How was Mercury's unusual spin-orbit coupling discovered?
A)Earth-based telescopes watched the movement of surface features on Mercury as it rotated.
B)It was determined by watching the phases of Mercury as observed from Earth.
C)It was not observed until a spacecraft was put into a permanent orbit around Mercury.
D)Radar waves were bounced off Mercury from an Earth-based radio telescope, and the Doppler shift of the reflections was measured.

Question

Quizplus · Accepted Answer

Radar astronomy was crucial in discovering Mercury's spin-orbit coupling. By measuring the Doppler shift of radar waves bounced off Mercury, astronomers were able to determine that Mercury rotates three times on its axis for every two orbits it makes around the Sun. This was a significant discovery that corrected earlier assumptions about Mercury's rotation period.

How Was Mercury's Unusual Spin-Orbit Coupling Discovered