Having just discussed whether humans – as opposed to their machines – will one day make interstellar journeys, it’s a good time to ask where we could get today with near-term technologies. In other words, assuming reasonable progress in the next few decades, what would be the most likely outcome of a sustained effort to push our instruments into deep space? My assumption is that fusion engines will one day be available for spacecraft, but probably not soon, and antimatter, that quixotic ultimate power source for interstellar flight, is a long way from being harnessed for propulsion.
We’re left with conventional rocket propulsion with gravity assists, and sail technologies, which not coincidentally describes the two large interstellar missions currently being considered for the heliophysics decadal study. Both JHU/APL’s Interstellar Probe mission and JPL’s SGLF (Solar Gravity Lens Focal) mission aim at reaching well beyond our current distance holders, the now struggling Voyagers. The decadal choice will weigh the same question I ask above. What could we do in the near term to reach hundreds of AU from the Sun and get there in relatively timely fashion?
A paper from the JPL effort in Experimental Astronomy draws my attention because it pulls together where the SGLF concept is now, and the range of factors that are evolving to make it possible. I won’t go into detail on the overall design here because we’ve discussed it in the recent past (see for example Building Smallsat Capabilities for the Outer System and Self-Assembly: Reshaping Mission Design for starters). Instead, I want to dig into the new paper looking for points of interest for a mission that would move outward from the Sun’s gravitational lens and, beyond about 650 AU, begin imaging an exoplanet with a factor of 1011 amplification.
Image: This is Figure 1 from the paper. Caption: The geometry of the solar gravity lens used to form an image of a distant object in the Einstein ring. Credit: Friedman et al.
Carrying a telescope in the meter-class, the spacecraft would reach its target distance after a cruise of about 25 years, which means moving at a speed well beyond anything humans have yet attained moving outward from the Sun. While Voyager 1 reached over 17 kilometers per second, we’re asking here for at least 90 km/sec. Remember that the focal line extends outward from close to 550 AU, and becomes usable for imaging around 650 AU. Our spacecraft can take advantage of it well beyond, perhaps out to 1500 AU.
So let’s clear up a common misconception. The idea is not to reach a specific distance from the Sun and maintain it. Rather, the SGLF would continue to move outward and maneuver within what can be considered an ‘image cylinder’ that extends from the focal region outward. This is a huge image. Working the math, the authors calculate that at 650 AU from the Sun, the light (seen as an ‘Einstein ring’ around the Sun) from an exoplanet 100 light years from our system would be compressed to a cylinder 1.3 kilometers in diameter. Remember, we have a meter-class telescope to work with.
Thus the idea is to position the spacecraft within the image cylinder, continuing to move along the focal line, but also moving within this huge image itself, collecting data pixel by pixel. This is not exactly a snapshot we’re trying to take. The SGLF craft must take brightness readings over a period that will last for years. Noise from the Sun’s corona is reduced as the spacecraft moves further and further from the Sun, but this is a lengthy process in terms of distance and time, with onboard propulsion necessary to make the necessary adjustments to collect the needed pixel data within the cylinder.
So we’re in continual motion within the image cylinder, and this gets further complicated by the range of motions of the objects we are studying. From the paper:
Even with the relatively small size of the image produced by the SGL, the spacecraft and telescope must be maneuvered over the distance of tens of kilometers to collect pixel-by-pixel all the data necessary to construct the image… This is needed as the image moves because of the multiple motions [that] are present, namely 1) the planet orbits its parent star, 2) the star moves with the respect to the Sun, and 3) the Sun itself is not static, but moves with respect to the solar system barycentric coordinates. To compensate for these motions, the spacecraft will need micro-thrusters and electric propulsion, the solar sail obviously being useless for propulsion so far from the Sun.
Bear in mind that, as the spacecraft continues to move outward from 650 AU, the diameter of the image becomes larger. We wind up with a blurring problem that has to be tackled by image processing algorithms. Get enough data, though, and the image can be deconvolved, allowing a sharp image of the exoplanet’s surface to emerge. As you would imagine, a coronagraph must be available to block out the Sun’s light.
What to do with the sail used to reach these distances? The mission plan is a close solar pass and sail deployment timed to produce maximum acceleration for the long cruise to destination. Solar sails are dead weight the further we get from the Sun, so you would assume the sail would be jettisoned, although it’s interesting to see that the team is working on ways to convert it into an antenna, or perhaps even a reflector for laser communications. As to power sources for electric propulsion within the image cylinder, the paper envisions using radioisotope thermoelectric generators, which are what will power up the craft’s communications, instruments and computing capabilities.
Image: This is Figure 4 from the paper. Caption: Trajectory of the mission design concept for a solar sailcraft to exit the solar system. Credit: Friedman et al./JPL.
Let’s clear up another misconception. If we deploy a sail at perihelion, we are relying on the solar photons delivering momentum to the sail (photons have no mass, but they do carry momentum). This is not the solar wind, which is a stream of particles moving at high velocity out from the Sun, and interesting in its own right in terms of various mission concepts that have been advanced in the literature. The problem with the solar wind, though, is that it is three orders of magnitude smaller than what we can collect from solar photons. What we need, then, is a photon sail of maximum size, and a payload of minimum mass, which is why the SGLF mission focuses on microsats. These may be networked or even undergo self-assembly during cruise to the gravity focus.
The size of a sail is always an interesting concept to play with. Ponder this: The sail mission to Halley’s Comet that Friedman worked on back in the mid-1970s would have demanded a sail that was 15 kilometers in diameter, in the form of a so-called heliogyro, whose blades would have been equivalent to a square sail half a mile to the side. That was a case of starting at the top, and as the paper makes clear, issues of packaging and deployment alone were enough to make the notion a non-starter.
Still, it was an audacious concept and it put solar sails directly into NASA’s sights for future development. The authors believe that based on our current experience with using sails in space, a sail of 100 X 100 square meters is about as large as we are able to work with, and it might require various methods of stiffening its structural booms. The beauty of the new SunVane concept is that it uses multiple sails, making it easier to package and more controllable in flight. This is the ‘Lightcraft’ design out of Xplore Inc., which may well represent the next step in sail evolution. If it functions as planned, this design could open up the outer system to microsat missions of all kinds.
Image: This is Figure 5 from the paper. Caption: Xplore’s Lightcraft TM advanced solar sail for rapid exploration of the solar system. Credit; Friedman et al./JPL.
Pushing out interstellar boundaries also means pushing materials science hard. After all, we’re contemplating getting as close to the Sun as we can with a sail that may be as thin as one micron, with a density less than 1 gram per square meter. The kind of sail contemplated here would weigh about 10 kg, with 40 kg for the spacecraft. The payload has to be protected from a solar flux that at 0.1 AU is 100 times what we receive on Earth, so the calculations play the need for shielding against the need to keep the craft as light as possible. An aluminized polymer film like Kapton doesn’t survive this close to the Sun, which is why so much interest has surfaced in materials that can withstand higher temperatures; we’ve looked at some of this work in these pages.
But the longer-term look is this:
Advanced technology may permit sails the size of a football field and spacecraft the size of modern CubeSats, and coming close to the Sun with exotic materials of high reflectivity and able to withstand the very high temperatures. That might permit going twice as fast, 40 AU/year or higher. If we can do that it will be worth waiting for. With long mission times, and with likely exoplanets in several different star systems being important targets of exploration we may want to develop a low cost, highly repeatable and flexible spacecraft architecture – one that might permit a series of small missions rather than one with a traditional large, complex spacecraft. The velocity might also be boosted with a hybrid approach, adding an electric propulsion to the solar sail.
It’s worth mentioning that we need electric propulsion on this craft anyway as the craft maneuvers to collect data near the gravitational focus. Testing all this out charts a developmental path through a technology demonstrator whose funding through a public-private partnership is currently being explored. This craft would make the solar flyby and develop the velocity needed for a fast exit out of the Solar System. A series of precursor missions could then test the needed technologies for deployment at the SGL We can envision Kuiper Belt exploration and, as the authors do, even a mission to a future interstellar object entering our system using these propulsion methods.
I recommend this new paper to anyone interested in keeping up with the JPL design for reaching the solar gravitational focus. As we’ve recently discussed, a vision emerges in which we combine solar sails with microsats that weigh in the range of 50 kilograms, with extensive networking capabilities and perhaps the ability to perform self-assembly during cruise. For the cost of a single space telescope, we could be sending multiple spacecraft to observe a number of different exoplanets before the end of this century, each with the capability to resolve features on the surface of these worlds. Resolution would be to the level of a few kilometers. We’re talking about continents, oceans, vegetation and, who knows, perhaps even signs of technology. And that would be on not one but thousands of potential targets within a ten light year radius from Earth.
The paper is Friedman et al., “A mission to nature’s telescope for high-resolution imaging of an exoplanet,” Experimental Astronomy 57 (2024), 1 (abstract).
NIAC’s award of a Phase I grant to study a ‘swarm’ mission to Proxima Centauri naturally ties to Breakthrough Starshot, which continues its interstellar labors, though largely out of the public eye. The award adds a further research channel for Breakthrough’s ideas, and a helpful one at that, for the NASA Innovative Advanced Concepts program supports early stage technologies through three levels of funding, so there is a path for taking these swarm ideas further. An initial paper on swarm strategies was indeed funded by Breakthrough and developed through Space Initiatives and the UK-based Initiative for Interstellar Studies.
Centauri Dreams readers are by now familiar with my enthusiasm for swarm concepts, and not just for interstellar purposes. Indeed, as we develop the technologies to send tiny spacecraft in their thousands to remote targets, we’ll be testing the idea out first through computer simulation but then through missions within our own Solar System. Marshall Eubanks, the chief scientist for Space Initiatives, a Florida-based startup focused on 50-gram femtosatellites and their uses near Earth, talks about swarm spacecraft covering cislunar space or analyzing a planetary magnetosphere. Eubanks is lead author of the aforementioned paper.
But the go-for-broke target is another star, and that star is naturally Proxima Centauri, given Breakthrough’s clear interest in the habitable zone planet orbiting there. The NIAC announcement sums up the effort, but I turn to the paper for discussion of communications with such swarm spacecraft. As Starshot has continued to analyze missions at this scale, it explores probes with launch mass on the scale of grams and onboard power restricted to milliwatts. The communications challenge is daunting indeed given the distances and power available.
If we want to reach a nearby star in this century, so the thinking goes, we should build the kind of powerful laser beamer (on the order of 100 GW) that can push our lightsails and their tiny payloads to speeds that are an appreciable fraction of the speed of light. Moving at 20 percent of c, we reach Proxima space within 20 years, to begin the long process of returning data acquired from the flybys of our probes. Eubanks and colleagues estimate we’ll need thousands of these, because we need to create an optical signal strong enough to reach Earth, one coordinated through a network that is functionally autonomous. We’re way too far from home to control it from Earth.
Image: Artist’s impression of swarm passing by Proxima Centauri and Proxima b. The swarm’s extent is ∼10 larger than the planet’s, yet the ∼5000-km spacing is such that one or more probes will come close to or even impact the planet (flare on limb). It should be possible to do transmission spectroscopy with such swarms. Green 432/539-nm beams are coms to Earth; red 12,000-nm laser beacons are for intra-swarm probe-to-probe coms. Conceptual artwork courtesy of Michel Lamontagne.
The engineering study that has grown out of this vision describes the spacecraft as being ‘operationally coherent,’ meaning they will be synchronized in ways that allow data return. The techniques here are fascinating. Adjusting the initial velocity of each probe (this would be done through the launch laser itself) allows the string of probes to cohere. The laser also allows clock synchronization, so that we wind up with what had been a string of probes traveling together through the twenty year journey. In effect, the tail of the string catches up with the head. What emerges is a network.
As the NIAC announcement puts it:
Exploiting drag imparted by the interstellar medium (“velocity on target”) over the 20-year cruise keeps the group together once assembled. An initial string 100s to 1000s of AU long dynamically coalesces itself over time into a lens-shaped mesh network 100,000 km across, sufficient to account for ephemeris errors at Proxima, ensuring at least some probes pass close to the target.
The ingenuity of the communications method emerges from the capability of tiny spacecraft to travel with their clocks in synchrony, with the ability to map the spatial positions of each member of the swarm. This is ‘operational coherence,’ which means that while each probe returns the same data, the transmission time is related to its position within the swarm. The result; The data pulses arrive at the same time on Earth, so that while the signal from any one probe would be undetectable, the combined laser pulse from all of them can become bright enough to detect over 4.2 light years.
The paper cites a ‘time-on-target’ technique to allow the formation of effective swarm topologies, while a finer-grained ‘velocity-on-target’ method is what copes with the drag imparted by the interstellar medium. This one stopped me short, but digging into it I learned that the authors talk about adjusting the attitude of individual probes as needed to keep the swarm in coherent formation. The question of spacecraft attitude also applies to the radiation and erosion concerns of traveling at these speeds, and I think I’m right in remembering that Breakthrough Starshot has always contemplated the individual probes traveling edge-on during cruise with no roll axis rotation.
Image; This is Figure 2a from the paper. Caption: A flotilla (sub-fleet) of probes (far left), individually fired at the maximum tempo of once per 9 minutes, departs Earth (blue) daily. The planets pass in rapid succession. Launched with the primary ToT technique, the individual probes draw closer to one another inside the flotilla, while the flotilla itself catches up with previously-launched flotillas exiting the outer Solar system (middle) ∼100 AU. For the animation go to https://www.youtube.com/watch?v=jMgfVMNxNQs (Hibberd 2022).
Figure 2b takes the probe ensemble into the Oort Cloud.
Image: Figure 2b caption: Time sped up by a scale factor of 30. The last flotilla launched draws closer to the earlier flotillas; the full fleet begins to coalesce (middle), now under both the primary ToT and secondary VoT techniques, beyond the Kuiper-Edgeworth Belt and entry into the Oort Cloud ∼1000–10,000 AU.
When we talk about using collisions with the interstellar medium to create velocities transverse to the direction of travel, we’re describing a method that again demands autonomy, or what the paper describes as a ‘hive mind,’ a familiar science fiction trope. The hive mind will be busy indeed, for its operations must include not just cruise control over the swarm’s shape but interactions during the data return phase. From the paper;
With virtually no mass allowance for shielding, attitude adjustment is the only practical means to minimize the extreme radiation damage induced by traveling through the ISM at 0.2c. Moreover, lacking the mass budget for mechanical gimbals or other means to point instruments, then controlling attitude and rate changes of the entire craft in pitch, yaw, roll, is the only practical way [to] aim onboard sensors for intra-swarm communications, interstellar comms with Earth and imagery acquisition / distributed processing at encounter.
I gather that other techniques for interacting with the interstellar medium will come into play in the NIAC work, for the paper speaks of using onboard ‘magnetorquers,’ an attitude adjustment mechanism currently in use in low-mass Cubesats in low Earth orbit. It’s an awkward coinage, but a magnetorquer refers to magnetic torquers or torque rods that have been developed for attitude control in a given inertial frame. The method works through interaction between a magnetic field and the ambient magnetic field (in current cases, of the Earth). Are magnetic fields in the interstellar medium sufficient to support this method? The paper explores the need for assessment.
A solid state probe has no moving parts, but it’s also clear that further simulations will explore the use of what the paper calls MEMS (micro-electromechanical systems) trim tabs that could be spaced symmetrically to provide dynamic control by producing an asymmetric torque. This sounds like a kludge, though one that needs exploring given the complexities of adjusting attitudes throughout a swarm. We’ll see where the idea goes as it matures in the NIAC phase. All this will be critical if we are to connect interswarm to create the signaling array that will bring the Proxima data home.
Interestingly, the kind of probes the paper describes may vary in some features:
We note for the record that although all probes are assumed to be identical, implicitly in the community and explicitly in the baseline study, there is in fact no necessity for them to be “cookie cutter” copies, since the launch laser must be exquisitely tunable in the first place, capable of providing a boost tailored to every individual probe. At minimum, probes can be configured and assigned for different operations while remaining dynamically identical, or they can be made truly heterogeneous wherein each probe could be rather different in form and function, if not overall mass and size.
There is so much going on in this paper, particularly the issue of the orbital position of Proxima b, which you would think would be known well enough by now (but guess again). The question of carrying enough stored energy for the two decade mission is a telling one. But the overwhelming need is to get information back to Earth. How data would be received from these distances has always bedeviled the Starshot idea, and having followed the conversation on this for some time now, I find the methods proposed here seriously intriguing. We’ll dig into these issues in the next post.
The paper is Eubanks et al., “Swarming Proxima Centauri: Optical Communication Over Interstellar Distances,” submitted to the Breakthrough Starshot Challenge Communications Group Final Report and available online.
Let’s break for a moment with interstellar issues to finish up a story I first covered at the beginning of the year. In 2022, members of the Interstellar Research Group led by Doug Loss began exploring the biological side of establishing a human presence on Mars. By ‘biological,’ what the team was looking at was how to create soil as opposed to regolith, soil with the microbial components needed to produce crops for human consumption on Mars. Alex Tolley wrote the idea up in MaRMIE: The Martian Regolith Microbiome Inoculation Experiment. Today’s post is the finalized document that has grown out of this effort, an attempt to foster further research by offering a framework for experiment. While the IRG lacks the means of executing these experiments itself, it offers this paper as a contribution to planetary studies to connect with those who can.
by Alex Tolley and Doug Loss*
* Contact: Doug Loss at email@example.com
The proposed designs for the settlement of Mars include various approaches for local food production. Food will most likely be based on traditional terrestrial crops to ensure that a variety of cuisines can be cooked for the well-being of the settlers. To farm on Mars, as well as provide an environment for plants and trees, will require establishing soils using the Martian regolith. The presence of (per)chlorates at levels toxic to plants and humans requires remediation of the regolith to remove the (per)chlorates. Prior work indicates that there is a knowledge gap in how to remediate the regolith to make it ready to support various crops for Martian agriculture. We propose a framework of experiments to help bridge the gap between the state of the regolith on the surface and the initial stages of soil creation.
With the renewed interest in settling Mars, there has been considerable attention on how to feed a base crew and its subsequent expansion into a larger settlement population. Unlike a human presence in low Earth orbit (LEO) and on the Moon where travel times are sufficiently short that food can be provided in regular shipments from Earth, the long 6 to 9-month, low-energy journeys to Mars that have 2-year gaps between flights, suggest that using local Martian resources to grow food would be a better option, both from economic and safety perspectives.
It should be noted that the flight times with current rocket transport technology are similar to that of the sailing ships traveling from England to the Botany Bay colony in Australia in the late 18th century. The resupply ship arrived 2 years later, with the colony starving from inadequate food supplies and an inability to successfully farm. Local food production on Mars would ensure that adequate, high-nutrition foods are available and avoid any supply problems from Earth.
The lower ambient light levels on Mars are sufficient for photosynthesis for a large range of plants from unicellular algae to many terrestrial crops . Additional light if needed can be supplied with mirrors or artificial lighting. The question then becomes what sort of plants should be cultivated? The simplest plants, such as cyanobacteria, have been proposed as they have short lifecycles and rapid growth, requiring small production areas and a few basic nutrients. However, anecdotal evidence from supplying astronauts to the ISS indicates that food quality is a very important factor in astronaut well-being [46,47,48]. Experiments with celebrity chef-developed meals have proven the popularity of meals that are similar to those on Earth and that are tasty, not just nutritious. For crew and settlers staying for long periods on Mars with minimum 2-year rotations, foods that can be prepared with different cuisines to be cooked by a base chef or personally would seem to be preferable. Cyanobacteria and algal species may grow quickly and be technically nutritious. However, algae is not a completely nutritious diet and only Spirulina  has been shown to be useful as a meal supplement, used for less than 1% of the diet, and therefore should be considered as feedstuffs for animals and as soil amendments.
Growing conventional food hydroponically  is often mooted as the means to grow conventional crops. It has the advantage of having a pedigree of experience in terrestrial farms as well as experimental success in space. Hydroponic food production can be carefully controlled which makes it attractive to those of technical expertise. However, hydroponics requires substantial inputs of nitrogen and phosphorus which are usually applied directly from external sources, and not all plants can be successfully grown hydroponically. In addition, an expansion for a growing settlement will require either transporting equipment from Earth or finding ways to manufacture at least the simple components locally on Mars. A more attractive approach has been to try growing conventional crops in the Martian regolith. Experiments using regolith simulants  have shown that given added nutrients and light, a number of common terrestrial leafy crops can be grown.
The advantage of using the Martian regolith as a medium to grow conventional crops is that it provides the needed anchorage and potentially water retention medium used by terrestrial plants. Martian agriculture would work like terrestrial agriculture which is done in a greenhouse. On Mars, the atmosphere and temperature would be controlled to maximize crop growth and it is feasible that some animal species might be transported to produce the high-protein foods. For example, fish eggs could be transported and herbivorous fish species such as Tilapia could feed on the algae and convert it for human consumption. However, it should be noted that soils are not simple, but include ecosystems with a large number of species including bacteria, fungi, and animals from annelid worms to insects..
Despite the research done to date, there are considerable gaps in our knowledge concerning how Martian agriculture should proceed. The Martian environment is very cold, and dry, with a thin atmosphere around 0.1% of Earth’s, composed mainly of carbon dioxide with a little nitrogen. While aqueous algal growth experiments have been done in conditions that approximate some of the Martian conditions, it is not known which conditions must be tightly
controlled for good growth of the algae. For complex plants that are to be grown in either regolith or hydroponically, what partial pressure of CO2 in the atmosphere and at what pressure is needed to ensure healthy growth? Crops grow in different soils on Earth, from near desert sandy soils for millet to rich dark loams and different acidities for different crops. We take for granted the quality of terrestrial soils, but on Mars, the regolith is considered sterile, with no organic carbon content to retain water and provide an environment for soil organisms.
Given that these conditions can be evaluated on Earth, the big gap in our knowledge is the issue of remediation of the toxic levels of (per)chlorates in the Martian regolith. All of the various experiments on growth conditions assume that none of these toxic compounds are present. Powdered terrestrial rocks and more carefully constructed Mars Regolith Simulants are free of (per)chlorates and therefore experiments on plant growth assume the (per)chlorates are removed. With (per)chlorate levels that are far higher than any found naturally on Earth, they are at levels found around sites that manufacture munitions where the compound is used as an oxidant. The US EPA has guidelines for the remediation of soils contaminated by (per)chlorates .
Soils can vary, with plants varying in requirements for water, nutrients, soil carbon, soil organisms, pH, climate, and weather conditions. Nutrients and organic carbon will need to be added, as well as soil organism inoculants to improve the regolith to become a soil capable of good crop production.
To get an agricultural food system working, which factors are critical? How best to detoxify the regolith? How best to amend its properties? Which crops are best suited and at which stages?
A low-mass approach is to employ bacteria that can metabolize (per)chlorates and grow locally. (Per)chlorate metabolizing organisms are proteobacteria of which there are more than 40 species known. Dechloromonas and Azospira genera appear to be ubiquitous on Earth. They have different pH tolerances and some can function in acidic conditions as low as pH 5  The Martian regolith has up to 1% of (per)chlorate  which is far higher than any uncontaminated place on Earth. (Per)chlorate reduction only occurs in anaerobic conditions . This suggests that regolith remediation may need to be kept isolated from the crop-growing areas. Experiments with Moorella sp show that these bacteria can grow on a variety of reduced carbon sources, optimally at neutral pH and warm temperatures (40-70C) . None of the experiments have tested the (per)chlorate-reducing metabolic rates and growth of the various potential bacterial inoculants under conditions between Mars and human habitation, such as lower atmospheric pressure, gas composition, and water requirements. As these bacteria need a carbon source, how would that source be provided by chemical means or by biological carbon fixation?
There is considerable interest in using cyanobacteria as carbon-fixing microorganisms. These hold promise to weather the regolith, release nitrogen and phosphorus for growth, create organic carbon to improve water retention, and allow a richer variety of solid organisms that may be needed for crop growth. These cyanobacteria have been tested in a variety of conditions to determine how they will fare under conditions closer to that on the surface of Mars. Resting states of cyanobacteria suitable for transport from Earth indicate that UV exposure is not tolerated, although survival in a vacuum is good . Cyanobacteria do require a lot of water to suspend the rock dust and particles, In CO2-dominant atmospheres, full terrestrial pressure reduces growth, partly because of the lowered pH of the aqueous media, while 100 mbar appeared more favorable. Temperatures need to be maintained between 15 and 30C. Most important is the finding that cyanobacteria cannot survive in (per)chlorate-contaminated conditions, requiring its removal before growth . Extensive testing of bacteria has shown that while a few can survive down to the 7 mbar of the Martian atmosphere, most require at least 25 mbar. Nitrogen-fixing bacteria can fix atmospheric nitrogen to as low as 1 mbar, but the 2.8% of nitrogen in the Martian atmosphere would require increasing the total local atmospheric pressure 50x.
From this prior work, it is clear that there is a difficulty in remediating the Martian regolith from its toxic state to a soil suited for crop growth. (Per)chlorate-reducing bacteria require reduced carbon sources with nitrogen and phosphorus for growth to detoxify the regolith. Ideally, this could be supplied by cyanobacteria that fix the CO2 in the atmosphere and can release the nitrogen and phosphorus from the regolith. The cyanobacteria can also provide the organic carbon in the soil to support crop growth. However, these cyanobacteria cannot tolerate the toxic (per)chlorates. Lastly, both the (per)chlorate-reducing bacteria and the cyanobacteria need to grow in aqueous conditions with the regolith particles separated to allow rapid microorganism growth. The regolith would then need to be drained and allowed to dry out before being suited to most crop growth, although rice might be able to grow in a “paddy field” of regolith that has settled. This suggests that there may need to be separate areas for removing the (per)chlorates, supplying needed nutrients for the (per)chlorate-reducing bacteria, by cyanobacteria growing in pre-treated regolith.
The following outline experiments are suggested to fill the gaps in treating the Martian regolith to make it suited for growing crops for the Martian settlement.
The exposed Martian regolith is both too cold and dry, as well as relatively airless, for bacteria to detoxify the (per)chlorates. Ideally, the detoxification would take place in optimal growth conditions for the bacteria. Given that maintaining atmospheric composition and pressure, as well as water and humidity conditions, incurs a mass penalty, it is important to determine what are the factors that can be reduced towards Mars’ conditions to reduce this cost. This will help decide whether the detoxification process must be carried out in a greenhouse suited to growing conventional crops, or whether simpler management of the regolith is sufficient. Other questions are also evident, such as the level of detoxification necessary before crops can be successfully grown in the treated regolith.
This suggests several experiments to test for these factors:
1. Composition of Bacterial inoculant
There are many known terrestrial (per)chlorate-metabolizing bacteria, e.g. Dechloromonas that can metabolize oxygenated chlorines. All are anaerobes and therefore may function with the existing composition of the Martian atmosphere. Questions to be considered are:
a. Should the inoculant be a single species or multiple?
b. Do other species need to be included to create a viable ecosystem, or are single-species populations both sufficient and effective?
2. Atmospheric pressure
Mars’s atmosphere is about 0.7% of that of Earth. While too low to support crop plants, how much pressure is needed for bacterial growth to be maintained? Unlike plants, the bacteria are aquatic, and therefore the needed atmospheric pressure need only be sufficient to prevent water from boiling off. In a sealed reactor, water vapor will provide the needed pressure to maintain the equilibrium. As the bacteria are anaerobes, the regolith would seem likely to be processed in separate areas from the crop plants, with the detoxified regolith then added to the agricultural area in the greenhouse to increase the cultivation area.
3. Atmospheric composition
Mars’ atmosphere is primarily CO2 with a little N2. This is not suitable for crop plants, but how much of a factor is this for the bacteria? Combined with atmospheric pressure, what composition is needed for the bacteria? For example, does the nitrogen partial pressure need to be increased to supply the needed nitrogen for bacterial growth, perhaps in combination with nitrogen-fixing bacteria in the inoculant, or just added as ammonia or nitrate? [c.f. Item 1 concerning species in the inoculant.]
Lastly, bacteria need wet conditions to grow and multiply. How wet does the regolith need to be for the bacteria? Do the bacteria survive and grow in an aqueous slurry, or would high humidity conditions be sufficient, saving water resources needed elsewhere?
To test these, experiments will need to be set up in conditions to test these various requirements, most probably in containers to maintain the conditions. It is assumed that surface UV and ionizing radiation do not need to be tested as simple shielding will be sufficient to mitigate these factors.
These experiments are primarily devoted to extending the existing work done on (per)chlorate removal by bacteria [2,10,13,22], extending prior work. If the regolith detoxification and preparation for traditional crops is to be the goal, the regolith will need additional preparation for crops, including nitrogen, phosphorus, and carbon supplements. Inoculants may be required to allow nitrogen-fixing bacteria to grow in association with the root nodules of crops like green beans. Prior experiments [30,34,40] with cyanobacteria have demonstrated the extraction of nitrogen from the regolith, suggesting this approach to fertilize the crop plants after the regolith has been cleared of the toxic chlorate and (per)chlorate.
A stretch goal might include gene splicing experiments to extend the capabilities of some microbial species. Can the (per)chlorate-reducing genes be added to cyanobacteria removing the need for the bacterial species? Conversely, can genes to extract the nitrogen and phosphorus from the regolith be inserted into the bacteria? Can the (per)chlorate genes be edited so that the oxygen is liberated safely in the organism, allowing the (per)chlorate to become an oxygen source for the Martian settlement? Suggestions as to possible ideas have been mooted [40, 49].
To start processing Martian regolith for food production on Mars, there is a substantial gap in our knowledge on getting this process underway in the volumes needed compared to the small-scale lab experiments. Firstly, the regolith must be detoxified to remove the (per)chlorates. While the lab experiments demonstrate that various species of bacteria can metabolize the (per)chlorates, there are two limitations. Firstly, the regolith needs to be in powdered form to expose the surfaces to the bacteria and be turned into an aqueous environment for the bacteria to survive. How wet the slurry needs to be is unknown and therefore the water requirements are also unknown. Secondly, the sterile regolith provides no useful food supplies for the bacteria to grow. How to supply the nutrients and from what source needs to be determined. Terrestrial starter kits may be inadequate for bulk regolith processing.
Cyanobacteria have been demonstrated in the lab to be able to fix the atmospheric CO2 and grow while extracting the needed nitrogen, phosphorus, and trace elements from the regolith, but only after the (per)chlorates have been removed.
Terrestrial crops are yet another step away as they need detoxified regolith, fertilizers, and organic carbon in the “soil” to grow successfully, suggesting that both the (per)chlorate-metabolizing bacteria and the cyanobacteria must preprocess the regolith.
While the bacterial cultures grow in aqueous conditions, terrestrial crops do not and are therefore subject to even more critical issues of the surrounding atmosphere: pressures, and composition.
Currently, it appears as if the regolith can be prepared by iteratively starting with (per)chlorate-metabolizing bacteria, followed by cyanobacteria to grow and produce the needed food for a larger amount of regolith to be detoxified so that large volumes of regolith can be prepared for conventional crops to be grown. Once the regolith has been prepared it is turned over to the agronomists to determine how best to provide the conditions and associated organisms to cultivate crops to feed the base crew or settlers. While hydroponics is favored for supplying small populations with food, more conventional agriculture using local resources including the regolith seems more likely to be the preferred approach once large settlements start to appear.
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Larry Niven played around with an interesting form of suspended animation in his 1966 Ballantine title World of Ptavvs. While the usual science fictional imagining is of a crew in some sort of cryogenic deep freeze, Niven went all out and envisioned a means of suspending time itself. It’s an ingenious concept based on an earlier short story in Worlds of Tomorrow, one that so aggressively pushes the physics that the more subtle delights of characterization and perspective come almost as afterthoughts. Niven fans like myself will recognize it as taking part in his ‘Known Space’ universe.
In the absence of time manipulation, let’s plumb more modest depths, though these can be tantalizing in their implications. In the last post, Don Wilkins described new work out of Washington University on inducing states of torpor – life processes slowed along with temperature – in laboratory experiments involving rodents. The spectrum from torpor to suspended animation has intervals that may suit our purpose. At the Interstellar Research Group’s Montreal symposium, John Bradford examined technologies with interplanetary as well as interstellar implications, though as he pointed out, they differ greatly from our cryogenic imaginings.
No deep freeze, in other words, and an experience for the crew that is far more like sleep – a very deep sleep – than the profound stoppage of metabolism that might keep a crew on ice for centuries. Bradford, who is CEO and principal engineer at Atlanta-based SpaceWorks, has explored the subject in two NIAC studies. His review for the Montreal audience took note of where we are today, when placing humans into a reduced metabolic state is relatively well understood, and therapeutic hypothermia is a medical treatment that can involve inhaled gases, cold saline injection and ice packs.
What particularly intrigued me in Bradford’s presentation (available here) is that there are numerous documented stories of human survival in extreme conditions, including one Anna Bagenholm, who survived submersion in an ice-covered lake for three hours. There are even some indications that some form of hibernation may have been present in early hominins, as evidenced by remains found in Spain involving changes in bone structure and density that imply cycles of use and disuse. Bradford’s NIAC work involved a mission design for Mars and Ceres in which the crew cycled through torpor states alternating with active periods. This notion of cyclic hibernation seems promising if we can discover ways of using it that maintain crew health both physical and mental.
Image: Suspended animation the ‘old-school’ way. This is an image from the 2016 film Passengers, involving a crew that is put into stasis inside hibernation pods.
Can we implement torpor in deep space? It’s a fascinating issue, one that may offer a path to manned interstellar travel if we can reduce trip times down to perhaps 50 years, a number Bradford chooses because, in conjunction with increases in human longevity, it offers missions where the balance of a human life is lived before and after the mission. The challenges are huge and already familiar from our space activities aboard the ISS. Muscle atrophy can be profound, as can bone loss and demineralization. An interstellar crew would confront exposure to galactic cosmic rays, while shielding exacts a mass penalty. There is also the matter of consumables.
Let’s face it, the human body is simply not designed for being off-planet, nor do we have even a fraction of the information we need to really assess such things as interplanetary missions with human crews from the physiological point of view. Until we have a dedicated research lab in orbit, we’re reduced to theorizing based on data from the ISS and manned missions that have never gone beyond the Moon.
If we can induce torpor states, we could drastically reduce the bulk of crew consumables needed for missions to another star, but let’s be clear about the kind of mission we’re talking about. A breakthrough in some sort of Alcubierre-like drive would eliminate the need for hibernation. On the other hand, absent the ability to take humans to 10 to 20 percent of lightspeed, we need to look to generation ships as the only viable way to move adult humans to an exoplanet, perhaps considering the possibility of using AI and human embryos raised at destination.
That’s why Bradford is really looking at ships moving fast enough that an interstellar crossing could be made within decades, and here we can look at such possibilities as induced ‘profound hypothermia’ that drops the body temperature below 20 ℃, a procedure used today in extreme cases and generally as a last resort. Perhaps more useful and certainly less drastic is gene-editing to enhance what may be in-built hibernation capabilities. Combine increased human longevity with some form of induced torpor and you come up with mission scenarios involving cycles of torpor and full wakefulness. Indeed, a 4-week cycle of torpor between periods of wakeful activity can reduce the perception of a 50-year transit to another star to a ten year period.
Plenty of work is going on in terms of longevity extension, ranging from research groups like the Methuselah Foundation and Altos Labs to drug trials involving replacement of molecules that tend to diminish with age and supplements like resveratrol and taurine that have promise in increasing lifespan. I’m not familiar with the details of this research, but Bradford said that there are voices in the scientific community arguing that 150 years is a reasonable goal for the average human, with the current record-holder (Jeanne Calment) having managed a startling 122.
Our scenario, then, is one in which we use induced torpor in a cyclical manner to reduce the time perception of an interstellar crossing. Therapeutic hypothermia (TH) currently involves periods of no more than three days, but as the chart above shows (drawn from Bradford’s slides), we can think in terms of two weeks in stasis with four or five days active as an achievable goal based on current research. Bradford’s NIAC work involved missions to Mars and Ceres using this cycle. Going beyond it raises all kinds of interesting questions about how the body responds to lengthy torpor states and, just as significantly, what happens to human cognition.
The question of infrastructure haunts the quest to achieve interstellar flight. I’ve always believed that we will develop deep space capabilities not only for research and commerce but also as a means of defense, ensuring that we will be able to change the trajectories of potentially dangerous objects. But consider the recent Breakthrough Starshot discussion. There I noted that we might balance the images we could receive through Starshot’s sails with those we could produce through telescopes at the Sun’s gravitational focus.
Without the infrastructure issue, it would be a simple thing to go with JPL’s Solar Gravitational Lens concept since the target, somewhere around 600 AU, is so much closer, and could produce perhaps even better imagery. But let’s consider Starshot’s huge photon engine in the Atacama desert not as a one-shot enabler for Proxima Centauri, but as a practical tool that, once built, will allow all kinds of fast missions within the Solar System. The financial outlay supports Oort Cloud exploration, fast access to the heliopause and nearby interstellar space, and planetary missions of all kinds. Add atmospheric braking and we can consider it as a supply chain as well.
Robert Freeland, who has labored mightily in the Project Icarus Firefly design, told the Interstellar Research Group’s recent meeting in Montreal about work he is doing within the context of the British Interplanetary Society’s BIS SPACE project, whose goal is to consider the economic drivers, resources, transportation issues and future population growth that would drive an interplanetary economy. That Solar System-wide infrastructure in turn feeds interstellar capabilities, as it generates new technologies that funnel into propulsion concepts. A case in point: In-space fusion.
To make our engines go, we need fuel, an obvious point and a telling one, since the kind of fusion Freeland has been studying for the Firefly design is limited by our current inability to extract enough Helium-3 to use aboard an interstellar craft. Firefly would use Z-pinch fusion – this is a way of confining plasma and compressing it. An electrical current fed into the plasma generates the magnetic fields that ‘pinch,’ or compress the plasma, creating the high temperatures and pressures that can produce fusion.
I was glad to see Freeland’s slides on the fusion fuel possibilities, a helpful refresher. The easiest fusion reactions, if anything about fusion can be called ‘easy,’ is that of deuterium with tritium, with the caveat that this reaction produces most of its energies in neutrons that cannot produce thrust. Whereas the reaction of deuterium with helium-3 releases primarily charged particles that can be shaped into thrust, which is why it was D/He3 fusion that was chosen by the Daedalus team for their gigantic starship design back in the 1970s. Along with that choice came the need to find the helium-3 to fuel the craft. The Daedalus team, ever imaginative, contemplated mining the atmospheres of the gas giants, where He3 can be found in abundance.
The lack of He-3 caused Icarus to choose a pure deuterium fuel (DD). Freeland ran through the problems with DD, noting the abundance of produced neutrons and the gamma rays that result from shielding these fast neutrons. The reaction also produces so-called bremsstrahlung radiation, which emerges in the form of x-rays. Thus the Firefly design stripped down what would otherwise be a significant portion of its mass in shielding by going to what Freeland calls ‘distance shielding,’ meaning minimal structure that allows the radiation to escape into space.
A starship using deuterium and helium-3 minimizes the neutron radiation, so the question becomes, when do we close the gap in our space capabilities to the point that we can extract helium-3 in the quantities needed from planets like Uranus? I see BIS SPACE as seeking to probe what the Daedalus team described as a Solar System-wide economy, and to put some numbers to the question of when this capability would evolve. The question is given point in terms of interstellar probes because while Firefly had been conceived as a starship that could launch before 2100, it seemed likely that helium-3 simply wouldn’t be available in sufficient quantities. So when would it be?
To create an infrastructure off-planet, we’ll need human migration outward, beginning most likely with orbital habitats not far from Earth – think of the orbital environments conceived by Gerard O’Neill, with their access to the abundant resources of the inner system. Freeland imagines future population growth moving further out over the course of the next 20,000 years until the Solar System is fully exploited. In four waves of expansion, he sees the era of chemical and ion rocketry, and perhaps beamed propulsion, to about 2050, with the second generation largely using fission-powered craft, in a phase ending in about 2200. 2200 to 2500 taps fusion energies (DD), while the entire Solar System is populated after 2500, with mining of the gas giants possible.
Let’s pause for a moment on the human population’s growth, because the trends noted in the image below, although widely circulated, seem not to be widely known. We’re looking here at the growth rate of our species and its acceleration followed by its long decline. As Freeland pointed out, the UN expects world population to peak at between 10 and 12 billion perhaps before the end of this century. After that, increase in the population is by no means assured. So much for the scenario that we have to go off-planet because we will simply overwhelm resources here with our numbers.
Image: In both this and the image below I am drawing from Freeland’s slides.
You would think this Malthusian notion would have long ago been discredited, but it is surprisingly robust. Even so, orbital habitats near Earth can potentially re-create basic Earth-like conditions while exploiting material resources in great abundance and solar power, with easy access to space for moving the wave of innovation further out. BIS SPACE looks with renewed interest at these O’Neill habitats in its first wave of papers.
The larger scenario plays out as follows: In the second half of our century, we move development largely to high Earth orbit, with materials drawn mostly from the Moon, using transport of goods by nuclear-powered cargo ships. The third generation creates orbital habitats at all the inner planets (and Ceres) and perhaps near-Earth asteroids using DD fusion propulsion, while the fourth generation takes in the outer planets and their moons. At this point we can set up the kind of aerostat mining rigs in the upper gas giant atmospheres that would enable the collection of helium-3. Here again we have to make comparisons with other technologies. Where will beamed spacecraft capabilities be by the time we are actively mining He-3 in the outer Solar System?
I’ve simplified the details on expansion greatly, and send you to Freeland’s slides for the details. But I want to circle back to Firefly. Using DD fusion, Firefly’s radiator and coolant requirements are extreme (480 tonnes of beryllium coolant!) But move to the deuterium/helium-3 reaction and you drop radiation output by 75 percent while increasing exhaust velocity. Beryllium can be replaced with less expensive aluminum and the physical size of the vessel is greatly reduced. This version of Firefly gets to Alpha Centauri in the same time using 1/5th the fuel and 1/12th the coolant.
In other words, the sooner we can build the infrastructure allowing us to mine the critical helium-3, the sooner we can drop the costs of interstellar missions and expand their capabilities using fusion engines. If such a scenario plays out, it will be fascinating to see how the population growth curves for the entire Solar System track given access to abundant new resources and the technologies to exploit them. If we can imagine a Solar System-wide human population in the range of 100 billion, we can also imagine the growth of new propulsion concepts to power colonization outside the system.
If we’re going to get to the stars, the path along the way has to go through an effort like Breakthrough Starshot. This is not to say that Breakthrough will achieve an interstellar mission, though its aspirational goal of reaching a nearby star like Proxima Centauri with a flight time of 20 years is one that takes the breath away. But aspirations are just that, and the point is, we need them no matter how far-fetched they seem to drive our ambition, sharpen our perspective and widen our analysis. Whether we achieve them in their initial formulation cannot be known until we try.
So let’s talk for a minute about what Starshot is and isn’t. It is not an attempt to use existing technologies to begin building a starship today. Yes, metal is being bent, but in laboratory experiments and simulated environments. No, rather than a construction project, Starshot is about clarifying where we are now, and projecting where we can expect to be within a reasonable time frame. In its early stages, it is about identifying the science issues that would enable us to use laser beaming to light up a sail and push it toward another star with prospects of a solid data return. Starshot’s Harry Atwater (Caltech) told the Interstellar Research Group in Montreal that it is about development and definition. Develop the physics, define and grow the design concepts, and nurture a scientific community. These are the necessary and current preliminaries.
Image: The cover image of a Starshot paper illustrating Harry Atwater’s “Materials Challenges for the Starshot Lightsail,” Nature Materials 17 (2018), 861-867.
We’re talking about what could be a decades-long effort here, one that has already achieved a singular advance in interstellar studies. I don’t have the current count on how many papers have been spawned by this effort, but we can contrast the ongoing work of Starshot’s technical teams with where interstellar studies was just 25 years ago, when few scientific conferences dealt with interstellar ideas and exoplanets were still a field in their infancy. In terms of bringing focus to the issue, Starshot is sui generis.
It is also an organic effort. Starshot will assess its development as it goes, and the more feasible its answers, the more it will grow. I think that learning more about sail possibilities will spawn renewed effort in other areas, and I see the recent growth of fusion rocketry concepts as a demonstration that our field is attaining critical mass not only in the research labs and academy but in commercial space ventures as well.
So let’s add to Atwater’s statement that Starshot is also a cultural phenomenon. Although its technical meetings are anything but media fodder, their quiet work keeps the idea of an interstellar crossing in the public mind as a kind of background musical riff. Yes, we’re thinking about this. We’ve got ideas and lab experiments that point to new directions. We’re learning things about lightsails and beaming we didn’t know before. And yes, it’s a big universe, with approximately one planet per star on average, and we’ve got one outstanding example of a habitable zone planet right next door.
So might Starshot’s proponents say to themselves, although I have no idea how many of those participating in the effort back out sometimes to see that broader picture (I suspect quite a few, based on those I know, but I can’t speak for everyone). But because Starshot has not sought the kind of publicity that our media-crazed age demands, I want to send you to Atwater’s video presentation at Montreal to get caught up on where things stand. I doubt we’re ever going to fly the mission Starshot originally conceived because of cost and sheer scale, but I’m only an outsider looking in. I do think that when the first interstellar mission flies, it will draw heavily on Starshot’s work. And this will be true no matter what final choices emerge as to propulsion.
This is a highly technical talk compressed into an all too short 40 minutes, but let’s just go deep on one aspect of it, the discussion of the lightsail that would be accelerated to 20 percent of lightspeed for the interstellar crossing. Atwater’s charts are worth seeing, especially the background on what the sail team’s meetings have produced in terms of their work on sail materials and, especially, sail shape and stability. The sail is a structure approximately 4 meters in diameter, with a communications aperture 1 meter in size, as seen in the center of the image (2 on the figure). Surrounding it on the circular surface are image sensors (6) and thin-film radioisotope power cells (5).
Maneuvering LEDs (4) provide attitude control, and thin-film magnetometers (7) are in the central disk, with power and data buses (8) also illustrated. A key component: A laser reflector layer positioned between the instruments that are located on the lightsail and the lightsail itself, which is formed as a silicon nitride metagrating. As Atwater covers early in his presentation, the metagrating is crucial for attitude control and beam-riding, keeping the sail from slipping off the beam even though it is flat. The layering is crucial in protecting the sailcraft instrumentation during the acceleration stage, when it is fully illuminated by the laser from the ground.
How to design lensless transmitters and imaging apertures? Atwater said that lensless color camera and steerable phased array communication apertures are being prototyped in the laboratory now using phased arrays with electrooptic materials. Working one-dimensional devices have emerged in this early work for beam steering and electronic focusing of beams. The laser reflector layer offers the requisite high reflectivity at the laser wavelength being considered, using a hybrid design with silicon nitride and molybdenum disulfide to minimize absorption that would heat the sail.
I won’t walk us through all of the Starshot design concepts at this kind of detail, but rather send you to Atwater’s presentation, which shows the beam-riding lightsail structure and its current laboratory iterations. The discussion of power sources is particularly interesting given the thin-film lightweight structures involved, and as shown in the image below, it involves radioisotope thermoelectric generators actually integrated into the sail surface. Thin film batteries and fuel cells were considered by Breakthrough’s power working group but rejected in favor of this RTG design.
So much is going on here in terms of the selection of sail materials and the analysis of its shape, but I’ll also send you to Atwater’s presentation with a recommendation to linger over his discussion of the photon engine, that vast installation needed to produce the beam that would make the interstellar mission happen. The concept in its entirety is breathtaking. The photon engine is currently envisioned as an array of 1,767,146 panels consisting of 706,858,400 individual tiles (Atwater dryly described this as “a large number of tiles”), producing the 200 gW output and covering 3 kilometers on the ground. The communications problem for data return is managed by scalable large-area ground receiver arrays, another area where Breakthrough is examining cost trends that within the decades contemplated for the project will drive component expenses sharply down. The project depends upon these economic outcomes.
Image: What we would see if we had a Starshot-class sailcraft approaching the Earth, from the image at two hours away to within five minutes of its approach. Credit for this and the two earlier images: Harry Atwater/Breakthrough Starshot.
Using a laser-beamed sail technology to reach the nearest stars may be the fastest way to get images like those above. The prospect of studying a planet like Proxima b at this level of detail is enticing, but how far can we count on economic projections to bring costs down to the even remotely foreseeable range? We also have to factor in the possibility of getting still better images from a mission to the solar gravitational lens (much closer) of the kind currently being developed at the Jet Propulsion Laboratory.
Economic feasibility is inescapably part of the Starshot project, and is clearly one of the fundamental issues it was designed to address. I return to my initial point. Identifying the principles involved and defining the best concepts to drive design both now and in the future is the work of a growing scientific community, which the Starshot effort continues to energize. That in itself is no small achievement.
It is, in fact, a key building block in the scientific edifice that will define the best options for achieving the interstellar dream. And while this is not the place to go into the complexities of scientific funding, suffice it to say that putting out the cash to enable these continuing studies is a catalytic gift to a field that has always struggled for traction both financial and philosophical. The Starshot initiative has a foundational role in defining the best technologies for interstellar flight that will lead one day to its realization.