union( sphere ) {

		if ( sphere.isEmpty() ) {

			return this;

		}

		if ( this.isEmpty() ) {

			this.copy( sphere );

			return this;

		}

		if ( this.center.equals( sphere.center ) === true ) {

			 this.radius = Math.max( this.radius, sphere.radius );

		} else {

			_v2$3.subVectors( sphere.center, this.center ).setLength( sphere.radius );

			this.expandByPoint( _v1$6.copy( sphere.center ).add( _v2$3 ) );

			this.expandByPoint( _v1$6.copy( sphere.center ).sub( _v2$3 ) );

		}

		return this;

	}
Expands this sphere to enclose both the original sphere and the given sphere.

@param {Sphere} sphere - The sphere to include.
@return {Sphere} A reference to this sphere.