Automated planning technologies show great promise of reducing operations costs by automating the spacecraft mission planning process. However, one of the bottlenecks is acquiring spacecraft operations knowledge from operations personnel and expressing it in a plan model. One of the primary design goals of the ASPEN planning language is to eliminate the knowledge acquisition bottleneck by making it easy for operations personnel to build plan models themselves. The ASPEN language is designed to be used by domain experts that have no knowledge of automated planning technology. Spacecraft operations knowledge can be expressed in ways that are natural to operations personnel. The language is intended to be intuitive and require almost no knowledge of how the planning algorithm works. ASPEN itself provides hooks for interfacing with existing planning support tools, and has a mixed-initiative planning mode and GUI that allow operations personnel to easily edit automatically generated plans if desired. A non-AI expert with an operations background was able to construct ASPEN models that are scheduled for use on two missions, EO-1 and UFO-1.