In the coming decades, space agencies from around the world will be venturing farther out into space than ever before. This includes returning to the Moon (perhaps to stay this time), exploring Mars, and maybe even establishing human settlements on both. Beyond that, there are even proposals for establishing habitats in space that could accommodate millions.
These plans build on decades of planning that go back to the dawn of the Space Age. In some cases, the plans are inspired by proposals made over half a century prior to that. While these grand visions for space exploration and colonization present many challenges, they also inspire innovative solutions.
In particular, missions to deep-space require fresh thinking about environmental control and life-support systems (ECLSS) that can provide self-sufficiency in terms of air, water, food, and protection from radiation and the dangers of space. These are essential since missions that take astronauts far from Earth cannot depend upon resupply missions from the surface to Low Earth Orbit (LEO).
More and more, researchers are looking to a particular type of ECLSS called a bioregenerative life support system (BLSS). A BLSS mimics the natural environment by utilizing biological (i.e., living) elements. The appeal of the BLSS to designers of space life support systems is that they can theoretically be designed to be sustainable.
Due to the threat of Climate Change, finding sustainable solutions on Earth is considered by many to be a matter of life and death. However, unlike Earth, the margin for failure in space and in hostile extraterrestrial environments is zero! By developing sustainable technologies and strategies for space environments that are hostile to life, the resulting applications are also likely to be useful on Earth.
Unfortunately, these efforts and their incorporation into mission architectures suffer from a particular problem. When it comes right down to it, there is a lack of clarity about "sustainability" and what it means for the future of space exploration. This problem dogs everything from creating long-duration habitats to plans for terraforming!
These issues were raised in a recent study titled "Terraform Sustainability Assessment Framework for Bioregenerative Life Support Systems." According to study authors Morgan Irons and Lee Irons, a "Terraform Sustainability Assessment Framework" (TSAF) is needed to evaluate technology and methods to ensure that ECLSS and BLSS are truly sustainable.
Soil science and space
Lee Irons is the Executive Director of the Norfolk Institute in Virginia, a research and development firm specializing in "human resiliency" solutions for Earth and space. He brings decades of experience in space plasma physics, energy production, hazardous environment decontamination and remediation, and large-scale engineering and construction projects.
Morgan Irons is a Ph.D. candidate in Soil & Crop Sciences at Cornell University, a Carl Sagan Institute Research Fellow, a 2020 National Science Foundation (NSF) Graduate Research Fellow, and the recipient of the 2019 Ken Souza Memorial Spaceflight Award.
Together, Lee and Morgan founded Deep Space Ecology Inc. (DSE) in 2016 to engineer and design agroecological systems for improving food sustainably on and away from Earth. The fruits of their work included pretreatments in Martian regolith to help plants grow in it, which Morgan developed during her undergraduate work at Duke University.
In 2018, Morgan also patented a model for a Closed-Ecological System (CES) model for a Martian habitat, consisting of a human habitation zone, an ecological buffer zone, and an agricultural zone. These efforts aimed to ensure sustainable agricultural practices and food security for farmers off-world and on Earth.
However, it quickly became apparent to Morgan and Lee that much more scientific understanding had to be developed to validate the engineering and design work of a CES. So, Morgan started working on her Ph.D. in soil science in 2018, and Lee launched Norfolk Institute in 2019. In 2020, they put together the team and the funding they needed to launch a soil experiment to the ISS which aims to determine gravitational effects on soil stability - a.k.a. the "Soil Health in Space" experiment.
Why soil, you might ask, when most plant experiments on the ISS involve hydroponics and aeroponics (water and air)? Morgan and Lee explain in their new paper that Earth’s life-sustaining environment is broadly the result of biogeochemical cycles acting through the soil, where water, air, geological minerals, organic matter, microbes, plants, and other organisms interact chemically and physically, driven by solar, gravitational, and geothermal energy.
The result on Earth is the ecosystems that constitute Earth’s natural environment. When environmental scientists refer to sustainability, they refer to the fundamental ability of a soil-based biosphere to sustain life, especially human life.
The implication is that, in order for a BLSS to provide the necessary services required for human sustainment, the BLSS must be based on the natural environment and the “root” soil basis from which the biological elements of the BLSS are derived. In other words, for a BLSS in space to be sustainable, it requires an Earth-like soil basis of its own.
Necessity & innovation
Before this decade is over, NASA plans to send the first crewed missions to the Moon since the Apollo Era - Project Artemis. Others, like the European Space Agency (ESA), Roscosmos (Russia), the CNSA (China), the ISRO (India), JAXA (Japan), and the CSA (Canada), all plan to send their first crewed missions to the Moon.
In all cases, these plans entail the creation of permanent infrastructure that will allow astronauts to remain there for long periods. This includes the ESA's International Moon Village, NASA's Artemis Base Camp, and the Lunar Gateway. To quote NASA, the goal is to create a "sustained program of lunar exploration."
By the 2030s, NASA and China intend to mount crewed missions to Mars, launching in 2033, 2035, and 2037. These windows coincide with what is known as a "Mars Opposition," something that occurs every 26 months or so when Earth and Mars are nearest to each other. Since this makes transit time much shorter, missions to Mars must launch during one of these windows.
For these and other plans, the need for sustainability and self-sufficiency is emphasized. Whereas the International Space Station (ISS) can be resupplied within a few hours from Earth, lunar habitats will have to wait days for resupply missions to arrive. For Mars, opportunities for resupply missions are even rarer, occurring roughly every 26 months with an Opposition.
To achieve this, NASA and other space agencies have designed their mission architectures around the principle of In-Situ Resource Utilization (ISRU). Roughly defined, this means using local resources to meet mission requirements and astronauts' needs - including food, water, air, propellant, building materials, etc.
But when it comes to sustainability, there is a lack of definition. NASA's Plan for Sustained Lunar Exploration and Development, released in 2020, lays out the basis for objectives and requirements for the Artemis Program. The term "sustainable" is often used in this docket, but the document doesn't define what that entails.
In the 2012 NASA report, Voyages: Charting the Course for Sustainable Human Space Exploration, "sustainability" is also used repeatedly. In the section titled "Habitation and Destination Capabilities," NASA provides a brief description of long-term habitation calls for:
"The long-duration habitation capability is a collection of technologies that supports a human crew as they travel through or explore space and live on planetary surfaces. Whether an in-space or surface habitat, this capability will integrate essential cross-cutting systems, including highly reliable environmental control and life support systems (ECLSS); food storage, preparation, and production; radiation protection; and technologies that support a crew's physical and mental health."
By definition, ECLSS life support systems are non-biological. These systems are designed to scrub the air of a pressurized vessel, be it a spacecraft or a space station. While NASA uses the term "regenerative" when describing the version used aboard the ISS, a concrete definition is lacking.
Towards a biological life support system
NASA used expendable versions of this technology for their Mercury, Gemini, and Apollo programs. A long-duration variant was developed for Skylab and is now used aboard the ISS. The ISS' ECLSS consists of the Water Recovery System (WRS) and the Oxygen Generation System (OGS).
The WRS provides clean drinking and irrigation water by recycling and purifying urine, cabin humidity, and other wastes with the help of chemicals. The OGS produces oxygen by electrolyzing water provided by the WRS, yielding oxygen and hydrogen as byproducts. In short, an ECLSS life-support system is dependent upon maintenance and replenishment over time.
Or, as Morgan and Lee Irons characterize these systems in their study, an ECLSS has no inherent ability in its nature to maintain or repair itself. Human intervention is required for this, the cost of which will eventually exceed the cost of replacing the ECLSS entirely. One metric by which the reliability of ECLSS systems can be measured is the Generalized Resilient Design Framework (GRDF).
This framework was developed by Dr. Jose Matelli - a visiting scientist with the NASA Ames Research Center. As Lee Irons explained to Interesting Engineering via Zoom:
"[I]t specifically addresses just disruptions of the nature of part failures. So you have a piece of hardware, and a part fails, and it causes a system to get less efficient or break down, and you have to repair it and get it moving again.
"This is an example of how the industry has been looking at sustainability more from a hardware resilience perspective and an engineered resilience perspective - how well you designed your system to maximize its runtime and minimize its downtime."
Looking to the future, NASA and other space agencies are working on Bioregenerative Life Support Systems (BLSS), which are defined by how they include one or more biological components. The benefit of these systems is that they are theoretically indefinite. Rather than replacing parts and requiring a supply chain to support that, a biological system regenerates itself over time.
Research into BLSSs currently includes conducting experiments aboard the ISS involving plants, microalgae, bacteria, and other photosynthetic organisms. NASA is also researching greenhouses that could provide food for crews and replenish life-support systems on missions to Moon, Mars, and other locations away from Earth. Examples include the Prototype Lunar/Mars Greenhouse project overseen by the Kennedy Advanced Life Support Research group at NASA's Kennedy Space Center, Florida.
To date, the vast majority of plant and bioregenerative systems have been performed aboard the ISS. As Morgan also explained to Interesting Engineering via Zoom:
"Most of the plant studies that have been done at this point were with the International Space Station. As we've seen, they've done a lot of hydroponics work, soilless systems, they have done some seed pillow work, that was prior to the hydroponics-based systems."
"So there's definitely been a lot of horticultural work on the International Space Station to understand fundamental biological system-processing and reproduction, but also to give the astronauts and cosmonauts an opportunity to have some fresh greens."
These experiments aim to create closed-loop systems that can support astronaut health and longevity by mimicking biological systems here on Earth. They are also a key component to future mission architectures, where the need for self-sufficiency is a must and "sustainability" is emphasized.
"Growing crops in space is one of the more obvious kinds of bioregenerative life support systems," added Lee Irons. "If you can grow crops and harvest some seeds to grow more crops, and keep that cycle going, then you effectively get into a bioregenerative process that can become self-sustaining - at least from the seed and food production perspective."
However, food production is merely one of hundreds or thousands of elements that need to be considered. To create a holistic, bioregenerative life-support system, one needs to consider all of the environmental factors here on Earth that humans depend on for their survival (and the very concept of sustainability). A metric to determine just how "sustainable" these systems are is all that is missing.
The term "sustainability" is a term that gained immense significance during the latter half of the 20th-century, a period of rapid industrialization and urbanization. During this time, environmental science and growing concern about the impact of human activities led many to question and reject traditional notions of "progress" and unlimited economic growth.
Jacobus Du Pisani, a professor of history with the School for Social and Government Studies at North-West University (South Africa), expounded on the subject in a 2006 paper ("Sustainable development – historical roots of the concept.") As he wrote:
"During the period of unprecedented industrial and commercial expansion after World War II, people became aware of the threats which rapid population growth, pollution, and resource depletion posed to the environment and their own survival as humans...
Anxiety was expressed in a growing body of academic literature that 'if we continue our present practices we will face a steady deterioration of the conditions under which we live' and about the real danger that humankind 'may destroy the ability of the earth to support life.'"
But as Morgan and Lee explain, it is important to understand how the definition of sustainability on Earth applies to proposals for human habitability in extraterrestrial environments. In this context, sustainability must be measured in terms of the resources that humans consume to survive. Sustainability is the short-term and long-term stability of such resources under nominal and occasionally abnormal human loads while being subject to an onslaught of expected and unplanned disturbances.
In their paper, Morgan and Lee combine numerous theoretical constructs of environmental science to apply the stability properties of resilience, resistance, persistence, and consistence. When applied to the resources provided by a BLSS in space for human consumption, these stability properties become sustainability measures. This now provides a way to quantify sustainability for any BLSS or ECLSS and measure the plans of NASA and commercial space companies against their claims and objectives.
But, as Morgan and Lee point out, ecosystems have another potential property that is poorly understood: variance. Said, Lee:
"It's this property that says that there are critical factors in ecosystems that over long periods of time don't necessarily stay constant. They vary. And they don't necessarily vary around a mean. They do large wandering. An entire ecosystem can evolve from a rocky substrate to grasslands, forests, and something else through an ecological succession process. So this property of variance seems to be a natural property of an ecosystem."
"As such, when you're thinking about variance, and you're thinking about calculating a resilience, which is a long-term factor of sustainability (or calculating persistence, which is also long-term). If you don't take into account the fact that those factors might be varying, you might appear to have a system that isn't sustainable, but really is, because it's just varying naturally."
The problem with measuring these properties is that they are difficult to quantify, partly because of a lack of clarity and understanding. "The danger is that we really don't - we think we do - but we really don't understand what it means to have a sustainable system," Lee added. "There are so many things going on here on Earth around us that we take for granted."
Towards a "Terraform" framework
For this reason, Morgan and Lee take their theoretical development one step further, presenting what they call the Terraform Sustainability Assessment Framework (TSAF). The basis for this framework is simple: if you can establish a bioregenerative system in space that is at least as sustainable as a similar system on Earth, then you've effectively formed an Earth-like system in space (i.e., you've "terraformed.")
Specifically, the TSAF means taking the values for resilience, resistance, persistence, and consistence and dividing them by the same values of a similar Earth system. In so doing, this framework effectively controls for the variance that is occurring in both systems and divides it out of the overall equation.
"If you get terraform-specific stabilities that are equal to one, then you have a bioregenerative system that is at least as sustainable as your similar Earth system," said Lee. "We don't expect to create a bioregenerative system in space that is theoretically more perfect than the Earth system, but if we can at least get it just as good as Earth, then that's what our goal is."
They also acknowledge that the only way to achieve such a system is to ensure that it is completely independent of Earth supply chains because such supply chains are inherently unsustainable. This is fitting since the goal of a BLSS is to ensure that humans can live in environments where resupply missions are irregular. In doing this, says Lee, scientists will be engaging in what looks like the science fiction of terraforming:
"You're actually taking a section of [the] surface of a planet that has gravity, and you're turning it into what humans like to call 'the Garden of Eden.' It has naturally-functioning biogeochemical cycles being driven by the solar power radiation coming in and by the gravitational and planetary dynamics that are involved. You get the whole physics, the whole chemistry, the whole biology, the whole geology, the whole meteorology of an environmental system functioning the way that it would function on Earth."
This description provides a pretty good idea of what the future of human space exploration will look like: domed enclosures where an entire life cycle, similar to what we see on Earth, has been engineered to ensure that nothing goes to waste. In other cases, it might look a little something like what we see in SF miniseries like The Expanse.
Like many works of SF, spaceships and stations have plants and trees that provide food and help produce oxygen for the crews. But to get a preview of what the future holds, one should look beyond the greenhouse concept or urban farms. As Morgan Irons explained:
"We need to continue to remind designers that plants are multi-functional. They're not just food. They can be used for breeding symbiotic relationships with other plants or microorganisms to do nitrogen-fixation - like legumes and rhizobium bacteria. They create a symbiotic relationship and fix the nitrogen that you need."
"You can use plants to use cooking oil, to create cloth. They can be used to have control over the atmospheric elements, whether that's oxygen, carbon dioxide, even temperature control. When we're looking at these systems, it's not only that we're eating them, but what other functions do they provide that are useful for humans, but also useful for creating a more stable, holistic environment."
Today, many advocates of space exploration stress that humanity's future depends upon its ability to expand beyond Earth. To do this, it's clear we need to "take Earth with us," which means establishing Earth-like environments wherever we plan on living long-term. This will not only allow for humans to live and thrive without having to be resupplied from Earth. It will also expand Earth's ecological presence alongside that of humanity.
What's more, testing our ability to terraform beyond Earth, where the margin for error is zero, will also have applications for life here on Earth. Studying how Earth ecology works at the most minute level, and reproducing those effects elsewhere, will ensure that future generations are armed with the knowledge to live sustainably on our home planet - what Frank Herbert called "Ecological Literacy."
As Morgan Irons summarized, the key to achieving this noble venture is to achieve a better understanding through cooperation:
"This is why having multidisciplinary collaborative teams is very important. You can't just have the engineer teams that you've traditionally had working on this. You need the soil scientists. You need the ecologist, the environmental scientists, the agricultural chemists, and the farmers. You need people who are actively researching this around Earth and people who are actively working in agricultural systems.
"So you really need these different perspectives, to bring in their knowledge of what they are working on, as well as for them to help contextualize the questions being asked, whether they are fundamental or applicational. Because people may not realize that the Earth question they are working on is actually also applicable to a space question and that there's this opportunity for crossover and development of knowledge and potential technology. That can help parallel both avenues of solving on Earth and solving for space."