AdvanceSpace

A spacecraft is acted upon by torques, the study of which is called attitude dynamics. Torque is the time rate of change of angular momentum. Momentum is the product of body mass and velocity. Linear momentum occurs in a straight line; angular momentum occurs in a curved line.

Attitude Dynamics

Disturbance torques are small, but applied over time add up to significant spacecraft orientation error. They are induced by the Earth's magnetic field, gravity gradient, atmospheric drag and solar radiation.

Magnetic fields generated by the Earth's molten core affect satellites most in Low Earth Orbit (LEO) and less in Geosynchronous Earth Orbit (GEO). Particles in the magnetosphere charge spacecraft. This charge creates a dipole, like a magnet. The spacecraft then responds to Earth's magnetic field as would two small magnets in close proximity.

Gravity gradient refers to the difference in gravitational force along a spacecraft. Although apparently trivial, this differential may destabilize or be used to stabilize a body in space depending on structural design.

Aerodynamic torques arise from atmospheric drag in LEO. Spacecraft have different drag coefficients across their body. When exposed to the atmosphere, these produce varying drag forces that twist the spacecraft about.

Solar radiation pressure is a result of photons emitted from the Sun. Photons are infinitesimal packets of energy that travel at the speed of light in waves from light sources. They are massless but have momentum. This momentum is transferred to the spacecraft as photons strike its surface generating disturbance torques.