Which module should be used to achieve a visual effect of asteroids shrinking as they approach a planet?

To achieve the visual effect of asteroids appearing to shrink as they approach a planet, the Size Over Lifetime module is the most suitable choice. This module allows you to manipulate the size of an object over its lifetime, enabling it to gradually decrease in scale as it moves closer to its target. By adjusting the size curves, you can create an effect where the asteroids appear to get smaller, enhancing the illusion of depth and distance in the scene.

The other modules, while useful for different visual effects, do not address the specific requirement of changing size. Speed Over Lifetime relates to the velocity of movement, Color Over Lifetime focuses on the color changes an object undergoes, and Rotation Over Lifetime deals with the rotation of an object rather than its size. Therefore, for the intended effect of asteroids shrinking visually, Size Over Lifetime is the appropriate module to utilize.