It is often the case that new technology for space needs to be taken out of the lab and proven in space. The primary goal of a technology mission is to prove the capabilities of newly developed space technology. Most technology missions have the challenge of discovering the limits and capabilities of new systems, and the Defense Advanced Research Projects Agency’s (DARPA) Orbital Express (OE) mission was no exception. Orbital Express launched March 8, 2007, and decommissioned on July 22, 2007. The Orbital Express mission demonstrated on-orbit servicing of spacecraft, including rendezvous, transfer of battery and CPU modules, and transfer of propellant, the actual duration of each being approximated but not known to high enough fidelity to commit to a communications and operations plan.
The challenging timeline for DARPA’s Orbital Express mission demanded a flexible, responsive, and (above all) safe approach to mission planning. Mission planning for space is challenging because of the mixture of goals and constraints. Every space mission tries to squeeze all of the capacity possible out of the spacecraft. For Orbital Express, this means performing as many experiments as possible, while still keeping the spacecraft safe. Keeping the spacecraft safe can be very challenging because we need to maintain the correct thermal environment (or batteries might freeze), we need to avoid pointing cameras and sensitive sensors at the sun, we need to keep the spacecraft batteries charged, and we need to keep the two spacecraft from colliding ... made more difficult as only one of the spacecraft had thrusters. /p>
The overall affect of using ASPEN has been approximated by the flight director as a 26 percent reduction in the execution time of the mission, a 50 percent reduction in the daily staff required to produce plans, and a 35 percent reduction in planning errors.
Mission planning had two primary roles for Orbital Express: (1) evaluate scenarios for feasibility early in the design of the mission, and (2) provide responsive communications and commanding planning and scheduling during the mission. To serve both roles, we modeled the mission scenarios using the ASPEN (Rabideau et al. 1999) planning system. OE required evaluation of many alternatives, so ASPEN was modified to accommodate reasoning about schema-level uncertainty. Rehearsals for operations indicated that the SRP needed to be very responsive to changes in the procedures. To accommodate this, we implemented a system for reading the procedures and interpreting these into ASPEN models.