Why must a geostationary satellite orbit Earth in exactly $1$ sidereal day?

Approximately how long does it take for a spacecraft using current technology, such as a $Hohman$ transfer orbit, to travel from $Earth$ to $Mars$?

In a two-body system consisting of a star and a planet orbiting their common center of mass, what is the period required for both objects to complete one full orbit around the center of mass?

Approximately how long does it take the Moon to complete one orbit around the Earth?

Which factor is not needed when calculating the velocity of a satellite in a stable circular orbit around a planet?

Why do satellites launched into low-Earth orbits not remain there indefinitely?

How does friction with the atmosphere affect the speed of an artificial satellite in low Earth orbit?

Which of the following would cause the greatest increase in the acceleration of a satellite in circular orbit around Earth ($a=\frac{v}{r^2}$ or $a=\frac{\mathrm{GM}}{r^2}$)?