Am 06.10.2018 um 09:47 schrieb Kenneth:
>> If free-fall without air resistance is what you want to model, you
>> should use a /single/ rotation about an arbitrary axis.
>>
>> This is because without external forces, angular momentum is conserved,
>> i.e. the axis of rotation doesn't change.
> 
> So an analogy would be a chicken on a spit, roasting over an open fire while the
> single rod is rotated? (I must be hungry at the moment...)

Yes.

> If we're on the same wavelength, that goes against what I *think* my eyes see
> when, for example, the ISS astronauts have some playful fun by spinning
> weightless objects for the camera. It looks like two-axis (POV-Ray) rotation to
> me.

That may be because they are not actually free-falling: They are falling
in a medium of air. And though the object in its entirety is stationary
with respect to that medium, the "ends" of the object are not, and thus
are experiencing aerodynamic forces.

> But that's only my recollection; I need to take another look at some of
> those videos. (BTW-- 2001:A SPACE ODYSSEY recently celebrated its 50th
> anniversary, and there are some space shots that have asteroids tumbling near
> the Discovery. I always thought they looked a bit fake-- because they are
> spinning around only one axis. Granted, Stanley Kubrick spared no expense in
> getting scientific details right; but my opinion is that the spinning of the
> asteroids (as special-effects models) had to be constrained, simply as a
> practical matter for filming. A chicken on a spit, in other words.)

Technically, free-falling objects in an inhomogenous gravitational field
(i.e. near another body of mass) deviate slightly from the
chicken-on-a-spit pattern. But in order for this to be noticeable the
object has to be large in dimensions, and even for an object as large as
Earth the precession period is in the order of 10k years.

Also, if 2-axis rotation of asteroits were more realistic, Kubrick would
have made his special FX team work overtime to make the impossible possible.

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