 Which term best describes the ease of space travel within the context of the passage?
The ice-cap, by its existence and circular shape, proved that the planet rotated at a not unreasonable rate. A water-ice ice-cap said that there were no poisonous gases in the planet's atmosphere. A large cap would have meant a large area with arctic and sub-arctic temperatures, with small temperate and tropical climate-belts. A small one like this meant wide tropical and sub-tropical zones. The fact was verified by the thick, dense cloud-masses which covered most of the surface,—all the surface, in fact, outside the ice-cap. But since there were ice-caps there would be temperate regions. In short, the ice-cap proved that a man could endure the air and temperature conditions he would find. Two of the Nadine's four-man crew watched out the same ports as the planet seemed to approach. They'd been on Coryus Three and they'd gotten departure clearance from its space-port. With clearance-papers in order, they could land unquestioned at any other space-port and take off again—provided the other space-port was one they had clearance for. Without rigid control of space-travel, any criminal anywhere could escape the consequences of any crime simply by buying a ticket to another world. Wherever the yacht landed, such a disparity between its documents and its crew would spark an investigation. It could have travelled at almost any speed in practically any direction for a length of time that was at least indefinite. A liner could re-locate itself without trouble. The process would be to find a planet and check its climate and relationship to other planets, and its flora and fauna against descriptions in the Directory. It marked a first-landing spot on some planet or other, so the people could find that spot again. But space-travel regulations are especially designed to prevent such escapes. One of the controls on space-traffic required a ship on landing to deposit its fuel-block in the space-port's vaults. The fuel-block was not returned until clearance for departure had been granted. It was not until six thousand feet above the surface that the planet-wide cloud-level seemed to begin. The yacht descended as if in the center of a sphere in which one could see clearly nearby, less clearly at a little distance, and not at all beyond a quarter-mile or so. Interplanetary drive is rugged and sure, but it does not respond to fine adjustment.
 strict
 complex  evolving  strict  flexible