In gameplay, all planets have a Circle of Influence, which is the area around the planet where its gravity begins to influence a spacecraft. This is depicted as a grey circle around the planet. Any spacecraft that crosses into the circle is subject to influence/capture by the planet's gravity.
A spacecraft will achieve a stable orbit if it has sufficient velocity (indicated by the central green central section in the Velocity Bar). A spacecraft will "fall" towards the planet and crash onto it if its velocity is too low (if it falls into the orange section at the bottom of the Velocity Bar). A spacecraft will leave orbit if its velocity is greater than the planet's escape velocity, or when attempting to enter orbit, the spacecraft will perform a slingshot maneuver and gain more speed (if it climbs into the red section at the top of the Velocity Bar).
In reality, the gravitational pull of a planetary body is weaker the further away an object is from it. This is difficult to represent in game mechanics, so planetary bodies are simply depicted as having a simple "circle of influence" around it.
In the generality of physics, the larger the size of a planetary body, the greater it's mass will be; and the greater it's mass, the stronger its gravity will be; therefore the further the distance its gravity will reach. Simply speaking, Jupiter's gravity is obviously "stronger" and larger than the Moon's gravity.
See sphere of influence on Wikipedia.
- A "Sphere of Influence" is a truer term in spacial representation, because a sphere (and our universe) is three-dimensional. Since Space Agency is two-dimensional, the more appropriate term to use for gameplay would be Circle of Influence.
- A planet's Circle of Influence should not be confused with its Hill Sphere. A planet's Hill Sphere represents the entire area in which its gravity has influence. Earth's Hill Sphere reaches very far beyond the moon, to over one million miles in distance. In gameplay, a Circle of Influence is only in the immediate area around a planet.