Mars completes one orbit around the Sun in approximately two Earth years. Mars orbits at an average distance to the Sun of about 1.5 AU, and Mars' mass is about 1/10 of Earth's mass. Therefore Mars' orbital speed is _____ the orbital speed of Earth.
A) about 0.8 times
B) 1.5 times larger than
C) about 1/10 of
D) about 3 times

Question

Quizplus · Accepted Answer

Mars' orbital speed is determined by its distance from the Sun. Since Mars is 1.5 AU from the Sun, its orbital speed is slower than Earth's, which is 1 AU from the Sun. Kepler's third law implies that the orbital speed is proportional to the inverse square root of the orbital radius, so Mars' speed is about 0.8 times that of Earth's.

Mars Completes One Orbit Around the Sun in Approximately Two