 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. If they left a beacon, they may have left an identification of the planet. 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. But his present companions got the drive dismantled two days later and once the yacht was out of overdrive they efficiently gave him his choice of surrendering or else. He surrendered, stipulating that he wouldn't be landed back on Coryus; he still clung to hope of avoiding return—which was almost certain anyhow.
 strict
 complex  evolving  strict  flexible