Friday, August 25, 2017

The Mother Ship

True space settlement depends on being able to have children out there, including pregnancy, childbirth, and the whole process of raising a happy, healthy child. We know very, very little about what it takes to do this in the harsh environment of space. What we do know suggests that the radiation protection and gravity required is far higher than for an adult. As a mental exercise, let us tackle a chunk of this question. Here follows a sketch of the mother ship - a ship compliant with conservative estimates of the protection needed to safely transport an expectant mother from the Moon to Earth.

First, the setting. There is a point on our timeline, after a pretty large, bustling colony has been set up at Lalande Crater on the Moon's equator, where a small number of young couples intending to have children are accepted into the colony's population. This step is taken after extensive research into pregnancy in higher mammals has been done. For now, let's posit that shows that very early pregnancy is alright in lunar gravity, but normal fetal development requires something close to normal gravity after that. Also, that only a very low level of cosmic radiation is permissible. This gives us a starting point for really thinking about the whole thing, which is what Moonwards is for.

Now, the very limited data there is on mammalian embryos in microgravity mostly show abnormal development. There is no information at all in low gravity like the Moon. A good review of current data is in Effects of Microgravity on Cell Cytoskeleton and Embryogenesis, by Susan J. Crawford-Young. Maybe a little gravity really helps, maybe the fact that experiments so far haven't controlled for other factors like elevated radiation and launch stresses means they've over-emphasized microgravity, maybe periodic exposure to higher gravity or other measures can offset problems... What can i say, you have to start somewhere. I have no good response to the low-gravity factor, in particular during the period before pregnancy is even detected. A woman known to be pregnant could be placed in higher simulated gravity for transport, except for a couple of short gaps. Before anyone knows she's pregnant, if a regular Moon lifestyle isn't enough, we're sort of stuck. It would be time to talk about orbital colonies for pregnancy. And i've even started thinking more about that... But first, the mother ship way, which addresses radiation.

The mother ship blocks cosmic rays well enough for a pregnant woman to be safe. She would be safe in the colony itself - it's been designed to have plenty of protection, even more so at this stage in its development. It would be like being on Earth in that respect. She would also be safe in the space station that anchors the equatorial tether complex. It is huge by the time these events are in play, with plenty of shielding. Same goes for the ship that ferries her from that station to the corresponding station on the other end, in Earth orbit. That ship shuttles between the Moon and Earth by being tossed between the upper tethers of each, it only needs enough engines and fuel for course corrections. (And i'm going to go ahead and start referring to them as toss ships, because it makes it a lot easier to explain things, and what the hell.) So it too is a big thing, designed to carry very heavy loads. And the Earth space station? That one gets started with an actual metal asteroid of worthy size, it's super shielded. We're talking about a time when there is major commerce in cislunar space and projects like this happen, thus the whole motivation to finally resolve this space-family thing. In all of these locations, our mother-to-be has space to stretch and move, decent medical facilities, and access to a full gravity environment in centrifuges. The full mother ship need only get her safely from the Moon's surface to the station in orbit, and the mother ship capsule provides shielding during other parts of the journey.

So, what level of shielding is needed to be pretty sure the embryo within her is safe? To answer that, let's talk about cosmic radiation, and embryonic development.

The data on exposure in the womb to the type of radiation we are talking about is limited to that resulting from the bombing of Hiroshima and Nagasaki - and it still isn't really the same. Below exposures of about 0.4 Gy (which means 0.4 J/kg of radiation) the clinical effects were limited to increased risk of cancer later in life. There was risk of mental retardation above 0.4 Gy, but we aren't contemplating anything close to that. Contrary to popular belief, there was no increased risk of stillbirths or birth defects among children exposed in utero to the a-bombs. Other than these case studies, the only data on exposure of embryos to ionizing radiation is from animal studies. I have found no such study that used radiation more energetic than gamma rays, which is so different there is no point talking about it.

The kind of damage that is done by cosmic radiation, especially high energy nuclei (HZEs), is in a different category. I really like this image from NASA's Space Radiation Cancer Risk Projections and Uncertainties, on page 5, as a way of getting it:

A comparison of particle tracks of different ions, from protons to iron, in nuclear emuisions, clearly showing the increased ionization density along the track by increasing the charge Z. The left panel shows three nuclei of human fibroblasts exposed to gamma rays and Si- or Fe-ions, and immunostained for detection of gamma-H2AX. Each green focus correspondsd to a DNA double-strand break (DSB). Whereas the H2AX foci in the cell that is exposed to sparsely ionizing gamma rays are uniformly distributed in the nucleus, the cells that are exposed to HZE particles present DNA damage along tracks (one Si- and three Fe-particles, respectively), and the spacing between DNA DSB is reduced at very high linear energy transfer.

And here are two quotes from the lead author of this model, Frank Cucinotta, from his interview on The Space Show on Aug 1 (quoted with permission from TSS). The first is at 17:17 of the show:

When you look at an x-ray, and you look at an important molecule like DNA... it only can ionize a few times with that volume of say 10 nanometers, but a heavy ion can ionize thousands of times in that same 10 nanometer volume. So it's a true qualitative difference. That's what we're worried about.

The second is from 25:52:

There is different kinds of damage to DNA called strand breaks, and you get single strand breaks all the time... your body knows how to handle that, and then there's a little bit more complicated, a double strand break meaning that you've broken both strands of the DNA within a short distance, say 10 base-pairs from each other, that also occurs somewhat in metabolism. But there is something else called a complex break, where you have double strand breaks, single strand breaks, and base damage all very close to each other and this is what ionizing radiation, this thing it can do, your metabolism won't do this. And from those events you have mis-repair that leads to mutations and chromosome aberrations.

Alright. So, we don't have any data, but that kind of damage sounds so awful, you'd figure that expecting parents would insist there be none of that. That's just off the table. Chromosome aberrations in an embryo that is dividing its little cells as fast as it can has got to be a really bad thing.

Therefore the mother ship will surround the mother-to-be on all sides with 3 m of water. That is equivalent to the atmosphere above you when you are in a jetliner at cruising altitude. Pregnant women can safely fly, so our hypothetical mom-to-be would be fine in this environment even for a day or two.

If we provide a capsule 2 m in diameter and 2.5 m long, surrounding that with 3 m of water amounts to 386 tons of the stuff. Add mass for the structure and systems, and let's call that 450 metric tons. Our shuttles use nuclear thermal rocket engines, ones that are a little fancier than usual because they have a system to inject oxygen into the nozzle, which combusts with the super-hot hydrogen exhaust and boosts thrust during launch and landing. This is known as a LANTR design. Water is, as it happens, also good shielding against neutron radiation from nuclear engines, so we can save some mass on shielding for the shuttle that is the designated mother ship.

With 4 engines and the fuel needed to get this ship to the foot platform of the equatorial tether, this ship masses 480 tons. The whole section composing the capsule, its shielding, and all its systems then separates from the ship, and is placed on the climber car to be taken to Gagarin Station, 5000 km above the surface. 450 metric tons would be the maximum payload that tether is designed for. Supposing the best material we have for making tether cables in that day is still Zylon, the mass of the whole cable up to the station would still be only 4000 tons, including safety margin and extra infrastructure like counterweights, the foot platform, and some maintenance stuff. For comparison, the main cables of the Golden Gate bridge each mass 11,000 tons.

If the climber car can manage an average speed of 1000 km/h, she gets to the station in 5 hours. There, she can stretch, and shower, and eat, maybe check in with a doctor, and get a good night's sleep all in a proper full gravity environment, in the O'Neill cylinder that extends from one side of Gagarin, across from the shipyard. Her capsule is moved around and docked to one of ferries that go up to the toss ships at the Earth departure platform part way up the upper tether. The next day she goes aboard her capsule again, and the ferry takes it up to the toss ship and loads it into its cargo bay. That trip takes another 5 hours or so. Then she can get out and go to the toss ship's centrifuge section for the 3 day trip to rendezvous with the upper tether of the Earth tether complex, Anshar Station.

Anshar Station is a whole different beast. It's still rather new when the first returning woman with-child comes aboard, and it can pamper her with a fancy centrifuge torus nestled under its metallic bulk, overlooking the Earth below. If we don't have enough experience by then to capture an asteroid with a suitably tempting metal content, and confidently maneuver it into orbit around Earth, we aren't really trying. To properly enjoy our new space station, built around this mine in the sky, we should drain the inner Van Allen belt with the scheme proposed by Robert Forward. Orbiting at an altitude of 3500 km, with an upper tether that reaches to 6500 km, that cable is hefty enough to safely dock cycler ships from Mars and Venus. If it was Zylon, it would be well over a megaton, so it would really be better if we had carbon nanotube cables by then. They don't have to be perfect nanotubes - not even close. Just good ones, manufactured in bulk. The lower cable would really benefit from that too, though it could be done with a few hundred kilotons of Zylon. Anshar is built to be Grand Central. It's foot moves at just 3.8 km/s relative to the Earth's surface and it can move a couple of kilotons along the lower cable at a time. But we can discuss these things another time...

The point is, our mom-to-be can be ferried down the lower cable still in her protective mother-ship capsule. She is then just 250 km from the cradle of humanity. The mother ship capsule is way too heavy to be used for re-entry into the Earth's atmosphere. Even if you could brake its descent for a soft landing, the cost of relaunching it for return to the Moon would be too high. Despite the tether foot platform reducing the speed a rocket needs to reach by half, it probably wouldn't make sense to have reusable rockets or space planes with payload capacities over 200 metric tons at most. The mother ship capsule goes back to the Moon for the next mom.

Anything dropped from the tether foot is immediately on a steep re-entry trajectory. I have asked about what sort of heating a reentry capsule would go through in that case. Being able to just drop things from the tether and have them safely reach the ground would be very advantageous, but we have to assume it won't be a good idea for our expectant mother.

She could descend in a space-plane sort of vehicle, that uses lift for a gentler re-entry. The process would take longer but be smoother, the heat of reentry is dispersed over a much larger area and mass, with much lower g-forces, and the plane can go straight to a landing strip instead of probably landing on water and being collected. The space shuttle took about a half hour to re-enter, so we could maybe reasonably suppose a space plane starting with half the forward speed could do the same sort of trajectory and accommodate twice the payload. Thus, if we wanted to, we could tuck our future mom in 50 tons of shielding for that trip, almost all of it on the skyward side, and protect her pretty well during that brief time. She lands at a convenient space port and is greeted as the first woman bearing a lunar child.

Okay, so this is a distant future. But this is doable. This is a matter of how bad we want it, which is really what the whole matter of space exploration comes down to. The question now is if the spaces of a few hours when she is in low gravity or microgravity during her trip are safe, and if conception and the days before pregnancy is detected can happen in low gravity. If so, this is something that would work. The mother ship capsule is pretty expensive, but highly reusable. The real expense is in the energy required to move around that great big thing. We now have a model for this we can start with. In due course, it will all be placed in Moonwards' virtual colonies.

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