+ double DistanceAt(const glm::dvec3 &p) const noexcept { return glm::length(p); }
+ /// acceleration due to gravity at given point
+ glm::dvec3 GravityAt(const glm::dvec3 &p) const noexcept { return NormalAt(p) * (-GravitationalParameter() / glm::length2(p)); }