Low-Earth Orbit Perturbations up to ~720 km Altitude – Dominant Effects Which of the following physical effects measurably perturb a satellite's orbit up to an altitude of about 720 km above Earth's surface?

Introduction / Context:
Low-Earth Orbits (LEO) are subject to several non-Keplerian perturbations. Mission design and orbit maintenance require understanding of which effects dominate at various altitudes. Around a few hundred kilometers up to roughly 700–800 km, multiple perturbations act concurrently with different time scales and magnitudes.

Given Data / Assumptions:

• Altitude regime: up to approximately 720 km.
• Typical LEO satellites with modest area-to-mass ratios.
• Keplerian two-body baseline with perturbations superposed.

Concept / Approach:

Earth’s gravity field (J2 and higher): Causes node regression and apsidal rotation; effects are significant throughout LEO. Third-body forces (Sun/Moon): Though weaker than J2 and drag, they produce secular and periodic changes in orbital elements, especially inclination and right ascension of ascending node. Aerodynamic drag: Residual atmosphere below ~800–1000 km induces semi-major axis decay and reduces perigee height; magnitude depends on solar activity and ballistic coefficient.

Step-by-Step Solution:

Verification / Alternative check:

Standard orbit determination models (e.g., SGP4/SDP4 families) include J2 and drag for LEO; high-fidelity propagators include Sun/Moon gravitation even at LEO altitudes.

Why Other Options Are Wrong:

(a), (b), (c) alone each omit other non-negligible contributors in this altitude band. (e) contradicts observed decay and nodal drift behavior of LEO satellites.

Common Pitfalls:

Assuming drag is negligible above 400–500 km regardless of solar cycle; ignoring J2-driven precessions that are pivotal for Sun-synchronous orbit design.

Final Answer:

All of the above