The space agency said tests at its Jet Propulsion Laboratory in Pasadena, Calif., that simulate Spirit's predicament have reinforced the understanding that getting Spirit to rove again will be a complex task.
To supplement those tests, NASA's rover team is refining a detailed computer model of rover mobility, calibrated with results from testing and measurements from Mars.
"The computer modeling will allow us to connect the results from tests performed in Earth gravity with what to expect from the rover in Mars gravity," said JPL's John Callas, project manager for Spirit and its twin rover, Opportunity.
Spirit is more than five years into its mission on Mars -- a mission that was originally scheduled to last three months.
"An additional round of testing was added to the September schedule to gain more detailed assessment of how to move Spirit while avoiding putting the rover's center of gravity directly over a rock that is touching or nearly touching the rover's underbelly," NASA said in a statement.
"We are proceeding very cautiously and exploring all reasonable options," Callas added. "There is a very real possibility that Spirit may not be able to get out, and we want to give Spirit the very best chance."