Space mission operations require flexible, efficient and reliable plan execution. In typical operations command sequences (which are a simple subset of general executable plans) are generated on the ground, either manually or with assistance from automated planning, and sent to the spacecraft. For more advanced operations more expressive executable plans may be used; the plans might also be generated automatically on board the spacecraft. In all these cases, the executable plans are received by a software system that executes the plan. This software system, often called an executive, must ensure that the execution of the commands and response of the fault protection system conforms to pre-planned behaviour.

PLEXIL is designed specifically for flexible and reliable command execution. It is designed to be portable, lightweight, predictable, and verifiable, and at the same time it is very expressive.

In order for the PLEXIL language to be useable, an execution system is required to interpret it. The Universal Executive is an execution system designed to facilitate reuse and inter-operability of execution and planning frameworks.

This project is in a collaborative effort between researchers at NASA Ames Research Center, NASA's Jet Propulsion Laboratory and Carnegie Mellon University.