intersectSphere( sphere, target ) {

		_vector$a.subVectors( sphere.center, this.origin );
		const tca = _vector$a.dot( this.direction );
		const d2 = _vector$a.dot( _vector$a ) - tca * tca;
		const radius2 = sphere.radius * sphere.radius;

		if ( d2 > radius2 ) return null;

		const thc = Math.sqrt( radius2 - d2 );

		// t0 = first intersect point - entrance on front of sphere
		const t0 = tca - thc;

		// t1 = second intersect point - exit point on back of sphere
		const t1 = tca + thc;

		// test to see if t1 is behind the ray - if so, return null
		if ( t1 < 0 ) return null;

		// test to see if t0 is behind the ray:
		// if it is, the ray is inside the sphere, so return the second exit point scaled by t1,
		// in order to always return an intersect point that is in front of the ray.
		if ( t0 < 0 ) return this.at( t1, target );

		// else t0 is in front of the ray, so return the first collision point scaled by t0
		return this.at( t0, target );

	}
Intersects this ray with the given sphere, returning the intersection
point or `null` if there is no intersection.

@param {Sphere} sphere - The sphere to intersect.
@param {Vector3} target - The target vector that is used to store the method's result.
@return {?Vector3} The intersection point.