The Hill Sphere Radius calculator computes the radius from a planet where an object is no longer under the dominant influence of the gravity of the planet but is now under the dominant influence of the gravity of the star.
INSTRUCTIONS: Choose units and enter the following:
Hill Sphere Radius (r): The calculator returns the radius in kilometers. However, this can be automatically converted to other distance units (e.g. miles) via the pull-down menu.
`r = a(1-e)(m / (3M))^(1/3)`
Thanks to Robert Frost, Instructor and Flight Controller at NASA for posting this formula on Quora.