Re: Planet X: Moon SWIRL

Nancy,
 
I have have carefully read your post in which the moons are
described to "spin in a slow whirlwind behind Planet X", but
cannot decipher what is meant in it by the word "spin".
 
The Earth spins, one rotation per 24 hours, causing day and
night.  Likewise, all the other planets in our Solar System
spin at their own rates, causing day and night cycles of
different lengths.  Jupiter, though very large, spins rapidly,
so that it completes a rotation in about ten hours.  Venus,
slightly smaller than Earth, rotates very, very slowly, and
in the opposite direction from most of the other planets.
The Moon spins slowly, too, rotating just once each time it
orbits Earth, in about 28 Earth days.
 
Artificial satellites always spin to some exent.  If they are
made to constantly face one side toward the Earth, as the Moon
does, then they will spin one rotation per orbit, just as the
Moon does.  Since a satellite in low Earth orbit takes about
100 minutes to go around the Earth, a satellite there which
constantly faces Earth would also rotate once per 100 minutes.
 
Hundreds of satellites and probes have been deliberately given
a spin once they are in Space, to maintain a constant attitude.
When an object is spinning, it keeps on spinning in that same
direction until something makes it stop spinning.  That is used
to great advantage in Space, where the lack of air friction
means a spacecraft can keep on spinning at a constant speed for
years or even decades.  The Pioneer 10 and 11 spacecraft were
each given an initial spin shortly after being launched in 1972
and 1973, and continue to spin now, far beyond Pluto's orbit.
 
Gyroscopes in spacecraft are used both to measure the attitude
and rotation of the spacecraft, and to control that rotation.
The gyroscopes themselves spin at very high speed.  The Hubble
Space Telescope is just one of hundreds of spacecraft which use
or have used gyroscopes for this purpose.  The Apollo flights
to the Moon used gyroscopes to measure the attitude of the
spacecraft, but not to directly control it, since it was easier
to use thrusters for all attitude control on a flight lasting
less than two weeks.  The Apollo spacecraft was given a slow
spin so that the heat of sunlight would be evenly distributed
over the entire surface.
 
>   This is not explored by man, who strives to move 
>   directly in space and treats any spin in an object 
>   under their control as a problem to be corrected 
>   promptly, as in "the probe has developed a spin and 
>   is threatening to spin out of control".  The reasons for
>   the spin having developed in the first place is treated 
>   as an irrelevancy, and the only issue whether or not 
>   the probe is under control.
 
Actually, the only time a spacecraft "spins out of control"
is when something specific causes it to do so, and the specific
cause is of great concern.  In 1966, astronauts Dave Scott and
Neil Armstrong were in orbit in a tiny Gemini capsule, docked
to a small Agena rocket upper stage which had been launched
into orbit for them to practice approach and docking.  Shortly
after the docking (the first ever), the spacecraft unexpectedly
began to slowly rotate.  When they couldn't stop the rotation,
they guessed that the problem was in the Agena, and undocked
from it, expecting to be able to move away from the Agena.
 
Instead, the spin immediately accelerated.  The astronauts had
guessed wrong: the problem was that one of the thrusters on the
Gemini capsule had stuck open.  They had to shut down the main
thruster system and use the much smaller re-entry thrusters to
stop the spin.  If they had known immediately *which* thruster
was malfunctioning, they'd have shut down only the thrusters in
that one cluster, and used the other main thrusters to stop the
spin while still attached to the Agena.
 
 
A second, colloquial meaning of the word "spin" is "the motion
of a body in a circle or loop", such as that of a car around a
racetrack, or the orbit of a satellite around the Earth, a moon
around a planet, or the planets around the Sun.  For example,
the Moon may be said to "spin around the Earth", and you can
"take a spin around the block" in your car.
 
Part of what is said in your message is about the first kind
of spin, and part of it is about the second kind of spin.  Yet
the writer was apparently unaware of the difference.
 
>   The spin is suppressed by the little jets that allow
>   man to control his probes when their trajectory needs
>   to be corrected, and this thus allows mankind to feel
>   smug about his knowledge of how things work.  The moons
>   of Planet X, which trail it like a string of pearls out
>   in space, have no such little jets, so nature, not man,
>   rules, and the full RESULT of a spin out in space can be
>   observed.  Why do the moons trail, and spin in a slow
>   whirlwind behind Planet X, rather than orbit the planet?
 
This begins using the word "spin" to mean "rotation of a body",
and ends using the word "spin" to mean "bodies moving along
circular or looping paths".  The writer was apparently unaware
that the definition changed in mid-paragraph.
 
  -- Jeff, in Minneapolis