What
you see in your "Heads-Up-Display" is the path followed
by the spacecraft pointing beam (the spin axis) in the Earth
nearby-space. Instead of the "red beam," you see its projection
on a giant imaginary screen which is traveling with the
Earth. Superimposed (in blue) you see the maximum offpointing
circle. Since your view is travelling with the Earth, stationary
objects like the stars, or the center of the rosette, appear
to drift away in the opposite direction.
The
center of the rosette (marked as a small red circle) is
what you actually move when you fire the thruster. This
centre happens to be the tip of the "Angular Momentum Vector",
and remains constant until you fire the thruster. When you
fire, it moves in the direction of the torque applied (the
orange arrow). In Ulysses case, both the thruster and its
corresponding torque, do continuously rotate with the rest
of the spacecraft.
The
trick is to move the "Angular Momentum Vector" (rosette
centre) on top of the elusive spin axis. If you do that,
you automatically set the spacecraft to "pure spin", and
reduce the nutation amplitude to zero!
Once
you have almost no nutation, the next time you fire you
will induce some. This happens because you push the
"Angular Momentum Vector" away from the spin axis, thereby
iniciating the oscillation. Think of it in this way: If
you push a swing at the right time you may be able to stop
it, but if you push it when at rest, you will set it to
oscillate. It is Ok to alternate between the following two
actions:
- Fire
to move towards the Earth (but inducing some nutation),
and then...
- Fire
to control nutation (but attempting to move towards the
Earth as well).
If left alone, the nutation (in this game) grows "exponentially".
This is just another way to say that the bigger the amplitude,
the faster it grows. Therefore you are better off by keeping
it rather small.
NOTE:
Understand that in real life, and under normal circumstances,
the "passive nutation dampers" (some clever friction devices)
will take care of the nutation, that will go down to zero
without "mission control" intervention. The "Nutation Anomaly"
only occurs in certain phases of the Ulysses Mission, when
the Sun's thermal energy is strong enough to make the flexible
booms oscillate (phenomenon discovered in-flight). The nutation
feeds on this oscillation, and the Sun "pumps" into the
system more energy than the nutation dampers can handle.
It is then, when we need to execute the real Nutation Control
Manoeuvres, which are based on the same principle than this
game (although they are slightly more complex).
Back
To Game
Nutation
Game copyright European Space Agency 1996
(Developed by Raul Garcia-Perez of the European Space Agency
office at JPL).
Send Comments to Bruce
E. Goldstein.
