by Andreas Hein
The immense problems of time, distance and life support invariably mean that when we talk about an interstellar mission, we talk about robotics. But the imaginative team at Icarus Interstellar, which is now setting up projects on everything from beamed lightsails (Project Forward) to pulse propulsion engines (Project Helios), has pushed into the biggest what-if of all, the question of manned missions. And as project leader Andreas Hein reminds us in the following article, a variety of approaches have been suggested for this over the years from which a new concept study can grow.
Andreas Hein received his master’s degree in aerospace engineering at the Technische Universität München, and is doing his PhD at the same university in the area of space systems engineering at the Institute of Astronautics. He has participated in several mission studies: a lunar gravity measurement mission by EADS and a cubesat mission analysis. During his internship at ESA-ESTEC, he participated in the joint ESA/industry lunar architecture study of the human spaceflight division, applying different systems engineering methodologies such as stakeholder analysis. Andreas is currently supervising a course on concurrent engineering of space systems at the Institute of Astronautics whose objective is to design an Earth observation mission. His particular interest is in the early phases of systems design, which he lists as requirements engineering, functional analysis, concept design/trade offs.
News of recently discovered exoplanets reaches us almost weekly and it seems only a matter of time until an “Earth 2.0” is found out there. If this happens, is there any possibility for humans to travel to this planet with today’s or foreseeable technologies? This is the main question Project Hyperion deals with.
Project Hyperion is a research project whose main purpose is to assess the feasibility of crewed interstellar flight with current or near-future technologies.
I will first give a brief overview of existing concepts for crewed interstellar flight and then dwell on challenges apart from crossing the distance between the stars.
Image: Andreas Hein (left) and Alan Bond, designer of the Bond/Martin World Ship concept, at the headquarters of the British Interplanetary Society. Credit: A. Hein.
Ideas for the Long Haul
The primary purpose of crewed interstellar flight is the long-term colonization of an exosolar system. What are the concepts that exist today to achieve this goal? The most well-known concept was introduced by Robert Forward in 1984 , who discusses the feasibility of a crewed trip to the nearest stars by a laser-pushed sail. Another concept is the fusion-propelled “colony ship” by Gregory Matloff, published in 1976  and the “world ship” concept of Alan Bond and Anthony Martin . A recent analysis for an antimatter rocket was conducted by Robert Frisbee in . Most of the existing “near-term” concepts can be put into four categories, based on the mode of crew transportation:
- Crew lives on the spacecraft: world ships, colony ships [2-5]
- Crew is in suspended animation or hibernates 
- Crew is transported as embryos or single cells 
- Crew is transported digitally 
We still know too little about each of these concepts. Nevertheless, some educated guesses can be made: From a) to d) an increasing level of technological sophistication is required in order to achieve the mission. On the other hand, the resources needed for transporting the crew decrease at the same time, which enables faster travel. Think of a lorry being slower than a Ferrari although they have the same horsepower. A world or colony ship is a massive spacecraft, several tens of miles-long. The crew lives and dies over generations on these small worlds, crossing the comic ocean between the stars taking centuries.
Image: A worldship dwarfs the imagination, offering a habitat for humans over the course of generations on a long, slow voyage to the stars. Credit: Adrian Mann.
A suspended animation / hibernation ship is still large but does not require a habitat as large as a world ship: most of time the crew “sleeps” during the trip like in the introductory sequence of the movie “Avatar”. However, some kind of habitat is probably still required for periodical “awakenings”. An embryo or single cell ship will likely have a payload the size of a house as no habitat is required and a digital crew spacecraft payload might have the size of a small car or even smaller.
Each concept is challenging in its own way: It is in principle possible to build a world ship or colony ship today, given the resources. However, how to design a habitat that enables human survival and comfort for centuries? This has analogies to people living on an island cut-off from the rest of the world. How will culture and technology develop under such conditions? Will people live like in the Stone-Age when they arrive at the target star system? How many people should be sent to maintain at least a certain level of cultural and technological diversity? How is the spacecraft maintained over centuries?
The feasibility of using a spacecraft with a crew in suspended animation or hibernation depends on the applicability of these concepts to humans. Whether or not it is possible to sustain a crew in this state for decades or even centuries is still an open question.
An embryo or single cell ship needs an approach to raise and educate them at the target system. Methods like android parenting have been mentioned in the literature. Furthermore, sophisticated automatic manufacturing is required to construct a shelter and to provide nourishment on the target planet. These are all technologies that are not available today. Whether or not a crew can be “resurrected” on the basis of digital data in the target system is an open question. If this becomes possible one day, this technology will not only make this special form of interstellar travel possible but will have a profound impact on our life on Earth.
Besides these technological challenges for each concept, different ethical questions have to be addressed as we are dealing with decisions that have far reaching consequences for the crew sent out to the stars and the ones that are yet unborn.
Building the Star Colony
We have now briefly covered the potential approaches for crossing the gap between the stars. But what happens once the crew arrives at the target star system?
Most of the scientific literature focuses on the trip between the stars. However, the establishment of a colony within the target star system is a neglected, but vital part for planning such a mission . We often assume that a habitable exoplanet is very similar to Earth and human life is possible on its surface. But there are myriads of potential obstacles to colonizing such a planet: An ecosystem that does not provide edible food and water, diseases to which the human immune system can not adopt adequately, toxic substances etc.
It is difficult to say whether it is possible to anticipate these difficulties in advance as many issues will remain undiscovered, even after an exploration with precursor probes. Alternative approaches like terraforming and building artificial colonies in space were also considered [2, 9]. Terraforming aims at changing a planet in a way that human life is possible on its surface for extended periods of time. Artificial colonies might be the last fall-back-option, in case human life is not sustainable on a planetary surface. The concept of large space colonies has been proposed by the physicist Gerard O’Neill in the 70s as a mode of human existence in the future . This might ultimately make colonization of another star system independent of the discovery of a habitable exoplanet. However, the immense difficulties associated with both approaches are difficult to anticipate today.
Image: The ISV Venture Star from James Cameron’s film Avatar. Credit: Ben Procter.
Most of the publications mentioned in this article have limited detail and are restricted to rough outlines and estimates without detailed engineering assessments. Think of the difference between a painter’s pencil sketch to prepare for a painting and the meticulous work that has to be put in to flesh out all the details on the canvas. Currently, only rough sketches exist.
With Project Hyperion we want to get from the sketch to the real painting! It is our conviction that today most of the required feasibility analysis for a crewed interstellar spacecraft can be done with current knowledge. Some readers will certainly think: “Ok, this is a nice exercise but won’t it take several centuries to realize such a mission? So why think about this today?” I agree that it is still a long time to go to see the first humans heading to the stars. However, we humans exist on Earth as a species for about 200,000 years. If one thousand years are one meter, this is a distance of about 200 meters, more than twice the length of a football field. If it takes about 300 years to send out the first humans to the stars, on this scale, this is about 30 cm. How insignificant in comparison to the distance in time we already traveled as a species and what a magnificent chance to take the first steps towards this goal today.
 Forward, R.L., “Roundtrip Interstellar Travel Using Laser-Pushed Lightsails”, Journal of Spacecraft and Rockets, Vol.21, No.2, March-April 1984.
 Matloff, G.L., “Utilization of O’Neill’s Model I Lagrange Point Colony as an Interstellar Ark”, Journal of the British Interplanetary Society, Vol. 29, pp. 775-785, 1976.
 Bond, A., Martin, A.R., “World Ships – An Assessment of the Engineering Feasibility”, Journal of the British Interplanetary Society, Vol. 37, pp. 254-266, 1984.
 Frisbee, R.H., “Limits of Interstellar Flight Technology”, Chapter 2 in Frontiers of Propulsion Science, ed. by Millis, M. and E. W. Davis, Progress in Astronautics and Aeronautics, 2009.
 Various articles on hibernation in JBIS, Vol. 59, pp.81-144.
 Crowl, A., Hunt, J., Hein, A., “How an Embryo Space Colonization (ESC) Mission Solves the Time-Distance Problem”, presentation at the 100 Year Starship Symposium, October 2011, Orlando, USA
 Tipler, F., The Physics of Immortality, Chapter 2, Doubleday, New York, 1994.
 Matloff, G.L., Mallove, E.F., “The First Interstellar Colonization Mission”, Journal of the British Interplanetary Society, Vol. 33, pp. 84-88, 1980.
 O’Neill, G.K., “The Colonisation of Space”, Physics Today, 27, No. 9, 32-40, September 1974.
Despite my best intentions, I still haven’t put my hands on the exchange between Robert Forward and Ian Crawford on lightsails that ran back in 1986 in JBIS, nor have I managed to come up with the source of the ‘lightsail on arrival’ illustration I mentioned last week. This was the one showing a battered and torn sail docked in what I assume was a repair facility at the end of its long journey, and the effects of passage through the interstellar medium were all too obvious. It was a great image and I was frustrated about not being able to find the magazine it was published in, but an email from James Early quickly changed my mood.
As opposed to the missing image that nagged at my memory, this was a case of having missed something perfectly obvious in the first place. I didn’t know about the paper Jim did with Richard London on lightsails and the interstellar medium — it was published in the Journal of Spacecraft and Rockets back in 2000, but somehow I didn’t find it in the research for my Centauri Dreams book back then. It’s a welcome addition to the literature, one Jim went on to present to the 100 Year Starship Symposium in Orlando last fall. Jim tells me the presentation tracks the earlier paper, and I’ll draw today from the more recent paper as we look at lightsail concepts.
Defining the Sail
Although Robert Forward came up with sail ideas that pushed as high as 30 percent of the speed of light (and in the case of Starwisp, even higher), Early and London are content with 0.1 c, which provides technical challenges aplenty but at least diminishes the enormous energy costs of still faster missions, and certainly mitigates the problem of damage from dust and gas along the way. Lightsails will be slow to accelerate, given that the light absorbed by the sail creates a thermal limit on the maximum laser power usable. In short, we can’t heat the sail beyond a certain point, and that gives us a maximum acceleration, so our sail may take a tenth of a light year to get up to cruise velocity. It’s noteworthy that the sail does not have to be deployed during cruise itself, but deceleration at the target star, depending on the methods used, may demand redeployment.
Geoffrey Landis’ work on the different types of sails that are feasible seems to be the standard. Landis divides sails into three broad categories:
- Extremely thin, low-density metal sails. Forward proposed using aluminum in his paper on a round-trip mission to Epsilon Eridani, but beryllium seems to be the material of choice.
- Heat-resistant thin metal sails made from niobium or tantalum. These offer higher temperatures and power densities.
- Dielectric thin films at one-quarter wavelength thickness for the laser light. Dielectric materials are poor conductors of electricity but can support electrostatic fields, as in a capacitor. These materials offer excellent reflectivity, requiring thicker sails, but the materials make for low absorption and offer good thermal performance, so higher accelerations are possible.
Early and London use beryllium sails as their reference point, these being the best characterized design at this stage of sail study, and assume a sail 20 nm thick.
The Sail and the Medium
In terms of the interstellar medium the sail will encounter, the authors say this:
Local interstellar dust properties can be estimated from dust impact rates on spacecraft in the outer solar system and by dust interaction with starlight. The mean particle masses seen by the Galileo and Ulysses spacecraft were 2×10-12 and 1×10-12g, respectively. A 10-12g dust grain has a diameter of approximately 1 µm. The median grain size is smaller because the mean is dominated by larger grains. The Ulysses saw a mass density of 7.5×10-27g cm-3. A sail accelerating over a distance of 0.1 light years would encounter 700 dust grains/cm2 at this density. The surface of any vehicle that traveled 10 light years would encounter 700 dust grains/mm2. If a significant fraction of the particle energy is deposited in the impacted surface in either case, the result would be catastrophic.
The question then becomes, just how much of the particle’s energy will be deposited on the sail? The unknowns are all too obvious, but the paper adds that neither of the Voyagers saw dust grains larger than 1 ?m at distances beyond 50 AU, while a 1999 study on interstellar dust grain distributions found a flat distribution from 10-14 to 10-12 g with some grains as large as 10-11 g. Noting that a 10-12 dust grain has a diameter of about 1-?m, the authors use a 1-?m diameter grain for their impact calculations.
The results are intriguing because they show little damage to the sail. Catastrophe averted:
At the high velocities of interstellar travel, dust grains and atoms of interstellar gas will pass through thin foils with very little loss of energy. There will be negligible damage from collisions between the nuclei of atoms. In the case of dust particles, most of the damage will be due to heating of the electrons in the thin foil. Even this damage will be limited to an area approximately the size of the dust particle due to the extremely fast, one-dimensional ambipolar diffusion explosion of the heated section of the foil. The total fraction of the sail surface damaged by dust collisions will be negligible.
The torn and battered lightsail in its dock, as seen in my remembered illustration, may then be unlikely, depending on cruise speed and, of course, on the local medium it passes through. Sail materials also turn out to offer excellent shielding for the critical payload behind the sail:
Interstellar vehicles require protection from impacts by dust and interstellar gas on the deep structures of the vehicle. The deployment of a thin foil in front of the vehicle provides a low mass, effective system for conversion of dust grains or neutral gas atoms into free electrons and ions. These charged particles can then be easily deflected away from the vehicle with electrostatic shields.
And because the topic has come up recently in discussions here, let me add this bit about interstellar gas and its effects on the lightsail:
The mass density of interstellar gas is approximately one hundred times that of interstellar dust particles though this ratio varies significantly in different regions of space. The impact of this gas on interstellar vehicles can cause local material damage and generate more penetrating photon radiation. If this gas is ionized, it can be easily deflected before it strikes the vehicle’s surface. Any neutral atom striking even the thin foil discussed in this paper will pass through the foil and emerge as an ion and free electron. Electrostatic or magnetic shields can then deflect these charged particles away from the vehicle.
Ramifications for Sail Design
All of these findings have a bearing on the kind of sail we use. The thin beryllium sail appears effective as a shield for the payload, with a high melting point and, the authors conclude, the ability to be increased in thickness if necessary without increasing the area damaged by dust grains. Ultra-thin foils of tantalum or niobium offer higher temperature possibilities, allowing us to increase the laser power applied to the sail and thus the acceleration. But Early and London believe that the higher atomic mass of these sails would make them more susceptible to damage. Even so, “…the level of damage to thin laser lightsails appears to be quite small; therefore the design of these sails should not be strongly influenced by dust collision concerns.”
The third type of sail, the thicker dielectric sail, is more problematic, suffering more damage from heated dust grains because of its greater thickness, and the authors argue that these sail materials need to be subjected to a more complete analysis of the blast wave dynamics they will be subjected to. All in all, though, velocities of 0.1 c yield little damage to a thin beryllium sail, and thin shields of similar materials can ionize dust as well as neutral interstellar gas atoms, allowing the ions to be deflected and the interstellar vehicle protected.
These are encouraging results, but the size of the problem is daunting, and I keep coming back to something Jean Schneider said in the exchange with Ian Crawford discussed here last Friday:
“The question is what probability of collision is acceptable. If a collision is lethal, this probability must be extremely close to zero for a several hundred billion € mission.”
The original paper on this work is Early, J.T., and London, R.A., “Dust Grain Damage to Interstellar Laser-Pushed Lightsail,” Journal of Spacecraft and Rockets, July-Aug. 2000, Vol. 37, No. 4, pp. 526-531. JBIS is planning to publish selected papers from the 100 Year Starship Symposium and I assume this is one of them. When this revised paper runs, I’ll post the complete citation.
Memories play tricks on us all, but trying to recall where I saw a particular image of a laser lightsail is driving me to distraction. The image showed a huge sail at the end of its journey, docked to some sort of space platform, and what defined it were the tears and holes in the giant, shredded structure. It presupposed long passage through an interstellar medium packed with hazards, and although I assumed I would have seen it on the cover of some science fiction magazine, I spent an hour yesterday scanning covers on Phil Stephensen-Payne’s wonderful Galactic Central site, but all to no avail.
The image must have run inside a magazine, then, but if so, I’m at a loss to identify it other than to say it would have appeared about twenty years ago. I had hoped to reproduce it this morning because our talk about starship shielding necessarily brought up the question of whether an enormous lightsail — some of these are conceived as being hundreds of kilometers in diameter — wouldn’t be impractical in denser areas of the galaxy. And that brought to mind a 1986 exchange between the British astronomer Ian Crawford (Birkbeck College, London) and Robert Forward, the physicist who did so much to awaken us to the possibilities of interstellar flight.
This morning I’m about eight miles away from the library where I can find back issues of the Journal of the British Interplanetary Society, but Gregory Matloff and Eugene Mallove wrote about this correspondence in The Starflight Handbook, which I do have right here in front of me. Forward’s position was that a laser lightsail would be so thin that dust grains would pass right through it without depositing a great deal of their kinetic energy as heat. So maybe the shredded lightsail isn’t a necessary outcome of a beamed sail mission. From The Starflight Handbook:
During a 10-ly journey at 0.2 c, only 1/500 of the area of a 0.0160 micron (160 Å or angstrom) thick light sail will be lost. However, Forward and we agree that a great deal of theoretical and experimental work on interstellar erosion must still occur before we can set off for the stars free of bad dreams.
Dust Grains Between the Stars
I’ve been thinking about Ian Crawford partially because of his recent paper on manned spaceflight and its virtues, but also because of another exchange he had about two years ago, this one with Jean Schneider (Paris Observatory), who had been examining our response to the detection of biosignatures on exoplanets, and in passing discussed how difficult it would be to get a probe to an exoplanet to investigate them. Schneider was worried about the interstellar medium too, and went to work on the possibilities assuming a spacecraft velocity of 30 percent of the speed of light. Moving at that pace, Schneider calculated that a 100-?m interstellar grain would have the same kinetic energy as a 100-ton body moving at 100 kilometers per hour.
The Schneider/Crawford exchange is up on the arXiv site (references below), and you can read about it in Interstellar Flight: The Case for a Probe as well as Interstellar Flight and Long-Term Optimism, the two articles I wrote about it back in 2010. It was Crawford’s position that interstellar dust grains could indeed present a hazard that will need to be factored into the design of the vehicle, but Crawford found several mitigating factors including speed, pointing out that 30 percent of lightspeed was a overly ambitious target, and certainly a more problematic one, owing to the scaling of kinetic energy with the square of the velocity.
Crawford finds that the situation at 0.1 c is considerably better. From the paper:
The issue of shielding an interstellar space probe from interstellar dust grains was considered in detail in the context of the Daedalus study by Martin (1978). Martin adopted beryllium as a potential shielding material, owing to its low density and relatively high speci?c heat capacity, although doubtless other materials could be considered. Following Martin’s (1978) analysis, but adopting an interstellar dust density of 6.2 x 10-24 kg m-3 (i.e., that determined by Landgraf et al., 2000), we ?nd that erosion by interstellar dust at a velocity of 0.1 c would be expected to erode of the order of 5 kg m– 2 of shielding material over a 6-light-year ?ight.
We clearly need shielding, then, and that adds to the mass of the interstellar probe, but Crawford does not find the problem insurmountable. Of course, what we have yet to learn is the true size distribution of dust particles in the nearby interstellar medium, which is one reason we need a mission like Innovative Interstellar Explorer, to make such measurements in situ. Crawford works out the spatial density of 100-?m grains at about 4 x 10-17 m-3 based on work by Markus Landgraf (Johnson Space Center) and colleagues in 2000. Here we find just how much work lies ahead:
…over the 6 light-year (5.7 x 1016 m) ?ight considered by Martin (1978), we might expect of the order of two impacts per square meter with such large particles, and the injunction by Schneider et al. (2010) may, after all, appear pertinent. On the other hand, it is far from clear that it is valid to extrapolate the distribution to such large masses, not least because of the dif?culty of reconciling the presence of such large solid particles in the LIC with constraints imposed by the cosmic abundances of the elements (as also noted by Landgraf et al., 2000, and Draine, 2009). Clearly, more work needs to be done to better determine the upper limit to size distribution of interstellar dust grains in the local interstellar medium.
More work indeed, and Schneider, in a response to Crawford’s own response to his earlier paper, notes that the matter comes down to what we are willing to live with in terms of probabilities:
The question is what probability of collision is acceptable. If a collision is lethal, this probability must be extremely close to zero for a several hundred billion € mission.
Searching for Solutions
We’re not yet able to make the definitive call on just how many large interstellar grains our probe may run into in the local interstellar medium, but Crawford thinks the problem can be addressed through the detection of incoming large grains and the use of either laser or electromagnetic means to destroy or deflect them before they impact the spacecraft. Again I turn back to Alan Bond’s idea of a dust cloud ejected from the vehicle and preceding it along its course. Remember that Bond was working with the Daedalus concept of an initial, multi-year period of acceleration followed by decades of coasting to reach Barnard’s Star. A dust cloud like this could destroy larger interstellar grains before they ever reached the main vehicle. Adds Crawford:
This concept was developed for Daedalus in the context of protecting the vehicle in the denser interplanetary environment of a target star system, but it would work just as well for the interstellar phase of the mission should further research identify the need for such protection.
A mature space exploration infrastructure here in our own Solar System is probably the prerequisite for the kind of interstellar probe Crawford is talking about, and he notes the value of building that infrastructure not only in terms of creating the technologies we’ll need to get to the stars, but also in terms of making possible the search for life not only on Mars but further out in the system. How long it takes us to build this framework plays directly to Schneider’s point that:
It is presumptuous to predict exactly what will happen after one century and into the future, but it is more than likely that development of the capacity to observe the morphology of meter-sized organisms on exoplanets will take several centuries, at least in the framework of present and forseeable physical concepts. Another optimistic possibility would be that, in a nearer future, we will detect pictures of extraterrestrials with a good resolution in SETI signals. The debate must still go on.
The initial paper by Jean Schneider is “The Far Future of Exoplanet Direct Characterization,” Astrobiology Vol. 10, Issue 1 (22 March, 2010), available as a preprint. Ian Crawford responded in “A Comment on ‘The Far Future of Exoplanet Direct Characterization’ — the Case for Interstellar Space Probes,” Astrobiology Vol. 10, Number 8 (2010), pp. 853-856 (preprint). Schneider’s follow-up response to Crawford is “Reply to « A Comment on ”The Far Future of Exoplanet Direct Characterization” – the Case for Interstellar Space Probes » by I. Crawford,” Astrobiology Volume 10, No. 8 (2010). I don’t have the page numbers on the latter but the preprint is available.
Interesting new ideas about asteroid deflection are coming out of the University of Strathclyde (Glasgow), involving the use of lasers in coordinated satellite swarms to change an asteroid’s trajectory. This is useful work in its own right, but I also want to mention it in terms of a broader topic we often return to: How to deal with the harmful effects of dust and interstellar gas on a fast-moving starship. That’s a discussion that has played out many a time over the past eight years in these pages, but it’s as lively a topic as ever, and one on which we’re going to need a lot more information before true interstellar missions can take place.
Lasers and the Asteroid
But let’s set the stage at Strathclyde for a moment. The idea here is to send small satellites capable of formation flying with the asteroid, all of them firing their lasers at close range. The university’s Massimiliano Vasile, who is leading this work, says that the challenge of lasers in space is to combine high power, high efficiency and high beam quality simultaneously. He adds:
“The additional problem with asteroid deflection is that when the laser begins to break down the surface of the object, the plume of gas and debris impinges the spacecraft and contaminates the laser. However, our laboratory tests have proven that the level of contamination is less than expected and the laser could continue to function for longer than anticipated.”
Vasile believes using a flotilla of small but agile spacecraft, each with a highly efficient laser, is more feasible than trying to deflect an asteroid with a single, large spacecraft carrying a much larger laser. One benefit is that the system is scalable — add as many spacecraft as needed for the job at hand. The other is that you have the redundancy afforded by multiple laser platforms. The Strathclyde work is also investigating whether a similar system could be used to remove space debris by de-orbiting problematic objects to avoid potential collisions.
Erosion Shields on the Starship?
If lasers can be used to alter an asteroid’s trajectory, we need to consider their uses in clearing out the space ahead of futuristic space probes. That the interstellar medium itself was going to be a problem became apparent as researchers began to study starship deceleration concepts in the early 1970s. Get your vehicle moving in the range of 0.3 c and any grain of carbonaceous dust a tenth of a micron in diameter it encounters carries a relative kinetic energy of 37,500,000 GeV, according to Dana Andrews (Andrews Space) in a 2004 paper. How that kinetic energy is dealt with is clearly a major issue.
By the late 1970s, aluminum and then graphite had been considered as possible erosion shields, with the preference going to graphite, but in 1978 Anthony Martin reviewed the literature and suggested a beryllium payload shield be deployed on the Project Daedalus probe, which would be moving at .12 c. It would be quite a large object, some 9 millimeters thick and 32 meters in radius, and even it didn’t completely solve the problem, for Daedalus would, upon arrival, be moving into a still denser gas and dust environment around Barnard’s Star. Daedalus designer Alan Bond suggested additional shielding in the form of a cloud of dust deployed from the probe, which would vaporize larger particles before they could damage the vehicle.
Image: Diagram of the Project Daedalus probe, developed by the British Interplanetary Society in the 1970s. Note the beryllium shield at upper left. Credit: Adrian Mann.
Clearing Out the Path
We’re still not through, though. What about particles larger than dust grains, up to hailstones in size? We are now talking about collisions that would be catastrophic, and must turn from passive to active measures to tackle the problem. Gregory Matloff and Eugene Mallove have suggested using a light or X-ray laser or a neutral particle beam firing ahead of the ship to deflect any objects detected in its forward-pointing radar. The Project Icarus team has looked at creating a bumper out of graphene, as discussed in this blog entry, and coupling it with a laser defense:
What I’m interested in for shielding is making a large, low-mass “bumper” which cosmic sand-grains run into before hitting the craft. After passing through several layers of graphene the offending mass is totally ionized and forms a high-energy spray of particles, but particles that can now be deflected by the vehicle’s cosmic-ray defences (akin to the mag-sail, but smaller with a higher current) and safely diverted away from sensitive parts.
The notion seems an adaptation of Conley Powell’s 1975 work on shields that move ahead of the ship, trapping ionized material on impact within a magnetic field. The earlier Daedalus researchers found that Powell’s ideas resulted in less erosion than other methods then being studied. This is an interesting shield, one placed perhaps 100 kilometers ahead of the spacecraft. Moreover, it is not passive but can signal the vehicle when grains have passed through it without being ionized:
This causes a signal to be sent back to the vehicle which then activates its final layer of defence, high-powered lasers. In microseconds the lasers either utterly ionize the target or give it a sideways nudge via ablation – blowing it violently to the side via a blast of plasma. Such an active tracking bumper would need to be further away than 100 km to give the laser defence time to react, though 1/600th of a second can be a lot of computer cycles for a fast artificial intelligence. The lasers might use advanced metamaterials to focus the beam onto a speck at ~100 km, without needing to physically turn the laser itself in such a split-second. Highly directional, high-powered microwave phased arrays exist which already do so purely electronically and an optical phased-array isn’t a stretch beyond current technology.
All of which takes me back to the University of Strathclyde work on laser deflection, and makes me wonder whether laser technologies first deployed against asteroids in our Solar System may one day be used to protect our interstellar voyagers.
Anthony Martin’s paper is “Bombardment by Interstellar Material and Its Effects on the Vehicle,” Project Daedalus Final Report (Journal of the British Interplanetary Society, 1978): S116–S121. Alan Bond discusses in-system shielding in “Project Daedalus: Target System Encounter Protection,” S123–S125 in the same publication. The Dana Andrews paper is “Things to Do While Coasting Through Interstellar Space,” AIAA-2004-3706, 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Fort Lauderdale, Florida, July 11-14, 2004.
The Kepler mission’s exoplanet discoveries have been so numerous that an extension of the mission seemed all but inevitable. At the same time, bureaucracies can be unpredictable, which is why it was such a relief to have the Senior Review of Operating Missions weighing in with an extension recommendation, one followed up by NASA with extensions not just for Kepler but also for the Spitzer telescope and the US portion of ESA’s Planck mission. Kepler’s extension runs through fiscal 2016 (subject to review in 2014), allowing for plenty of time to home in on Earth-sized planets in the habitable zone around stars like our Sun.
While Kepler’s scheduled mission duration was 3.5 years, the mission was intended to be extendible to 6 years or more and this news is more than satisfying. But of course while we continue to monitor the Kepler work, we’re following numerous other exoplanet stories including the European Southern Observatory’s observations of the prolific star HD 10180, a Sun-like star about 127 light years away in the constellation of Hydrus. The ESO work, performed with the HARPS spectrograph attached to the 3.6 meter telescope at La Silla in the Chilean Andes back in 2010, revealed the presence of at least five and possibly seven planets. Now astronomer Mikko Tuomi (University of Hertfordshire) has performed further data analysis on the HARPS radial velocity data, concluding that the system may contain as many as nine planets.
Image: The circle shows the location of the class G near-solar star HD 10180, which may be orbited by as many as nine planets. Credit: Jim Kaler/UIUC.
The two new worlds Tuomi’s work reveals range from 1.9 to 5.1 Earth masses respectively, allowing them to be classified as probable super-Earths with orbits of 10 and 68 days. If the findings are confirmed, this would make the HD 10180 system more fecund than our own, at least in terms of number of planets. Tuomi also reports he has verified the inner planet signature first announced in 2011, indicating a planet on a 1.18 day orbit with a minimum mass as low as 1.3 Earth masses. He has also revised the orbital parameters of the other six planet candidates around the star. Add it all up and you get nine planets. From the paper:
As noted by Lovis et al. (2011), the star is a very quiet one without clear activity-induced periodicities, which makes it unlikely that one or some of the periodic signals in the data were caused by stellar phenomena. Also, the periodicities we report, namely 9.66 and 67.6 days, do not coincide with any periodicities arising from the movement of the bodies in the Solar system. Therefore, we consider the interpretation of these two new signals of being of planetary origin to be the most credible explanation.
If this is borne out, then we have exceeded our Solar System’s planet count for the first time in exoplanet studies. The paper notes the need for additional high-precision radial velocity studies to confirm these findings. And we may not be through. There appear to be stable orbits for a low-mass companion in or near the habitable zone of HD 10180, one whose mass would be unlikely to exceed 12.1 Earth masses based on Tuomi’s samplings. All that should keep this star among our targets for some time to come as the data are mined for confirmation of these worlds.
The paper is Tuomi, “Evidence for 9 planets in the HD 10180 system,” accepted for publication in Astronomy and Astrophysics (preprint).