Look at an intersting example of circular motion. Gravity acts like a string to hold satellites in a circle. When spacecraft arrive at their destination planet they generally must slow down to enter orbit. The rockets used for orbital maneuvers are typically fired at full throttle, so the amount of force they deliver is constant. Inertial guidance systems integrate the accelerations experienced by the vehicle, but small errors in measurement cause increasingly large errors in estimated position as the length of time increases.
Spacecraft typically carry some sort of optical system for viewing stars.
By measuring the angle between the spacecraft and several distant stars, trigonometry can be used to determine location. The figure shows the optical system and associated computer for calculating spacecraft position relative to the stars. Once in orbit around the destination planet, the man-made satellite must maintain a safe distance from the planet's surface. The satellite must now counter the effect of the planet's gravitational force which is constantly pulling it into an ever increasing lower orbit. To do so, the orbiter will occasionally fire its rockets boosting it away from the planet, but not so far away that it free falls outside the planet's gravitational force.