Our troubleshooting today eliminated some suspected causes of propulsion system problems. We do not think any of the valves are malfunctioning," Keith Cowing with the ISEE-3 Reboot Project wrote in a statement Wednesday (July 9). "Right now we think there is a chance that the nitrogen used as a pressurant for the monopropellant hydrazine propulsion system may have been depleted. That said, we still have a number of troubleshooting options yet to be explored." [See images of the ISEE-3 spacecraft]