Gravity, as the old joke goes, sucks.
It drags us down, pulls on our weary limbs, makes our feet tired, makes parts of us droop. But it's also a critical factor for our long term well-being. Astronauts and cosmonauts circling the Earth over the past 60 years have discovered that zero-g, or microgravity, is really not very good for you.
The human body has evolved in a piece of curved space-time where objects experience a close to uniform 9.81 meters per second per second acceleration. Blood and fluids are pressurized accordingly and arteries and veins are squeezed by muscles, so as not to all pool inconveniently in our feet. Eyeballs are tensioned so as to retain an optically proper shape. And our microbiome is adapted to an environment with a definite up and down - especially when it comes to digestion.
Put one of us in zero-g or microgravity, and things get tricky. Our cardiovascular system gets confused quickly, and so fluids accumulate in places they don't usually - hence the puffy faced appearance that spacefarers can get. Eyes have to accommodate to unfamiliar forces. And to further confound things, we're all different, so our physiological changes can happen to varying degrees depending on the individual.
The absence of gravity for extended periods is even more serious. The growth and maintenance of skeletons is compromised, and the minerals that would otherwise be used on bones ends up clogging our blood plasma and ultimately putting us at risk for kidney stones and other delights. Red blood cell counts drop, and immune systems show signs of being compromised.
So it's not surprising that we've long thought about how to mitigate these effects. One way is to try creating artificial gravity anytime we're away from a nice massive planet.
A very early example of these ideas was in 1896 when the extraordinary Russian scientist Konstantin Tsiolkovsky described the use of rotating structures in space to exploit centrifugal 'forces' to simulate gravitational acceleration. Spin things around and, just like the schoolyard experiment of whirling a bucket filled with water on a string until the water stays put in the bottom, you produce a steady acceleration on objects.
The iconic movie 2001: A Space Odyssey, released in 1968, presented one of the best visualizations of how a rotating space habitat might actually work (copyright issues complicate a reproduction here, but the image below from NASA's archives is pretty close in character).
The first bona fide experiment on artificial gravity took place in September 1966, when NASA's Gemini 11 mission, crewed by Pete Conrad and Richard Gordon, rendezvoused in low Earth orbit with the Agena Target Vehicle (a 7,000 pound modified rocket stage).*
One of their activities was the attachment of a 30-meter (100 foot) nylon tether between the Gemini capsule and the Agena. The idea was that first, by stationing the Gemini 'above' the Agena, the tether would be kept taught by the difference in gravitational pull over this distance (that didn't work so well), and second, that the two ships could move like a set of 'bolas' around each other - held together by the tether and generating artificial gravity towards one 'floor' of the Gemini capsule.
You can watch the results here, starting at around 10 minutes and 30 seconds. Despite the oscillations in the tether and other spacecraft issues, these did settle down temporarily after 20 minutes or so, and for a brief time a teeny, tiny bit of artificial gravity was observed in the Gemini capsule. How much gravity? About 0.0005 g with 0.15 revolutions per minute. Some time later the tether was released.
It's a little nerve-wracking to witness. It's even more nerve-wracking to read the details of what was going on, which you can here.
The acceleration produced in a spinning habitat presents other challenges too, because your occupants aren't going to remain stationary.
When you're standing on the 'floor' in a spinning environment your head is always closer to the spin-axis, which means that your skull is actually moving slower than your feet. What happens when you bend over to pick something up or tie your shoelace? Equally, what happens when you try to walk? In these moments your body has to overcome that velocity differential, and your inner ear - with its little fluid filled motion detectors - feels some very strange things happening.
The Earth has a large enough radius for its mass that the gradient in the gravitational field is shallow, and the difference in gravitational acceleration between our feet and heads is only about 0.000006 meters per second per second, or about 0.00006% (ignoring complications of planetary rotation). We're adapted to this tiny shift and don't notice it. The same is not necessarily true in a rotating spacecraft.
It doesn't stop there either. What about lateral motions? Just turning your head in a spinning habitat involves changing which bits of you are tangential or perpendicular to the spin direction, with similarly unpleasant possibilities for your inner ear.
Many of these questions have been considered over the years, although most of the work was done back in the 1960's and 1970's. A particularly thorough investigation was made by Theodore Hall for his PhD thesis in the 1990's and lots of this material is available online - including a handy artificial gravity calculator called SpinCalc!
The conclusion is that you need to keep the spinning environment large compared to the size of a human body (100 meters radius would be nice), to minimize the absolute velocity differences between head and feet, and to keep the number of revolutions per minute relatively low (probably less than 3-4). You also need to configure it so that the actual velocity of the circular motion is significantly larger than the typical speeds that humans are going to walk or run at. Otherwise, if you run in an anti-spin direction you risk negating the very acceleration that's keeping you pushed to the floor!
The bottom line is that, as the Gemini 11 astronauts got a little taste of, not only is setting up an artificial gravity system a bit of an engineering challenge, it's also a challenge to tune things to adequately accommodate what humans (or any other organisms) can cope with. It is not as simple as just spinning up your space wheels...
*Editor's Note: (4/17/18): This sentence was edited after posting to correctly identify Gemini 11 pilot Richard Gordon.