What do we need astronauts for?by Joe Carroll
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| We can only settle the Moon or Mars if they have enough gravity for good long-term human health. And we don’t know if lunar or Martian gravity are enough! |
Astronauts lost that contest decades ago. And since then, robotic spacecraft have continued to evolve far faster than humans. Artificial intelligence should further speed that evolution. We will make faster progress in exploring space if we move our human space exploration funding to a wider range of uncrewed missions. Don’t redo Apollo.
If space exploration doesn’t need astronauts, then what else might they be useful for?
Since Apollo, their main tasks have been living, working, and testing microgravity effects for longer and longer times, in low Earth orbit. I suggest a challenging extension of those tasks: Identify realistic paths for human settlement beyond Earth.
This task inherently needs astronauts, because we can only settle the Moon or Mars if they have enough gravity for good long-term human health. And we don’t know if lunar or Martian gravity are enough! Besides testing health in lunar and Martian gravity, crews can tackle other settlement challenges, like reducing dependence on Earth. This can make settlements more realistic, by cutting costs and risks.
Human health as a constraint on settlement
Surface gravities in this solar system between 0.09 and 2.3g all fit into three fairly tight clusters, near Moon, Mars, or Earth gravity. Venus, Saturn, Uranus, and Neptune each have within 12% of 1g. But all four pose many serious challenges, including extreme temperatures and pressures. And all but Venus have escape velocities of 21 to 35 kilometers per second. (Forget about ever leaving.) The only other bodies with surface gravities between 0.09 and 2.3g are Mars and Mercury, each with 0.38g, and the six largest moons, each of which has 0.13 to 0.18g.
These numbers matter, because if lunar gravity isn’t high enough to allow good life-long health, forget about settling any moon. And if Martian gravity isn’t high enough, then we must master the challenges of either settling Venus, Saturn, Uranus, or Neptune, or developing spinning free-space settlements that provide enough gravity.
All that we know now is that we have normal health in 1g, but health deteriorates in microgravity, despite decades of refining “countermeasure” diets, medications, and exercise (now about two hours a day). And as more crews stay 6 to 12 months in microgravity, we keep finding new health problems!
Hence human health, specifically in lunar or Martian gravity, is a key constraint on space settlement destinations and types. Space settlers will need good health for the full human lifecycle. We have no data on human health in sustained low gravity. We should be healthier in 0.13 to 0.38g than in microgravity, but we don’t know if degradation of health will stop or just slow down, what microgravity health issues will persist, what countermeasures we will still need or how well they will work and with what side effects, or how well we might tolerate 1g later—or even if we will grow up normally, from conception to adulthood.
I know of no way to determine the health impacts of sustained low gravity without people actually living in sustained low gravity. One option is to keep crews on the Moon and Mars long enough (about one year and then longer?) But another option seems far better and cheaper.
It’s time for artificial gravity (“AG”)
A spinning dumbbell with 7:3 mass ratios can give artificial gravity at both lunar and Martian gravity levels, in LEO. Testing at both levels allows sharing of staff and analysis equipment. Adding a microgravity node even allows tests of commuting from AG to microgravity. (for more details on this concept, see “How to clarify human futures beyond Earth,” The Space Review, December 6, 2021.)
| As we go further from Earth, minimizing Earth supplies becomes a key part of making a profit. It’s not the full story, but it’s a key part of half of the full story. |
Most proposed AG designs use fast spins (3 to 10 rpm) to reduce facility size and cost. These rates are based on reactions of people in rotating rooms. Many people have negative initial reactions even at 3 rpm, but felt “spin effects” in rotating rooms stay consistent when you turn around. So, you adapt to them. But in AG, the felt effects reverse direction every time you turn around. We won’t know how we will adapt to that until we do relevant tests. My companion article describes how we can do AG spin tests on Crew Dragon.
But longer radii do reduce spin rates and their effects. This argues for the longest practical radius, not the shortest tolerable one. A dumbbell shape gives the longest affordable radius and slowest spin.
Monitoring crew health and even lab-animal health and life cycles need not take much of the crew’s time. They can also take on other settlement challenges, like recycling, growing food, and hosting tourism as a step towards settlements. I discuss these tasks below.
Recycling as a settler technology
On the ISS, food is less than half the total crew supplies. That may also occur in settlements. Settlers must greatly reduce dependence on all external supplies. Recycling is not as sexy as mining celestial materials, but it may be far easier to recycle the midden (which has the right mix of all elements you need) than to find, refine, transport, and process celestial materials into forms as useful as the midden.
We need to recycle much more even on Earth, but recycling will be far more critical for living affordably in space. We can test and refine automated recycling faster and cheaper in LEO than beyond LEO.
But space settlers will need some Earth-based supplies for a long time, and must trade things of enough value to cover all delivery costs. Details may vary wildly from site to site. In contrast, recycling will have far more commonality over all sites, even including Earth.
But while recycling may sustain a settlement, it cannot expand it. Traffic can let it grow. In the last 20 years, the total mass of cargo vehicles that departed ISS and then burned up is over twice the 420-tonne mass of ISS. Most of it was aerospace alloys. And even most of the rest would be considered a high-grade ore, if we found it near a Moon or Mars settlement.
This focus on recycling may be jarring to those who envision paying for supplies from Earth, out of profits from selling water or helium-3. But as we go further from Earth, minimizing Earth supplies becomes a key part of making a profit. It’s not the full story, but it’s a key part of half of the full story. Recycling is a critical settler technology.
Growing food
We recycle most of the air and water on the ISS. But food from ISS plant growth tests is probably less than 0.1% of the crew diet. All other food comes from Earth. Settlers will also need Earth-supplied food at first. But each settler will need about half a kilogram per day, even freeze-dried. That is about twice the settler mass per year. This will drive settlers to grow as much of their food as possible.
| LEO should cost about a tenth as much as on the Moon or Mars for testing human health in low gravity, developing settler technologies, and developing commercial space resorts. |
The many issues found in testing Biosphere 2 suggest that we should do more iterations on Earth, perhaps at Biosphere 2. Then we can do tests in LEO, at both lunar and Martian levels of AG. The task is not just growing crops, but also managing a microbiome, worms, pests, and more, all in new gravity levels. Doing these tests in LEO will cost far less than on the Moon or Mars. And AG spin in LEO lets us sling sample capsules back to Earth for more analysis. It will make sense to try to settle beyond LEO only after we close most of the supply loops.
LEO tourism as a step towards space settlements
If an AG facility spins slowly enough to prevent unpleasant spin effects, space tourists may prefer it to a microgravity-only facility, because AG offers experiences at Moon and Mars gravity as well as microgravity. Tourists may be attracted more to microgravity, but may spend more time in lunar and Martian gravity, especially if that assists the awkward transition to microgravity. An AG facility with serious health diagnostics and staff for long-term crew can also provide useful medical support for space tourists and can even assess the health implications of low gravity on the elderly.
A spinning resort does complicate views of Earth. But a clear dome on the spin axis allows a de-spun view in microgravity. And de-spun cameras can support hi-res steerable zoomable displays all over the resort.
An AG resort cannot host true microgravity-class lab or fab operations, but can support them indirectly. Traffic to and from an AG resort can reduce times and costs for delivering supplies and technicians to and from a co-orbiting microgravity facility. A local taxi can transfer technicians and cargo between resort and microgravity facility, for service and swapping out supplies.
Graphics of spinning cylindrical settlements with open interiors look appealing, but waste usable space. Some inboard volume can be used for farming and recycling, and some can host low-gravity activities. In AG facilities, an elevator allows easy access to a range of gravity levels. Real gravity can’t offer that. That may be a key attraction in AG settlements, and perhaps even more so in AG resorts in LEO.
LEO remains the place to beat (for now)
LEO should cost about a tenth as much as on the Moon or Mars for testing human health in low gravity, developing settler technologies, and developing commercial space resorts. This is based just on costs of delivering mass to and from LEO versus the Moon and Mars. Simpler LEO crew vehicles further reduce costs, and we can even share them and technicians and equipment with both ends of one AG facility.
Modified Starship stages may launch only once, without fins or TPS, but with an integrated payload. They can be used earlier, more often, and more productively in LEO than on the Moon or Mars.
LEO also allows quicker return to earth in emergency, and can allow much lower radiation doses, particularly in low inclination orbits. This is very relevant if some people plan to stay more than a year.
Beyond LEO
Even on celestial bodies, keeping a space settlement working will depend more on recycling than mining. And recycling is common to all space settlements. What will differ from site to site is the business that pays for continued dependence on Earth or other settlements.
The criticality and complexity of life support and recycling suggest that all free-space AG settlements be checked out in LEO, before going elsewhere. One can boost a spinning dumbbell to escape by thrusting parallel to the spin axis from the heavier end. Thrust changes a flat spin into a conical spin, somewhat like a tether ball.
Trips to the Moon take just a few days. But it may be useful to pair vehicles together in LEO, to test critical hardware at Moon-level AG.
Trips to Mars will require vehicles to enter and land separately. But during a roughtly six-month passenger transit, it seems useful to tether them to provide Mars-level AG with an acceptably slow spin rate.
We may start with million-tonne-class NEOs, which are about 100 meters in diameter. They are bright enough to see, but there are enough of them (about 100,000) to be useful. Some will occasionally allow low-ΔV access. We can send unmanned spacecraft to survey candidates. Later we can send “settlement seeds” to convert suitable NEOs into settlements.
Some theses for discussion
Let me digest the above discussion into seven theses:
- Surface gravities near those of Moon, Mars, and Earth are the only ones available for human settlement in our solar system.
- A key unknown in where and how we can settle beyond Earth is our health in Moon and Mars gravity, from conception to death.
- It is faster, better, safer, and cheaper to test low-g human health and settler technologies in AG in LEO, than on the Moon and Mars.
- We should shift US human space exploration funds to robotic space exploration plus human health tests in low gravity in LEO.
- Settlers will reduce dependence on supplies from Earth more by recycling, than by mining and processing celestial resources.
- It is faster, better, safer, and cheaper to reduce dependence on Earth supplies by developing recycling in LEO, not beyond LEO.
- Artificial gravity may be more appealing than real gravity, since it allows easy “elevator access” to a range of gravity levels.
Now let me add four more theses, without elaboration:
- We want a long slow-spinning dumbbell-shaped crew facility, to minimize all negative AG spin effects on human health and comfort.
- What we learn about human health in lunar and Martian gravity will significantly improve future exploration and settlement plans.
- It will be much more complex and costly to keep people alive and supplied on Mars than to transport them to Mars.
- Settling all of Mars just doubles our settleable land. Free-space settlements can vastly exceed that. And we can even start in LEO.
A scenario to consider
If you accept many of the above theses, you may be interested in this sequence of six initial steps:
- Find crew responses to a range of AG spin rates by spin tests in Crew Dragons, as described in my companion article.
- Test the sustained health of crews in Moon and Mars gravity in LEO, as I proposed in an earlier article.
- Do more AG tests in LEO, at Moon, Mars, and even Earth gravity: longer crew stays, primate life cycles, crop growth, and recycling.
- Expand that facility into a microgravity-Moon-Mars-Earth AG LEO resort, with a long-term staff large enough to support resort operations.
- Gradually grow facility capabilities, sustainability, and economic viability, as a test facility, resort, and embryonic settlement.
- Use all data on health, sustainability, economics, etc., to refine plans for larger space settlements, both in and beyond LEO.
I doubt that anyone can give good estimates of the cost, schedule, or chance of completing such a sequence. But it is a serious alternative to “human space exploration” that can do far more to clarify realistic human futures beyond Earth. It may also allow faster and broader commercialization of human space, and earlier and broader work on a range of space settler technologies.
If is this is not the best new role for astronauts, what may be better?
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