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Homing in on the Gravity Prescription

Image of the JAXA Kibo module on the International Space Station which houses the MARS short arm centrifuge for artificial gravity studies. Credit: NASA/JAXA

The Gravity Prescription (GRx), a term first coined by Dr. Jim Logan, refers to the minimum “dosing” of gravity (level and duration of exposure) to enable healthy conception, gestation, birth and normal, viable development to and throughout adulthood as a human being…over multiple generations. It should be noted that the GRx can be broken down into at least three components: the levels needed for healthy pregnancy (conception through birth), early child development, and adulthood.

The Japan Aerospace Exploration Agency (JAXA) has been studying the adult gravity prescription for mitigation of mammalian physiological issues that arise in space due to microgravity using mice for about a decade. To conduct this research, they’ve been using the Multiple Artificial-gravity Research System (MARS) short arm centrifuge in the Kibo module on the International Space Station (ISS). This will help us understand what dosing level of gravity is required to prevent the myriad of health issues (e.g., serious reduction in bone and muscle mass, ocular changes, weakening of the immune system – there are many more) that arise in mature adults when exposed for long periods to microgravity, to inform countermeasures for long-duration spaceflight and settlement.

By way of background, I reported back in 2021 on JAXA’s first long-term mouse study comparing mice reared under microgravity conditions to a cohort raised under 1g artificial gravity in the MARS centrifuge, which found that Earth-normal artificial gravity appears to prevent the negative health effects of microgravity. In the same post, I provided an update upon publication of the results of a second experiment executed a couple of years later, which tested mice in Moon gravity and found that 1/6g prevents muscle atrophy in mice, with the downside that this level of artificial gravity cannot prevent changes in muscle fiber (myofiber) and gene modification induced by microgravity. There appeared to be a threshold between 1/6g and Earth-normal gravity, yet to be determined, for skeletal muscle adaptation. Then in 2023 we got a few more data points helping us zero in on the right dose, at least for the adult GRx, between Moon and Earth levels of gravity.

I mentioned this new preliminary data in my presentation at ISDC 2024 on how the GRx may impact the future of space settlement. The results came from a NASA-funded experiment in cooperation with JAXA, headed by Dr. Mary Bouxsein and collaborators, that studied adult mice launched to the ISS on SpaceX CRS-27. The mice were split into four groups and dosed in the MARS centrifuge at four levels of gravity (microgravity, 0.33g, 0.67g, and 1g). The results were presented at the American Society for Gravitational and Space Research (November 2023). Still, no formal paper had been published at that time, only an abstract from the conference talk. Preliminary results showed that hindquarter muscle strength increased commensurate with the level of artificial gravity, indicating that spaceflight-induced atrophy can be mitigated with artificial gravity – more is better. Now the full study has finally been published in Scientific Advances with further details.

After each of the cohorts were individually treated to the four gravity levels in the MARS centrifuge and returned to Earth, the mice were euthanized and the researchers examined their soleus muscle (the calf muscle which is very sensitive to gravity levels), along with strength tests and blood biomarkers.

Key findings included:

  • Muscle shrinkage (atrophy): In microgravity, the soleus muscle fibers shrank noticeably. At 0.33g, the muscle cross-sectional area was largely preserved—meaning it didn’t lose much mass. Higher gravity (0.67g and 1g) also protected muscle size.
  • Muscle fiber type: Muscles have different fiber types—slow-twitch (good for endurance) and fast-twitch. Microgravity causes slow fibers to switch to fast ones. 0.33g partially prevented this switch, but 0.67g fully stopped it, keeping the muscle composition closer to normal.
  • Muscle function: Strength tests (like grip strength) and electrical measurements showed that 0.67g was enough to maintain overall muscle performance. Lower gravity levels did not fully protect function.

The study also found 11 blood metabolites (small molecules that are intermediate or end products of metabolism) that changed depending on gravity level. These could serve as future biomarkers to monitor astronauts’ health without invasive tests.

Why does this matter? This is the first experiment to identify the adult GRx thresholds for muscle health. It suggests that 0.33g (close to Mars-level) helps prevent muscle weakening, but titrating to 0.67g is needed to fully maintain strength and normal muscle condition. For future space stations or vehicles with rotating sections to create artificial gravity, this information is valuable: partial gravity helps, but at least 0.67g is better for long-term missions. Crucially, these results could inform the spin-rate specifications for a Mars Cycler to generate artificial gravity to maintain muscular function for travelers to and from Mars.

In short, the paper shows that even moderate artificial gravity could mitigate the musculoskeletal problems astronauts face in space, helping keep crews healthier on deep-space journeys. The research used careful controls, multiple measurement methods, and advanced analysis to reach these clear conclusions.

The bigger question is what is the GRx for reproduction? If permanent settlements are to be established in space, they should be (in the long run) at destinations that enable biological self-sustainability, meaning we will want to have healthy children and raise families there as we expand out into the solar system.

One study conducted by Japanese researchers in 2019 aboard the ISS suggested that mouse embryonic growth may be possible in microgravity. But this experiment was only 4 days of embryo development, which took place after conception. Another study of pregnant mice from the shuttle era found serious issues with brain development after exposure to microgravity. These are just snapshots of embryo and later fetus development, and only in microgravity. There still have not been rigorous scientific experiments covering the full mammalian reproductive cycle through all its phases under variable gravity conditions mimicking the Moon, Mars, or higher levels below that of our home planet.

Conceptual AI generated image of an expectant mother in Earth orbit under appropriate GRx dosing conditions after mammalian reproduction has been validated via higher animal models through all stages of pregnancy for a safe level of artificial gravity. An appropriate level of radiation shielding would also be required and is not shown in this illustration. Credit: MS Designer

It is vitally important as the private space stations begin to replace the ISS and space tourism takes off (including maybe even hotels on the Moon), that we understand the risks and implications for having babies off Earth in lower gravity environments. Alex Layendecker, the founder of the nonprofit Advanced SpaceLife Research Institute (ASRI), told me the following in an interview about his Green Paper on Sex in Space: Consideration of uncontrolled human conception in emerging space tourism:

“With mass access to space, we’ll soon have groups of individuals going up solely for vacation/leisure purposes, and you can be assured some of them will be engaging in sexual activity.  While it would be absurd to try to implement or enforce laws preventing sexual activity in those environments, the dangers associated with potential conception still exist.  What is critically needed at this point is a better collective understanding of those dangers, their mitigation, and for space companies to be able to provide those paying customers with enough information that informed consent can be established – space is inherently dangerous already, and people launching into space are briefed on that.”

Besides JAXA and some limited research by NASA, there are just a couple of organizations dedicated to studying the GRx for reproduction. ASRI based at Cape Canaveral and a Dutch company called SpaceBorn United. SpaceBorn has space missions planned in the next few years using their ARTIS (Assisted Reproductive Technology in Space) platform, focused on mammalian conception and early embryo development, with a prototype “space-embryo-incubator” capable of treating samples with adjustable artificial gravity. Initial missions will send this device to space with male and female mouse gametes, where in vitro conception and 5-6 day embryo development will proceed under artificial gravity conditions. The embryos will then be cryogenically frozen and returned to Earth where, if approved, they will be implanted in natural wombs, where the pregnancy will progress on Earth. If successful, subsequent phases over the next couple of decades will focus on increasing stages of pregnancy in space and early embryo development on the Moon.

But SpaceBorn’s platform is a small, automated self-contained centrifugal device operated in microgravity. To really study the GRx for mammalian reproduction, a fully equipped (preferably crewed) variable gravity biolab large enough to rear lower organisms from conception through pregnancy to birth, maturing into adulthood – over multiple generations and progressing successively in higher mammals – would be required. A very tall order! But there are reasonable and well-thought-out concepts for such facilities I’ve explored here previously.

Currently, there are only a couple of companies with rotating space stations in their strategic plans to provide larger-scale artificial gravity laboratories that could be used for reproductive studies. Vast Inc., a well funded, privately held startup, explicitly lists an Artificial Gravity Station targeted for around 2035 in its official product roadmap, a long cylindrical design that will rotate end-over-end at ~3.5 RPM to create artificial gravity for long-term habitation. It would build on their modular (non-rotating) Haven-1 module currently planned for launch in 2027 and the follow-on Haven-2 station which would gradually add units, eventually leading to a 9-module configuration slated for 2032.

Conceptual illustration of Vast Space Haven-2 nine module configuration space station that could be placed in Earth orbit as soon as 2032. Credit: Vast Space

Above Space, with aspirations for rotating wheel type space stations in the 2030s, has significant engineering and funding hurdles, but is the only other player that has plans for variable gravity facilities in space.

In my presentation at ISDC2024 linked earlier in this post, I advocated for determining the GRx for reproduction sooner rather than later, especially given Elon Musk’s timelines for colonizing Mars. As Musk has made clear for years and with his recently revealed SpaceX compensation package tied to it, his vision for the company is to make humanity multiplanetary and build a city on Mars with a million people. Up until recently, he was projecting mid-century to achieve that milestone. His shifting focus from Mars to the Moon aside, does he expect the population of his Martian colony to be composed of just adults? Musk, who has warned of population collapse and low fertility rates here on Earth as one of humanity’s biggest challenges, has been quoted as saying, “People are going to have to revive the idea of having children as a kind of social duty. If you can, and are so inclined, you should. Otherwise civilization will just die.”

Image of Elon Musk holding his son X AE X-II after Grimes gave birth to him in May 2020. Credit: Elon Musk

I’m pretty sure he envisions that the people occupying his city on Mars will want to have children there. And he’d probably want to know the GRx for reproduction well before significant numbers of people began migrating there to stay. A variable gravity research facility in low Earth orbit dedicated to studying mammalian physiology, including reproduction in less than 1g, could be built using SpaceX hardware and would not cost much more than Musk’s pocket change, but I’ve seen no indication that this is a priority for him.

Of course, there are others, affectionately called O’Neillians, who say we know 1g works, so let’s not waste time and just get started building rotating space settlements like Kalpana One, a 500m diameter starter unit located in equatorial low Earth orbit. Shielded by Earth’s magnetic field and therefore requiring less mass for radiation shielding, it would be much easier to accomplish than the enormous miles-long settlements O’Neill envisioned to be placed out at Earth-Moon Lagrange Point L5. The attractiveness of this approach (in LEO) may be waning with the rise of AI and what will likely be a proliferation of orbital data centers in the next few years. Cowboy Space has just filed with the FCC for a literal Stampede of 20,000 satellites adding to the queue of companies seeking regulatory approval for their megaconstellations, including Blue Origin’s Project Sunrise (51,600), Starcloud (88,000), and SpaceX’s 1 million satellites, all in sun-synchronous orbits ranging from 500 to 2000 kilometers. It may be getting pretty crowded up there soon, increasing the risk of collision with large cross-section space stations that are not easily maneuvered out of the way.

Personally, my view has evolved on the urgency for determining the GRx for reproduction on the Moon or Mars. I still hold that healthy human reproduction in lower-gravity environments is highly uncertain, given millions of years of evolution in Earth’s gravity, and carries a substantial risk of complications at every stage of reproduction. Being inspired by O’Neill, I was admittedly biased toward his approach to mitigate these risks and had a preference for spacious artificial gravity worlds. This led me to believe that if we confirmed through studies of the GRx for reproduction that having children in less than 1g could lead to dangerous complications and, therefore, be morally wrong, the information might bend the arc of space development toward free space settlements. Knowing we could not have children on the Moon or Mars might start to change the mindset of many space settlement advocates, whom the great science fiction author Isaac Asimov called planetary chauvinists, leading to diversion of resources toward building the infrastructure needed to construct rotating space settlements like Kalpana. I’m old enough to remember that O’Neill invented one of the key technologies for building these huge structures: a mass driver intended to be placed on the Moon to launch massive amounts of lunar regolith into space so that it could be processed into radiation shielding for these colonies. Advances in robotics, in situ resource utilization (ISRU), in-space assembly, life support systems, and many other technologies are needed to enable O’Neill colonies. Many of them are proceeding at pace anyway, but I thought that if we found the GRx for reproduction was not less than 1g, it would put a sense of urgency around these efforts.

However, it looks like market forces may be changing all this. I did not anticipate Elon Musk advocating for mass drivers on the Moon. Though he says they would be used for launching SpaceX’s AI satellites, the infrastructure for ISRU and electromagnetic launchers looks like it may be coming to Luna sooner than we expected and can certainly be retooled to fling regolith into space for shielding rotating space settlements, even larger ones, if the drive (and markets) are there.

But rather than the GRx for reproduction being the determining factor for where we settle space – planetary surfaces or free space colonies – a split life cycle may be the solution. This approach, credited to Kelly and Matt Weinersmith in their book A City on Mars, suggests using both destinations! As clarified in a presentation at ISDC2024 by Dale Skran, Chief Operating Officer & Senior Vice President of the National Space Society, this new way of thinking (not an official position of NSS) acknowledges that we probably need Earth normal gravity for having children and suggests that a rotating space station birthing center be placed in orbit above surface colonies on the Moon or Mars. Couples would conceive, bear children, and raise them in a healthy artificial gravity crèche, allowing them to mature to early adulthood, after which families or individuals may choose to remain in space or relocate to the low-gravity surface communities below.

So research by Dr. Bouxsein et al. may be homing in on the adult GRx, with the results from their study paving the way for practical countermeasures against health issues adult astronauts will face in microgravity on long-duration missions in space. But the artificial gravity dose level for reproduction needs much further study before we can safely say biologically self-sustaining space settlements in less than 1g will be possible. Until then (and after, if the answer is no), split life cycle communities may be the strategy for an expanding population migrating out into the solar system and beyond.

Artist’s impression of the interior of Kalpana One, a cylindrical rotating colony providing Earth normal artificial gravity for healthy living in space. With the coming proliferation of AI orbital data centers later this decade, this approach may lose favor in LEO, but could be one possible in-space component of a split life cycle solution for settlement of the Moon or Mars. Credits: Bryan Versteeg / spacehabs.com

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Split life cycle approach to settling the solar system

Left: Artist impression of the inside of Kalpana One, a free space settlement providing artificial gravity. Credits: Bryan Veerseeg / Spacehabs.com; Right: Conceptual illustration of a colony on the surface of Mars. Credits: SpaceX.

Until recently, space settlement advocates have typically split into two camps: those who favor building colonies on the surfaces of the Moon or Mars, and those who prefer constructing O’Neill cylinders in free space, spinning to provide artificial gravity outside of planetary gravity wells. Readers of this blog know I lean toward the latter, mainly because colonies on worlds with gravity lower than Earth’s could pose problems for human physiology, particularly reproduction. Truthfully, we won’t know if humans can reproduce in less than 1g until we conduct long-term mammalian reproduction experiments under those conditions. It would be far cheaper and quicker to perform these experiments in Low Earth Orbit (LEO) rather than waiting for sufficient infrastructure to be established on the Moon or Mars for biological research.

Another approach involves not sending humans into space at all, instead entrusting space colonization to human-level artificial general intelligence (HL-AGI) and conscious machines—a non-biological strategy. With recent advancements in AGI and automation, conscious HL-AGI robots may become feasible in the near future (though the exact timeline—whether decades or longer—remains a matter of debate). This prospect might disappoint many space advocates who view migration beyond Earth as the next phase of natural biological evolution hopefully starting within our lifetimes. Deploying sentient machines would effectively remove humanity from the equation altogether.

If you’ve been following space colonization in the press you’ve most likely heard of the book A City on Mars by Kelly and Matt Weinersmith. I have not purchased the book but I’ve read several reviews and heard the authors interviewed by Dr. David Livingston on The Space Show to get an understanding of the Wienersmith’s overall viewpoint, which is at the very least skeptical, and to some space advocates downright anti-settlement. The book is very pessimistic taking the position that the science and engineering of space settlements for large populations of people is too challenging to be realized in the near future.

Peter Hague, an astrophysicist in the UK, wrote an excellent three part review setting the record straight correcting some of the critical facts that the Wienersmith’s get wrong. But in my opinion the best critique by far was written by Dale Skran, Chief Operating Officer & Senior Vice President of the National Space Society (NSS). In a recent post on the NSS blog, he links to a 90 page Critique of “A City on Mars” and Other Writings Opposing Space Settlement in the Space Settlement Journal where he provides a chapter-by-chapter, section-by-section response to the entire book as well as rebuttals to a few other naysayer publications [“Dark Skies” (2021) by Daniel Deudney; “Why We’ll Never Live in Space” (2023) in Scientific American by Sarah Scholes; “The Case against Space” (1997) by Gary Westfahl].

However, Skran credits the Weinersmiths with an innovative idea he hadn’t encountered before, one that addresses the challenge of human reproduction in low gravity. They suggest establishing orbital spin-gravity birthing centers above surface colonies on the Moon or Mars, where children would be born and raised in an artificial gravity environment—essentially a cosmic crèche. Skran builds on this concept, proposing that the life cycle of Moon or Mars colonists could be divided into phases. The first phase would take place in space, aboard rotating settlements with Earth-normal gravity, where couples would conceive, bear children, and raise them to a level of physical maturity—likely early adulthood—determined by prior research. Afterward, some individuals might opt to relocate to the low-gravity surfaces of these worlds. There, surface settlements would focus on various activities, including operations to extract and process resources for building additional settlements.

Skran elaborated on this split life cycle concept and outlined a roadmap for implementing it to settle low-gravity worlds across the solar system during a presentation at the 2024 International Space Development Conference. He granted me permission to share his vision from that presentation and emphasized that the opinions expressed in his talk were his own and did not reflect an official position or statement from the NSS.

Taking a step back, the presentation summarized research that has been performed to date on mammalian physiology in lower gravity, e.g. studies SSP covered previously on mice by JAXA aboard the ISS in microgravity and in the Kibo centrifuge at 1/6g Moon levels. The bottom line is that studies show some level of gravity less then 1g (artificial or otherwise) may be beneficial to a certain degree but microgravity is a horrible show stopper and much more research is needed in lower gravity on the entire reproduction process, from conception through gestation, birth and early organism development to adulthood. The question of reproduction in less then 1g is the elephant in the space station living room. In my presentation at ISDC last year, I took the position that the artificial gravity prescription for reproduction could impact the long term strategy for where to establish biologically self-sustaining space settlements leading to a fork in the road: a choice between O’Neill’s vision of free space rotating settlements vs. lower gravity surface colonies (because outside of the Earth all other solar system worlds where it is practical to establish surface settlements have less then 1g – e.g. the Moon, Mars, Asteroids and the moons of the outer planets – I exclude cloud settlements in Venus’s atmosphere as not realistic). I’ve been swayed by Skran’s proposal and have come to the realization that we don’t need to be faced with a choice between surface settlements or free space artificial gravity habitats – we can and should do both with this split life cycle approach.

How would Skran’s plan for settling the solar system work? He suggests we start small with rotating space settlements in LEO like Kalpana Two, an approach first conceived by Al Globus and popularized in his book coauthored by Tom Marotta The High Frontier: an Easier Way. Locating the habitats in LEO leverages the Earth’s protective magnetic field, shielding the occupants from radiation caused by solar particle events. This significantly reduces their mass and therefore costs because heavy radiation shielding does not need to be launched into orbit. In addition, the smaller size simplifies construction and enables an incremental approach. Kasper Kubica came up with a real estate marketing plan for Kalpana in his Spacelife Direct scenario.

Skran promoted a different design which won the Grand Prize of the NSS O’Neill Space Settlement Contest, Project Nova 2. The novel space station, conceived by a team of high school students at Tudor Vianu National High School of Computer Science, Bucharest Romania, slightly resembles Space Station V from the film 2001: A Space Odyssey. Many other designs are possible.

Project Nova 2 rotating space settlement, one possible design of a rotating space settlement initially built in LEO then moved out to the Moon and beyond. Credit: Tudor Vianu National High School Research Centre Team / NSS O’Neill Space Settlement Contest 2024 Grand Prize Winner

But to get there from here, we have to start even smaller and begin to understand the physics of spin gravity in space. To get things rolling Kasper Kupica has priced out Platform 0, a $16M minimum viable product artificial gravity facility that could be an early starting point for basic research.

Conceptual illustration of Platform 0, a habitable artificial gravity minimum viable product. Credits: Platform 0 – Kasper Kubica / Earth image – Inspiration4

These designs for space habitats will evolve from efforts already underway by private space station companies like Vast, Above, Axiom Space, Blue Origin (with partner Sierra Space) and others. Vast, which has for years had AG space stations on its product roadmap, recently revealed plans to use its orbital space station Haven-1 to be launched in 2026 to study 1/6g Moon level AG in a few years, albeit without crew. And of course let’s not forget last month’s post which featured near term tests proposed by Joe Carroll that could be carried out now using a SpaceX Falcon 9 as an orbital laboratory where researchers could study human adaptation to AG.

Illustration depicting a SpaceX Crew Dragon spacecraft tethered to a Falcon 9 second stage which could be spun up (in direction of down arrow) to test centrifugal force artificial gravity. Credit: Joe Carroll

Back the plan – once the rotating space habitat technology has been proven in LEO, a second and third settlement would be built near the Moon where lunar materials can be utilized to add radiation shielding needed for deep space. The first of these facilities becomes a factory to build more settlements. The second one becomes a cycler, the brilliant idea invented by Buzz Aldrin, initially cycling back and forth in the Earth Moon system providing transportation in the burgeoning cislunar economy just around the corner. The next step would be to fabricate three more copies of the final design. Two would be designated as cyclers between the Earth and Mars. Building at least two makes sense to establish an interplanetary railroad that provides transportation back and forth on a more frequent basis then just building one unit.

Here’s the crown jewel: the third settlement will remain in orbit around Mars as an Earth normal gravity crèche, providing birthing centers and early child development for families settling in the region. Colonists can choose to split their lives between rearing their young in healthy 1g habitats until their offspring are young adults then moving down to live out their lives in settlements on the surface of Mars – or they may choose to live permanently in free space.

This approach enhances the likelihood that settlements on the Moon or Mars will succeed. The presence of an orbiting crèche significantly reduces the risks associated with establishing surface communities by providing an orbital station that can support ground settlements and offer a 1g safe haven to where colonists can retreat if something goes wrong. This alleviates the pressure on initial small crews on the surface, meaning they wouldn’t have to rely solely on themselves to ensure their survival. Finally, an incremental strategy, involving a series of gradual steps with technology readiness proven at each stage through increasingly larger iterations of orbital settlements, offers a greater chance of success.

The final step in this vision for humanity to become a truly spacefaring civilization is to rinse and repeat, i.e. cookie cutter duplication and dispersal of these space stations far and wide to the many worlds beyond Mars with abundant resources and settlement potential. There’s no need to choose between strategies focused solely on surface communities versus spin-gravity colonies in free space. We can pursue both, as they will complement each other, providing families with split life cycle settlement options to have and raise healthy children while tapping the vast resources of the solar system.

Images of resource rich lower gravity worlds beyond Mars with potential for split life cycle settlement (not to scale). Top: the asteroid Ceres. Middle: Jupiter’s Moons, from left to right, Io, Europa, Ganymede, and Callisto. Bottom left: Saturn’s moon Titan. Bottom right: Neptune’s moon Triton. Credits: NASA.
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The case for free space settlements if the Gravity Rx = 1g

Cutaway view of interior of Kalpana One, an orbital settlement spinning to produce 1G of artificial gravity. Credits: © Bryan Versteeg, Spacehabs.com / via NSS

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 scaled down version of O’Neill’s concept called Kalpana One.  But they go even smaller with Kalpana Two, a cylinder roughly 100 meters in diameter and the same in length, spinning at 4 rpm to create 1g of artificial gravity. The facility would be 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.

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Coming soon: the $10M orbital condominium

Living space in a Kalpana orbital space settlement. Credits: Bryan Versteeg

Kasper Kubica presents an optimistic business case for space tenants moving in (er, up) to deluxe condominiums orbiting the Earth within 10 years. Initially for the ultra rich, the price tag is comparable to high end real estate currently on the market. Of course the devil is in the details, so lets dive in.

In a post on Medium, Kubica uses the rotating habitat Kalpana as an illustrative example of his “Spacelife Direct” approach for an orbital settlement spinning to create 1G of artificial gravity and hosting north of 400 condominiums in LEO. Such a facility would be shielded from radiation by Earth’s magnetosphere if it were located in low equatorial orbit and therefore could be constructed with less shielding. This results in a significant reduction of mass driving costs way down. Running the numbers on this scenario opens up exciting possibilities with the amazing capabilities of Elon Musk’s Starship currently under development by SpaceX.

Using the scaled down Kalpana Two version as discussed in Tom Marotta and Al Globus’ book, The High Frontier, an Easier Way, the cylindrical habitat is sized at just over 100 meters in diameter and the same in length, weighing in at 16, 800 metric tons. Kubica estimates that it would take 140 launches to loft the required mass to LEO. Assuming costs keep coming down as Starship launch cadence increases (a safe bet), at $10M/launch the cost of just the materials to LEO would be $1.4B. Of course there are many more expenses associated with design, development and fabrication, not to mention insurance of such an orbital condo complex. For the sake of argument Kubica triples that figure arriving at a total price tag of $4.2B.

But would there be a market for real estate in LEO? Kubica provides comparable examples of skyscrapers with similar costs and over 200 condominiums recently selling for over $10M in Manhattan.

“The clamor for earthside luxury condos is massive and growing. Orbital condos — representing an exclusive experience far beyond that available to anyone on earth — could generate astronomical demand.”

With the economics of Starship opening up limitless possibilities, Kubica lays out a roadmap over the next 10 years to realize the Spacelife Direct opportunity. First would come financing the venture though a team of visionary entrepreneurs and investors (are you listening Dylan Taylor?). Design and development would come next including the robotic systems that would be required for assembly in space. Laying the groundwork for this infrastructure may be completed soon by Orbital Assembly Corporation which could potentially be leveraged as a Spacelife Direct supplier. To keep labor costs down much of the facility would be fabricated on Earth in launchable modules that would be assembled in orbit. The final stages would activate life support systems and finish out the interiors for the occupants to begin moving in.

So what about the rest of us? As history has shown in the aerospace industry at the beginning of the last century and we see unfolding in the space tourism market today, the rich help pave the way so that mass production and economies of scale will drive down costs eventually making space settlement affordable for the masses.

“We don’t want to live in space because it’s an economic necessity, we want to live in space because we are explorers and adventurers, and space is humanity’s next frontier!”