Peri

 Gemini Ai:

Gravity-assist; all fine and good that Artemis will be speeding up thanks to gravity-assist from the Moon, but won't the Moon be pulling Artemis back once the latter starts returning to Earth??

That is a sharp observation. It seems counterintuitive that a celestial body can "give" speed to a spacecraft without immediately "taking it back" as the craft pulls away.

The resolution lies in the fact that the Moon is not stationary; it is moving in its own orbit around the Earth at approximately 1,022 meters per second. A gravity assist is less about "falling" into a gravity well and more about hitching a ride on a moving target.

The Frame of Reference

To understand why the speed isn't lost, we have to look at the maneuver from two different perspectives:

Relative to the Moon: As Artemis approaches, it speeds up due to gravity. As it departs, it slows down by the exact same amount. In this local frame, there is no net gain in speed—only a change in direction.

Relative to the Earth (or Sun): This is where the magic happens. Because the Moon is moving, Artemis "steals" a tiny fraction of the Moon's orbital momentum.

The "Tennis Racket" Analogy

Think of the Moon as a moving tennis racket and Artemis as a tennis ball.

If you throw a ball at a stationary racket, it bounces back at the same speed it arrived.

If you swing the racket toward the ball, the ball leaves the strings with its original speed plus the speed of your swing.

When Artemis flies "behind" the Moon in its orbital path, the Moon’s gravity pulls the spacecraft along with it. By the time Artemis begins to move away, it has already been accelerated forward by the Moon’s own orbital velocity.

Why the Moon doesn't "pull it back"

While the Moon does exert a backward pull as Artemis departs, it is pulling on a spacecraft that is now traveling much faster relative to the Earth than it was before the encounter.

The "cost" of this maneuver is technically a slowdown of the Moon. Because the Moon is trillions of times more massive than the Artemis capsule, the Moon slows down by a distance roughly equivalent to the width of a single atom over billions of years, while the light spacecraft gains several hundred meters per second in velocity.

Key Factors in the Artemis Return

For the return trip, NASA uses a Powered Flyby. This involves:

Vectoring: Aiming the craft so the Moon’s gravity bends the trajectory toward Earth.

The Oberth Effect: Firing engines at the "Periapsis" (the closest point to the Moon). Burning fuel while moving at high speeds deep in a gravity well is significantly more efficient than burning it in empty space, allowing Artemis to maximize the "kick" it gets for the trip home.

                                                                                  

                                                            Source: Wikipedia