Safe Two Hundred Person L5 Space Station

Pekka Janhunen, who designed the electric sail, has proposed a 200 person L5 space station that would have safe long term radiation shielding and rotate for one G of gravity.

The proposed station would weigh 89000 tonnes and provides 60 square meters (646 square feet) of floorspace per person. The radiation shield is made of asteroid rock, augmented by a water layer with 2 % of the mass for neutron moderation, and a thin boron-10 layer for capturing the thermalized neutrons.

Nextbigfuture notes that just bundling 12 Starships in place of each sphere would have a similar volume. The old proposals for using Space Shuttle Fuel Tanks again show how a Starship shortcut for minimal construction could be done. SpaceX Starships are made out of steel, so welding and cutting the steel could be manageable. The outer Starships could be filled with supplies, water and fuel which would make it easy to create the radiation shielding.

Al Globus and Strout used the OLTARIS tool to simulate the GCR (Galactic cosmic rays ) equivalent radiation doses (millisievertrates) behind various thicknesses of different materials. They recommend 20mSv/year as the equivalent dose level during the solar minimum when the galactic radiation is at maximum.

20 mSv/year shield using 2 % of water.
Material   Thickness   kg/m2          Role   
Iron         2.3 cm    180         Structural wall
Dry regolith 3.34 m    8683        Shield
Water        17.7 cm    177        Neutron moderator
Boron-10      0.4 mm    1          Neutron absorber
Total         3.5 m     9041

Globus and Strout also require that the absorbed dose for pregnant women be less than 6.6 mGy/year, or 5 mGy per pregnancy. This condition is satisfied since the absorbed dose at the center of the sphere during solar minimum is 4.0 mGy/year.

If SpaceX Starship reaches $30/kg, the first 200-person settlement could probably be built without space manufacturing. Each settler might need initial capital of∼$21M. Space Manufacturing and space resources can reduce the cost of the station by a factor of four.

SOURCES- Arxiv Pekka Janhunen, SpaceX Starship alternive by Brian Wang
Summary Written by Brian Wang,

34 thoughts on “Safe Two Hundred Person L5 Space Station”

  1. Your floors would only have to be as stiff as similar sized floors on Earth. After all it’s the same level of gravity.
    And as pointed out, it isn’t any bigger than an medium commercial building, so the floors aren’t anything special.

    Except the degree of curvature would be visible given the planned quite small radius of the entire structure.

  2. Yes, I touched on that, but strictly speaking, they’d more likely be shape of a catenary (https:\en.wikipedia.orgwikiCatenary). (repl with slash) But close. And more parabolic should there be compressional stiffness also in the floor structures.  

    However, while you might think you’d want them to be flexible, moving masses about the floors would — like the famous ‘if gravity were an elastic sheet’ animations have shown — have dimples proportionate to the mass and distance from an edge, that’d move with.  

    Objects with wheels would tend to be ‘attracted to each other’. How bizarre!

    Yet another problem to solve.  Maybe the solution is to make the floors quite stiff and ‘suspend them’ from the ceiling, which in this case would be the closest-to-hub wall. Lots of internal tethers. Keep it stiff!  

    Whew… exhausting.
    ⋅-=≡ GoatGuy ✓ ≡=-⋅

  3. What? … the tether. the hub… the centripetal balance of masses … arranging for a sun-facing solar panel that doesn’t spin … and finding a way to get its power to the hubs and spokes in some klever way. … and having to deal with centripetal tension on the starships. And how to move between them. And scheduling who moves where, when. And where to put the kitchen?

  4. Yah, I suppose so.  

    The moving-internal-mass-causing-rotational-precession problem isn’t really that bad in the context of 90 kilotons of total mass, with one must assume, most of it ‘out there’ in pod-on-tether-land.  

    Having a central hub (I see ‘ideally like the pods’), but with substantial structural strength, would go a long way to ‘fixing up’ the mass-moment issue.  

    Spokes not directly normal to the central axis, but more tangential off the hub (kind of like spokes on a bicycle wheel) would very likely do quite nicely in transferring momentum moment changes to the other pod(s) hooked up to the hub.  

    The problem is ‘size’, and trying to maintain that 1 G.

    If 1 km DIAMETER is chosen as the optimum compromise, and 0.5 G as the target centripetal ‘gravity’, then:

    F = ω²r
    ½ 9.81 = ω² × ½ 1,000 m
    4.40 ÷ 500 = ω²
    ω = 0.0938 radian/s
    2π/ω = 67 sec/revolution

    Moving 20 tons out of 90,000 with a hub of (r = 15 m), … calc … calc … the critical spoke off-center separation need only be 0.7 m without overstretching the outer tethers. 

    Those 30 m separated tether lines would be able to handle in excess of 50× that amount of mass-movement.  

    The real gotcha would be ‘rising (as in anti-centripetal direction)’ against ‘g’ to comport to the central hub. Not just moving 30 m. A half klik, or 16× as much. So, 16× the momentum transfer per leg.

    ⋅-=≡ GoatGuy ✓ ≡=-⋅

  5. DoctorPat has a point.

    volume = ⁴⁄₃πr³
    r = ½d
    d = 33 m

    volume = 14,130 m³

    Which ain’t 150,000 m³. Even if I multiply by 2, for 2 of ’em.
    Y’know what I mean?

    So, let’s say (as we should) that the 2 pods aren’t spherical.
    Cylindrical, yes.  Spherical, and the math doesn’t work. 

    At (r = 15 m), needing 75,000 m³ per, remembering the formula:

    V = ⁴⁄₃πr³ + Lπr² — working backwards
    75,000 = ⁴⁄₃ × 3.14159 × 15³ + L × 3.14159 × 15²
    75,000 = 14,137 + 707 L
    L = 86 m

    Well, there we are, I guess. ( L + 2r = 116 m ) long all told, with an 86 meter cylindrical section. Doesn’t seem particularily outrageous.  

    A ‘thing’ that’d be nice (given my other comments re: centripetal precession), would be to have a 15 m radius “at the center”, of more-or-less the same type as the pods out on the 3 km tethers.  

    For safety, many tethers to each pod, preferable. Dozens. So, the ((n – 1) / n) resilience problem is well addressed.

    With enough, even ((n – 2) / n) or ((n – 3) / n) isn’t really that bad.  
    ‘n’ needs to be 20+ though.  

    Multiple tethers addresses the moving-mass rotational moment problem by coupling momentum changes to the central hub. And it in turn transfers those to the other pods attached.  

    Its workable, just not obvious how to do it in practice.

    ⋅-=≡ GoatGuy ✓ ≡=-⋅

  6. Wait! That are two “spheres”?? Thought it would be a torus.

    How will the floor be shaped? Like lenses? Because you need each point of the floor to be perpendicular to the center of rotation!

  7. “Freedom” isn’t a place & even if you define it as such, Mars is no different than the Moon, an asteriod or space. The same applies to a “crucible for the development of advanced technology”, except that the costs for that development are exorbitant. Tourism is a possibility, but still, a very expensive one.
    Why go back down the gravity well? Space has more easily discoverable & accessible resources, 5 times the solar power at Earth’s orbit & any gravity you want.

  8. You don’t HAVE to re-enter when you are sick of the wall colour.

    You can attach an ion drive and move it up to a higher orbit. Sure, it now gets irradiated but if everyone has moved out already (the bathroom decore is SO 2030. I mean rotary taps! It is literally like the stone age…)

  9. Dunno – the mass of 200 humans would only be on the order of 20 tons, vs the whole station being around 89000 tons, and the majority of movements inside would tend to cancel out so long as you don’t encourage people to march around in synch or something odd like that.
    Some motion damping would probably be a good idea though.

  10. One nice thing about sliderules – they gave you a gut-level understanding that your results are at best approximate.

    Every modern digital calculator should calculate an error range for every calculation it does.

  11. It’s still a pretty big object – so space junk might actually be a problem, meaning you might need some shielding anyhow.

  12. Well, it’d have a pretty limited clientel, since a single vacation to the moon-casino would probably cost at least $200K/person. I guess they could hype it up by changing the house odds so nearly everyone goes home with an average of $10K in ‘winnings’, to fit the pyramid-scheme theme…
    And of course, that’s on the moon, where there’s abundant cheap construction material and shielding mass, not spinning at L5 where everything has to be imported. A construction pyramid scheme might last an extra ‘level’ there.

  13. Another of the problems is “centripetal precession” with masses (AKA “people and their junk”) moving about.  Consider a figure ice-skater. He’s skating along, and decides to do a pirouette (spin).  Enters the spin, then pulls in his arms and leg, increasing the spin-rate markedly.  3× to 1, often.  

    Every time ANY mass moves aboard, on ANY floor, the movement induces centripetal counter-force against the whole structure. If both polar living blobs are directly opposite each other, spinning in synchrony, taking the stairs upward (or downward) will impart Δv to the rotational rate of the blob.  

    IF the blobs are tethered, the flexibility of the cable won’t immediately transfer that Δv to the other blob. Yack!  

    The probably-obvious answer is to have a system of tanks, pumps and valves, along with optical distance sensors to convey to both (or all) of the blobs relative position.  If ‘our blob’ is moving toward the others, then ‘we’ take action by pumping water hubward or away, to counteract.  

    It probably makes sense to also have some co-spinning reference beacons on completely separate tethers, that don’t ‘suffer’ the precession problem. This way, all living blobs can sync to a highly stable physical reference. 

    ⋅-=≡ GoatGuy ✓ ≡=-⋅

  14. Wow… I find myself singularly unimpressed. It is like 1961 all over again, Science Fiction wise. At least back then there were enough slipstick savants (slide-rule calculating hacks) with decent Physics background, to flesh out such proposals past what a drawing in some open source graphics program can do.  

    № 1 — synthetic gravity — is ω²⋅r
    № 2 — oblate spheroid — is optimal cross section, not cylindrical or spherical
    № 3 — ‘floors’ ‘hang in tension’ 
    № 4 — shielding doesn’t follow № 2 closely.
    № 5 — scaling?
    № 6 — failures, fires, floods, famine?

    Fc (centripetal ‘gravity’ force) = ω²⋅r, varies linearly with ‘r’ from center-of-mass:

    9.81 = ω² (rmin + 33);
    ω = √( 9.81 / (rmin + 33) );

    What to guess? rmin = 100 meters?  

    ω = √( 9.81 ÷ 133 );
    ω = 0.272 radian/sec, 
    ω = 0.0432 rps
    1/ω = 23 sec per rotation

    Building on the international space hub in “2001 Space Odyssey”, a rotation rate of once every 2 minutes, or 120 seconds is better.  

    1/ω = 120 sec
    ω = 0.008333 rps;
    ω = 0.0524 radian/sec

    9.81 = ω² ( rmin + 33 )
    9.81 ÷ 0.0524² = ( rmin + 33 )
    3,575 m = rmin + 33

    Obviously the “33” is of little consequence.  3.5 kilometers. One LONG tether, 2r’s or 7+ kms worth.

    Nice thing is, if we ignore the mass of the tether line(s), the strength and cross-section of tether is a constant for any particular blob-at-the-end living space.  

    Anyway, math.
    Sucks to be old enough to remember depending on that slipstick.

    ⋅-=≡ GoatGuy ✓ ≡=-⋅

  15. On Earth, Ecuador can become a real state hot spot in the late XXIth century for this very reason.

    Along with Colombia, Brazil, Sao Tome and Principe, Gabon, Republic of the Congo, Democratic Republic of the Congo, Uganda, Kenya, Somalia, Maldives, Indonesia and Kiribati…

    But on the Moon, there is only one sub-planetary point that takes you directly to L1 indeed.

  16. Casino developers could build a giant pyramid-shaped casino on the Moon. They already built one on Earth, so it is not totally crazy. Revenue would be the usual for a casino- gambling, hookers, and hotel services. This would be a literal pyramid scheme 🙂

  17. Metallic asteroids are already a decent iron alloy (90% Fe, 9% Ni, 1% Co). If you add a little carbon from the Carbonaceous type asteroids, you get a decent steel.There wouldn’t be any “least valuable materials” unless it has rocky inclusions.
    Think in terms of shielding doing double-duty as raw materials storage, structural shell, farm-area soil, etc. It can start out as bulk unprocessed ore, but as it gets turned into useful stuff, that stuff can be the shielding, and you backfill where needed with more ore.
    L-5 isn’t the best location, rather a high orbit near the Moon. The Moon has different “ores” than the asteroids do, and by using both, you get a wider range of things you can make. With low escape velocity and no atmosphere, it is fairly easy to “throw” bulk ore off the Moon with an electric catapult.
    Also, the Moon can be used for a gravity assist to slow down incoming asteroid loads. You can’t do that at L-5

  18. Coming from someone who was captivated by my first Space Shuttle launch as a child, just to watch everything stagnate since then, these are exciting times! Thank you Elon Musk and NASA COTS! Now, can we please maximize our NASA budget purchasing power to go big on everything space, especially drives, lunar infrastructure and large ships? I promise the support, in the US and out of it, is there from the public.

  19. “Don’t be so small minded.” Mars is a tiny planet. The things you mention can be done near Earth, and benefit Earth. Visit Mars later.

  20. 150,000 cubic meters of space so 750 cubic meters per person. You have to spend space on infrastructure but it doesn’t seem too bad.

  21. The easier non-rocket options that have been proposed here are a solar-powered rotary catapult (slow charging, low throughput) followed by a mass driver for higher throughput (needs more complex power systems).

  22. “One could build an orbital settlement in equatorial low Earth orbit (ELEO) with much lower mass than elsewhere, because in ELEO the Earth’s magnetic field protects rather well against cosmic rays and solar pro-tons [3]. However, in LEO there is the risk of orbital. For example, recently the insurance company Assure Space stopped offering collision risk coverage policies in LEO [4]. There is also the issue of having to perform a targeted reentry when done with the facility.Letting it fall freely would be a public safety issue for people living near the equator.”

  23. The focus on people first is a big problem. We need robotic ISRU building of much more stuff! People will be needed for this, but as an aid to the robots until things are cheap enuf for real estate purchase. Cash flow.

  24. I don’t see the business model for this.
    Is there demand for L5?
    Any science to be done there that can’t be done elsewhere?
    Why not build closer at Geosynchronous?
    Even Lunar orbit would be preferable then L5

  25. There is an opportunity to self-finance by doing something useful for the Earth, unlike the situation on Mars.

  26. For a small station like that, it makes more sense to do what Globus suggests and put it in LEO. Then you barely need any shielding.

  27. 33 metres in diameter? So this entire 200 person space colony will be 2 spheres, each the size of one medium suburban house block?

    Hope they don’t mind crowding.

  28. The new “giant” FH fairing have more than 250 m3 of internal volume. Making a space station of “fairing shape” elements, can get a larger ISS equivalent with 4 FH launching.

  29. This is cool and all – but the authors’ suggestion that after bootstrapping it might self-finance by building more living space sounds like a gigantic pyramid scheme in space.

    There has to be something else, even if once you’re there everything other than housing is free.

  30. Yeah you just need bulk regolith powder for radiation shielding. If only there was some way to get bulk goods up from the moon’s surface, kind of like an elevator. You could make Fe containers from regolith and ship up moon dust.

    Note to interested national parties: there are only so many places you can make a space elevator on the Moon because all the elevators go to the same place. So space elevators are highly valuable real estate.

  31. We should focus upon how Starships can be converted into mass drivers and smelters. Not end product that has to be further launched just to get *more*.

  32. A nickel-iron near earth asteroid should be moved to L-5, mined, and the least valuable materials used as shielding, if not for the first habitat, for later ones.

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