by Paul Gilster | Apr 3, 2024 | Missions |
Eugene Parker, after whom the Parker Solar Probe was named, seems to have been the first to have accurately predicted the stream of particles emitted by the Sun that forms the ‘solar wind.’ Parker made the call in a 1958 paper, when solar sailing was just being noised about for the first time, so it wouldn’t have struck him that the term was a bit incautious. Today, when solar sailing is operational, people often assume the solar wind drives solar sails, when in fact the operating principle for solar sails is the momentum generated by photons, which are themselves massless. But streaming particles are indeed a kind of ‘wind,’ and there are magnetic sail concepts tailored for them too.
As always, we have to be careful about terminology, especially given the significance of the solar wind in defining our Solar System’s environment. Solar transients likewise have to be considered, because in addition to solar flares, we have to factor in coronal mass ejections (CMEs) and the particles accelerated by both of these. All of this as well as the interplanetary magnetic field shapes our star’s interactions with the local interstellar medium, creating the plasma envelope called the heliosphere. Only the Voyager spacecraft have returned data from the LISM, and they’re not very far into it.
We’ve looked at the Interstellar Probe concept in these pages before. The result of countless hours and numerous contributors at the Johns Hopkins Applied Physics Laboratory and elsewhere, the study was created for the Solar and Space Physics Decadal Survey and extensively analyzes everything from the launch vehicle to instrumentation for a mission that would exit the heliosphere and reach as far as hundreds of AUs within 50 years. Given that our knowledge of the realm beyond the heliosphere is almost entirely the result of remote sensing and indirect measurements, having an actual spacecraft on the scene would take us far beyond our modeling.
Image: The SWAP instrument aboard New Horizons has confirmed that the solar wind slows as it travels farther from the Sun. This schematic of the heliosphere shows that the solar wind begins slowing at approximately 4 AU radial distance from the Sun and continues to slow as it moves toward the outer solar system and picks up interstellar material. Current extrapolations reveal the termination shock may currently be closer than found by the Voyager spacecraft. However, increasing solar activity will soon expand the heliosphere and push the termination shock farther out, possibly to the 84-94 AU range encountered by the Voyager spacecraft. Credit: Southwest Research Institute; background artist rendering by NASA and Adler Planetarium.
Because I’m talking about the outer boundaries of the heliosphere today, a quick word about how remote sensing ‘sees’ them is appropriate. We have data from the IBEX satellite (Interstellar Boundary Explorer) covering an entire solar cycle from 2009 through 2019. Although it’s in Earth orbit, IBEX detects energetic neutral atoms (ENAs) from the outer regions of the heliosphere, the zone where solar wind particles begin to collide with those of the even less understood interstellar wind. We are in essence mapping a region by sending a signal – actually using the Sun’s ‘signal,’ the particles of the solar wind – deep into the edge of the system and trying to make sense out of the return echoes. The IMAP (Interstellar Mapping and Acceleration Probe), set for launch in 2025, will further examine this region from its own vantage at the L1 Lagrange point.
We can draw a lot of conclusions from the data from such missions, but not enough. Sarah Spitzer (University of Michigan), lead author of a paper on a just released study that analyzes how best to exit the heliosphere, notes the significance of the JHU/APL work: “Without such a mission, we are like goldfish trying to understand the fishbowl from the inside.”
Indeed. Considered as a kind of shield, the heliosphere acts as a brake on galactic radiation, but predicting its effects, and even more significantly its shape and size over time, is all but impossible. We tend to refer to the heliosphere as a ‘bubble,’ but it’s a poor term given that the shape changes with solar output and interactions with the LISM. A variety of potential shapes for the heliosphere remain in play in the literature. Have a look at the figure below, which represents just one of the numerous possibilities.
Image: One recent study of the heliosphere’s shape posited a croissant-shape or small spherical shape with tail lobes. Credit: Figure adapted from Merav Opher et al., 2020.
As Spitzer and colleagues note:
Small variations in model parameters and properties measured in the nose of the heliosphere, the leading edge in the direction of the Sun’s motion through the LISM, lead to significant differences in the projected shape. The global response of the heliosphere is additionally expected to fluctuate with solar activity and therefore solar cycle…, which is yet another element in the interplay of heliosphere–interstellar interactions and another factor in our uncertainty of the overall shape. The main features of the shape of the heliosphere include the nose; the tail, which can be defined relative to the Sun as the region of the heliosphere found in the direction opposite the Sun’s motion through the LISM; the size of the heliosphere; and the behavior of the magnetic field lines at the heliospheric boundaries. All of these properties, including the size of the heliosphere, vary vastly in different proposed shapes and models of the heliosphere, which range in size anywhere from hundreds to thousands of au and in shape from comet- or magnetosphere-like or otherwise with extended tails… to spherical to croissant-shaped with multiple tail lobes…
Bear in mind that the heliosphere is a moving target. As recently as 60,000 years ago, the Sun entered what is called the Local Interstellar Cloud, and is now considered to be at that cloud’s edge or perhaps beyond it and in contact with surrounding clouds. The JHU/APL Interstellar Probe study notes that within 2000 years, our system will likely be within a completely different interstellar environment. The heliosphere will adjust. Let me quote the Interstellar Probe report on this, because the effects are startling when we consider the Sun’s 20 revolutions around the galactic core since its formation:
The orders-of-magnitude differences in interstellar properties have had dramatic consequences for the penetration of interstellar gas, dust, and galactic cosmic rays (GCRs) that have affected elemental and isotopic abundances, chemical atmospheric evolution, and perhaps even biological evolution. Along the evolutionary path, high interstellar cloud densities and ionization fractions have likely compressed the heliosphere down to below 25 au… Evidence is emerging for supernovae explosions as recent as 3 million years ago at only 20–50 pc from the Sun that probably compressed the heliosphere even below the orbit of Saturn and perhaps more, exposing the terrestrial planets to almost the full force of interstellar material and GCRs…
The GCR’s referenced above are galactic cosmic rays, high energy particles and heavy nuclei that can prove lethal to biology. The shielding effects of the heliosphere are all too easy to take for granted until we consider its malleable nature, so the more we learn about what affects it the better. The JHU/APL work highlights a probe trajectory that will not only sample the local interstellar medium but give us the ‘look back’ capability to see it as a whole, with the probe exiting the heliosphere at approximately 45 degrees off the heliopause nose direction.
This trajectory is attractive because it allows the heliopause and local interstellar medium to be reached within a reasonable timeframe, which in this context means something less than 50 years with current propulsion technologies. It also offers what the authors call “somewhat of a side view of the heliopause,” though one in the direction of the nose, and it allows the IBEX ribbon, a still puzzling region of enhanced Energetic Neutral Atom (ENA) emission in the outer heliosphere, to be probed.
But a trajectory through the IBEX ribbon center is only one of those that Spitzer and team analyze, ranging from the heliosphere nose to various angles out of the heliotail, the trailing part of the heliosphere in relation to the Sun’s motion through the medium. Here again I’m reminded of how little we know of the heliosphere’s shape, for some estimates of the heliotail have it extending more than 5000 AU downwind of the Sun. Hence the value of this paper, which assembles what our indirect observing methods have so far produced by way of data on the various defined parts of the heliosphere.
To facilitate mission planning, the paper proposes continued indirect measurements of ENA and the pickup ions (PUI) that facilitate a stronger ENA flux, emphasizing the heliotail, and measurements of interstellar ions that penetrate the heliosphere, including cosmic rays in the heliotail region. For the in situ measurements, the authors point out that the largest differences between the suggested shapes of the heliosphere would appear in the tail region. Our probe would thus do best to exit through the side of the heliosphere’s tail. Better still, the authors say, would be a two-spacecraft Interstellar Probe mission option reminiscent of the twin Voyager missions, one moving toward the nose, the other in the direction of the heliotail.
A great idea, but try to get that through the various funding entities… Even so:
Only a study of the tailward region of the heliosphere will give definitive evidence for the complete shape, which impacts how the heliosphere interacts with the LISM and therefore how the LISM impacts the composition of the heliosphere. Therefore, complementary in situ and indirect interstellar measurements must be made tailward within the heliosphere. These measurements can be made through the use of intentional instrumentation requirements for outer heliosphere missions. Additionally, it would be beneficial for the Interstellar Probe mission to either consider a trajectory through the heliotail, via the flank, or to comprise a unified mission of two spacecraft, in which a second Interstellar Probe would be launched with a tailward trajectory, perhaps intersecting one of the proposed…tail lobes.
The paper is Spitzer et al, “Complementary interstellar detections from the heliotail,” Frontiers in Astronomy and Space Sciences (08 February 2024). Full text.
by Paul Gilster | Feb 21, 2024 | Missions |
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).
by Paul Gilster | Jan 16, 2024 | Missions |
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.
by Paul Gilster | Nov 10, 2023 | Missions |
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 douglas.loss@irg.space
Abstract
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.
Introduction
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 [28]. 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 [30] 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 [28] 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 [4] 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 [36].
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 [10] The Martian regolith has up to 1% of (per)chlorate [13] which is far higher than any uncontaminated place on Earth. (Per)chlorate reduction only occurs in anaerobic conditions [3]. 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) [2]. 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 [34]. 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 [30]. 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.[40]
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.
Suggested experiments
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.]
4. Hydration
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].
Conclusion
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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