Identify the main idea of this paragraph:
it was a law not of men's choosing but made imperative by the circumstances of the space frontier. galactic expansion had followed the development of the hyperspace drive and as men scattered wide across the frontier there had come the problem of contact with the isolated first-colonies and exploration parties. the huge hyperspace cruisers were the product of the combined genius and effort of earth and were long and expensive in the building. they were not available in such numbers that small colonies could possess them. the cruisers carried the colonists to their new worlds and made periodic visits, running on tight schedules, but they could not stop and turn aside to visit colonies scheduled to be visited at another time; such a delay would destroy their schedule and produce a confusion and uncertainty that would wreck the complex interdependence between old earth and the new worlds of the frontier.