With the external force from gravity, the direction of the body is changed. If the velocity and gravitational force have just the right balance, the body can move into orbit around the planet. If the gravitational force is too great, the body will be pulled down to the planet's surface. If the gravitational force is too low, the body will escape from the pull and move away from the planet. 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.