astro.Function.hillRadius

@elite-dangerous-almanac/core / astro / hillRadius

Function: hillRadius()

hillRadius(body, primary): number | null

Defined in: src/astro/body-physics.ts:261

The radius of a body's Hill sphere, in metres — how far its own gravity wins against the body it orbits.

Parameters

body

BodyProperties

The body whose sphere is wanted. Needs SemiMajorAxis and a mass.

primary

BodyProperties

The body it orbits. Needs a mass.

Returns

number | null

The Hill radius in metres, or null when a mass or the orbit is missing.

Remarks

r_H = a · (m / 3M)^(1/3). Anything orbiting the body must stay inside this radius to remain bound to it, so it is the reach a moon has over nearby material — which is what makes a small moon a shepherd of a ring it does not sit in.

The semi-major axis is used as written, so the figure is the mean-distance Hill radius. On an eccentric orbit the sphere shrinks at periapsis; recompute with orbitExtents(body)?.periapsis in place of the axis when that matters.

Throws

If either argument is not an object.

Example

import { hillRadius } from '@elite-dangerous-almanac/core/astro/body-physics';

// Earth about the Sun: about 1.5 million km.
hillRadius({ MassEM: 1, SemiMajorAxis: 1.496e11 }, { StellarMass: 1 }); // -> 1496417737.0…