier this year. Using planetary radii determined from Kepler data and masses found by precision radial velocity measurements and analysis of TTVs, it was found that the density of super-Earths tended to rise with increasing radius as would be expected of rocky planets. But somewhere around the 1.5 to 2.0 RE range, a transition is passed where larger planets tended to become less dense instead. The interpretation of this result is that planets with radii greater than about 1.5 RE are increasingly likely to have substantial envelopes of various volatiles such as water (including high pressure forms of ice at high temperatures) and thick atmospheres rich in hydrogen and helium that decrease a planet’s bulk density. As a result, these planets can no longer be considered terrestrial or rocky planets like the Earth but would be classified as mini-Neptunes or gas dwarfs depending on the exact ratios of rock, water and gas.
Image: It now appears that many of the fanciful artist depictions of super-Earths are wrong and that most of these planets are more like Neptune than the Earth (NASA Ames/JPL-Caltech).
A detailed statistical study of this transition was submitted for publication this past July by Leslie Rogers (a Hubble Fellow at the California Institute of Technology) who is also one of the coauthors of the PH3 c discovery paper. In her study, Rogers confined her analysis to transiting planets with radii less than 4 RE whose masses had been constrained by precision radial velocity measurements. She excluded planets with masses determined by TTV analysis since this sample may be affected by selection biases that favor low-density planets (for a planet of a given mass, a large low-density planet is more likely to produce a detectable transit event than a smaller high-density planet). Rogers then determined the probability that each of the 47 planets in her Kepler-derived sample were rocky planets by comparing the properties of those planets and the associated measurement uncertainties to models of planets with various compositions. Next, she performed a statistical analysis to assess three different models for the mass-radius distribution for the sample of planets. One model assumed an abrupt, step-wise transition from rocky to non-rocky planets while the other two models assumed different types of gradual transitions where some fraction of the population of planets of a given radius were rocky while the balance were non-rocky.
Rogers’ analysis clearly showed that a transition took place between rocky and non-rocky planets at 1.5 RE with a sudden step-wise transition being mildly favored over more gradual ones. Taking into account the uncertainties in her analysis, Rogers found that the transition from rocky to non-rocky planets takes place at no greater than about 1.6 RE at a 95% confidence level. Assuming a simple linear transition in the proportions of rocky and non-rocky planets, no more than 5% of planets with radii of about 2.6 RE will have densities compatible with a rocky composition to a 95% confidence level. PH3 c, with a radius of 2.7 RE, exceeds the threshold found by Rogers and, based on its density, is clearly not a terrestrial planet.
An obvious potential counterexample to Rogers’ maximum rocky planet size threshold is the case of Kepler 10c, which made the news early this year. Kepler 10c, with a radius of 2.35 RE determined by Kepler measurements and a Neptune-like mass of 17 ME determined by radial velocity measurements, was found to have a density of 7.1±1.0 g/cm3. While this density, which is greater than Earth’s, might lead some to conclude that Kepler 10c is a solid, predominantly rocky planet, Rogers counters that its density is in fact inconsistent with a rocky composition by more than one-sigma. Comparing the measured properties of this planet with various models, she finds that there is only about a 10% probability that Kepler 10c is in fact predominantly rocky in composition. It is much more likely that it possesses a substantial volatile envelope albeit smaller than Neptune’s given its higher density.
While much more work remains to be done to better characterize the planetary mass-radius function and the transition from rocky to non-rocky planets, one of the immediate impacts of this work is on the assessment of the potential habitability of extrasolar planets. About nine planets found to date in the Kepler data have been claimed by some to be potentially habitable. Unfortunately, all but two of these planets, Kepler 62f and 186f, have radii greater than 1.6 RE and it is therefore improbable that they are terrestrial planets, never mind potentially habitable planets.
This still leaves about a dozen planets that have been frequently cited as being potentially habitable that were discovered by precision radial velocity surveys whose radii are not known. However, we do know their MPsini values where MP is the planet’s actual mass and i is the inclination of the orbit to our line of sight. Since this angle cannot be derived from radial velocity measurements alone, only the minimum mass of the planet can be determined or the probability that the actual mass is in some range. Despite this limitation, the MPsini values can serve as a useful proxy for radius.
Rogers optimistically estimates that her 1.6 RE threshold corresponds to a planet with a mass of about 6 ME assuming an Earth-like composition (which is still ~50% larger than the measured mass of PH3 c, which is now known to be a non-rocky planet). About half of the planets that some have claimed to be potentially habitable have minimum masses that exceed this optimistic 6 ME threshold while the rest have better than even odds of their actual masses exceeding this threshold. If the threshold for the transition from rocky to non-rocky planets is closer to the 4 ME mass of PH3 c, the odds of any of these planets being terrestrial planets are worse still. The unfortunate conclusion is that none of the planets discovered so far by precision radial velocity surveys are likely to be terrestrial planets and are therefore poor candidates for being potentially habitable.
Please do not get me wrong: I have always been a firm believer that the galaxy is filled with habitable terrestrial planets (and moons, too!). But in the rush to find such planets, it now seems that too many overly optimistic claims have been made about too many planets before enough information was available to properly gauge their bulk properties. Preliminary results of the planetary mass-radius relationship now hints that the maximum size of a terrestrial planet is probably about 1½ times the radius of the Earth or around 4 to 6 times Earth’s mass. Any potentially habitable planet, in addition to having to be inside the habitable zone of the star it orbits, must also be smaller than this. Unfortunately, while recent work suggests that planets of this size might be common, our technology is only just able to detect them at this time. With luck, over the coming years as more data come in, we will finally have a more realistic list of potentially habitable planet candidates that will bear up better under close scrutiny.
The discovery paper for PH3 c by Schmitt et al., “Planet Hunters VII: Discovery of a New Low-Mass, Low Density Planet (PH3 c) Orbiting Kepler-289 with Mass Measurements of Two Additional Planets (PH3 b and d)”, The Astrophysical Journal, Vol. 795, No. 2, ID 167 (November 10, 2014) can be found here. The paper by Leslie Rogers submitted to The Astrophysical Journal, “Most 1.6 Earth-Radius Planets are not Rocky”, can be found here.
For a fuller discussion of how Rogers’ work impacts the most promising planets thought by many to be potentially habitable, please refer to Habitable Planet Reality Check: Terrestrial Planet Size Limit on my website Drew Ex Machina.
Tennessee Valley Interstellar Workshop
I’m at the Tennessee Valley Interstellar Workshop in Oak Ridge for the next few days. As I’ve done at past conferences, I’ll need to spend my time taking the notes that will be turned into next week’s entries here. That means no further posts until Friday, though I’ll try to keep the comment moderation going, perhaps with a few delays. TVIW 2014 has lined up a good group of speakers including, besides MSFC’s Les Johnson himself (TVIW’s founder), exoplanet hunter Sara Seager, beamed sail specialist Jim Benford, the SETI League’s Paul Shuch and TZF founder Marc Millis, along with a healthy representation from Icarus Interstellar. I’m also looking forward to the workshop tracks and will be participating in one called “Language as Reality: A Near-Term Roadmap for Exploiting Opportunities and Natural Experiments Here on Terra Firma to Inform *C*ETI.” Expect a complete report when I get back.
Interstellar Arrival: Slowing the Sail
Some final thoughts on hybrid propulsion will wrap up this series on solar sails, which grew out of ideas I encountered in the new edition of the Matloff, Johnson and Vulpetti book Solar Sails: A Novel Approach to Interplanetary Travel (Copernicus, 2014). The chance to preview the book (publication is slated for later this month) took me in directions I hadn’t anticipated. Solar Sails offers a broad popular treatment of all the sail categories and their history, as you’d expect, but this time through I focused on its four technical chapters on sail theory that helped me review the details.
And because I kept running into the idea of multiple modes of propulsion, my thoughts on avoiding doctrinaire solutions continue to grow. In fact, I’d venture to say that probing into the possibilities of multimodal propulsion may offer a serious opportunity for insights. Centauri Dreams regular Alex Tolley came up with one of these yesterday, asking whether a sail mission to Jupiter space might deploy the planet’s huge magnetic field as an assist. Alex invokes Pekka Janhunen’s ideas about electric sails. Let me quote from the Solar Sails book on what Janhunen has in mind:
Similar to the magsail, this concept uses the solar wind for producing thrust. However, different from the magsail, this sail interacts with the solar plasma via a mesh of long and thin tethers kept at high positive voltage by means of an onboard electron gun. In its baseline configuration, the spacecraft spins and the tethers are tensioned by centrifugal acceleration. It should be possible to control each wire voltage singly, at least to within certain limits.
We get thrust out of this when protons from the solar wind, positively charged, are repelled by the positive voltage of the spacecraft’s tethers, while electrons are captured and ejected — otherwise, their growing numbers would neutralize the voltage in the tether mesh. But Alex also brings to mind Mason Peck’s interesting work at Cornell on miniaturized ‘Sprites,’ tiny chip-like spacecraft that could use the Lorentz force to accelerate in directions perpendicular to the magnetic field. Remember that Jupiter’s magnetic field is 18,000 times stronger than Earth’s, a useful resource if we can tap it even so far as to adjust the orbits of planetary probes.
Alex’s thoughts on the matter deserve to be quoted:
We often think of sail ships as clipper ships – i.e. using large surfaces to capture or direct the wind to move. But modern ships use screws. There have also been numerous wind turbine designs that offer advantages over canvas sails, even if they are not as aesthetic to the eye. (Clipper ships were possibly the most pleasing ship designs ever built). Might we be thinking too much in terms of sails that mimic the romance of traditional sails, rather than designs that might offer better performance, albeit with some aesthetic loss?
Interstellar Arrival
A sense of aesthetics produces pleasing designs but what looks best isn’t always what we need. Back in my flying days some of us used to talk about (and in a few cases actually fly) some of the great aircraft designs of the 1930s and later, and although I never got my hands on the controls of one, a great favorite was the Beech Staggerwing, a gorgeous design with a negative wing stagger, meaning that the lower wing is farther forward than the upper. Designs like this could be sleek and lovely because of the medium they worked in. But spacecraft don’t need wings and streamlined fuselages, and our Voyagers and Cassinis look nothing like the wilder designs of early science fiction because they don’t need to, never encountering a planetary atmosphere.
Image: The Beech Model 17 Staggerwing, first produced in 1932. Credit: Wikimedia Commons.
A beamed lasersail on its way to Alpha Centauri may be anything but a thing of beauty. Once the mission enters its cruise phase, the sail can be safely stowed, and one good use for it would be to shroud the payload to offer additional protection against radiation. We’re always trying to think of ways to get more value out of existing assets, which is what extended missions are all about. Or think about the Benfords’ JPL work that revealed desorption. No one with an eye for design would come up with painting a desorption layer on a sailcraft, but it’s conceivable that desorption, which is the release of CO2, hydrocarbons and hydrogen from within the manufactured sail as it heats up under the beam, could give an added kick to interplanetary sails being pushed by powerful microwave beams.
Mentioning Forward brings me back around to the ‘staged sail’ concept that he worked out for stopping at another star. The sail has three divisions, as shown in the diagram below, which is taken from his paper on a manned mission to Epsilon Eridani. ‘Staging’ the sail means losing first the outer ring, then the middle one, until only the inner ring is left. In sequence, the spacecraft slows down by using laser light beamed from our Solar System, reflected off the now separated outer sail as it approaches the star — the light is directed back at the two remaining sail segments with payload. Ingenious tinkering let Forward use the second sail detachment as the way the crew got home, with laser light boosting the much smaller inner sail by reflection from the middle segment.
Image: Robert Forward’s staged sail concept. What he calls a ‘paralens’ in the diagram is an enormous Fresnel lens in the outer Solar System, made of concentric rings of lightweight, transparent material with free space between the rings. Credit: Robert Forward.
Staged sails are hard to see as anything but a longshot — the success of the mission depends not only upon perfect execution of the staging process but, crucially, upon the laser beam from Earth being able to illuminate the sail segments effectively. Forward was fully aware of the possibilities here, and you can find discussion in places like his novel Rocheworld (Baen, 1990) about how politics on Earth might affect the use of the expensive beam. I for one wouldn’t want to put my life in the hands of a design like this, which depends so crucially upon decisions made far from the spacecraft.
Interestingly, like Mason Peck, Forward had some thoughts on how we might use the Lorentz force as well. Remember that a charged object moving through a magnetic field experiences this force at right angles to its direction of motion and the magnetic field itself. Out of this you get ‘thrustless turning,’ which both Forward and Philip Norem thought could be used for deceleration. Instead of staged sails, you get an electrostatically charged probe — think of Janhunen’s electric sail tethers — on a trajectory that goes well beyond the target star. The spacecraft’s interactions with the galactic magnetic field bend its trajectory so that it approaches the target from behind.
Once it’s inbound to the destination system, a laser beam from Earth can be turned upon it to slow it down for arrival. The idea is anything but aesthetic, just as the Janhunen sail would look like something closer to a porcupine than the silvery lozenge of an early SF starship. It’s also hampered by the fact that mission times, already measured in decades at minimum, are tripled with the use of this maneuver. I should mention that Solar Sails authors Gregory Matloff and Les Johnson have also explored the uses of electrodynamic tethers to supply power to an Alpha Centauri expedition, even if a Norem-style arrival seems too lengthy.
Creative thinking about these matters often springs from putting two or more solutions together to see what can happen. What I’ve always admired about the interstellar community is its ability to re-examine older concepts to look for interesting cross-pollination of ideas. As we move into the era of increasingly tiny components, it’s heartening to think how many designs will be affected by new nanotechnological possibilities. Mason Peck has talked about using Jupiter’s magnetic field to spew thousands of ‘Sprites’ out on interstellar trajectories. What else can we imagine as we look for extended uses of existing tech and ponder where they might lead us?
Forward’s paper on staged sails is “Roundtrip Interstellar Travel Using Laser-Pushed Lightsails,” Journal of Spacecraft and Rockets 21 (1984), pp. 187-195. The Norem paper is “Interstellar Travel: A Round Trip Propulsion System with Relativistic Capabilities,” AAS 69-388 (June, 1969). Forward’s paper on Lorentz force turning is “Zero-Thrust Velocity Vector Control for Interstellar Probes: Lorentz Force Navigation and Circling,” AIAA Journal 2 (1964), pp. 885-889. Matloff and Johnson discuss electrodynamic tethers in “Applications of the Electrodynamic Tether to Interstellar Travel,” JBIS 58 (June, 2005), pp. 398-402.
Hybrid Strategies for Deep Space
On Monday I touched on the topic of multi-modal spacecraft, wondering whether future deep space missions might carry twin or even triple systems of propulsion. The example I want to tinker with is an interstellar craft driven by beamed energy, akin to some of Robert Forward’s designs in the 1980s. Forward went through enormous challenges trying to decelerate at the destination, though as we’ll see, he did come up with more than one solution.
A beamed laser sailcraft runs into this problem because the power source is in a close solar orbit, while the craft is reaching speeds that make a human crossing to another star possible. How to slow it down from behind? Deceleration is going to take a long time no matter what the method, but if we factor in a second mode of propulsion, a magnetic sail, we can brake against the destination star’s stellar wind. I mentioned on Monday as well that the Venture Star, the starship that got James Cameron’s crew to Alpha Centauri in the film Avatar, was depicted as a hybrid craft, with a Forward-style beamed lightsail to reach cruise and antimatter engines to slow upon arrival.
The Venture Star’s braking strategy involves the annihilation of matter and antimatter to heat up hydrogen propellant for thrust. Cameron’s use of enormous heat radiators also marks this craft, an indication that he tried for scientific accuracy where he could — Robert Frisbee is only one of the scientists who have noted the need to dissipate the heat from matter/antimatter reactions, and he was echoing the prescient Les Shepherd, who wrote about the issue back in 1952.
Image: The Venture Star wins points for accuracy in the details. Winchell Chung has a fine breakdown of its systems on his Project Rho site. Credit: 20th Century Fox.
Near-Term Steps
Hybrid ideas for spacecraft don’t have to wait for science fictional futures, because we’re already seeing interesting steps in this direction. When the Japanese space agency JAXA successfully launched and operated the IKAROS space sail, it demonstrated a unique system of spacecraft control. The liquid crystal films located near the edges of the sail were installed for attitude control. Their reflectivity could be altered by applying a voltage to the strips. Experimenting with the method in 2010, JAXA successfully demonstrated attitude control torque using the method, an adjustment of reflectivity that bodes well for future hybrid methods.
There are various ways of manipulating a sailcraft’s attitude, all of them analyzed mathematically in Giovanni Vulpetti’s book Fast Solar Sailing (Springer, 2012), one of them being the change in reflectance used on IKAROS. I won’t get into the details, though you can find them in the book I’ve been tapping this week for ideas, Solar Sails: A Novel Approach to Interplanetary Travel, soon to be released by Springer in its second edition.
Manipulating vanes on a segmented sail, using small sails at the boom ends of the spacecraft, even the use of small rockets has been suggested. But sail reflectance and its alteration point us in new directions. Let me quote from Solar Sails on this:
The concept of thrust maneuvering for solar-photon sails is…more general than sail attitude control via mechanical actuators of conventional and/or advanced type. The above-mentioned experiment on IKAROS appears as a special device opening a new ‘seam’ of very advanced spacecraft.
How we can use changes in reflectance to alter solar sail trajectories in space is now under investigation at the University of Rome, a goal being to develop a comprehensive thrust model. That’s interesting stuff, because multiple attitude control systems give every indication of being both efficient and more fault-tolerant, since you’re carrying a backup system on-board. The suspicion here is that we’ve only begun to realize how these methods may affect future propulsion strategies as well. But it may be that JAXA is thinking well ahead on this matter.
Image: The IKAROS sail, a hybrid design with attached solar cells. Note the solar cells in blue, used to change the spacecraft’s attitude. Credit: JAXA.
For IKAROS has already demonstrated sail deployment, attitude control and maneuverability, as well as showing — with its on-board gamma-ray burst detector — the ability of sails to serve as a platform for science missions. Scaling up such missions for further testing lies ahead, and the success of IKAROS has spurred the agency to continue with an ambitious sail mission to Jupiter that could be launched as early as 2020. Here we’re talking about another hybrid design, one that would use gravitational assists, a solar sail, and an ion engine to explore Jupiter’s magnetosphere, with an additional task of a flyby of at least one of Jupiter’s Trojan asteroids.
So we’re combining propulsive methods in this mission, one that should build our experience with the kind of maneuvering that may one day be used on ‘Sundiver’ missions that perform close flybys of the Sun. Called the Jupiter Magnetosphere Orbiter (JMO), the mission’s sail will upgrade the original IKAROS. Based on what has been published so far, it will be a square sail that measures about 100 meters to the side and like IKAROS will use a 7.5 micron polyimide for the sail material. The mass of sail and associated structure including inflatable booms should be in the range of 150 kilograms, with a total mass (including payload) in the range of 250 kilograms.
The Interstellar Hybrid
Giovanni Vulpetti’s name has kept coming up with respect to multi-modal propulsion because as I’ve investigated the concept, I’ve realized he has been studying what he calls ‘multiple propulsion mode’ for many years. At the end of this post I give references for two papers on the matter, the first (“Multiple Propulsion Concept: Theory and Performance”) dating back as far as 1979. In 1992, Vulpetti discussed a deep space vehicle driven by nuclear ion propulsion and a solar sail, much like the JMO spacecraft, at the first World Space Congress in Washington, DC.
The concept here is staging, but instead of coupling stages that each use the same propulsion strategy, we’ll use entirely different propulsion techniques. Staged propulsion spacecraft (this is Vulpetti’s term) are more or less forced upon us as we ponder the complexities of interstellar missions. Gregory Matloff, a fellow author of Vulpetti and Les Johnson on the Solar Sails book, was able to get mission times to Alpha Centauri — for a solar sail using a sundiver approach to the Sun — down to roughly 1000 years in papers he wrote for the Journal of the British Interplanetary Society in the 1980s. But solar sails are efficient only near a star. Can we use a solar sail by itself for needed deceleration?
More likely is the scenario where a sail or fusion-powered starship at least supplements its primary propulsion with something like a magnetic sail for years-long braking against the stellar wind in the destination system. [Addendum: As noted in the comments, braking against a stellar wind won’t be a ‘years-long’ process because at these speeds the probe would cross the heliosphere within days. I was really thinking about braking against the interstellar medium, an idea that grows out of Bussard’s ramscoop ideas and the drag they have been found to create]. Tomorrow I’ll mention some of Robert Forward’s notions about the deceleration dilemma including staged sails, and explore other options, which hybrid missions seem to trump.
Giovanni Vulpetti’s papers on hybrid spacecraft include “Multiple Propulsion Concept: Theory and Performance,” JBIS 32 (June, 1979), pp. 209-214; and “Multiple Propulsion Concept for Interstellar Flight: General Theory and Basic Results,” JBIS 43 (December, 1990).
A Near-Term Sail Niche
When Les Johnson spoke to a session on sail technologies at the 100 Year Starship symposium in Houston last September, he startled some in the audience by going through a list of how many solar sail missions are now in the works. The European Space Agency’s Gossamer program accounts for one of these, which is already built and waiting for launch, but three are in the pipeline. The University of Surrey (UK) is a surprisingly active entrant, with three CubeSat sails set for flight in the next three years. We also have the Planetary Society’s LightSail to contend with, a CubeSat design with a 32 square meter sail when deployed.
There are other missions as well, with names like NEA [Near Earth Asteroid] Scout, Lunar Flashlight, and although it is now in limbo at least for several years, NASA’s Sunjammer. The surge in interest in CubeSats is hard to miss here. They’re cheap, small, and ideal for trying out sail experiments as we try to figure out how best to use this technology in space. Master it and we have the possibility of sail-driven CubeSat missions sent deep into the Solar System, carrying miniaturized payloads and perhaps flown in ‘swarm’ configuration. And of course the lessons we learn should scale to the larger sails we hope to fly as we build expertise.
Image: The European Space Agency’s Gossamer sail in an artist’s visualization. Credit: ESA/DLR.
Johnson is deputy manager for NASA’s Advanced Concepts Office at the Marshall Space Flight Center in Huntsville, AL. He’s also an active writer of science fiction, most recently portraying — in a novel called Rescue Mode — a Mars mission’s crew on a crippled ship locked in a struggle for survival. Somehow he also finds time to write non-fiction, bringing his formidable background in space technology to bear in Solar Sails: A Novel Approach to Interplanetary Travel, which I’m examining this week as part of a series on sailcraft and their uses. A look at his Amazon offerings illustrates just how prolific Johnson has been in the past decade. [Note: I’m not going to link to the Solar Sails book again until the new edition is available, which should be soon].
Missions in the Near Term
Creating public interest in space is crucial for beginning the gradual spread into the Solar System that some of us think will lead to an infrastructure that can support eventual interstellar missions. After all, we’re asking government, which that same public funds, to play a major role here, even as we also explore what commercial initiatives can achieve. So it’s helpful to keep in mind that technologies have their particular niches. As we talk about multi-modal technologies tomorrow, we’ll consider the fact in greater detail. We’ll never get off the ground with a solar sail — we need rockets for that — but there are missions that even the best rocket design cannot fly.
We know from Tsiolkovsky’s rocket equation that carrying more and more propellant gets to be a non-starter, because soon we’re adding propellant just to push more propellant. Here a solar sail can offer unique opportunities. Suppose, for example, that we want to observe what’s going on at the Sun’s poles. Missions we’ve launched Sunward have tended to stay in the ecliptic because that’s where Earth is, which means we have limited views even of the mid-latitude regions.
From the long-term perspective that Centauri Dreams takes, the solar wind might offer propulsive possibilities for various magnetic sail designs. But we have to reckon with the fact that we don’t understand it well. This stream of charged particles can flow outward from the Sun’s equatorial regions at 400 kilometers per second, but much faster streams, up to 800 kilometers per second, seem to originate in the mid to upper latitude regions. This is just one of the things we’d like to study in addition to the factors that lead to solar plasma outbursts.
A highly inclined orbit around the Sun is a tough challenge for chemical rockets, but it’s realistic to design a Solar Polar Imager sailcraft that can take advantage of the sharp increase in photon ‘push’ available to it at 0.5 AU. Remember, it’s an inverse square law, so that if we halve the distance between Earth and the Sun, we get four times the solar flux on the sail. One mission concept under study calls for a square, 3-axis stabilized sail about 150 meters on the side.
Many sail concepts involve close study of the Sun, including missions to warn of solar storms. Consider that coronal mass ejections (CMEs) can crank out accelerated particles with a speed of ejection up to 1000 kilometers per second, a serious ‘gust’ in the solar wind. Protecting our current infrastructure involves shielding the satellite assets we rely on, including the GPS system and the telecommunications satellites that drive cable TV systems. Adequate warning means these can be powered down in the event of a solar storm, and reoriented to put as much onboard spacecraft mass as possible between key systems and incoming radiation.
Image: Solar flares and CMEs are currently the biggest “explosions” in our solar system, roughly approaching the power in one billion hydrogen bombs. Fast CMEs occur more often near the peak of the 11-year solar cycle, and can trigger major disturbances in Earth’s magnetosphere. Credit: NASA/GSFC.
The Advanced Composition Explorer (ACE) spacecraft is located at the L1 Lagrange point some 1.5 million kilometers from Earth, where it monitors the Sun for dangerous events. But the lead time for any warning is short, which emphasizes the need for a sailcraft that can be positioned between the Sun and the Earth. The current mission concept is called Heliostorm. It would use a square sail 70 meters to the side. A more advanced version with a circular sail 230 meters in radius could orbit the Sun at 0.70 AU, maintaining a direct line between Sun and Earth. Our solar storm warning time would go from the current one hour to between 16 and 31 hours.
Or consider the Mars sample return. It’s a high-priority item for astrobiology as we continue to learn about the Red Planet’s interesting past, but the fuel required not only to get our spacecraft to Mars but also to land, launch and return the sample is a demanding barrier to overcome. Instead, we can consider a lander sent to Mars by conventional rocket, one that collects the needed samples and returns them to Mars orbit. There we rendezvous with a sailcraft above Mars, eliminating the propellant mass we’d have otherwise needed for the return.
The list could go on, from ‘pole sitters’ that use solar photons to maintain a position over one of the Earth’s poles — useful for weather and environmental monitoring — to near earth asteroid reconnaissance, where sail propulsion allows a spacecraft to visit multiple NEA’s. The latter is a technology that couples nicely with the growing use of cubesats and miniaturized components, helping us characterize nearby asteroids in large numbers with swarm missions. I should mention too that a mission called L-1 Diamond is being examined that would use four sailcraft to study the Sun, orbiting it in a triangular formation with the fourth looking down at the pole.
So the range of missions is wide, and it extends as we gain expertise with sailcraft into ‘Sundiver’ missions of the sort that Gregory Matloff began writing about in the 1980s (an idea originally hatched by David Brin and Gregory Benford), which could lead to missions to the gravitational lens (550 AU) and even the Oort Cloud. One of the Solar Sails authors, Giovanni Vulpetti, has explored fast outer system missions in a 2012 book called Fast Solar Sailing (Springer). As our infrastructure builds, sail ‘clippers’ to Mars and other closer destinations may begin to supply distant colonists with the supplies they need.
One day, too, a large solar sail could conceivably be deployed near an asteroid on a dangerous trajectory, changing its reflectivity enough to alter its orbit over the course of decades. It’s a thought that takes me back to the Russian Znamya missions, ostensibly tests of a space mirror that might light Siberian cities at night, but for all intents and purposes a deployment shakeout of sail technologies. The potential of sails for interstellar missions continues to intrigue me, and I’ll be talking about sails and multi-modal propulsion designs for starships tomorrow. But building up our near-Earth infrastructure may be most economically served by near-term sails that can return data and haul supplies to support a civilization with a very deep frontier in mind.
Sailcraft: Concepts, Design, Lab Work
Although we can trace the growth of research into interstellar flight all the way back to the days of Konstantin Tsiolkovsky, the effort has often operated outside of government channels. Scientists and engineers whose day job might take in aspects of rocketry were hard pressed to find time for studying trips to the stars when the proximate needs were better communications satellites or improved designs for reaching low Earth orbit. Nonetheless, work continued, marked by the enthusiasm of the practitioners for what was clearly the ultimate mission. Official or unofficial, small groups hammering on ideas have continued to debate the core concepts.
When the Jet Propulsion Laboratory in Pasadena turned Aden and Marjorie Meinel loose on a mission concept aimed at reaching 1000 AU back in the 1970s, the duo looked at two propulsion options. As the new edition of Solar Sails: A Novel Approach to Interplanetary Travel (Copernicus, 2014) points out, the first of these was a nuclear-electric ion drive using xenon. But the Meinels also evaluated the use of a solar sail unfurled near the Sun. It’s interesting to read here that Chauncey Uphoff, senior analyst on the propulsion phase, was unable to publish the results of the sail study, which wound up circulating only as an internal memo within NASA. [Note: The link above goes to the first edition of this book. The new edition is scheduled for publication within the next few weeks. I advise waiting for it.]
Uphoff’s memo considered the solar sail as an alternative propulsion method, and even if this particular deep space sail concept remained out of view, the efforts of Gregory Matloff and Eugene Mallove soon brought interstellar missions using solar sails to the attention of the community. One of the three authors of Solar Sails, Matloff went to work on what might be done with a close solar pass and sail deployment (partial or complete) at perihelion. He and Mallove evaluated space-manufactured metal sails closing to within 0.04 AU of the Sun’s center, with cables approximating the tensile strength of industrial diamond.
Much of this work was published in the 1980s in the Journal of the British Interplanetary Society, focusing not on beamed laser sails (Robert Forward’s theme) but solar sails using solely the momentum imparted by solar photons. A sail like this, once outbound, could wind cable and sail around the habitat section to provide shielding against cosmic rays. Even a large payload might be accelerated to speeds allowing a trip to Alpha Centauri in 1000 years. Matloff’s 1984 paper “Solar Sail Starships – The Clipper Ships of the Galaxy” (JBIS 34, 371-380) is a classic that he and Mallove would soon update not only in optimization studies in JBIS but also in popular texts like The Starflight Handbook (1989).
The Italian Sail Effort
But back to the theme of small-scale collaborations, from which the interstellar community has for so long benefited. It was back in 1993 at the International Astronautical Congress in Graz, Austria that a small group of solar sail enthusiasts gathered to organize a study of the technology. The study group that emerged was dubbed the Aurora Collaboration, a nod to Greek mythology, in which Aurora was the younger sister of Helios, the god of the Sun. Matloff was one of the core seven behind this collaboration, as was Giovanni Vulpetti, who became team coordinator. Other names familiar to Centauri Dreams readers will be FOCAL mission advocate Claudio Maccone and engineer and author Giancarlo Genta.
Image: Artist’s conception of a solar sail in space. Credit: Rick Sternbach.
Excellent work can come out of highly motivated small groups like these, and I think the Aurora effort deserves greater attention than it has received. Fifteen published papers emerged from its labors, with three presentations to European space agencies and a workshop held at the University of Rome. Computer code for optimizing sail trajectories, experimental work on layered sail construction (a plastic substrate that can be detached once the sail has been constructed in space), and a number of deployment concepts resulted. The collaboration also studied telecommunications systems, analyzed aluminum sail optical properties, and optimized trajectories for potential missions to near interstellar space and the Sun’s gravitational lens.
The latter deserves a note: The gravitational well created by the Sun’s mass causes light to curve as it grazes the Sun from an object directly behind it (as seen by the observer). The resulting lensing effect is promising for observations at various wavelengths, which is where we get the idea of a FOCAL mission to the focus beginning at 550 AU. What the Aurora team did — with results presented at a meeting of the International Academy of Astronautics in Turin, Italy in 1996 — was to produce preliminary results for a less demanding mission, a sail to the heliopause. The team members presented a thin-film 250-meter square sail and analyzed ways of reducing the sail areal mass thickness, as well as exploring communications options and offering a structural analysis.
The Aurora team’s sail would act as a bridge between the Voyager probes ( the Aurora spacecraft would exit the system about three times faster than Voyager) and later deep space designs. Massing 150 kg, it would use a close solar pass for acceleration and its target was a more manageable (in the near term) 50 to 100 AU. Without benefit of press coverage or large amounts of funding, the Aurora Collaboration moved the ball forward through serious volunteer efforts of the kind the interstellar community has always relied on.
Beamed Sail Experiments
As we go through the papers groups like these create, it’s easy to think of the interstellar effort as being almost entirely theoretical. But laboratory work on some of these technologies goes back a long way, and we can trace early sail studies in the lab to the work of Russian physicist Peter Lebedev in 1899, who experimented with metal sheets of differing levels of reflectivity to measure the effect of the exchange of momentum from photons. I mentioned above the Aurora Collaboration’s experimental work on layered sail construction, and in the early years of the 21st Century, Gregory and James Benford studied beamed sail technologies in a JPL lab.
Their findings are important as we move from straightforward solar sailing to the beamed variant that Robert Forward studied both for microwave and laser designs. As president of Microwave Sciences in Lafayette, CA, James Benford’s experience with microwaves led him to join with brother Gregory, a physicist and well-known science fiction writer, to use advances in materials technologies to attempt these laboratory experiments. Temperature was a key here: Working on Earth’s surface, a sail would have to overcome gravity, and to do that, the sail materials would need to be heated to temperatures higher than 1500 degrees Celsius.
Aluminum can’t handle that kind of punishment (its melting temperature is 660 degrees Celsius), and the problem persists with many potential metal sails. But carbon undergoes sublimation at temperatures above 3000 degrees Celsius, making the emergence of lightweight carbon structures as potential sail experiments the key. The Benfords used a 10-square centimeter sail in a vacuum chamber, demonstrating acceleration under a 10-kilowatt, 7 GHz microwave beam. Their sails remained intact after experiencing temperatures up to 1725 degrees Celsius.
The carbon microtruss used in the JPL work, developed by San Diego’s Energy Science Research Laboratories and ten times thinner than a human hair, handled the heat requirement with ease. In fact, the Benfords were able to observe accelerations of several gravities in their tests. They also saw a phenomenon known as ‘desorption,’ in which the rapid heating from the microwave beam evaporates molecules — CO2, hydrocarbons and hydrogen — incorporated into the sail during the manufacturing process, adding an interesting second source of acceleration to a sail. A carefully applied layer of compounds painted onto a sail thus creates a propulsive layer of its own.
Image: Carbon disk sail lifting off of truncated rectangular waveguide under 10 kW microwave power (four frames, 30 ms interval, first at top). Credit: James and Gregory Benford.
Solar Sails notes the desorption findings, but the Benfords produced a result that I consider far more valuable. Their experiments have demonstrated that the pressure of a microwave beam will keep a concave-shaped sail in tension. The beam is itself producing a sideways restoring force. The terminology here is ‘beam-riding,’ and in the case of future sail designs, it means that a properly shaped sail will be stable under the intense beam that drives it.
Moreover, it becomes clear from this laboratory work that the beam can carry angular momentum which it can communicate to the sail. We have, then, a mechanism for allowing ground-based controllers to stabilize a beamed sail against yaw and drift. This important finding grows out of comparatively inexpensive experiment and meshes with ongoing efforts to study the deployment and control of conventional solar sails. What we are seeing is a technology track that holds the promise for space missions on both an interplanetary and interstellar scale.
Tomorrow I’ll continue this series on solar sail and beamed sailcraft with a look at near-term sail concepts discussed in Solar Sails and the mission needs that drive them.
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