Which orbital perturbation is primarily caused by Earth's oblateness?

• A. Atmospheric drag causing semi-major axis decrease.
• B. Solar radiation pressure causing mean anomaly drift.
• C. Tidal effects causing inclination changes.
• D. Precession of orbital elements, including RAAN drift (Omega) and argument of perigee.

Answer: (D)

Earth’s equatorial bulge makes its gravity field non-spherical, and the dominant non-uniform term (often called J2) pulls on satellites in a way that slowly rotates the orbit itself rather than just shrinking or shifting it. This causes a steady, long-term drift or precession of certain orbital elements: the right ascension of the ascending node (Ω) and the argument of perigee (ω) both change over time. The rates of these drifts depend on the satellite’s orbit (including inclination, semi-major axis, and eccentricity) and on Earth’s size and the J2 value, while the semi-major axis stays nearly constant in the absence of other forces. That combination—precession of the orbital plane and of the ellipse orientation within that plane—is the hallmark of oblateness.

So the correct choice describes precession of orbital elements, including Ω drift and the argument of perigee, caused by Earth's oblateness. The other options describe perturbations dominated by other forces: atmospheric drag primarily reduces the semi-major axis, solar radiation pressure mainly alters other elements depending on area-to-mass, and tidal effects are not the primary oblateness-driven mechanism for near-Earth orbits.