Going Beyond Reliability to Robustness and Resilience in Space Life Support Systems

The words reliability, robustness, and resilience are often used interchangeably to describe
tough and dependable systems but the distinctions between them suggest how to design more
serviceable space systems. Reliability is simply the quality of consistently performing well. A
system that dependably meets its design requirements in the specified environment is reliable.
The designers may not consider themselves responsible for failures under unanticipated
conditions. Robustness is the capability of performing without failure under a wide range of
conditions, which can go beyond the expected range to include possible off-nominal conditions.
Resilience is the ability to recover from or adapt to unanticipated damaging events, such as
failures, accidents, external disruptions, and repurposing. Such changes can invalidate the
usual operating assumptions and cause system failure.

Reliability, robustness, and resilience describe dependable performance under
increasingly difficult conditions, first the specified environment, then a wider possible
environment, and finally unanticipated damaging conditions. These three qualities are
increasingly desirable and increasingly difficult to achieve.

Engineering for resilience would design systems that can ignore or repair failures, survive
accidents, and recover from unanticipated disruptions. Increasing the resilience of space
systems would greatly increase space crew safety. Improving reliability and robustness
requires dealing with known problems, but improving resilience requires implementing a
general approach to reducing the impact of unknown future events.

The need for robustness and resilience has been stated for decades but little has been done.
Systems designers often assume that they understand everything they need to know. The
potential failures caused by changes, failures, accidents, unknown environments, and
unknown unknowns can be ignored. Such overconfidence can lead to neglect of reliability,
robustness, and resilience.