Worst case » for a rotating space station located at L1 of the Earth-Moon system.
We will consider the station during its orbital evolution, the radiation it receives, and the radiation it is exposed to. It must simultaneously receive maximum sun radiation on the solar panels (energy) and minimum sun radiation on the habitat, whilst also dissipating excess heat. Let’s examine energy collection, then the necessary protection against heat, and finally against radiation. Note: (1) « Orbits » is plural because there are two orbits to consider: an « active » orbit closest to the Moon (L1) and a « passive » orbit alongside the Moon during its journey around the Earth; (2) SePs are « Solar Energetic Particles”, and GCRs are Galactic Cosmic Rays. Unlike SePs, GCRs are omnidirectional and constant. SePs are almost exclusively protons. Among GCRs, 2% are HZEs (High, Z – atomic number – Energy). These are atomic nuclei with an atomic number greater than that of helium, moving at speeds close to that of light. Today I’m presenting the ‘worst case’; next week it will be the ‘best case’.
1. Energy collecting.
Energy collecting by the station will be carried out by a flat, ring-shaped disk, slightly larger than the diameter of the torus (6m), or 4107 m² (the torus’s circumference is 703m), covered with solar panels and attached to the torus. This disk will be designed (1) to capture the maximum amount of energy with minimal interruptions from eclipses and (2) to protect the torus and its inhabitants. Note: As a reminder, the 703m height results from a 112m distance to center, distance necessary to achieve with a rotation rate of 2 rpm (round per minute), a head-to-toe gravity gradient of <2% within the torus for a 180 cm tall human being.
I reiterate my choice of an ‘active’ orbit. This is a halo orbit around the L1 Lagrange point of the Earth-Moon system. Note: Halo orbits are considered more stable than other Lissajous orbits and offer better long-term predictability. Furthermore, since the L1 point will follow the Moon around the Earth (passive orbit), there will also be a constant evolution of the angle of exposure to the Sun!
To collect the maximum amount of energy, the station’s axis (orthogonal to the plane of the torus) must constantly point towards the Sun. This requires very precise control involving constant adjustments to its orientation and therefore its attitude. This can be achieved with gyroscopic actuators, reaction wheels, and, of course, propulsion (xenon).
To better understand this challenge, I need to say a few words about the halo orbit. It’s not a simple perpendicular circle, but a complex 3D trajectory (shaped like a kidney bean) with significant variations in distance, both to the Moon and to Earth. On the chosen trajectory, centered on the Earth-Moon axis with a radius of 25,000 km, the station’s distance will evolve between 33,000 km and 83,000 km from the Moon, and between 301,000 km and 350,000 km from the Earth.
The estimated need for trajectory adjustments is 5-10 m/s/year of Δv. Fortunately, the propellant requirement for these adjustments via gas propulsion will be manageable. Xenon is chosen for its stability and ease of storage i.e. 50kg in cylinder 50cm diameter and 50cm high. For the station’s 7880-ton mass, some 200 kg will be needed. To actuate the motors, we will need electricity (500-850 kW including for operation and living conditions inside the station), but we will have plenty thereof, thanks to the surface area of the solar panels. The 4107 m² can provide approximately 1.6 MW with an assumed panel efficiency of 30%.
2. Protection from Solar Heat
At the Earth-Moon system’s distance from the Sun, the average irradiance is 1,360 W/m². This continuous irradiance will obviously be very beneficial for energy production (as said, 1.6 MW estimated power). However, without an atmosphere to dissipate the heat, it would be catastrophic if its thermal effects were not managed. Therefore, the torus, radial tubes, and central axis head must be protected by a reflective surface and layers of insulating materials, and heat evacuated.
Fortunately, the proposed annular disk for collecting solar energy can also be used for thermal protection of the torus. However, it will need to be reinforced on three levels. The first level, beneath the disk, will be an insulator of approximately 30 cm and up to the torus, made up of two components: over 2 to 4 cm, some 15 to 30 layers of aluminized films (Mylar/Kapton) separated by spacers (‘MLI’ for ‘Multi-Layer Insulation’); over 25 cm, a layer of hydrogen-enriched high-density polyethylene (hereinafter referred to as ‘polypropylene’). Below the insulation, a gap of 5 to 10 cm will be managed to have piping evacuate residual heat to radiators placed under the radial tubes. These radiators will be panels with ammonia or propylene glycol tubes.
For the tubes, a reflective plate made of aluminum or composite material, 112 meters long by 6 meters wide, will be placed above each one. Below this plate will be the same insulating system as for the torus (MLI, polyethylene, gap allowing the passage of a heat transfer fluid piping to dissipate residual heat through radiators). Since there will be no solar panels above the tubes or the axis (not needed), the surface of the plates will be coated with ‘Z93’. This very thin-film product, used on the ISS, has a reflectivity of approximately 82-86%. It is also highly emissive (approximately 92%), meaning that it very efficiently radiates internal heat (infrared) into space. The radiators, located below the radial tubes (shielded from solar radiation), will have a surface area of 350 to 400 m².
For the central axis’s ‘cap’, an active thermal shield will be better. It will include a reflector disc, made of aluminum or composite material, with a diameter of 10 m (to protect the various modules located below), or 314 m². There will be a 20-40 layer MLI behind the reflector, and of course a 25 cm layer of polypropylene. Below this, another ventilated gap of 5-10 cm (as for the radial tubes) will be managed with a heat transfer fluid circulating in a closed loop to dissipate residual heat to other lateral radiators (100 to 150 m²).
Note that the solar radiative pressure on the surface area considered (the 4107 m² of the disc protecting the torus, and the 314 m² of the cap protecting the central axis) will be quite low (easily corrected by the propulsion system).
3. Radiation protection (SeP and GCR):
Beyond thermal protection, the station must also protect its occupants from space radiation, primarily SePs, and limit exposure time to GCR HZEs.
The polyethylene layers already planned for the thermal protection of the torus and radial tubes play a crucial role in this protection. The aforementioned thickness of 25 cm and the station’s constant axis orientation towards the Sun will reduce the SeP radiation dose by 85%. Water, i.e. ‘water wall’ concept, can also be used and will even be slightly more effective (higher hydrogen density). Both will be necessary (water is more massive) because the same thickness of polypropylene and water spread all around the torus, will also be effective against omnidirectional GCRs. Unfortunately, the effect against those will be much weaker (a 12 to 18% reduction). Indeed, it must be acknowledged that against GCRs, especially HZEs, the effects of passive shielding remain limited. But it will be ‘better than nothing’. If we insisted upon a significant attenuation, several meters of material would be required, and that would be utterly prohibitive in terms of mass. Note that the polypropylene layer can be placed on the outside of the modules. This will free up internal space (the water must, of course, be inside).
The water tanks would cover not only the walls of the torus but also those of the Bigelow modules. The water would serve a triple purpose: life-support (recyclable), radiation shielding, and mass for heat regulation. It would, of course, be compartmentalized (by module) to facilitate its management.
Storm shelter:
Despite these protections, major solar flares (relatively rare events, very dangerous but predictable 12-48 hours in advance) can generate proton fluxes extremely harmful on account of their density. It is therefore essential to have a storm shelter – a strongly shielded refuge module.
This shelter will be located on the station’s central axis, near the junction central sphere. It will be a cylinder 5m in diameter and 10m long, divided into three sections, with sanitary facilities. This will give a free volume of approximately 150 m³, sufficient to accommodate 30 people (each enjoying about 5m3) for a few hours up to a few days. Its walls will consist of a water wall of 65 cm inside, and 35cm polypropylene outside. Total mass will reach some 150 tons (of which water 94,3 tons and polypropylene 45,7 tons). It will provide a near-full protection against SEP (dose reduction of 98-99%) and will reduce the effect of GCR by 35% to 45%. It will be equipped with autonomous life support systems (air, water, emergency food), communications, and medical devices.
Radiation monitoring:
The station will be equipped with radiation detectors (active and passive dosimeters) distributed throughout all modules, as well as a connection system with terrestrial solar warning networks (NOAA Space Weather, ESA Space Weather). Apart from protecting from solar storm, it could be used to accommodate people suffering from an excess of radiation.
Expected annual dose:
With these protections, the annual radiation dose for occupants will be on the order of 290 to 310 mSv/year, down to a few tens of mSv during stays in the storm shelter during solar flares. This dose remains acceptable for stays of a few months in the station. Note: NASA’s recommended ‘ALARA’ dose (3% increased risk of cancer) is 250 mSv per month, 500 mSv per year, and (until 2022) 1 to 4 Sv for a career (depending on the age and sex). However, in 2022 this last limit was reduced to 600 mSv regardless of age or sex (in anticipation of deep space missions where radiation is more intense than around Earth).
4. Services and Operations by Remotely Operated Humanoids:
The annual radiation dose aboard the station makes the use of permanent human staff impossible. Short stays will be quite acceptable for tourists and most scientists but poses a problem for maintenance and continuous operation.
The solution lies in the use of remotely operated robotic humanoids from Earth (such as Tesla’s Optimus, Boston Dynamics’ Atlas). Thanks to the short distance between Earth and L1 (326,000 km, or about 1 light-second), the round-trip communication latency is only 2.2–2.4 seconds, allowing for near-direct control by operators on Earth. The operators, stationed in a control center on Earth and working in shifts 24/7, will see through the eyes of the humanoids and direct their actions: routine maintenance, inspections, repairs, cargo handling, assistance to human visitors, and even extravehicular activities (EVAs).
A fleet of 10-20 permanently stationed humanoid robots would be adequate. Five humanoid depots could be set: the main one on the central axis, and four more in the torus (Bigelow modules), each near a radial tube, as different adjustments of the robots to gravity, and emergency interventions, may be required. Operating costs would be considerably reduced: no recurring personnel transport, no extended life support, and ground-based salaries for operators. This configuration would also ensure that the expertise of top ground-based technicians would be constantly available on board, without the constraints and risks of repeated manned flights.
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An interesting caveat regarding the amount of energy comes from the reflection of sunlight by the Moon, i.e., its albedo:
The station will reflect moonlight variably depending on its position around the Earth during the lunar month and its position in its halo orbit around L1. This reflection is not entirely negligible given the perceived size of the lunar disk in L1 on account of its proximity. In effect, this disk will be seen 23 to 113 larger than from Earth. This will result in an irradiance of 34 to 80 W/m². This is not much compared to the 1360 W/m² of direct solar irradiance in the area, but it must still be considered for two reasons. First, it will give some light on the station’s walls, which would be otherwise always in the dark (since the axis will always point towards the Sun). Second reason: the energy received from this albedo will be much more variable than that received directly from the Sun, depending on the distance to the Moon. It will therefore be necessary to manage thermal fluctuations: (1) the size of the radiators will have to take this into account (done in the 550m² already mentioned); (2) a selective-emissivity coating was already useful (for the surface exposed to the Sun) but it becomes essential for the whole station even if the albedo contribution to radiation is marginal. As mentioned above, it will be Z93.
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Problem:
The dominant problem with the project is its mass, and therefore the number of flights and the time required to assemble it, and thus the cost. Let me remind you of the situation: The polypropylene alone has a mass of 1300 tons. The water mass is even worse: for the torus the 15 cm means 1320 tons of water; for the radial tubes, 530 tons; for the 70 Bigelow modules, 670 tons; for the central axis, 310 tons; for the life support tanks, 60 tons. The panels above the tubes, the cap, and the MLI, have a mass of 50 tons. Total is 4980 tons, hence 50 Starship deliveries. Remember that the structure of the station has a mass of about 3,000 tons. Altogether we’ll reach 8000 tons. That is really a lot!
You’ve seen the worst-case scenario. But there is a solution to this problem: resizing the station. It would have some drawbacks, but above all, very significant advantages. I’ll tell you what I think, next week.
copyright Pierre Brisson
