Satellites: The moons of Mars, Phobos (Fear) and Deimos (Terror), are very close to the planet compared to Earth's Moon. Their orbital radii are 9,378 km and 23,459 km respectively. What is the ratio of the orbital speed of Phobos to that of Deimos?
A) 0.2528
B) 0.3998
C) 1.582
D) 2.858
E) 3.956

Question

Quizplus · Accepted Answer

The orbital speed of a satellite is inversely proportional to the square root of its orbital radius. Therefore, the ratio of the orbital speeds of Phobos to Deimos is the square root of the inverse ratio of their orbital radii: sqrt(23,459 / 9,378) = 1.582.

Satellites: the Moons of Mars, Phobos (Fear) and Deimos (Terror)