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A recent question back in Physics asks whether there is a way for an astronaut to rotate when in microgravity and without touching anything else, while still conserving angular momentum.

One way to do this is easily demonstrated using a heavy book and a swivelling office chair. You hold out the book in front of you and rotate it about a vertical axis, bringing it closer to, and away from, your body when it is going to your left and right, respectively. To conserve angular momentum, your body also rotates slightly, but due to the difference in moment of inertia of the book when close/far from your body, the angular displacement of your body is different for the two stages and the final state is a displaced attitude.

This approach, and other similar ones - including the proverbial cat turning in midair -, have been worked to bits in Physics and most other outlets. However, the usual approaches sound too cumbersome to be used in space, but there may be cleverer ways to move one's body to achieve the same effect.

In practice, how do astronauts change their orientation in space? Do they regularly perform free-body manoeuvres while within their spacecraft, or do they simply grab onto the craft? During EVAs, do they use their thrusters, grab onto something, or just wave around until they're in position? If the latter, what are common ways to achieve such rotations? Does this depend on what axis you want to rotate about?

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    $\begingroup$ I've seen a video of astronauts doing the cat trick, turning around various axes without touching anything. But damn if I can find it now. (High divers do this too.) I doubt that they use those techniques other than for fun, since the quarters are cramped enough that there's pretty much always something in reach to grab. Even on EVAs they try to stay within reach of a surface. $\endgroup$ – Mark Adler Nov 29 '13 at 19:43
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This video published on YouTube on Zero-G: "Movement in Microgravity: Skylab to Space Shuttle" 1988 NASA Weightlessness Footage, starting at 2:10 into it, shows a Skylab astronaut doing a front roll and a spiral roll in the Skylab Orbital Workshop without touching anything to push against to change his orientation. And the same video from 5:45 to 6:00 shows astronauts wiggling from one direction to another to attention (fun video!):

As you will see in it, astronauts did all kinds of zero-g / microgravity stunts like that, here's one such fun photograph:

   enter image description here

    Astronaut Gerald P. Carr, Commander for the Skylab 4 mission, jokingly demonstrates weight training in zero-gravity as he
    balances astronaut William R. Pogue, pilot, upside down on his finger. (Source: Wikimedia Commons)

I've also frequently seen International Space Station (ISS) astronauts use such movement to change their orientation on the station, for example by watching Space Station Live or video recordings of it on YouTube, albeit while they would mostly first push against some surface to gain velocity towards their next destination. These movements wouldn't be much different than what swimmers do on a turn in a swimming pool, or as previously mentioned, a cat falling and reorienting to land on its feet. For a more direct demonstration, here's a Smarter Every Day video #85 on How Astronauts Turn In Space from March 2013 with ISS crew demonstrating change of orientation while not touching anything and of course preserving angular momentum:

During Extravehicular Activity (EVA) though, I doubt that they have much need for such stunts, or that they would be an easy feat to do after donning their EVA gear, with mobility units (latest one is Simplified Aid for EVA Rescue or SAFER) somewhat impairing their ability to change orientation like that, prohibiting free flexing of the body, while at the same time making them unnecessary, since the change in orientation can be provided by the mobility unit itself, if there isn't any surface to push against.

Astronauts are now also tethered to the space station and use on the station's outer hull mounted safety grips during EVA, so not only would such movement be cumbersome due to their EVA suit, but could result in the astronaut entangling in the tether. Some more about the astronaut propulsion and mobility units is described in the second half of this answer.

   Astronaut Rick Mastracchio working with a SAFER system attached.

   Astronaut Rick Mastracchio working with a SAFER system attached. (Source: Wikipedia on SAFER: Simplified Aid for EVA Rescue)

On the ISS itself, astronauts use footholds to fix themselves at a work location so their own body movement doesn't continuously move them around, and they push against all kinds of surfaces with their feet and hands (and sometimes, for fun, even tips of their hair, like I believe Sunita Williams did first) to make their way through the station. For some examples, I recommend watching some video tour of the ISS, like for example this Sunita Williams one, or an ISS tour by André Kuipers. Footholds will also be used by Robonaut 2 once it gets its legs, which I believe should be this month or at the latest in January 2014.

   enter image description here

   One of the footholds on the International Space Station. (Source: Ghost In The Machine on Observation Deck)

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    $\begingroup$ Yes, the clip from 5:45 to 6:00 is one of the ones I remember. $\endgroup$ – Mark Adler Dec 1 '13 at 4:28
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Although this has indeed "worked to bits" on the Physics and other SE sites it's worth looking at, for the sake of Space Exploration, the interesting history behind the analysis of the falling cat. For the fully rigorous description of the cat's righting reflex - perfectly in keeping with conservation of angular momentum - only came about because it was prompted precisely by research that was done in the late 1950s and early 1960s into how the human body would deal with the environment it met in outer space.

The main researcher here was Professor Thomas Kane, who

  1. Found the correct theoretical equations describing the cat's shift in orientation in response to its "shape shifting" during the righting reflex;
  2. Experimentally showed that humans could, with some training, make the righting reflex motions and flip over exactly like a cat.

The primary reference here is:

T. R. Kane and M. P. Scher, “A Dynamical Explanation of the Falling Cat Phenomenon“, Int. J.Solids and Structures, 5, pp663-670, 1969

Indeed Thomas Kane trained people to do this in 1968 in Apollo spacesuits, as shown below.

Cat and Astronaut

The left falling cat sequence was taken from the work of physiologist Étienne-Jules Marey (1830-1904) (famous for the development of motion photography for the study of high speed movements); the one on the right was taken during Thomas Kane's 1968 experiments with a trampolinist in an Apollo like spacesuit. Étienne-Jules Marey was a physiologist who did some of the little serious research into the cat's righting reflex before the outer space prompted research of Thomas Kane. Marey, unlike many of his contemporaries, clearly understood that the cat's motion was torque free (see footnote) and indeed used his photography to rule out a commonly held theory that the cat pushes off whatever it falls from. The collage was taken from

Alexis C. Madrigal, “Video: Deducing the Physics of How Cats Fall“, The Atlantic Magazine, 9th September, 2011

Now to clear up some popular misconceptions about the cat righting reflex, particularly applied to astronauts.

The rollover motion is not particularly burdensome or clumsy for humans to do, as Thomas Kane's experiments showed. It's very much like a hula hoop motion. OK so my CGI skills are crap - this is the best cat animation I can make with basic solids in Mathematica, but this movement will roll you over in space, whether you be cat or human, with or without a tail.

Hula Hoop Motion

The only differences between cat and human for this motion are (1) the cat's spine is much bendier than ours, so that the flipover can be done in fewer "hula hoop" cycles by the cat and (2) the exquisite sensitivity of the cat's vestibular system and lightning swift reflexes compared to ours. Point (2) is irrelevant when one is making a planned rotation in a freefall (gravity free) state in space, as opposed to flipping oneself over in limited time as one falls.

Another common misconception is that the cat needs its tail to flip over: this is wrong as shown by Thomas Kane's experiments that show tailless humans can make the righting motion. Indeed for our housecats, the tail is actually not used much for the reflex at all. I cite here solid observational evidence of my own here: my own cat has been tailless since she was run over by a car in 2004 and has no difficulty whatsoever righting herself when she falls off things, which she does often owing to her somewhat clumsy nature – typically when she falls asleep with her head drooping too far over the edge of our bed. Even though twelve years old, she manages to rouse herself and make the righting reflex within the time it takes her to fall the 40cm or so to the ground from off our bed. Moreover, just after the time she had the accident, I saw her make the righting reflex falling asleep in this way when she had healed barely well enough to walk properly. So it would seem that she needed very little "retraining" to adjust for her new lack of tail. This also comes out of a theoretical analysis, as I show in my article cited below. Indeed the analysis of the video below from Wikipedia, showing the isosceles triangle addition of the two angular momentum vectors is valid only for a "symmetric" cat (owing to the isosceles shape of the vector addition diagram) with no tail (i.e. a cat whose hinder half has the same inertia tensor about the origin as the forward half).

Falling Cat Explanation

(Source: Wikipedia "Cat Righting Reflex" Page)

Some wild cats, notably the Asian Clouded Leopard and the Asian Marbled Cat have huge tails, much more like a club than the elegant, slender (and very small mass moment of inertia) tail of the housecat (Felis Sylvestris) and this is indeed very much used to control the animal's orientation in space, but the tail lets the animal reorient itself freely about all three axes i.e. it can pitch and yaw as well as roll pretty much at will - in contrast to simply flipping over in the cat righting reflex, which is essentially a one-axis motion. This is a useful ability for tree dwelling predators as they leap from tree to tree and also precision dive-bomb their prey.

I discuss all this in much more detail in my article (including the analysis of "symmetric" (tailless) cats on my website:

"Of Cats and their most Wonderful Righting Reflex"


Footnote: The primary source for Marey's 1894 studies is the following:

Étienne-Jules Marey, “Des mouvements que certains animaux exécutent pour retomber sur leurs pieds, lorsqu’ils sont précipités d’un lieu élevé“, La Nature, 1119, 10 Novembre 1894

Near the end of this article he makes the following definitive statement (translation mine, so apologies to French speakers): "First of all, the inspection of these figures [photos of falling cats] rules out the notion that the animal imparts a rotational motion on itself by thrusting against the hands of the experimenter. [This conclusion follows] because the first frames of the two series [of photos of a falling cat] show that in the first instants of it its fall, the cat as yet has no tendency to turn from one side nor the other. Its rotation only begins with the twisting of its waist."

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    $\begingroup$ This answer has 4 spinning animated cats, and yet only 7 upvotes? This is a GREAT answer! $\endgroup$ – corsiKa Sep 16 '15 at 16:03
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Conservation of angular momentum would apply if the astronaut was a still rod. Which to a first approximation they might be.

However, since they can rotate their body, similar to how a falling cat can rotate to land feet down, they can rotate their upper half, while attempting to keep lower half still, and when they capture some stationary point, would then release the momentum to the lower half.

And if they never capture anything stationary, then all the twisting in the world is just whistling in the wind. This of course is inside the vehicle. Should they be outside, this is whistling in the vaccum.

While outside the vehicle they are ALWAYS attached to something. (MMU's on one or two shuttle flights being the exceptions that make the rule. They stopped using them after a few uses). Letting go, is a horribly bad idea while on EVA. They are also clipped via a cable. (There is the SAFER pod they wear, that are like low performance, baby MMU's for emergency fly back if they did get disconnected).

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    $\begingroup$ Apologies, but conservation of angular momentum always holds unless you grab onto something else, regardless of how much you twist. It is a standard 'paradox' that astronauts - and cats - can still change their attitude whilst conserving angular momentum; please see the answers on the Physics thread to see some methods. This question is about whether such manoeuvres, or similar ones, are actually used in space. $\endgroup$ – E.P. Nov 29 '13 at 18:10
  • $\begingroup$ @EmilioPisanty Thus my point. Because they can rotate half their body, when they finally connect with something stationary (such that it, in an orbiting vehicle) the rest of their body follows along, maintaining the conservation of momentum. $\endgroup$ – geoffc Nov 29 '13 at 18:22
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    $\begingroup$ You are missing my point. The manoeuver is to turn without touching anything else, and it is indeed possible. The astronaut's total angular momentum will be preserved, ideally at zero, but by changing the relative angular momenta of different parts of her body she may still achieve a net rotation. Do read the link, or my extended question. $\endgroup$ – E.P. Nov 29 '13 at 18:27
  • $\begingroup$ @EmilioPisanty One of the easiest ways to do that is by stretching one arm while holding the other on your chest, and then fast moving the first one to your chest and stretching the one that was previously on your chest. Some exercise tools will have you swing like that too. I wouldn't know if astronauts actually use such movement (could be done differently too, this is just one example), likely not during EVA since they have mobility units and are attached by a cable, but they have some funny ways inside the station, S. Williams and K. Nyberg used their hair tips to push against even. $\endgroup$ – TildalWave Nov 29 '13 at 18:33
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    $\begingroup$ @EmilioPisanty is correct. The answer should be edited, since the conservation of angular momentum always applies. The first paragraph is very misleading. While angular momentum is conserved (always!), the angular orientation of a flexible object is not. That allows cats, high divers, and astronauts to change their orientation without touching anything or pushing against the air. $\endgroup$ – Mark Adler Nov 29 '13 at 19:57

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