Review: How Much Gravity Do We Need, and Why Do We Care?

One of my hobby horses that I’ve blogged about a few times is the question of how much gravity do humans need to be healthy? As I’ve pointed out in the past, we know microgravity is awful for people long-term, and 1G is fine, but we really don’t know what number between 0 and 1G is the minimum that a typical person needs to avoid unacceptable health degradation.

Why do we care? This topic came again today in the context of a twitter conversation about Mars colonization. Obviously, if the magic gravity level people need is higher than 0.38G (Mars gravity), Mars colonization is going to be harder than if it is lower than 0.38G. If the minimum required gravity level is more than Mars levels, you’ll need to come up with some sort of countermeasure on Mars, or face potentially severe health issues over time. This may involved a mix of biochemical countermeasures (drugs), exercise, and even small centrifuges–all of which have large potential drawbacks. If it turns out that the minimum required gravity level is less than 0.38G though, life becomes a lot easier. You might want to do something about the microgravity on the flight out, but wouldn’t have to deal with countermeasures after you landed. And if you went the artificial gravity route on the way out, you wouldn’t need to provide as much of it, which would make the system size a lot smaller and more manageable–for a given max spin rate, the centrifuge radius is inversely proportional to the gravity level required–for lunar gravity you’d only need 8m radius at a 4rpm rotation rate.

But while many in the space community have pointed out the importance of answering this question, and joked about how nice it would be to have a national space program to answer questions like this, no real progress has been lately. Most of the potential solutions have been too expensive to raise the money for. What is needed is a way to start getting the data as cheaply as possible. Hopefully well less than $50M if we want a realistic chance of getting NASA or private donors to fund it.

Dragon V2 as an xGRF Platform

While the idea came to me based on some of the ISS visiting vehicle post-mission reuse ideas we’ve been looking at at Altius, when doing a little research for this blog post, I found that this idea was originally suggested by A. M. Swallow and googaw in comments to my previous post. I usually blow those two off, but they were right a lot sooner than I this time around. The idea is basically using the pressurized volume of a repurposed Dragon V2 as the habitat for a 1-person (and many mouse) version of Kirk Sorensen’s xGRF concept. While there is a lot more info in the previous blog post on xGRF, the high level version is you start with a habitat connected via a variable length tether to a counterweight. As shown below, when the tether is all the way out, the system will settle into a gravity gradient orientation, completing one rotation per orbit. By winching in the cable in the right manner, conservation of angular momentum causes the rotational rate to spin up, creating higher levels of artificial gravity up to a peak level with the tether at its minimum length.

For my Dragon V2 variant, here are some key points in the concept (which is still only partly baked):

You would use a Dragon V2 after it has launched a crew to the station. All but one member would board the station while the last crew member stayed aboard for the experiment. After successful completion of the mission, the Dragon V2 would return to the station to pick up the crew for return to Earth. You’d have mice on board as well as the human for two reasons: to give the person something useful to do so they don’t go crazy being by themselves for a few weeks or months at a time, and because you can get a lot more data points in a small volume with mice than you can with humans. While the human data is very useful, the mice might give you a better idea of the variability of the effects. You’d try to locate the Dragon V2 xGRF experiment as close to the station as you could get while still factoring in the risk of tether breakage–Kirk’s paper shows that at station altitudes a tether break wouldn’t lead to immediate reentry, but you’d want to make sure it also had a negligible probability of hitting the station. But if possible it would be great if you could find a position close enough that you could visibly see the station. Being alone for long periods of time might not be so bad if there are people within easy visual range that you can communicate with with no delay. You’d probably leave the xGRF kit attached to the Falcon 9 upper stage, but tucked into the trunk volume. This kit would include a dumb docking port, the tether/winch system, any required solar panels (if the solar panels on the trunk of Dragon V2 aren’t enough), and possibly inside the docking port some extra life support equipment/consumables (if you can’t cram enough into Dragon itself). Once the crew going to ISS were offloaded (along with most of the launch couches, and the vehicle configured with the mouse and its one human inhabitant for the experiment), it would leave ISS, re-rendezvous with the upper stage, dock with it, maneuver it to the experiment flight position, and then deploy the xGRF system. You’d probably want to make sure you had way to do a lot of telepresence, to keep the volunteer from getting too lonely. Two people might be psychologically nicer, but a lot harder to cram into a 10m^3 room for long periods of time. Telepresence coupled with being in visible range of ISS might mitigate the issues with having one person flying solo on a mission like this. You’d probably want to pick an astronaut who had flown a few times before, so you’d have pre-existing data on how their body responds to microgravity, to use as a comparison point. It’s not perfect–long term you’d want as many human data points as you can get, but at least with someone who has flown a few times already, you wouldn’t be dealing with a completely unknown quantity as far as space physiology reactions. If they weren’t already burned out, having twins like Mark and Scott Kelly do the experiment, with one on ISS and one on the xGRF platform might also be an interesting way of screening-out some potential genetic effects. The most interesting data is if the minimum required gravity level is less than Martian gravity levels (less than lunar would be even better). If it turns out it’s higher than that, Venus and Earth become the only realistic solar system destinations that you could live at without expensive countermeasures. So it might be worth intentionally designing the system for a maximum of say 0.4G at say a little under the 4RPM max limit that people like to stay under. With the upper stage potentially being a larger chunk of the xGRF mass than Kirk’s original paper suggested, this would greatly reduce the required tether length, making the system lighter and more manageable.

Some potential concerns include:

Is 10m^3 enough volume for one person for long durations? I don’t know for sure, but if I’m not reading it wrong, a quick skim of this reference suggests that 10m^3/person might work for durations up to 3-6 months. Can the Dragon V2 without structural mods handle the loads in question? I would think so–the docking port is usually resisting a pressure load on the order of 15psi acting on the cross-section of the passageway, which is at lest 30in in diameter–yielding pressure loads of >10,000lbf they’re designed to resist. My guess is the loading would be similar in this situation, so probably something that could be handled without modification. Can Dragon V2 support 1 person on-board for long durations (>3wks) without expensive modifications? I’m less confident on this one, especially since the life support would be acting in partial gravity instead of zero-gravity. Everything I’ve heard suggests that even a little bit of gravity (for fluid settling and natural convection) can make many things a lot easier, but I’m not sure how much could be done leveraging existing hardware and interfaces, and how much would have to be done semi-custom. The less mods the less development cost. Fortunately, aborts to Earth or aborts to ISS can likely be done quickly enough that if life support stuff starts breaking down, there are plenty of quick rescue options.

The biggest question of all is how much would something like this cost, and would it be cheap enough to cross the line into something that could actually get funded? By reusing a Dragon V2 that’s already going to the ISS, you might only have to pay the delta-cost of operations and of the xGRF module. Could you keep that below $50M? Below $10M?

My guess is you’re almost positively above the $2M-ish limit of what is demonstrably crowd-fundable. But are you too high for a wealthy philantrocapitalist? I’ve heard that part of why Dennis Tito suggested Inspiration Mars was that he wanted a way to invest some of his money into making a lasting difference in the development of humanity in space. Could you get the cost low enough that Dennis, or someone like him, could chip in the money? Could you do this with NASA as a partner without NASA’s safety culture turning this into something so expensive it never flies?

Anyway, it’s some food for thought.