Conceptual illustration of an oxygen pipeline located at the lunar south pole. Credits: Peter Curreri
This year’s list of NASA Innovative Advanced Concepts (NIAC) Phase I selections include a few awards that look promising for space development. For wildcatters (or their robotic avatars) drilling for water ice in the permanently shadowed craters at the lunar south pole and cracking it into hydrogen and oxygen, Peter Curreri of Houston, Texas based Lunar Resources, Inc. describes a concept for a pipeline to transport oxygen to where it is needed. Clearly oxygen will be a valuable resource to settlers for breathable air and oxidizer for rocket fuel if it can be sourced on the Moon. The company, whos objective is to develop and commercialize space manufacturing and resources extraction technologies to catalyze the space economy, believes that a lunar oxygen pipeline will “…revolutionize lunar surface operations for the Artemis program and reduce cost and risk!”.
Out at Mars, Congrui Jin from the University of Nebraska, Lincoln wants to augment inflatable habitats with building materials sourced in situ utilizing synthetic biology. Cyanobacteria and fungi will be used as building agents “…to produce abundant biominerals (calcium carbonate) and biopolymers, which will glue Martian regolith into consolidated building blocks. These self-growing building blocks can later be assembled into various structures, such as floors, walls, partitions, and furniture.” Building materials fabricated on site would significantly reduce costs by not having to transport them from Earth.
A couple of innovations are highlighted in this NIAC grant. First, Jin has studied the use of filamentous fungi as a producer of calcium carbonate instead of bacteria, finding that they are superior because they can precipitate large amounts of minerals quickly. Second, the process will be self-growing creating a synthetic lichen system that has the potential to be fully automated.
In addition to building habitats on Mars, Jin envisions duel use of the concept on Earth. In military logistics or post-disaster scenarios where construction is needed in remote, high-risk areas, the “… self-growing technology can be used to bond local waste materials to build shelters.” The process has the added benefit of sequestration of carbon, removing CO2 from the atmosphere helping to mitigate climate change as part of the process of producing biopolymers.
Graphical depiction of biomineralization-enabled self-growing building blocks for habitats on Mars. Credits: Congrui Jin
To reduce transit times to Mars a novel combination of Nuclear Thermal Propulsion (NTP) with Nuclear Electric Propulsion (NEP) is explored by Ryan Gosse of the University of Florida, Gainesville.
Conceptual illustration of a bimodal NTP/NEP rocket with a wave rotor enhancement. Credits: Ryan Gosse
NTP technology is relatively mature as developed under the NERVA program over 50 years ago and covered by SSP previously. NTP, typically used to heat hydrogen fuel as propellant, can deliver higher specific impulse then chemical rockets with attractive thrust levels. NEP can produce even higher specific impulse but has lower thrust. If the two propulsion types could be combined in a bimodal system, high thrust and specific impulse could improve efficiency and transit times. Gosse’s innovation couples the NTP with a wave rotor, a kind of nuclear supercharger that would use the reactor’s heat to compress the reaction mass further, boosting performance. When paired with NEP the efficiency is further enhanced resulting in travel times to Mars on the order of 45 days helping to mitigate the deleterious effects of radiation and microgravity on humans making the trip. This technology could make an attractive follow-on to the NTP rocket partnership just announced between NASA and DARPA.
Finally, an innovative propulsion technology for hurling heavy payloads rapidly to the outer solar system and even into interstellar space is proposed by Artur Davoyan at the University of California, Los Angeles. He will be developing a concept that accelerates a beam of microscopic hypervelocity pellets to 120 kilometers/s with a laser ablation system. The study will investigate a mission architecture that could propel 1 ton payloads to 500 AU in less than 20 years.
Artist depiction of pellet-beam propulsion for fast transit missions to the outer solar system and beyond. Credits: Artur Davoyan
Conceptual illustration of a lunar base in Mare Tranquilitatis Hole, believed to be an entrance to a lava tube about 100 meters below the lunar surface. Credits: Dipl.-Ing. Werner Grandl
In a new paper in Acta Astronautica Raymond P. Martin, a propulsion test engineer at Blue Origin and Haym Benaroya, a professor of mechanical and aerospace engineering at Rutgers describe the former’s research he carried out as a graduate student under the latter analyzing the structural integrity of lunar lava tubes after pressurization with breathable air. As reported previously on SSP, subterranean lava tubes on the Moon and Mars hold much promise as naturally occurring enclosures that are believed to be structurally sound, thermally stable and would provide natural protection from micrometeoroids as well as radiation. If they could be sealed off for habitation and filled with breathable air, life could be simplified for colonists as they would not have to don space suits for routine activities.
“This paper makes the argument that … lunar lava tubes present the most readily available route to long-term human habitation of the Moon”
Two views of a lunar skylight revealing a potential subsurface lava tube in Mare Ingenii. Credit: NASA/Goddard Space Flight Center/Arizona State University
Martin opens the paper with a history of the discovery and physical characteristics of lunar lava tubes tapping geological data dating back to the Apollo program. The existence of a lava tube is sometimes revealed by the presence of a “skylight”, a location where the roof of the tube has collapsed, leaving a hole that can be observed from space. Using an engineering simulation software called ANSYS, he developed a computer model to assess the structural integrity of these formations when subjected to internal atmospheric pressure.
Martin creates a model for his simulation based on the morphology of a relatively small lava tube known to exist from imagery taken by the Chandrayaan-1 spacecraft, the first lunar probe launched by the Indian Space Research Organisation . This structure averages 120 meters in diameter and was chosen because it has a rille-type opening level to the surface and could be sealed off at two locations. This approach makes sense as a starting point because the cavern would be easy to access and less energy would be be required to pressurize a smaller enclosure. Thus, the amount of infrastructure needed to establish early settlements would be minimized.
The goal of the simulation was to assess the integrity of the enclosed space under varying roof thicknesses and pressurization levels. Failure conditions were defined using commonly employed methods of assessing stability of tunnels in civil engineering and based on lunar basaltic rock general material properties known from testing of samples brought back from the Moon in the Apollo program and lunar meteorites. Finally, a formula was derived for safety factors associated with the failure conditions to ensure robustness of the design.
When running the simulation over various roof thicknesses and internal pressures, an optimum solution was found indicating that it is possible to pressurize a lava tube with a roof thickness of 10 meters with breathable air at nearly a fully atmosphere while maintaining its structural integrity. This would would feel like sea level conditions to people living there.
Being able to pressurize a lava tube for habitation could significantly simplify operations on the Moon as the infrastructure needed to make surface dwellings safe from radiation, micrometeorite bombardment and thermal extremes would be extensive adding costs to the settlement.
“A habitat within a pressurized tube would offer large reductions in weight, complexity, and shielding, as compared to surface habitats.”
Once a permanent settlement has been established and engineering knowledge advances to enable expansion into larger lava tubes, we can imagine how cities could be built within these spacious caverns, and what it would be like to live and work there. SSP explored just this scenario with Brian P. Dunn, who painted a scientifically accurate picture of such a future in Tube Town – Frontier, a hard science fiction book visualizing life beneath the surface of the Moon. Dunn envisions a thriving cislunar economy with factories producing spacecraft for Mars exploration.
Conceptual illustration of a spacecraft manufacturer inside a lava tube. Credit: Riley Dunn
Martin and Benaroya dedicated their paper to the memory of Brad Blair, a mining engineer who was a widely recognized authority on space resources.
Conceptual illustration of ESA’s SOLARIS space based solar power system. Credits: ESA
This year there were a lot of announcements and commentary regarding government support for studies that may lead to actual development activities for space solar power. These events, as well as some efforts by private companies, have been prompted by global initiatives to reduce carbon emissions toward net zero by midcentury in the hope of mitigating climate change.
Last January Japan codified into law an aggressive timetable to launch an end-to-end space solar power demonstration flight in LEO by 2025. From an English translation of Japan’s Basic Space Law provided by the National Space Society, the exact text reads “Each ministry will work together to promote the realization of space solar power generation. Concerning microwave-type space solar power generation technology, the aim will be to demonstrate by 2025 energy transmission from low Earth orbit to the ground.” If implemented on time, this would be the first such technical demonstration to be performed from space. Also, the fact that the initiative is codified into Japan’s laws means they are serious.
At a Royal Aeronautical Society conference last April in London called Toward a Space Enabled Net-Zero Earth, chairman of the Space Energy Initiative Martin Soltau outlined a 12-year timeline that would provide gigawatts of power from space for the UK by 2035. The Initiative, which is a collection of over 50 British technology organizations, has selected a space solar power satellite design called CASSIOPeiA after a cost benefit analysis performed by Frazer-Nash Consultancy initially covered by SSP. Incidentally, links to the final report by Frazer-Nash Consultancy completed in September 2021 and to the CASSIOPeiA system are available on the SSP Space Solar Power page.
At the International Space Development Conference in Washington D.C. last May, Nickolai Joseph of the NASA Office of Technology Policy, and Strategy (OTPS) announced an effort by the space agency to reexamine space based solar power. The purpose of the study is to assess the degree to which NASA should support its development. Joseph said the report was to be completed by the end of September but as this post goes to press, it had not been released. Head of the OTPS, Bhavya Lal, tweeted last month that the report was in final review but this Tweet has been deleted without explanation. We are still waiting.
Three items on space solar power came up in September. First, John Bucknell returned to The Space Show to give an update on Virtus Solis, his space-based power system that SSP covered previously in an interview. With the novel approach of a Molynia sun-synchronous orbit, Bucknell claims that Virtus Solis will provide baseload capacity at far lower cost. In addition, the choice of orbits allow sharing orbital assets globally enabling solutions for multiple countries and regions. Bucknell hopes to have a working prototype to test in space within the next few years.
Schematic illustration of a three-array Virtus Solis constellation in Molniya orbits serving Earth’s Northern Hemisphere and a two-array constellation serving the Southern Hemisphere of Luna. Credits: Virtus Solis
Later in the month, the American Foreign Policy Council published a position paper on space based solar power in the organization’s publication Space Policy Review. From the introduction, author Cody Retherford writes that space solar power “…satellites are a critical future technology that have the potential to provide energy security, drive sustainable economic growth, support advanced military and space exploration capabilities, and help fight ongoing climate change.”
Overview of Space-based Solar Power from Figure 1 in American Foreign Policy Council report. Credits: AFPC and U.S. Department of Energy.
Also in September, the European Space Agency proposed a preparatory program called SOLARIS to inform a future decision by Europe on space-based solar power. The proposal was submitted for consideration in November at the ESA Council at Ministerial Level held in Paris.
The goal of SOLARIS, conceptualized in the illustration at the top of this post, would be to lay the groundwork for a possible decision in 2025 to move forward on a full development program to realize the technical, political and programmatic viability of a space solar power system for terrestrial needs.
Upon the conclusion of the ESA Council at Ministerial Level meeting SOLARIS was approved as a program. The Council confirmed full subscription to the General Support Technology Programme, Element-1, which requested funding for SOLARIS development. The activities performed under Element 1 support maturing technologies, building components, creating engineering tools and developing test beds for ESA missions, from engineering prototype up to qualification. Still to be determined: how much funding will be allocated by each member of the EU.
Then in October an article published in Science asks the question “Has a new dawn arrived for space-based solar power?” The authors bring to light what many advocates have already realized: that better technology and falling launch costs have revived interest in the technology. Also in October, MIT Technology Review issued a report “Power Beaming Comes of Age”. Based on interviews with researchers, physicists, and senior executives of power beaming companies, the report evaluated the economic and environmental impact of wireless power transmission to flush out the challenges of making the technology reliable, effective and secure.
China announced in November that it plans to test space solar power technologies outside its Tiangong space station. Using the robotic arms attached to the station, they plan to evaluate on-orbit assembly techniques for a space-based solar power test facility which will eventually then orbit independently to verify solar energy collection and wireless power transmission. The China Academy of Space Technology has already articulated plans for development of their own space solar power system culminating in a 2 Gigawatt facility in geostationary orbit by 2050.
To cap off the year, aerospace engineer and founder of The Spacefaring Institute Mike Snead published a four-part series on evaluation of green energy alternatives including space solar power which he calls Astroelectricity. In the first part, he covers the history of humanity’s energy use and the dawn of fossil fuel use over the last century pointing out the fragility of the current system with respect to energy security. A gradual transition to fossil fuel free alternatives is needed to provide enough time for technology development and conversion over to green energy sources while not creating shocks to an economy based mostly on coal, oil and gas.
Next, nuclear power is addressed (and dismissed) as a green alternative with the next generation of smaller modular fission nuclear reactors currently under development. Due to waste heat challenges and nuclear weapons proliferation issues plus problems with scaling up enough of these power plants as base load supply to supplement intermittent wind and solar, this alternative is rejected as a viable green alternative. No mention is made of some the numerous fusion energy development activities in process or the promise of thorium molten salt reactors, so some readers may take issue with Snead’s position on this point.
In the third installment, if it is assumed that nuclear power is not a viable option, Snead examines to what extent wind and terrestrial based solar power has to be scaled up to replace fossil fuels in the latter part of this century given population growth and resulting energy needs. Not surprisingly, given the intermittent nature of wind and solar he finds these sources lacking, and they “… are not practicable options for America to go green.” Enter space solar power to fill the void.
In the last article in his series, Snead provides guidance for establishing a national energy security strategy for an orderly transition to green energy. He concludes that, “With America’s terrestrial options for going green not providing practicable solutions, the time for America to develop space solar power-generated astroelectricity has arrived. America now needs to pursue space solar power-generated astroelectricity to ensure that our children and grandchildren enjoy an orderly, prosperous transition to green energy.”
Finally, we close out the year with this: Northrop Grumman announced plans for an end to end space to ground demo flight in 2025 of their Space Solar Power Incremental Demonstrations and Research (SSPIDR) project funded by the Air Force Research Laboratory. SSP reported on the SSPIDR system previously. This development sets up a race between Japan, Virtus Solis (both mentioned above) and the U.S. government to be the first to beam power from space to the ground by the middle of this decade.
Conceptual illustration of Olympus, a lunar construction system based on in situ resource utilization. Credits: ICON
In a press release, the Austin based company reports how the Phase III award under NASA’s Small Business Innovation Research (SBIR) program will be used to adapt its existing additive manufacturing process for home building on Earth to the Olympus system using lunar regolith for fabrication of structures on the Moon. ICON envisions the system to be integrated into a rover that will be delivered to the Moon via a lander. The rover will then autonomously drive to a target site where the Olympus laser 3D printer will process lunar regolith into useful structures. The system can be used for fabricating roads, landing pads and habitats out of local resources without having to bring building materials from Earth, thereby significantly lowering costs. Once the system is proven on the Moon, perhaps in the later stages of Artemis, the same technology can be applied on Mars as well.
ICON plans to test the system “…via a lunar gravity simulation flight” although no details were revealed on such a mission. Presumably, this would be a parabolic flight in the Earth’s atmosphere. The company would use samples of lunar soil brought back during the Apollo missions and lunar regolith simulant to tune the process variables of their laser 3D printing equipment operating under these conditions. Once optimized, Olympus would be placed on the Moon “…to establish the critical infrastructure necessary for a sustainable lunar economy including, eventually, longer term lunar habitation.”
“The final deliverable of this contract will be humanity’s first construction on another world, and that is going to be a pretty special achievement.”
Artist’s rendering of a farming settlement on Mars. Credits: HP Mars Home Planet Rendering Challenge via International Business Times.
Space settlement will eventually require space farming to feed colonists and to provide life support. It’s clear that we will replicate our biosphere wherever we go. In that spirit, Bryce L. Meyer envisions Mars as the breadbasket of the outer solar system. In a presentation at Archon 45, a science fiction and fantasy convention held annually by St. Louis area fans, he makes the case for why the fourth planet would be the ideal spot to grow crops and feed an expanding population as part of the roadmap to agriculture in space.
Carbon dioxide and subsurface water ice are plentiful on Mars, critical inputs for crop photosynthesis. There is also evidence of lava tubes there which could provide an ideal growing environment protected from radiation, micrometeorite bombardment and temperature extremes. The regolith should provide good nutrients and there is already research on methods to filter out perchlorates, a toxic chemical compound in the Martian soil.
Image of Lava tubes on the surface of Mars as photographed by ESA’s Mars Express spacecraft. Credits: ESA/DLR/FU Berlin/G Neukum / NewScientist
Another advantage that Mars holds as a food production hub for the asteroids and beyond is its placement further out in the solar system. Since it is higher up in the sun’s gravity well, Meyer calculates that it would take less than 43% of the fuel needed to transport goods from Mars outward than from Earth. He even suggests that with its lower gravity and recent advancements in materials research, a space elevator at Mars could be economically feasible to cheaply and reliably transport foodstuff off the planet.
Meyer keeps a webpage featuring space agriculture, terraforming, and closed cycle microgravity farming where he poses the question “Why settle space?” I like his answer: “Trillions of Happy Smiling Babies!!!”
Basic input/output diagram of an environmental control and life support system like what would be expected in a space farm. Credits: Bryce L. Meyer
Meyer is the founder and CEO of Cyan React, LLC, a startup that designs compact photobioreactors and provides expertise in closed-cycle farming and life support especially for space settlement and space habitats. He is also a National Space Society Space Ambassador doing his part to educate the public about the potential benefits to humanity through the use of the bountiful resources in space. In a presentation at this year’s International Space Development Conference, he describes his research on bioreactors explaining how settlers will grow food and recycle waste sustainably on the high frontier.
Diagram depicting the flow of materials in a closed space farm habitat utilizing bioreactors. Credits: Bryce L. Meyer
Complete closure and stability of an environmental control and life support system (ECLSS) is challenging and not without limitations. As launch and space transportation costs come down in the near future and off-Earth supply chains become more reliable, complete closure will not be required at least initially. In situ resource utilization will provide replacement of some ECLSS consumables where available for colonists to live off the land. As missions go deeper into space reaching the limits of supply chain infrastructure and even out to the stars, closure of habitat ECLSS and resource planning will become more important. Meyer has done the math for farms in space to provide food and air for trillions of smiling babies…and their families as they move out into the solar system.
Artist concept of a cutaway view of the Stanford Torus free space settlement. Credits: Rick Guidice / NASA
Can humanity explore and develop space responsibly by learning from some of the mistakes made throughout history while settling new lands? In an article called “To Boldly Go (Responsibly)” on LinkedIn, CEO of Trans Astronautica Corporation Joel Sercel provides a vision for how we should conscientiously manage space settlement in a manner that respects human rights and the rule of law, but also maintains stewardship of the space environment.
“Through space settlement, we have a chance to show that humanity has learned from history and is evolving morally and culturally”
Sercel warns of the “Elysium effect”. In the words of Rick Tumlinson, who coined the term in an article on Space.com, “…as entrepreneurs like Elon Musk, Jeff Bezos and Richard Branson spend billions to support a human breakout into space, there is a backlash building that holds these projects as icons of extravagance.” Ironically, these New Space pioneers actually have the opposite goals of lowering the cost of access to space for average citizens and preserving the Earth’s environment by moving “dirty” industries outside it’s biosphere.
Public space agencies and private space companies can help open the high frontier responsibility through cooperation on development of common standards and international agreements in accordance with the Outer Space Treaty. Sercel believes that an urgent need in this area would be establishment of salvage rights for defunct satellites and dormant orbital debris like spent upper stages which under the OST are the responsibility of the nation that launched the payloads.
“That’s a legal impediment for companies developing satellites to clean up orbital debris and firms eager to recycle abandoned antennas and rocket bodies.”
Some work in the area of orbital debris mitigation has already been started by the Space Safety Coalition, an ad hoc coalition of companies, organizations, and other government and industry stakeholders, through establishment of best practices and standardization for space operations. And just last month the Orbital Sustainability Act of 2022 was introduced in the U.S. Senate that will “require the development of uniform orbital debris standard practices in order to support a safe and sustainable orbital environment.”
Good progress on interagency cooperation in space has also been made with the creation of the Artemis Accords, Principles for a Safe, Peaceful, and Prosperous Future. Signed by seven nations thus far, the agreement provides a legal framework in compliance with the OST for humans returning to the Moon and establishing commercial mining rights.
Sercel thinks that before establishing a permanent human presence on Mars we should first thoroughly explore the planet robotically for signs of life to ensure that we do not disrupt any indigenous organisms if a biosphere is found to be present there.
Another example of space ethics, discussed on SSP in previous posts, is determination of the gravity prescription, especially the human gestation component. The answer to this critical factor may drive the decision on where to establish permanent long term settlements so colonists can raise families. It may turn out that having children in less than 1G may not be biologically possible and therefor, for ethical reasons, may change the long term strategy for human expansion in the solar system favoring free space settlements with Earth normal artificial gravity over surface settlements. Sercel believes that determination of the gravity Rx should be a high priority and suggested on The Space Show recently a roadmap of mammalian clinical reproduction studies starting with rodent animal models producing offspring over multiple generations progressing to primates and then, only if these are successful, initiating limited human experiments. Such studies would prevent ethical issues that may arise from birth defects or health problems during pregnancy because we don’t know how lower gravity would effect embryos during gestation.
Dylan Taylor of Voyager Space Holdings has advocated for a sustainable approach to space commercial activities to ensure “…that all humanity can continue to use outer space for peaceful purposes and socioeconomic benefit now and in the long term. This will require international cooperation, discussion, and agreements designed to ensure that outer space is safe, secure and peaceful.”
Sercel is calling for the National Space Council “…to coordinate private organizations to include think tanks, advocacy groups, and the science community to work together to define the field of space ethics…to guide the development of laws and regulations that will ensure the rapid and peaceful exploration, development and settlement of space.”
Eventually we will get to the stars. It may not happen in our lifetime but its going to happen some day. Adam Crowl has provided a nice historical review of the interstellar pioneers from the last century that worked out the physics of the starships that will take us there. He does this in a chapter he wrote for James and Gregory Benford’s ground-breaking anthology Starship Century which was based on the findings of the 100‐Year Starship Symposium seeded by a DARPA solicitation and executed by NASA back in 2011.
Crowl begins the story with the early days of rocketry pioneered by Tsiolkovsky determining the rocket equation and Goddard and others experimenting with liquid fueled rockets. Tsiolkovsky was the first to come up with the idea of a generation starship (sometimes referred to as a worldship) when he realized that existing chemical propellants would be insufficient to fuel a space ship for interstellar travel.
Artist depiction of an interstellar worldship. Credits: Michel Lamontagne / Principium, Issue 32, February 2021
More practical interstellar craft don’t come on the scene until after WWII when advanced propulsion concepts really take off. The possibility of harnessing light to “push” a rocket, feasible because photons carry momentum, first appeared in science fiction. As it turned out, physicists realized that to generate the needed thrust with light pressure would require enormous amounts of energy, the waste heat of which would vaporized the vessel. Nevertheless, the photon rocket was still being discussed as late as 1972 when I first saw the rendering at the top of this post by David Hardy in the book he coauthored with Patrick Moore called Challenge of the Stars. Fast forward to today, Dr. Young K. Bae’s Photonic Laser Thruster shows great promise if it can be scaled up for interstellar travel.
Diagram depicting the layout of the Photonic Laser Thruster. Credits: Young K. Bae, Ph.D.
In the latter half of the last century, as the physics of nuclear energy and laser technology progressed, we see a proliferation of many concepts for star travel, including various forms of fusion rockets, laser sails, antimatter propulsion and my personal favorite, the Bussard ramjet. Conceived by the physicist Robert Bussard in 1960, the ship eliminates the need to carry fuel by collecting hydrogen from the interstellar medium using a magnetic field as a ram scoop and compresses the gas to fusion temperatures to create thrust. Crowl summarizes some of the physical limitations of the original concept and discusses several physicist’s alternative designs to address them.
One concept that didn’t make it into Crowl’s piece was developed recently by Leif Holmlid and Sindre Zeiner-Gundersen. Called the laser induced annihilation drive, it uses a pulsed laser to initiate “antimatter-like” annihilation reactions in hydrogen fuel producing high velocity K meson elementary particles at relativistic speeds to generate thrust.
Diagram of a laser-induced annihilation generator for space propulsion. Credit: Leif Holmlid and Sindre Zeiner-Gundersen, Acta Astronautica 23 May 2020
When I asked Crowl if he had any updates to some of the starship propulsion concepts he sent me an article penned by an unknown author for Medium that came up with another alternative to address the limitations of the original Bussard Ramjet. The author, who goes by the pseudonym “deepfuturetech”, reminds us like Crowl discussed in his piece, that the cross section ( i.e. the probability that a given atomic nucleus or subatomic particle will undergo a nuclear reaction in relation to the species of the incident particle) of the Bussard ramjet proton-proton fusion reaction is too low to be useful. Deepfuturetech proposes a different fusion mechanism via (p,n) reactions which involve a nucleus capturing a proton and subsequently emitting a neutron. These type of reactions have higher cross sections and could be tested in reactors in the near future. Further analysis is needed to confirm whether these reactions could produce neutrons at sufficiently low energy cost to enable profitable hydrogen fusion.
Artist depiction of a Bussard ramjet. Credits: NASA
Incidentally, Crowl talked about many of these starship concepts at a subsequent Starship Century Symposium held in 2013 by the Arthur C. Clarke Center for Human Imagination in collaboration with the Benford brothers who shared the highlights from their Starship Century anthology summarizing scientific results from the 100‐Year Starship project. You can also get a “Deeper Future View” of his independent research on interesting items not typically covered by the mainstream science media at his blog Crowlspace.
SSP has addressed the gravity prescription (GRx) in previous posts as being a key human factor affecting where long term space settlements will be established. It’s important to split the GRx into its different components that could effect adult human health, child development and reproduction. We know that microgravity (close to weightlessness) like that experienced on the ISS has detrimental effects on adult human physiology such as osteoporosis from calcium loss, degradation of heart and muscle mass, vision changes due to variable intraocular pressures, immune system anomalies…the list goes on. But many of these issues may be mitigated by exposure to some level of gravity (i.e. the GRx) like what would be experienced on the Moon or Mars. Colonists may also have “health treatments” by brief exposures to doses of 1g in centrifuge facilities built into the settlements if the gravity levels in either location is found to be insufficient. We currently have no data on how human physiology would be impacted in low gravity (other then microgravity).
The most important aspect of the GRx with respect to space settlement relates to reproduction. How would lower gravity effect embryos during gestation? Since humans have evolved in 1g for millions of years, a drastic change in gravity levels during pregnancy could have serious deleterious effects on fetal development. Since fetuses are already suspended in fluid and can be in any orientation during most of their development, it may be that they don’t need anywhere near the number of hours of upright, full gravity that adults need. How lower gravity would affect bone and muscle growth in young children is another unknown. We just don’t know what would happen without a clinical investigation which should obviously be done first on lower mammals such as rodents. Then there are ethical questions that may arise when studying reproduction and growth in higher animal models that could be predictive of human physiology, not to mention what would happen during an accidental human pregnancy under these conditions.
Right now, we only know that 1g works. If space settlements on the Moon or Mars are to be permanent and sustainable, many space settlement advocates believe they need to be biologically self-sustaining. Obviously, most people are going to want to have children where they establish permanent homes. If the gravity of the Moon or Mars prevents healthy pregnancy, long term settlements may not be possible for people who want to raise families. This does not rule out permanent settlements without children (e.g. retirement communities). They just would not be biologically self-sustaining.
SSP has suggested that it might make sense to determine the GRx soon so that if we do determine that 1g is required for having children in space, we begin to shape our strategy for space settlement around free space settlements that produce artificial gravity equivalent to Earth’s. Fortunately, as Joe Carroll has mentioned in recent presentations, the force of gravity on bodies where humanity could establish settlements throughout the solar system seems to be “quantized” to two levels below 1g – about equal to that of the Moon or Mars. All the places where settlements could be built on the surfaces of planets or on the larger moons of the outer planets have gravity roughly at these two levels. So, if we determine that the GRx for these two locations is safe for human health, we will know that we can safely raise families beyond Earth in colonies on the surfaces of any of these worlds. Carroll proposes a Moon/Mars dumbbell gravity research facility be established soon in LEO to nail down the GRx.
But is there an argument to be made for skipping the step of determining the GRx and going straight to an O’Neill colony? After all, we know that 1g works just fine. Tom Marotta thinks so. He discussed the GRx with me on The Space Show recently. Marotta, with Al Globus coauthored The High Frontier: An Easier Way. The easier way is to start small in low Earth orbit. O’Neill colonies as originally conceived by Gerard K. O’Neill in The High Frontier would be kilometers long in high orbit (outside the Earth’s protective magnetic field) and weigh millions of tons because of the amount of shielding required to protect occupants from radiation. The sheer enormity of scale makes them extremely expensive and would likely bankrupt most governments, let alone be a challenge for private financing. Marotta and Globus suggest a step-by-step approach starting with a far smaller version of O’Neill’s concept called Kalpana. This rotating space city would be a cylinder roughly 100 meters in diameter and the same in length, spinning at 4 rpm to create 1g of artificial gravity and situated in equatorial low Earth orbit (ELEO) which is protected from radiation by our planet’s magnetic field. If located here the settlement does not require enormous amounts of shielding and would weigh (and therefore cost) far less. Kasper Kubica has proposed using this design for hosting $10M condominiums in space and suggests an ambitious plan for building it with 10 years. Although the move-in cost sounds expensive for the average person, recall that the airline industry started out catering to the ultra-rich to create the initial market which eventually became generally affordable once increasing reliability and economies of scale drove down manufacturing costs.
What about all the orbital debris we’re hearing about in LEO? Wouldn’t this pose a threat of collision with a free space settlement given their larger cross-sections? In an email Marotta responds:
“No, absolutely not, I don’t think orbital debris is a showstopper for Kalpana.
… First, the entire orbital debris problem is very fixable. I’m not concerned about it at all as it won’t take much to clean it up: implement a tax or a carbon-credit style bounty system and in a few years it will be fixed. Another potential historical analogy is the hole in the ozone layer: once the world agreed to limit CFCs the hole started healing itself. Orbital debris is a regulatory and political leadership problem, not a hard technical problem.
Second, even if orbital debris persists, the technology required to build Kalpana…will help protect it. Namely: insurance products to pay companies (e.g. Astroscale, D-Orbit, others) to ‘clear out’ the orbit K-1 will inhabit and/or mobile construction satellites necessary to move pieces of the hull into place can also be used to move large pieces of debris out of the way. In fact, I think having something like Kalpana…in orbit – or even plans for something that large – will actually accelerate the resolution of the orbital debris problem. History has shown that the only time the U.S. government takes orbital debris seriously is when a piece of debris might hit a crewed platform like the ISS. Having more crewed platforms + orbital debris will drastically limit launch opportunities via the launch collision avoidance process. If new satellites can’t be launched efficiently because of a proliferation of crewed stations and orbital debris I suspect the very well-funded and strategically important satellite industry will create a solution very quickly.”
To build a space settlement like the first Kalpana, about 17,000 tons of material will have to be lifted from Earth. Using the current SpaceX Starship payload specifications this would take 170 launches to LEO. By comparison, in 2021 the global launch industry set a record of 134 launches. Starship has not even made it to orbit yet, but assuming it eventually will and the reliability and reusability is demonstrated such that a fleet of them could support a high launch rate, within the next 20 years or so there will be considerable growth in the global launch industry. If larger versions of Kalpana are built the launch rate could approach 10,000 per year for space settlement alone, not to mention that needed for rest of the space industry. This raises the question of where will all these launches take place? Are there enough spaceports in the world to support it? Marotta has an answer for this as well. As CEO of The Spaceport Company, he is laying the groundwork for the global space launch infrastructure that will be needed to support a robust launch industry. His company is building distributed launch infrastructure on mobile offshore platforms. Visit his company website at the link above for more information.
Conceptual illustration of a mobile offshore launch platform. Credits: The Spaceport Company
For quite some time there has been a spirited debate among space settlement advocates on what destination makes the most sense to establish the first outpost and eventual permanent homes beyond Earth. The Moon, Mars or free space O’Neill settlements. Each location has its pros and cons. The Moon being close and having ice deposits in permanently shadowed craters at its poles along with resource rich regolith seems a logical place to start. Mars, although considerably further away has a thin atmosphere and richer resources for in situ utilization. Some believe we should pursue all the above. However, only O’Neill colonies offer 1g of artificial gravity 24/7. With so many unknowns about the gravity prescription for human health and reproduction, free space settlements like Kalpana offer a safe solution if the markets and funding can be found to make them a reality.
Hubble Space Telescope image of Mars showing clouds in atmosphere near the poles and the extinct volcano Olympus Mons at right. The primary constituents of the Martian air are carbon dioxide (95%) and nitrogen (~3%). Credits: NASA
A new technology funded by ESA is under development in Belgium and Portugal that could produce breathable air, oxidizer for rocket fuel and nitrogen for fertilizer out of thin air on Mars. Using a high energy plasma, researchers at the University of Antwerp and the University of Lisbon published independent results that look promising as a source of oxygen for life support and propulsion, plus nitrogen oxides as fertilizer to grow crops.
Team Antwerp heated simulated Martian atmosphere with microwaves in a plasma chamber. The electrical energy cracked the carbon dioxide and nitrogen in the gas into highly reactive species generating oxygen which, in addition to creating breathable air and oxidizer for fuel, was combined with the nitrogen to create useful fertilizer.
The scientists in Lisbon used direct current to excite the gases into a plasma state, literally creating lightning in a bottle. This team focused only on the production of oxygen.
The efficiency of these processes is quite impressive. For example, when compared to the Mars Oxygen In Situ Resource Utilization Experiment (MOXIE) on NASA’s Perseverance rover, the Antwerp system uses the same input power, about 1kWh, but produces 47 g per hour which is about 30 times faster. MOXIE uses solar energy to electrochemically split carbon dioxide into oxygen ions and carbon monoxide, then isolates and recombines the oxygen ions into breathable air.
Image of the toaster-sized Mars Oxygen In Situ Resource Utilization Experiment (MOXIE) being installed on the Perseverance rover at the Jet Propulsion Laboratory prior to launch. Credit: NASA/JPL-Caltech
The research is in early days but has the potential for benefits on Earth too. The amount of energy needed to fix nitrogen in fertilizer for terrestrial crops is significant and releases considerable amounts of carbon dioxide to support worldwide agriculture. This plasma technology, if it can be commercialized, has the potential to reduce the carbon footprint of Earth-based fertilizer production. The fact that the process has duel-use provides a profit motive for development of the equipment and scaling up production, which could lead to improvements in efficiency and reduction in the mass for space applications.
We love ISRU technology that facilitates production of consumables using local resources at space destinations, thereby reducing the mass that needs to be transported to support space settlements and enabling them to become self-sustaining.
Schematic of the Thorium Molten Salt Reactor for space propulsion applications. Credits: Ajay Kothari / The Space Review
President Joe Biden recently signed into law a sweeping climate bill that will have very little (if any) impact on addressing global warming (a reduction of 0.028 degrees F by 2100). While there are tax credits in the bill for construction of new nuclear power plants over the next 10 years, only two are planned to add to the existing 93 facilities operating today which provide 18% of the U.S. energy production. Most of the funding in the bill is targeted at tax credits for EVs and incentives for renewable sources such and wind and solar which are subject to interruption. Nuclear energy holds enormous promise to offset the carbon emissions associated with fossil fuel energy production and can provide reliable base load power, but it is still plagued by negative public perceptions related to safety and the potential for weapons proliferation.
Is it time to reimagine our approach to sourcing clean energy in general, and nuclear power in particular while at the same time addressing climate change? Ajay Kothari thinks so – by research and development and eventual commercialization of nuclear power plants fueled by thorium rather than uranium. Dr. Kothari describes his vision in the August 1, 2022 issue of The Space Review. He believes that this powerful and sustainable power source “…will solve the world’s energy problem a thousand times over with zero carbon dioxide emission during operation, and it may be the cheapest form of energy production for us.”
“One ton of thorium is roughly equivalent to five million barrels of oil”
Thorium is abundant in the Earths crust making it relatively cheap and therefore, more affordable. It is only slightly radioactive, far less so then uranium and does not contain fissile material making it much safer and easier to moderate (i.e. switch off) in the case of an accident. This would prevent meltdowns unlike conventional reactors which have coolants that operate at much higher pressures and need far more complicated engineering safeguards to prevent disasters.
Thorium molten salt reactors are inherently safe. Flibe Energy is designing a Liquid Fluoride Thorium Reactor (LFTR) and according to the company’s website, “…any increase in operating temperature reduces the density of the salt which in turn, causes the reaction to slow and the temperature to fall. LFTR is also designed with a simple frozen salt plug in the bottom of the reactor core vessel. In the event of power loss to the reactor, the frozen salt plug quickly melts and the fuel salt drains down into a storage tank below – causing a termination in the fission process.”
Once developed for energy production on Earth, the same technology has applications in space. While it would not be used in a booster during launch, a molten salt thorium reactor upper stage, like that shown in the illustration above, could provide an efficient 700 second specific impulse by heating hydrogen as fuel for advanced propulsion for the next few decades until fusion energy comes on line. An added benefit would be that the upper stage reactor could also be used to provide energy at the destination, for example on the Moon or Mars.
“One kilogram of thorium taken from Earth [to the Moon] … can support a 2.6 thermal megawatt plant for a year.”
A thorium reactor was developed at Oak Ridge National Laboratory (ORNL) back in the 1960s but was never commercialized after the then Atomic Energy Commission favored plutonium fast-breeder reactors.
Diagram of the thorium fuel cycle in molten salt reactors. Credits: Flibe Energy
There are challenges to overcome. For example, the thorium fuel cycle is complicated and still produces some radioactive waste, but far less and with much shorter half life when compared to conventional uranium nuclear reactors. But the benefits of this clean, abundant and affordable energy source could make investment by the public and private sector worth the effort.
“With US reserves at 595,000 tons of thorium, we have enough to last us 600 years at current rates.”
Kothari has been a long time proponent of Thorium reactors. He recently gave a talk on the molten salt thorium reactor via Zoom for the University of Maryland now available on YouTube. You can also hear an in-depth discussion of the technology on The Space Show when he was a guest back in October 2021 and when he returns to the show September 13, 2022.
Dr. Kothari agreed to take a deeper dive with SSP into what he calls “Thor – The Life-Saver” through an email interview. If you have questions I didn’t cover about thorium molten salt reactors please leave a comment.
SSP: Dr. Kothari, thank you for taking the time to answer my questions. With respect to the public’s fear of nuclear power in general, the safety of thorium molten salt reactors is certainly an argument in favor to the technology. But aren’t there still risks of nuclear proliferation?
AK: We have more than 400 reactors in more than 40 countries worldwide. We found ways to have countries develop their reactors but have proliferation controls. This idea, the TMSR, creates no Plutonium, and would be easier to monitor. Besides, whether we want [to] or not, other countries WILL do it. Many are. Also we can develop the technology for ourselves and [for] friendly countries OR at the very least, USE IT FOR OURSELVES! How can we deny this incredible opportunity for our (US) populace? Is that fair?
SSP: Flibe Energy appears to be the only U.S. company pursuing LFTR technology. Chicago based Clean Core is focusing on thorium-based fuels to be used in existing pressurized heavy-water reactor designs. What do you think of these two company’s approaches and are you aware of any other thorium reactor development efforts in the U.S, either in private industry or academia?
AK: MIT is developing tech to resolve some of the TMSR issues that would be quite helpful [SSP found this story from MIT Nuclear Reactor Laboratory on deployment of its “…nuclear reactor (MITR) and related testing apparatus as a proving ground for the materials and processes critical to molten-salt-cooled reactors.”]. Others are shown in the chart below with some of them being US based (bottom right).
Color coded map showing global molten salt reactor technology development activities and the sponsoring country/entities. FHR= Fluoride salt-cooled high-temperature reactor. LEU = Low Enriched Uranium. HEU= Highly Enriched Uranium. TRU=Transuranic wastes, i.e. heavier elements than Uranium. Credits: Oak Ridge National Laboratory
SSP: How difficult would it be to adapt this technology for space propulsion and power applications and is it so far off that fusion energy may be available by the time development efforts come to fruition?
AK: In my opinion, …. controlled fusion may be 100, 200 or 50 years away. We have a valley of death … between now and then. This TMSR can fill the gap but can also be used for space propulsion as my diagram above shows. Sure, the TRL of it needs to be brought up, but that’s what we are here for. It would be less heavy than [the] NERVA idea, especially if the chemical processing plant is separated and U233 is used for space propulsion rather than Th232. This would be the idea. The rate of fission is then controlled by the graphite rod moderators/controllers.
SSP: China has been working on a LFTR since 2011 and was recently cleared to start operating the reactor which is a direct descendent of the original experimental design that ORNL studied in the 1960s. It would appear that the Chinese have a significant head start. Is this concerning?
AK; Absolutely. All I can say is that we are idiots.
SSP: One of the disadvantages of thorium reactors is that large upfront costs are needed due to the significant amount of testing and licensing work for qualification of commercial reactors. The reactors also involve high fuel fabrication and reprocessing costs. How would you address these issues to attract investors?
AK: This idea really is a golden nugget, so to speak. The way to attract investors is to bring the TRL up with government (DoE, NASA and DoD) funds. When the light at the end of the tunnel is seen by investors, they will jump in with both feet. It may still be 5-10 years away but if we do not do it soon, (1) it will always remain so, and (2) some other country (China) or many other countries will DEFINITELY move ahead of us!
SSP: Another disadvantage is the presence of a significant level of gamma ray emissions due to Uranium-232 in the fuel cycle. How will this be dealt with safely?
AK: The Gamma ray radiation occurs from Protactinium 233 absorbing another neutron (before it Beta decays) to become Pa234 If it is separated in a chemical processing plant, it would remain easier to handle. From Wiki[pedia]: “The contamination could also be avoided by using a molten-salt breeder reactor and separating the 233Pa before it decays into 233U)”.
SSP: What regulatory and policy changes are needed to realize this technology in the U.S.?
AK: [The] NRC and DoE should allow smaller (~2 MW) size experimental reactors at Universities and research institutions right now.
SSP: On a related note, what efforts can leaders in private industry, academia and government undertake to begin the research and technology needed to commercialize thorium molten salt reactors.
AK: There are a few uncertain items in this nuclear process that Universities, small businesses and government research institutions can resolve. Government agencies need to fund SBIR/STTR type of initiatives to address the following technical issues:
The sustainability of the heat exchangers whether they are to be made of Hastelloy-N or some other composite. This characterization is needed w.r.t. neutron flux intensity, temperature reached and time exposed (in months to years)
The same as above for reactor containment vessels and pipes carrying the hot molten salt.
Chemical separation for in-line or off-line work for Protactinium and U233.
Tritium mitigation ideas (probably using CO2 in closed loop for electricity generation) or sequestration of it for later use in fusion when and if available. Designing and demonstrating tritium separators are key elements of DOE’s solid fuel MSR program at both universities and national laboratories
Gamma ray mitigation or reduction
Thorium doesn’t spontaneously undergo fission – when an atom’s nucleus splits and releases energy that can generate electricity. Left to its own devices it decays very slowly, giving off alpha radiation that can’t even penetrate human skin, so holidaymakers don’t need to worry about sunbathing on thorium-rich beaches.
We don’t have as much experience with Thorium. The nuclear industry is quite conservative, and the biggest problem with Thorium is that we are lacking in operational experience with it. When money is at stake, it’s difficult to get people to change from the norm.
Irradiated Thorium is more dangerously radioactive in the short term. The Th-U cycle invariably produces some U-232, which decays to Tl-208, which has a 2.6 MeV gamma ray decay mode. Bi-212 also causes problems. These gamma rays are very hard to shield, requiring more expensive spent fuel handling and/or reprocessing.
Thorium doesn’t work as well as U-Pu in a fast reactor. While U-233 an excellent fuel in the slow-neutron regime, it is between U-235 and Pu-239 in the fast spectrum. So for reactors that require excellent neutron economy (such as breed-and-burn concepts), Thorium is not ideal.
Proliferation Issues
Thorium is generally accepted as proliferation resistant compared to U-Pu cycles. The problem with plutonium is that it can be chemically separated from the waste and perhaps used in bombs. It is publicly known that even reactor-grade plutonium can be made into a bomb if done carefully. By avoiding plutonium altogether, thorium cycles are superior in this regard.
Besides avoiding plutonium, Thorium has additional self-protection from the hard gamma rays emitted due to U-232 as discussed above. This makes stealing Thorium based fuels more challenging. Also, the heat from these gammas makes weapon fabrication difficult, as it is hard to keep the weapon pit from melting due to its own heat. Note, however, that the gammas come from the decay chain of U-232, not from U-232 itself. This means that the contaminants could be chemically separated and the material would be much easier to work with. U-232 has a 70 year half-life so it takes a long time for these gammas to come back.
The one hypothetical proliferation concern with Thorium fuel though, is that the Protactinium can be chemically separated shortly after it is produced and removed from the neutron flux (the path to U-233 is Th-232 -> Th-233 -> Pa-233 -> U-233). Then, it will decay directly to pure U-233. By this challenging route, one could obtain weapons material. But Pa-233 has a 27 day half-life, so once the waste is safe for a few times this, weapons are out of the question. So concerns over people stealing spent fuel are largely reduced by Th, but the possibility of the owner of a Th-U reactor obtaining bomb material is not.
One especially cool possibility suitable for the slow-neutron breeding capability of the Th-U fuel cycle is the molten salt reactor (MSR), or as one particular MSR is commonly known on the internet, the Liquid Fluoride Thorium Reactors (LFTR). In these, fuel is not cast into pellets, but is rather dissolved in a vat of liquid salt. The chain reaction heats the salt, which naturally convects through a heat exchanger to bring the heat out to a turbine and make electricity. Online chemical processing removes fission product neutron poisons and allows online refueling (eliminating the need to shut down for fuel management, etc.). None of these reactors operate today, but Oak Ridge had a test reactor of this type in the 1960s called the Molten Salt Reactor Experiment [Wikipedia] (MSRE). The MSRE successfully proved that the concept has merit and can be operated for extended amounts of time. It competed with the liquid metal cooled fast breeder reactors (LMFBRs) for federal funding and lost out. Alvin Weinberg discusses the history of this project in much detail in his autobiography, The First Nuclear Era [amazon.com], and there is more info available all over the internet. These reactors could be extremely safe, proliferation resistant, resource efficient, environmentally superior (to traditional nukes, as well as to fossil fuel obviously), and maybe even cheap. Exotic, but successfully tested. Who’s going to start the startup on these? (Just kidding, there are already like 4 startups working on them, and China is developing them as well).
Loss of coolant accident consequences are significantly different than for Light Water Reactors
– Low driving pressure and lack of phase change fluids
– Guard vessels employed on some designs
– Planned vessel drain down to cooled, criticality-safe drain tanks on some designs
