Gravitational Pull (Gravity Wells)

Lore & Background

In the early days at KSC, junior engineers would stare at the space map and wonder why the faint circles around Kerbin and Mun looked so arbitrary. The answer, as the flight-dynamics team explained over cold coffee, was one of pragmatism: the simulation needed a clean cutoff so that a craft parked near Jool wouldn't also be tugged by Eeloo's distant pull. The three-radius rule became the unofficial 'handshake' between a body and everything orbiting it. Cross the line, and that body simply stops caring about you.

Kerbals who have made the long haul to the outer system speak of the gravity wells almost reverently. Leaving Kerbin's well is the rite of passage—every rookie feels the moment the navball's prograde arrow stops fighting them, the instant Kerbin's invisible hand releases. But the outer wells are deceptively generous; Jool's well is enormous, and a miscalculated periapsis can trap a craft in a slow, fuel-draining spiral for days of in-game time. Veterans say you never truly 'escape' a well; you merely trade one for another, and the trade is never free.

There is a quiet irony the KSC community loves to point out: in the real universe, gravity never truly ends. The well is a kindness the game extends to the player—a merciful boundary where an otherwise infinite tug simply stops. For a species that has only just learned to leave its home world, that mercy is everything.

Reader's Guide

RULE: Each body's gravity well extends to exactly three times its physical radius. Inside that circle, the body exerts gravitational acceleration on your craft. Outside it, that body's pull is zero—regardless of how close it looks on the map. This is a hard cutoff, not a gradual falloff.

WHY IT MATTERS: The well boundary is the line between 'bound orbit' and 'free coast.' You cannot plan a transfer, a capture, or a flyby without knowing where that line sits. A craft that is technically outside Kerbin's well is no longer in Kerbin's orbit; it is a free-flying object until the next well claims it.

COMMON FAILURE MODES: (1) Running out of propellant inside a well, leaving you in a decaying orbit with no escape energy. (2) Assuming the well edge is a soft gradient and mis-timing your burn, so you overshoot the target body's well and miss capture entirely. (3) Ignoring the well radius of a third body during a complex transfer, causing an unexpected perturbation right when you need a clean trajectory.

PRO TIPS: Before any interplanetary burn, open the target body's info panel and note the well radius. Use the space map's circle as a visual checkpoint: your periapsis on the target should sit comfortably inside that circle. For capture, aim your periapsis well within the well (roughly 1.5–2× the body's radius) to give yourself a generous burn window. When planning a flyby, remember the well edge is where the body's influence starts, so your closest approach should be inside it. Finally, if you find yourself outside every well with no target in range, you are in the 'open'—the most fuel-efficient part of any trajectory. Use that coast to reposition before the next well grabs you.

Did You Know?

• The space map's translucent circles are not just decorative; they are the primary planning tool for every interplanetary transfer, and veteran Kerbals often sketch their burn sequences directly onto those circles.
• In real orbital mechanics, a body's sphere of influence is determined by the mass ratio between it and its parent (e.g., the Sun), not by a simple multiple of its radius.