Final answer:
The size of the smallest feature visible in Saturn's rings, as seen from Earth at closest approach with a resolution of 0.09', can be calculated using the small-angle formula. The distance from Saturn to Earth would need to be approximately 152,715 kilometers to make out detail in its rings as small as 20 km.
Step-by-step explanation:
The size of the smallest feature visible in Saturn's rings, as seen from Earth at closest approach with a resolution of 0.09', can be calculated using the small-angle formula. With the given angular resolution of 0.09' and assuming the smallest feature size to be 20 km, we can use the formula: angular resolution = 206265 * (feature size / distance).
Rearranging the formula, we find that the distance from Saturn to Earth would need to be approximately 152,715 kilometers to make out detail in its rings as small as 20 km.