The group of civilian scientists who launched a crowdfunded attempt to resurrect a 1970s spacecraft circling the sun may still manage to get the probe working again, but not the way it initially hoped.

Attempts to shift the course of the International Sun-Earth Explorer 3 into an orbit around the earth are at best in flux, with controllers now suspecting the spacecraft may have run out of fuel.

The robotic probe, sent to space first to study solar wind and later to chase down and analyze comets, was expected to fire its positioning thrusters several times Tuesday to move into a more better position to communicate with Earth.

And the first burn went off, apparently without any problems. But then the propulsion system suddenly stopped working during the second series of burns -- for reasons still not fully understood.

"Our troubleshooting today eliminated some suspected causes of propulsion system problems. We do not think any of the valves are malfuctioning," Keith Cowing, one of the project's leaders, said in a statement posted at the recovery effort's Website. "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 ... One of our volunteers, Karl-Max Wagner from Germany has an interesting idea. Did the nitrogen pressurizing gas dissolve in the hydrazine in the tanks? This is something that we would like to research and for efficiencies sake and to get the job done quicker, we would like our project fans out there to help us in this research. I am reading an old USAF document on this now and it may be nothing, but it also may be something."

If the ISEE-3 is not redirected soon, it will loop around the moon and settle into a path that would make communications between it and Earth significantly more difficult.

Yet, even in that situation, probe's aspiring rescuers -- who, through a first-of-its-kind agreement with the National Aeronautics and Space Administration, are controlling the ISEE-3 through the Arecibo Observatory in Puerto Rico and also tracking its location with the space agency's Deep Space Network -- could still find ways to glean new information from the awakened robot.

A pre-scheduled session using the DSN today, Cowing said, would help the group "recalibrate our location information and trajectory plans for ISEE-3."

Even if the original "halo orbit is no longer an option, we do have plans to use ISEE-3 for science in other locations within the inner solar system after the lunar flyby on 10 August," he continued in the statement.

Launched in 1978 and and all but completely shut down in 1997, the ISEE-3, before this week's course corrections, hadn't fired its engines since 1986.

The reboot initiative has so far raised $150,000 in funds from the public.