Warp wrote:
> scott <sco### [at] laptopcom> wrote:
>> "Each turn robs the planet of angular momentum"
> 
>> Unfortunately not...
> 
>   I don't see, technically speaking, why not.
> 
>   Let's assume we have a big object in a weightless space in vacuum
> rotating. Can this rotation be stopped by the object itself without
> applying external forces?
> 
>   The answer is yes. The most obvious way to do this is to fire up some
> rockets in specific directions. This causes the rotation to slow down,
> basically by expelling material at high speed from the object.
> 
>   A slightly less obvious way is to rotate some significant part of the
> object at a different speed. Why does this slow down the overall rotation?
> It's because part of the angular momentum is converted into heat due to
> friction, and this heat dissipation is taken away from this angular momentum.
> 
Nope.

As the part is spun up, the larger part is spun in the opposite 
direction to a degree determined by their relative masses.  The net 
angular momentum of the system stays constant assuming that this is a 
closed system.

When the spin is removed as you suggest by friction, slowing the 
'rotating part' applies an equal but opposite force to the main body. 
This results in the assembly as a whole returning to the original rate 
and direction of spin (or rest).  It can however be left pointing in a 
different direction.

Angular momentum is conserved.

The heat dissipated is simply the energy used to spin up the system 
being returned.

What is useful is that the system - say a satellite - can be pointed to 
and kept at different directions and that unwanted rotation can be 
countered by precisely controlling a number of gyroscopes.

Real satellites are subject to external forces like drag, outgassing and 
uneven light pressure that can give them unwanted spin.  Gyroscopes can 
be used to soak this up.  Eventually if the external forces keep 
building up in the same direction the gyroscopes can't be spun up safely 
any more to counter the rotation.  The satellite operator uses thrusters 
to really counter the rotation with off-centre thrust.  At the same time 
the gyroscopes are spun down and returned to a rate of rotation that is 
within a comfort zone and correct to match any residual spin after the 
thruster stops.

When the gyroscopes seize or when the thruster propellant is exhausted 
the satellite could no longer be controlled.  Usually some time before 
this the satellite is retired to a safer parking orbit or in some cases 
is even de-orbited.  The idea is to do this while the operator can 
control the orientation.

Hubble is a good example.  It has 6 large gyroscopes.  Originally 3 were 
used to be able to point and hold very steady in any given orientation. 
   The others were provided as spares.  Quite a few gyroscopes have 
failed and had to be replaced during servicing missions.  IIRC it was 
out of action for a while when 4 gyros failed but they developed a way 
to operate with only 2 but with reduced capability.  At the moment it is 
operating with close to no redundancy and NASA has concluded that a 
shuttle won't be used to perform another service mission.  The proposal 
was to de-orbit it while they have control.  This caused furore and NASA 
was reconsidering.  Last I heard a 5th servicing mission was back on for 
next year.  Pessimistically I'd guess that no mission will happen and 
that good old Hubble is either going to be brought down or otherwise 
become unusable when the next gyro fails.

Another aside - There is some research going on to develop 
micro-thrusters that could potentially do away with gyroscopes in future 
satellites.  Each one provides a once-off and very small amount of 
thrust.  A very large number are built into a grid.  Imagine something 
like a chip built using semiconductor fabrication holding 1000x1000 
little electronically fired cells.  Each contains a microgram of solid 
propellant.  Place lots of these chips at strategic points on the 
outside of the satellite.

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