Centauri Dreams
Imagining and Planning Interstellar Exploration
Project Dragonfly: The case for small, laser-propelled, distributed probes
Andreas Hein is a familiar figure in these pages, having written on the subject of worldships as well as the uploading of consciousness. He is Deputy Director of the Initiative for Interstellar Studies (I4IS), as well as Director of its Technical Research Committee. He founded and leads Icarus Interstellar’s Project Hyperion: A design study on manned interstellar flight. Andreas received his master’s degree in aerospace engineering from the Technical University of Munich and is now working on a PhD there in the area of space systems engineering, having conducted part of his research at MIT. He spent a semester abroad at the Institut Superieur de l’Aeronautique et de l’Espace in Toulouse and also worked at the European Space Agency Strategy and Architecture Office on future manned space exploration. Today’s essay introduces the Initiative for Interstellar Studies’ Project Dragonfly Design Competition.
by Andreas Hein
2089, 5th April: A blurry image rushes over screens around the world. The image of a coastline, waves crashing into it, inviting for a nice evening walk at dawn. Nobody would have paid special attention, if it were not for one curious feature: Two suns were mounted in the sky, two bright, hellish eyes. The first man-made object had reached another star system.
Is it plausible to assume that we could send a probe to another star within our century? One major challenge is the amount of resources needed for such a mission. [1, 2]. Ships proposed in the past were mostly mammoths, weighing ten-thousands of tons: the fusion-propelled Daedalus probe with 54,000 tonnes and recently the Project Icarus Ghost Ship with over 100,000 tonnes. All these concepts are based on the rocket principle, which means that they have to take their propellant with them to accelerate. This results in a very large ship.
Another problem with fusion propulsion in particular is the problem of scalability. Most fusion propulsion systems get more efficient when they are scaled up. There is also a critical lower threshold for how small you can go. These factors lead to large amounts of needed propellant and large engines, for which you need a large space infrastructure. A Solar System-wide economy is probably needed, as the Project Daedalus report argues [3].
Image: The Project Icarus Ghost Ship: A colossal fusion-propelled interstellar probe
http://www.spaceanswers.com/futuretech/ghost-ship-to-alpha-centauri/
However, there is a different avenue for interstellar travel: going small. If you go small, you need less energy for accelerating the probe and thus less resources. Pioneers of small interstellar missions are Freeman Dyson with his Astrochicken; a living, one kilogram probe, bio-engineered for the space environment [4]. Robert Forward proposed the Starwisp probe in 1985 [5]. A large, ultra-thin sail which rides on a beam of microwaves. Furthermore, Frank Tipler and Ray Kurzweil describe how nano-scale probes could be used for transporting human consciousness to the stars [6, 7].
At the Initiative for Interstellar Studies (I4IS), we wanted to have a fresh look at small interstellar probes, laser sail probes in particular. The last concepts in this area have been developed years ago. How did the situation change in recent years? Are there new, possibly disruptive concepts on the horizon? We think there are. The basic idea is to develop an interstellar mission by combining the following technologies:
- Laser sail propulsion: The spacecraft rides on a laser beam, which is captured by an extremely thin sail [8].
- Small spacecraft technology: Highly miniaturized spacecraft components which are used in CubeSat missions
- Distributed spacecraft: To spread out the payload of a larger spacecraft over several spacecraft, thus, reducing the laser power requirements [9, 10]. The individual spacecraft would then rendezvous at the target star system and collaborate to fulfill their mission objectives. For example, one probe is mainly responsible for communication with the Solar System, another responsible for planetary exploration via distributed sensor networks (smart dust) [11].
- Magnetic sails: A thin superconducting ring’s magnetic field deflects the hydrogen in the interstellar medium and decelerates the spacecraft [12].
- Solar power satellites: The laser system shall use space infrastructure which is likely to exist in the next 50 years. Solar power satellites would be temporarily leased to provide the laser system with power to propel the spacecraft.
- Communication systems with external power supply: A critical factor for small interstellar missions is power supply for the communication system. As small spacecraft cannot provide enough power for communicating over these vast distances. Thus, power has to be supplied externally, either by using laser or microwave power from the Solar System during the trip and solar radiation within the target star system [5].
Image: Size comparison between an interplanetary solar sail and the Project Icarus Ghost Ship. Interstellar sail-based spacecraft would be much larger. (Courtesy: Adrian Mann and Kelvin Long)
Bringing all these technologies together, it is possible to imagine a mission which could be realized with technologies which are feasible in the next 10 years and could be in place in the next 50 years: A set of solar power satellites are leased for a couple of years for the mission. A laser system with a huge aperture has been put into a suitable orbit to propel the interstellar, as well as future planetary missions. Thus, the infrastructure can be reused for multiple purposes. The interstellar probes are launched one-by-one.
After decades, the probes start to decelerate by magnetic sails. Each spacecraft charges its sails differently. The first spacecraft decelerates slower than the follow-up probes. Ideally, the spacecraft then arrive at the target star system at the same point in time. Then, the probes start exploring the star system autonomously. They reason about exploration strategies, exchange and share data. Once a suitable exploration target has been chosen, dedicated probes descend to the planetary surface, spreading dust-sized sensor networks onto the pristine land. The data from the network is collected by other spacecraft and transferred back to the spacecraft acting as a communication hub. The hub, powered by the light from extrasolar light sends back the data to us. The result could be the scenario described at the beginning of this article.
Image: Artist’s impression of a laser sail probe with a chip-sized payload. (Courtesy: Adrian Mann)
Of course, one of the caveats of such a mission is its complexity. The spacecraft would have to rendezvous precisely over interstellar distances. Furthermore, there are several challenges with laser sail systems, which have been frequently addressed in the literature, for example beam collimation and control. Nevertheless, such a mission architecture has many advantages compared to existing ones: It could be realized by a space infrastructure we could imagine to exist in the next 50 years. The failure of one or more spacecraft would not be catastrophic, as redundancy could easily be built in by launching two or more identical spacecraft.
The elegance of this mission architecture is that all the infrastructure elements can also be used for other purposes. For example, a laser infrastructure could not only be used for an interstellar mission but interplanetary as well. Further applications include an asteroid defense system [20]. The solar power satellites can be used for providing in-space infrastructure with power [18].
Image: Artist’s impression of a spacecraft swarm arriving at an exosolar system (Courtesy: Adrian Mann)
How about the feasibility of the individual technologies? Recent progress in various areas looks promising:
- The increased availability of highly sophisticated miniaturized commercial components: smart phones include many components which are needed for a space system, e.g. gyros for attitude determination, a communication system, and a microchip for data-handling. NASA has already flown a couple of “phone-sats”; Satellites which are based on a smart phone [13].
- Advances in distributed satellite networks: Although a single small satellite only has a limited capability, several satellites which cooperate can replace larger space systems. The concept of Federated Satellite Systems (FSS) is currently explored at the Massachusetts Institute of Technology as well as at the Skolkovo Institute of Technology in Russia [14]. Satellites communicate opportunistically and share data and computing capacity. It is basically a cloud computing environment in space.
- Increased viability of solar sail missions. A number of recent missions are based on solar sail technology, e.g. the Japanese IKAROS probe, LightSail-1 of the Planetary Society, and NASA’s Sunjammer probe.
- Greg Matloff recently proposed use of Graphene as a material for solar sails [15]. With an areal density of a fraction of a gram and high thermal resistance, this material would be truly disruptive. Currently existing materials have a much higher areal density; a number crucial for measuring the performance of solar sails.
- Material sciences has also advanced to a degree where Graphene layers only a few atoms thick can be manufactured [16]. Thus, manufacturing a solar sail based on extremely thin layers of Graphene is not as far away as it seems.
- Small satellites with a mass of only a few kilograms are increasingly proposed for interplanetary missions. NASA has recently announced the Interplanetary CubeSat Challenge, where teams are invited to develop CubeSat missions to the Moon and even deeper into space (NASA) [17]. Coming advances will thus stretch the capability of CubeSats beyond Low-Earth Orbit.
- Recent proposals for solar power satellites focus on providing space infrastructure with power instead of Earth infrastructure [18, 19]. The reason is quite simple: Solar power satellites are not competitive to most Earth-based alternatives but they are in space. A recent NASA concept by John Mankins proposed the use of a highly modular tulip-shaped space power satellite, supplying geostationary communication satellites with power.
- Large space laser systems have been proposed for asteroid defense [20]
In order to explore various mission architectures and encourage participation by a larger group of people, I4IS has recently announced the Project Dragonfly Competition in the context of the Alpha Centauri Prize [21]. We hope that with the help of this competition, we can find unprecedented mission architectures of truly disruptive capability. Once this goal is accomplished, we can concentrate our efforts on developing individual technologies and test them in near-term missions.
If this all works out, this might be the first time in history that there is a realistic possibility to explore a near-by star system within the 21st or early 22nd century with “modest” resources.
References
[1] Millis, M. G. (2010). First Interstellar Missions, Considering Energy and Incessant Obsolescence. Journal of the British Interplanetary Society, 63(11), 434.
[2] Hein, A. M. (2012). Evaluation of Technological-Social and Political Projections for the Next 100-300 Years and the Implications for an Interstellar Mission. Journal of the British Interplanetary Society, 65, 330-340.
[3] Martin, A. R. (Ed.). (1978). Project Daedalus: The Final Report on the BIS Starship Study. British Interplanetary Soc.
[4] Dyson, F. J. (1979). Disturbing the universe. Basic Books.
[5] Forward, R. L. (1985). Starwisp-An ultra-light interstellar probe. Journal of Spacecraft and Rockets, 22(3), 345-350.
[6] Tipler, F. (1994), The Physics of Immortality, Chapter 2, Doubleday, New York.
[7] Kurzweil, R. (2005). The singularity is near: When humans transcend biology. Penguin.
[8] Forward, R. L. (1984). Roundtrip interstellar travel using laser-pushed lightsails. Journal of Spacecraft and Rockets, 21(2), 187-195.
[9] Mathieu, C., & Weigel, A. L. (2005, August). Assessing the flexibility provided by fractionated spacecraft. In Proc. of AIAA Space 2005 Conference, Long Beach, CA, USA.
[10] Brown, O., & Eremenko, P. (2006). Fractionated space architectures: a vision for responsive space. Defense Advanced Research Projects Agency, Arlington, VA.
[11] Colombo, C., & McInnes, C. (2011). Orbital Dynamics of” Smart-Dust” Devices with Solar Radiation Pressure and Drag. Journal of Guidance, Control, and Dynamics, 34(6), 1613-1631.
[12] Andrews, D., & Zubrin, R. (1990). Magnetic sails and interstellar travel. Journal of the British Interplanetary Society 43, 265-272.
[13] Wikipedia, Phonesat: http://en.wikipedia.org/wiki/PhoneSat
[14] Golkar, A. (2013, April). Federated Satellite Systems: an Innovation in Space Systems Design. In 9th IAA Symposium on Small Satellites for Earth Observation, IAA, Berlin, Germany.
[15] Matloff, G. L. (2012). Graphene, the Ultimate Interstellar Solar Sail Material? Journal of the British Interplanetary Society, 65, 378-381.
[16] Paton, K. R., Varrla, E., Backes, C., Smith, R. J., Khan, U., O’Neill, A., … & Coleman, J. N. (2014). Scalable production of large quantities of defect-free few-layer graphene by shear exfoliation in liquids. Nature Materials, 13(6), 624-630.
[17] NASA Interplanetary Cubesat Challenge: http://sservi.nasa.gov/articles/interplanetary-cubesat-challenge/
[18] Mankins, J., Kaya, N., & Vasile, M. (2012). Sps-alpha: The first practical solar power satellite via arbitrarily large phased array (a 2011-2012 nasa niac project). In 10th International Energy Conversion Engineering Conference.
[19] Mankins, J.C. (2014). The Case for Space Solar Power, Virginia Edition Publishing.
[20] Hughes, G. B., Lubin, P., Bible, J., Bublitz, J., Arriola, J., Motta, C., … & Pryor, M. (2013, September). DE-STAR: Phased-array laser technology for planetary defense and other scientific purposes. In SPIE Optical Engineering+ Applications (pp. 88760J-88760J). International Society for Optics and Photonics.
[21] I4IS Project Dragonfly Design Competition: http://i4is.org/news/dragonfly
Laniakea: Milky Way’s Address in the Cosmos
Science fiction writers have a new challenge this morning: To come up with a plot that takes in not just the galaxy and not just the Local Group in which the Milky Way resides, but the far larger home of both. Laniakea is the name of this supercluster, after a Hawaiian word meaning ‘immense heaven.’ And immense it is. Superclusters are made up of groups like the Local Group — each of these contain dozens of galaxies — and clusters that contain hundreds more, interconnected by a filamentary web whose boundaries have proven hard to define.
Where does one supercluster begin and another end? As explained in a cover story in the September 4 issue of Nature, an emerging way to tune up our cosmic maps is to look at the effect of large-scale structures on the movements of galaxies. A team under R. Brent Tully (University of Hawaii at Manoa) has been using data from radio telescopes to study the velocities of 8000 galaxies, adjusting for the universe’s accelerating expansion to create a map of the cosmic flow of these galaxies as determined by gravitational effects.
The boundaries between superclusters, such as those between Laniakea and the Perseus-Pisces Supercluster, are where the galactic flows diverge and neighboring structures shear apart. As this National Radio Astronomy Observatory news release points out, within the boundaries of the Laniakea Supercluster, the motions of galaxies are directed inward. In other superclusters, the flow of galaxies goes toward a different gravitational center.
This is how our horizons get adjusted. Previously we thought of the Milky Way as part of the Virgo Supercluster, but now we see even this region as just part of the far larger Laniakea Supercluster. We’re talking about a structure some 520 million light years in diameter that contains the mass of one hundred million billion suns across a staggering 100,000 galaxies. And just as the Sun is in the galactic ‘suburbs’ of the Milky Way, a long way from the galaxy’s teeming center, so the Milky Way itself lies on the outskirts of the Laniakea Supercluster.
Image: A slice of the Laniakea Supercluster in the supergalactic equatorial plane — an imaginary plane containing many of the most massive clusters in this structure. The colors represent density within this slice, with red for high densities and blue for voids — areas with relatively little matter. Individual galaxies are shown as white dots. Velocity flow streams within the region gravitationally dominated by Laniakea are shown in white, while dark blue flow lines are away from the Laniakea local basin of attraction. The orange contour encloses the outer limits of these streams, a diameter of about 160 Mpc. This region contains the mass of about 100 million billion suns. Credit: SDvision interactive visualization software by DP at CEA/Saclay, France.
Those of us with an interest in Polynesia will love the name Laniakea, which was chosen to honor the Polynesian sailors who used their deep knowledge of the night sky to navigate across the Pacific. If you look through the essays in Interstellar Migration and the Human Experience (University of California Press, 1985), you’ll find several that dwell on the historical example of the Polynesian navigators as a way of examining future migration into the stars. The theme resonates and I invariably hear it mentioned at the various conferences on interstellar flight.
The diagrams below offer another way of viewing the gravitational interactions that pull together the immense supercluster. You’ll notice the Great Attractor, a gravitational focal point that influences the motion of galaxy clusters including our own Local Group. The NRAO refers to it as a ‘gravitational valley’ whose effects can be felt across the Laniakea Supercluster.
Image: Two views of the Laniakea Supercluster. The outer surface shows the region dominated by Laniakea’s gravity. The streamlines shown in black trace the paths along which galaxies flow as they are pulled closer inside the supercluster. Individual galaxies’ colors distinguish major components within the Laniakea Supercluster: the historical Local Supercluster in green, the Great Attractor region in orange, the Pavo-Indus filament in purple, and structures including the Antlia Wall and Fornax-Eridanus cloud in magenta. Credit: SDvision interactive visualization software by DP at CEA/Saclay, France.
Have a look at this video from Nature to see the whole supercluster set in motion.
So now we know that our home supercluster is actually 100 times larger in volume and mass than we previously thought. In an article summarizing these findings in Nature, Elizabeth Gibney points out that a somewhat different definition of a supercluster is being used by Gayoung Chon (Max Planck Institute for Extraterrestrial Physics, Germany) and colleagues, who base their definition on structures that will one day collapse into a single object, something that cannot be said for Laniakea because some of its galaxies will always move away from each other. Clearly, the definition of a supercluster is a work in progress, but let’s hope the name sticks.
The paper is Tully et al., “The Laniakea supercluster of galaxies,” Nature 513 (4 September 2014), 71-73 (abstract).
Red Dwarf Planets: Weeding Out the False Positives
For those of you who, like me, are fascinated with red dwarf stars and the prospects for life around them, I want to mention David Stevenson’s Under a Crimson Sun (Springer, 2013), with the caveat that although it’s on my reading list, I haven’t gotten to it yet. More about this title after I’ve gone through it, but for now, notice that the interesting planet news around stars like Gliese 581 and GJ 667C is catching the eye of publishers and awakening interest in the public. It’s easy to see why. Planets in the habitable zone of such stars would be exotic places, far different from Earth, but possibly bearing life.
At the same time, we’re learning a good deal more about both the above-mentioned stars. A new paper by Paul Robertson and Suvrath Mahadevan (both at Pennsylvania State) looks at GJ 667C with encouraging — and cautionary — results. The encouraging news is that GJ 667Cc, a super-Earth in the habitable zone of the star, is confirmed by their work. The cautionary note is that stellar activity can mimic signals that we can interpret, wrongly, as exoplanets, and not every planet thought to be in this system may actually be there.
Image: The view from GJ 667Cc as presented in an artist’s impression. Note the distant binary to the right of the parent red dwarf. New work confirms the existence of this interesting world in the habitable zone. Credit: ESO/L. Calçada.
Remember that this is a system that was originally thought to have two super-Earths: GJ 667Cb and GJ 667Cc. The complicated designation is forced by the fact that the red dwarf in question, GJ 667C, is part of a triple star system, a distant companion to the binary pair GJ 667AB. It was just last year that the first two planets around GJ 667C were announced, followed by results from a different team showing five more super-Earths around the same star. See Gliese 667C: Three Habitable Zone Planets for my discussion of the apparent result, which at the time seemed spectacular.
The new paper from Robertson and Mahadevan takes a critical look at this system, examining the amount of stellar activity found in the host star and finding ways to study the average width of the star’s spectral absorption lines, which should flag changes to the spectrum being produced by magnetic features like starspots. Using these methods the team was able to remove the stellar activity component from the observed signals, allowing the signature of the real planets to remain while suggesting problems with the other candidates.
GJ 667Cc survives the test, a happy outcome for those interested in the astrobiological prospects here. GJ 667Cb also makes the cut, but Robertson and Mahadevan believe that planet d in this system, originally thought to be near the outer edge of the habitable zone, is a false positive created by stellar activity and the rotation of the star. As for the other planet candidates in this system, Paul Robertson has this to say in an online post:
The signals associated with them are so small that they cannot be seen with “industry-standard” analysis techniques, regardless of whether we have corrected for activity. However, considering how successful our activity correction has been at boosting the signals of real planets, the fact that we see no sign of any of these planet candidates after the activity correction leads us to strongly doubt their existence.
All of this should remind us not to jump too swiftly to conclusions about planet candidates, particularly given the sensitivity involved with the spectrographs used in our planet-finding work. Radial velocity data that looks strong can actually be the result of magnetic events on the surface of the star being observed. When Robertson and Mahadevan looked at Gliese 581, another highly interesting system because of planets possibly in the habitable zone, they found no sign of Gliese 581g, a controversial candidate whose existence is still being debated. In Gliese 581 and the Stellar Activity Problem, Robertson has this to say:
With an orbital period of 33 days, the controversial “planet g” also lies at an integer ratio of the stellar rotation period. Sure enough, no sign of g remains after our activity correction, revealing that it too was an artifact of magnetic activity. While this outcome is certainly disappointing for anyone hoping to find signs of life in the GJ 581 system, it is heartening to finally put the confusion and dispute surrounding this system to rest.
Moreover, another candidate potentially in the habitable zone, Gl 581d, falls back into the measurement noise, meaning that it was another signature of stellar activity rather than an actual planet. The red dwarf Gliese 581 is thus reduced to three planets in its system, and we’ve lost the best candidates for life. That may sound discouraging, but I think we can take heart from the fact that work like this shows we’re getting much better at eliminating false positives. Using these methods, real planets stand out in the data, which means we can more readily identify habitable zone planets as our spectrographic instrumentation improves.
The paper is Robertson and Mahadevan, “Disentangling Planets and Stellar Activity for Gliese 667C,” accepted for publication at Astrophysical Journal Letters (preprint). For Gliese 581, see Robertson and Mahadevan, “Stellar Activity Masquerading as Planets in the Habitable Zone of the M dwarf Gliese 581,” published in Science Express (3 July 2014).
Streamers of Gravel near Orion Nebula?
I have a soft spot in my heart for the Green Bank Telescope in West Virginia. It’s not just that Frank Drake started Project Ozma on the site in 1960, or that Benjamin Zuckerman and Patrick Palmer ran an Ozma follow-up there in the mid-1970s. I was tracking SETI closely by 1980 or so and knew of these observations, but it was my friend Mike Gingell whose yearly trips to Green Bank kept the place firmly in mind. Like me, Mike was a member of the Society of Amateur Radio Astronomers, and unlike me, he was a highly qualified engineer.
Mike died just last year and I went out to his house to look through a collection of old radio books his wife thought I might be interested in. There in the back yard were three radio dishes, all tuned not for television but for the radio astronomy work Mike was so engaged in. Seeing them already beginning to succumb to foliage — Mike had been ill for some time and couldn’t keep up with them — reminded me strongly of some of J.G. Ballard’s fiction, like 1968’s “The Dead Astronaut,” in which the launch gantries and control rooms of Cape Canaveral have all been abandoned, succumbing to time and spreading sawgrass.
Fortunately for us, the Green Bank Telescope is vibrantly alive, and has just reported on findings that would have had Mike poring over the Monthly Notices of the Royal Astronomical Society, in which they will shortly appear. The big radio dish, the world’s largest fully steerable radio telescope, has been turned to the Orion Molecular Cloud Complex, a star-forming region that is home to the famous Orion Nebula. Star-forming material here is found to be filled with planetary building blocks the size of pebbles.
Image: Radio/optical composite of the Orion Molecular Cloud Complex showing the OMC-2/3 star-forming filament. GBT data is shown in orange. Uncommonly large dust grains there may kick-start planet formation. Credit: S. Schnee, et al.; B. Saxton, B. Kent (NRAO/AUI/NSF), acknowledging the use of NASA’s SkyView Facility located at NASA Goddard Space Flight Center.
If we can confirm the finding, it will show that objects up to a thousand times larger than the dust grains normally found around protostars may be a new class of mid-sized particles that could make planet formation that much easier to begin. What astronomer Scott Schnee (National Radio Astronomy Observatory) and team have found is an unusually nurturing environment for planets in which some protostars evidently form. The star-forming material here exists in the form of dust-rich filaments dotted with dense knots known as cores.
We’re looking at what, in a million years and perhaps less, will begin to evolve into a star cluster, all within a region called OMC-2/3 in the northern part of the Orion Molecular Cloud Complex. The Green Bank Telescope revealed that the region was shining much brighter than expected in millimeter-wavelength light, based on earlier studies at the IRAM 30 meter radio telescope in Spain. Says Schnee:
“This means that the material in this region has different properties than would be expected for normal interstellar dust. In particular, since the particles are more efficient than expected at emitting at millimeter wavelengths, the grains are very likely to be at least a millimeter, and possibly as large as a centimeter across, or roughly the size of a small Lego-style building block.”
But just how unusual is the finding? The paper on the work comments:
…it will be important to determine if OMC-2/3 is unique in exhibiting large grains or if this is a common feature of star-forming filaments. Although OMC-2/3 is unique in that it has a higher density of starless and protostellar cores than other regions within ~500 pc of the Sun, there are no other properties (mass, density, temperature, etc.) that would lead one to suspect that the dust grains in OMC-2/3 ought to have properties significantly different than those found in other nearby molecular clouds.
Image: Zoom in of the OMC-2/3 region. Credit: S. Schnee, et al.; B. Saxton, B. Kent (NRAO/AUI/NSF), acknowledging the use of NASA’s SkyView Facility located at NASA Goddard Space Flight Center.
If the star-forming filaments, by virtue of their lower temperatures, higher densities and lower velocities (compared to molecular clouds) are themselves the cause of the formation of these large grains, then we may be looking at a population of rocky particles not previously identified at this juncture in stellar evolution, what NRAO astronomer Jay Lockman calls a ‘vast streamer of gravel.’
The other possibility, noted in this NRAO news release, is that the rocky particles observed in OMC-2/3 may have emerged inside earlier protoplanetary disks and have simply escaped back into the surrounding molecular cloud. Whatever the case, the paper points out that there may be other explanations for the bright signature of the OMC emissions, which is why the work continues. This region contains a high concentration of protostars that serve as a laboratory for our study of star formation and the molecular clouds from which they emerge.
The paper is Schnee et al., “Evidence for Large Grains in the Star-forming Filament OMC-2/3,” accepted at Monthly Notices of the Royal Astronomical Society (preprint).
Remembering Voyager: Triton’s New Map
I’m glad to see Ralph McNutt quoted in a recent news release from the Johns Hopkins Applied Physics Laboratory. McNutt has been working on interstellar concepts for a long time, including the Innovative Interstellar Explorer mission that could become a follow-up to New Horizons. But he’s in the news in late August because of Voyager, and in particular Voyager 2, which made its flyby of Neptune on August 25, 1989, some 25 years ago. McNutt recalls those days, when he was a member of the Voyager plasma-analysis team:
“The feeling 25 years ago was that this was really cool, because we’re going to see Neptune and Triton up-close for the first time. The same is happening for New Horizons. Even this summer, when we’re still a year out and our cameras can only spot Pluto and its largest moon as dots, we know we’re in for something incredible ahead.”
I can only envy someone who was up close with the Voyager outer planet flybys and is now a key player on New Horizons, for which McNutt leads the energetic-particle investigation team. The image below is a long way from the much closer views Voyager gave us of Neptune, but it’s what New Horizons could make out with its Long-Range Reconnaissance Imager in mid-July. It’s what NASA’s Jim Green calls a ‘cosmic coincidence’ that New Horizons crossed the orbit of Neptune on the 25th anniversary of the Voyager flyby.
Image: The New Horizons spacecraft captured this view of the giant planet Neptune and its large moon Triton on July 10, 2014, from a distance of about 3.96 billion kilometers — more than 26 times the distance between the Earth and sun. The 967-millisecond exposure was taken with the New Horizons telescopic Long-Range Reconnaissance Imager (LORRI). New Horizons traversed the orbit of Neptune on Aug. 25, 2014 — its last planetary orbit crossing before beginning an encounter with Pluto in January 2015. In fact, at the time of the orbit crossing, New Horizons was much closer to its target planet — just about 440 million kilometers — than to Neptune.
I can remember staying up late the night of the Neptune encounter, being most curious not about Neptune itself but its moon Triton. We had already learned to expect surprises from Voyager — Io alone made that point — and Triton did not disappoint us with its unanticipated plumes, signs that the frozen world was active, and its odd ‘cantaloupe’ terrain. A bit larger than Pluto, Triton serves as a rough guide for what to expect at Pluto/Charon, but it’s also a point of departure, given its evident capture by Neptune and the resulting tidal heating.
Remember, this is a world that follows a retrograde orbit, moving opposite to Neptune’s rotation. The odds are strong that we’re looking at an object captured from the Kuiper Belt. Gravitational stresses would account for melting within this ice world, and explain the fractures and plume activity, evidently geysers of nitrogen, that Voyager saw. A newly restored Triton map, produced by Paul Schenk (Lunar and Planetary Institute) has a resolution of 600 meters per pixel and has been enhanced for contrast.
Image: The best-ever global color map of Neptune’s large moon Triton, produced by Paul Schenk. This map has a resolution of 600 meters per pixel. The colors have been enhanced to bring out the contrast but are a close approximation to Triton’s natural colors. Voyager’s “eyes” saw in colors slightly different from human eyes, and this map was produced using orange, green and blue filter images. Credit: Paul Shenk/LPI.
The video using the same data is a bit breathtaking. Have a look.
Keep in mind the limitations of the imagery. In 1989, the year of the Voyager flyby, Triton’s northern hemisphere was swathed in darkness, allowing the spacecraft to have a clear view of only one hemisphere during its closest approach. Now we wait to see what views New Horizons will generate of Pluto/Charon next summer. Given that Triton and Pluto are similar in density and composition, with carbon monoxide, carbon dioxide, nitrogen and methane ices on the surface, we may see some similar features. Will there be plumes on Pluto?
Thinking about Magnetic Sails
Magnetic sails — ‘magsails’ — are a relative newcomer on the interstellar propulsion scene, having been first analyzed by Dana Andrews and Robert Zubrin in 1988. We saw that the particle beam concept advanced by Alan Mole and discussed this week by Jim Benford would use a magsail in which the payload and spacecraft were encircled by a superconducting loop 270 meters in diameter. The idea is to use the magnetic field to interact with the particle beam fired from an installation in the Solar System toward the departing interstellar craft.
Within our own system, we can also take advantage of the solar wind, the plasma stream flowing outward from the Sun at velocities as high as 600 kilometers per second. A spacecraft attempting to catch this wind runs into the problem that sunlight contains far more momentum, which means a magnetic sail has to deflect a lot more of the solar wind than a solar sail needs to deflect sunlight. A physical sail, though, is more massive than a spacecraft whose ‘sail’ is actually a magnetic field, so the magsail spacecraft can be the less massive of the two.
Science fiction began exploring basic solar sails in the 1960s through stories like Clarke’s “Sunjammer” and Cordwainer Smith’s “The Lady Who Sailed the Soul.” In fact, SF writers have done an excellent job in acquainting the public with how solar sails would operate and what their capabilities might be. But magsails are hard to find in science fiction, and the only novel that springs readily to mind is Michael Flynn’s The Wreck of the River of Stars, whose haunting title refers to a magsail passenger liner at the end of its lifetime.
Here’s Flynn in ‘Golden Age’ Heinlein style introducing the tale:
They called her The River of Stars and she spread her superconducting sails to the solar wind in 2051. She must have made a glorious sight then: her fuselage new and gleaming, her sails shimmering in a rainbow aurora, her white-gloved crew sharply creased in black-and-silver uniforms, her passengers rich and deliciously decadent. There were morphy stars and jeweled matriarchs, sports heroes and prostitutes, gangsters and geeks and soi-disant royalty. Those were the glamour years, when magsails ruled the skies, and The River of Stars was the grandest and most glorious of that beautiful fleet.
Image: There are few science fiction stories involving magsails, and even fewer visual depictions. The cover art for Michael Flynn’s book, by the artist Stephan Martiniere, is a striking exception.
The novel takes place, though, many years later, when the grand passenger liner has become no more than an obsolete freighter whose superconducting sail structure has been decommissioned in favor of newly developed fusion drives. What happens when she needs to power up the sail again because of a fusion emergency makes up the bulk of the tale. The Wreck of the River of Stars is not about an interstellar journey but a highly developed infrastructure within the Solar System that, for a time, used the solar wind. It will be interesting to see what science fiction tales grow out of the current interstellar thinking.
For magsails emerged in an interstellar context, and if it was Robert Zubrin and Dana Andrews who worked through the equations of what we conceive today as a magsail, it was Robert Bussard who first brought life to the idea through his notion of an interstellar ramjet that would use magnetic fields to scoop up fuel between the stars. Both Zubrin and Andrews saw the potential uses of a magsail for deceleration against a stellar wind. If beam dispersal cannot be prevented to allow an interstellar magsail to be accelerated by particle beam, we might still consider equipping a beamed laser sailcraft with magsail capabilities for use upon arrival.
And when it comes to magsails closer to home, one cautionary note is provided by a 1994 paper from the Italian physicist Giovanni Vulpetti, who describes the problems we may have operating superconductors within the orbit of Mars. The paper notes that superconductivity can be lost this close to the Sun unless massive thermal shielding is applied and that, of course, ramps up the spacecraft mass. This evidently does not preclude outer system work, but it could serve as a brake on using magsails near the Earth, at least until we make considerable advances in superconductor technology.
The Vulpetti paper is “A Critical Review on the Viability of Space Propulsion Based on the Solar Wind Momentum Flux,” Acta Astronautica 37 (1994), 641-642.
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