Space Forum / Space History / May 2004

No. An orbiting craft has to have some kind of pointing control, either
passive (spin stabilization) or active (thrusters, gyros, or momentum
wheels) to stay pointed in a constant direction. Otherwise, any kind of
small perturbing torque will cause it to drift away from the desired
attitude. For more detail, read on.

Let's break this problem into a couple of simpler ones first. First, we'll
consider the theoretical case where ISS is subject to zero perturbing
torques and starts out pointed perfectly parallel to the tangent of its
flight path. In the absence of disturbances, it should continue pointing in
the same direction in space. However, its flight path is a circle because
it is in orbit around the Earth. So a quarter orbit later, ISS will still
be pointed in the same direction, but its flight path will have rotated 90
degrees, so it will be pointing perpendicular to its flight path (away from
the Earth). A half orbit later, it will be pointed opposite its flight
path, and so on, until one orbit later, when it will be pointing parallel
to the flight path again.

OK, now we throw the perturbing torques into the mix. In low Earth orbit
where ISS flies, the main perturbing torques are gravity gradient torques
and aerodynamic torques. Gravity gradient torques occur because gravity
gets weaker with distance, so the force of gravity is stronger on the parts
of a spacecraft that are closer to the Earth. Aerodynamic torques occur
because the Earth's atmosphere does not end abruptly; its density
exponentially decreases to zero so that even in low Earth orbit, there is
still a very trace amount of residual gas molecules left.

ISS has flight attitudes, called torque equilibrium attitudes (TEAs), where
the gravity gradient and aero torques balance out. NASA likes to fly ISS in
these attitudes because it minimizes the amount of Russian thruster firings
required to desaturate ISS' gyros. And it happens that the flight attitude
you describe (ISS pointed along its flight path) is close to a TEA
attitude.

However, not all TEAs are created equal. The TEA described above is
unstable; in other words, any small perturbation away from the TEA will
cause ISS' attitude to diverge. So without active attitude control, ISS
could not stay pointed tangent to its flight path. It would drift out of
attitude until it reached a stable TEA (in the current configuration, this
would have the long axis of modules pointed at the center of the Earth,
with the solar arrays edge-on to the velocity vector), and then it would
slowly oscillate about that attitude.