While geostationary satellites monitor rapid meteorological events, sun-synchronous platforms are utilized for compiling global land-cover inventories. What is the defining orbital characteristic of a sun-synchronous satellite?
- AIt maintains a constant altitude exactly matching Earth's escape velocity.
- BIts orbital plane precesses eastward at a rate that matches Earth's revolution around the Sun crossing the equator at the same local solar time.Correct
- CIt orbits exactly at the Earth's rotation rate, but in a highly elliptical path to focus continuously on the polar ice caps.
- DIt remains stationary relative to the sun, meaning the dark side of the Earth is never imaged.
Solution
Concept
Orbital nodal precession of sun-synchronous satellites
Formula
dOmega / dt = - (3/2) × J2 × (Re / p)2 × n × cos(i) approx 0.9856 deg/day
Step-by-step solution
1. Earth's equatorial bulge (oblateness J2) exerts a gravitational torque causing retrograde orbital precession. 2. Selecting an inclination slightly greater than 90 degrees (retrograde orbit) causes the plane to precess eastward. 3. Tuning this precession rate to match Earth's mean orbital motion (~0.9856 deg/day) ensures equator crossing at constant local solar time. Answer: B
Explanation
The defining orbital characteristic of a sun-synchronous satellite is that its orbital plane precesses eastward at the exact rate at which Earth revolves around the sun (approx 0.986 degrees per day). By matching this nodal regression to Earth's annual rotation, the orbital plane maintains a constant angle relative to the sun, crossing the equator at the identical local solar time on each pass. Thus, option B is correct.
Why the other options are wrong
A) Satellite speed in low Earth orbit is ~7.5 km/s, well below Earth escape velocity (~11.2 km/s). C) Orbiting at Earth rotation rate describes geosynchronous orbits, not sun-synchronous polar orbits. D) The satellite orbits the rotating Earth and images both day and night sides.
Shortcut
Sun-synchronous = Precession matches Earth revolution (~0.986 deg/day) = Constant local solar time.
Common mistake
Confusing sun-synchronous orbits with geostationary orbits or believing the satellite stays fixed relative to the sun.
Correct answer: B) Its orbital plane precesses eastward at a rate that matches Earth's revolution around the Sun crossing the equator at the same local solar time.