In satellite dynamics, which bodies primarily cause long-term orbital disturbances of a geostationary (geosynchronous) spacecraft—lunar gravity, solar gravity, Earth's oblateness/drag, or a combination?

Introduction / Context:
Geostationary satellites do not remain perfectly fixed without active station-keeping. Small but continuous perturbing forces gradually pull the spacecraft away from its assigned “box,” requiring corrective maneuvers. This question checks awareness of the main disturbance sources.

Given Data / Assumptions:

• A satellite is parked in the geostationary belt near 35,786 km altitude.
• We consider long-term, secular and periodic perturbations.
• Atmospheric drag is negligible at GEO, but Earth’s non-spherical gravity field is not.

Concept / Approach:

There are three principal drivers: (1) Third-body gravitational forces from the Moon, (2) Third-body gravitational forces from the Sun, and (3) Earth’s oblateness (J2 and higher harmonics) and geopotential asymmetries. Solar radiation pressure can also contribute to drift in inclination and longitude.

Step-by-Step Solution:

Verification / Alternative check:

Station-keeping budgets in GEO always allocate propellant for north–south (inclination) and east–west (longitude) control to counter lunar/solar gravity and Earth’s geopotential effects.

Why Other Options Are Wrong:

• Moon or Sun alone: incomplete—both act.
• Earth alone: incomplete—non-spherical gravity is real, but not the only source.
• None: contradicts operational experience with routine station-keeping.

Common Pitfalls:

• Assuming geostationary means truly motionless; in reality, continual corrections are needed.

Final Answer:

All of the above