Periodic crises in the supplies of food and air, caused by blights in the greenhouses or breakdowns in equipment, took away people whose strength had been sapped to begin with. The journey entailed thousands of traversals of the Van Allen radiation belts. Rather than passing through these but one or two times, as might have been the case in a more traditional space voyage, they had to do it twice on each orbit; and during the first year they were, for all practical purposes, never out of them. They sheltered as much as they could in shielded parts of the ship. But no shelter was perfect. Some of the crew were obliged by duty or by happenstance to remain in exposed locations. And the mere fact of being crammed together in a confined space for a significant fraction of the time was a drag on health.