Reliability and maintainability of both terrestrial and space systems have too long been needlessly sacrificed because man's role within each system has been an afterthought. To overcome this short-sighted approach, man's capabilities as well as limitations must be clearly understood at the outset of system development by those who are responsible for design.This paper presents the results of recent research in human engineering which should aid system designers to optimize man's role as a control element in space systems. In particular, five human characteristics necessary for spacecraft control are shown to exceed the capabilities of any known or planned machines.The maximum role of which man is capable should be defined for each particular space system. When this maximum role is known, then tradeoffs become possible between human and automatic control. The line of demarcation between men and equipment, known as the man-machine interface, becomes deliberately established rather than drawn accidentally. Reliability is thereby enhanced because man is performing only those functions in which his superiority over equipment is clearly confirmed. Furthermore, the converse also holds--man is excluded from performing functions more reliably executed by machines.