The very first paper I dove into from the new NASA public server covered topics near and dear, primarily the fact that microgravity is not survivable over the long term. There was, shall we say, a very enthusiastic embrace of the term 'omics' but otherwise some very interesting points.
In a nutshell, we know almost nothing about the effects of any level of pseudogravity (referred to as AG from here on) between microgravity and 1 g. This is important. NASA is considering human missions beyond LEO in less than a decade. We can and should do some kind of testing between now and then, and the only way I see to do that affordably is to launch a dedicated orbiting laboratory.
Read on for more. All costs are in current US dollars and are assumptions based on very little data.
We know a lot more about levels higher than 1 g because we can access that regime here on Earth with centrifuges. In short, even levels as low as 1.5 g for more than a few hours is dangerous for humans. Slightly elevated levels lead to clear signs of stress in nearly all higher organisms (though there are microbes that thrive under absurdly high AG). We can withstand enormous g-loads for short periods but we do not seem to adapt to higher gravity over the long term.
Microgravity research is the point of the ISS and it has been churning out data in abundance, virtually all of which shows strong negative consequences for living tissue and for most microbes. There are species that can adapt; certainly many potential crops seem to do ok and many microbes can handle the stress. Mammals have a universally terrible response, leading to lasting and to some extent permanent damage after weeks or months of exposure. Coupled with the additional negative effects of radiation on bone mass and inflammation (among a host of others), this means space is an extraordinarily hostile environment.
As for the in-between region, all the places we want to go (Mars, Luna, maybe eventually others) have gravity fields in this range. We also want to use AG for long-duration missions, but that means complex engineering problems like spinning parts, rotary vacuum seals, rotary unions, etc. It is important that we learn how much AG is enough for normal human health. Does it absolutely have to be 1 g or are we going to be ok to colonize Mars? What about the Moon? Is a tenth of a g enough to keep a deep-space crew in shape for a year? What about three years? Nobody knows.
There have been two experiments on animals in orbit, both Soviet and done in the 70's. The first was on turtles and fish while the second was on rats. Spin rates were very high (50+ RPM) as were gradients (r = 32 to 37 cm), leading to coordination problems in the subject animals. (In other words, the rats were so dizzy they wanted to die and every tiny motion led to debilitating nausea. The turtles apparently were fine.) However, their data suggested that AG levels as low as 0.3 g had protective effects against microgravity wasting.
There has been one experiment on humans in orbit, conducted by NASA in the form of a centrifuge chair that was flown on the Shuttle. (STS-90, 1998) Four members of the flight crew experienced 1.0 g and 0.5 g spins for 20 minutes at a time, spinning either sideways or lying down. (See here for a diagram.) These short-duration tests were tolerable (for astronauts) and did show some benefits. That suggests that a sort of hamster wheel exercise device might be enough to stay healthy if you can tolerate the high rotation.
The only way to get good data between 0 and 1 g is to do the research in space. We've already done quite a bit of work on microbes on ISS, but scaling up to animals requires a lot of cage space and big equipment. ISS is not a suitable platform for this kind of work; there isn't enough 'elbow room' for low rotation testing and the vibration from larger equipment at high RPM interferes with other research.
The simple solution is to launch a dedicated laboratory. This would be an environment focused on human factors and biology, allowing the ISS to stay focused on microgravity, materials science and physics. Ideally the whole facility would be under spin gravity so no complex centrifuge gear is required. Commercial or existing hardware should be used wherever possible but experimental gear should be included wherever that is prudent. The only viable options right now would be Bigelow BA-330 modules or ESA ATV modules.
Consider two BA-330 modules with a central node similar to Harmony (ISS node 2), spinning end over end (tumbling pigeon). The effective spin radius would be about 16 meters, requiring 7.5 RPM for 1 g at the two ends. (Probably ok for astronauts with strong stomachs. Hopefully.) Every gravity value from 1 g to near-zero would be accessible, with over 700 m³ of volume for running experiments. Lunar gravity experiments would have to be done inside the central node, meaning limited volume available. Mars gravity and Earth gravity would be available near the top and bottom of each expandable module. Experiments would be run in parallel, with sets run on Earth for a baseline and compared to 1 g and fractional g sets in orbit. That would allow us to see any differences between Earth gravity and medium-radius spin gravity at 1 g as well as sampling behavior at a variety of AG levels all at once. As an alternative, a spin rate of 4.6 RPM would yield Mars and Lunar gravity at roughly the same places as Earth and Mars gravity under higher spin and with less potential for nausea.
Life support, environment, attitude control, power and propulsion would be housed in the central node. Living space would be roughly in the middle of the two expandable modules. Cargo could be stored in pods attached to the node and in lockers throughout the station but the mass would have to be carefully balanced. The initial spin-up would eat a fair amount of propellant, so the craft should point its spin axis at the Sun to minimize stationkeeping. Solar panels would be deployed on the sunward side, minimizing thermal stresses on the modules themselves. Radiator panels would be deployed on the spaceward side. Visiting vehicles would have to be able to dock either sunward or spaceward since the crew would have to have a lifeboat available at all times. The ability to dock with a spinning target would have to be developed; in physics terms it's quite straightforward but in real-world engineering and software terms it's probably a huge pain in the ass.
Much of the scientific equipment could be off the shelf as there will be at least some gravity throughout the facility. That won't apply to everything, but at least we won't have to redesign every single part from the ground up to operate in microgravity. Existing ISS technology could be paired with prototype gear like ECLSS components for rapid deployment, redundancy and early flight experience on the new stuff. Aside from a variable-gravity research lab this facility could serve as a proving ground for new tech with fewer hoops, red tape and paperwork than ISS.
Bigelow claims to be able to produce several modules in short order, and Thales Alenia Space (the manufacturer of Harmony) could probably put together a Harmony-based node fairly quickly as well. If not, there are several satellite manufacturing companies that would be up to the task. We could have this thing in orbit in three to five years. Interestingly, NASA could specify the deep-space variant of the modules so the whole facility could be relocated elsewhere (such as in lunar orbit) if the opportunity should arise down the road.
Since we wouldn't need to rely on Russian cargo launches the station could be in a low-inclination orbit for maximum launcher performance. The three main components would fit on a Falcon 9 or Ariane 5. Crew trips would lean on the CCDev teams of Boeing (CST-100) and SpaceX (Crew Dragon), probably crews of 4 to 6 on 6 to 12 month rotations. Each crew would ride the same capsule up and back. Cargo trips would lean on the commercial resupply teams and anyone else willing to compete for a contract. Docking hardware would be identical to ISS (universal docking ports), so the same cargo craft could service both stations.
As for cost, no question this would be an expensive project. Some of the costs could be bartered in exchange for lab time as was done with the ISS. This would be a US project, not subject to the regulatory framework and treaty environment of ISS. I see two ways to go about it:
- Use the traditional NASA approach. Pay contractors and internal teams to build the hardware, launch it into space, own it and operate it. When the research program has yielded the desired data either deorbit the station or seek follow-on research projects. This gives NASA the most control over what happens and when, but NASA is paying the entire bill.
- Use the new COTS philosophy. Lease space on the station from a private operator (Bigelow), buy commercial crew and cargo service and only use what you need. Let the operator lease out additional space as they see fit and make sure that station management is their problem. This probably costs NASA the least, and if they decide to stop research after a few years then they're not tied to this massive, expensive facility in orbit when they would rather be putting people on the Moon. The operator assumes the risk of having to find other business opportunities for their facility. NASA may have to help guarantee the loan and may also have to pay a sizeable chunk up front, but they would still have a lot of leverage when it comes to details of the design and operations. At the end of the day a US company would own an orbital laboratory, NASA would get a lot of urgently-needed science done and the commercial space ecosystem would get a big stimulus shot just in time to work on things like ISRU and better life support that NASA also wants.
Bigelow wants to lease a BA-330 at $450 million per year. That seems a bit steep given that the estimated cost of a module in 2014 was $125 million. Perhaps they can compromise and allow NASA to pay a significant up-front cost in exchange for permanent access to that facility. (That might mean NASA uses most of the space early on while there is urgent science to do, but downsizes later and leaves room for other paying customers.) Let's assume a flat $1 billion (thousand million) for the two expandables. Let's also assume that the central node is outrageously expensive due to the rush job and the very complex equipment inside and call it another half billion. Launch costs would be around $450 million for 3x Falcon Heavy, Ariane 5 or Atlas 5 or some combination of the three. Let's just call that two billion dollars so far.
The three main components would rendezvous and dock autonomously, expand, pressurize, deploy solar panels and radiators and complete self-checks before the first crew launches. Once everything checks out, three cargo flights would deliver about 10 metric tons of equipment, food and experiment supplies. A crew capsule would launch and dock with the station, then the crew would set up and test the various equipment. After a check-out period the science would begin in earnest. This initial cycle would cost about $650 million.
Ongoing operations would require around four cargo flights ($600m) and one crew flight ($200m) per year. At the end of the first operational year the project will have cost about $3.5 billion (about $1.6 million per crew-day), not counting ground operations and the parallel experiments and probably a dozen other cost centers I've omitted. Each additional year would cost $800 million or so for 2,190 crew-days, only $365k per crew-day. Contrast this with a rough estimate of ISS costs at $7.5 million per crew-day and it's downright affordable.
Also consider that NASA could back off of this commitment at any time. They might choose to send fewer cargo missions, running longer-duration studies and only sending crew consumables and other essentials. They might choose to send a smaller crew, though that wouldn't really save any money. They might decide to simply skip a year, or join forces with ESA, JAXA or other partners to share the bill. The public won't complain, and Bigelow's costs would have been covered before any hardware launched. In the other direction, NASA might choose to raise the tempo and cycle experiments more quickly. As long as they can get the cargo to the station and back they can spend as much or as little as they want on transport.
This would buy us a lot of science yield on topics of critical importance to human spaceflight, answers that can only be obtained in orbit and only with a large-scale artifical gravity system of some kind. This spinning station is the simplest, safest way to solve that problem and get the answers we need as we look towards the Moon in the coming decade and Mars in the next. This also gives NASA a way to get out of the space station business while still having access to LEO for science; a smooth transition rather than an abrupt end to one of the pinnacles of human achievement.