Lists the space systems visible to this client.

        Space systems are returned in lexicographical order.

        :rtype: :class:`.SpaceSystem` iterator

def list_space_systems(self, page_size=None):
        """
        Lists the space systems visible to this client.

        Space systems are returned in lexicographical order.

        :rtype: :class:`.SpaceSystem` iterator
        """
        params = {}

        if page_size is not None:
            params['limit'] = page_size

        return pagination.Iterator(
            client=self._client,
            path='/mdb/{}/space-systems'.format(self._instance),
            params=params,
            response_class=mdb_pb2.ListSpaceSystemsResponse,
            items_key='spaceSystem',
            item_mapper=SpaceSystem,
        )