What is the slingshot and why does NASA use it?

Answer 1

"Sling Shot" is a NASA slang term for Gravity Assist maneuvers to help propel satellites further into space and at more speed. This is due to the fact that deep space probes have limited, finite amounts of propellant aboard for maneuvering operations once they reach their destinations, and they don't want to use it all up just getting there.

It's used to get a spacecraft to a particular point in space at a particular time. Space probes are launched at the best windows of opportunity to enable them to get to their target locations at a particular time, when a planet, comet, moon, etc., is at its closest orbit or approach, or the best position for a spacecraft rendezvous. Using gravity assist enables NASA to get satellites to their destinations within that window of opportunity; otherwise, in some cases, it could be months, years, or decades before such a window presents itself again.

Essentially, a spacecraft is ordered to head toward a planetary body or other mass with significant gravity. As the spacecraft encounters the gravity pull effects, it increases in speed. At a calculated moment, the spacecraft is ordered into a trajectory around the planetary body that will propel it at increased speed toward its next destination in space,and at a significant amount of velocity that will enable it to break free of gravitation pull of the body it used for an assist. That could either be its final destination, or a rendezvous with another planet for another gravity assist.

The long acceleration and swing around a planet and subsequent increase in speed coming out of the assist maneuver is how it earned the name "Sling Shot".