The Spirit rover, one of two twin explorers sent to Mars in 2004, suffered a bit of confusion over the weekend. The rover mysteriously did not respond to its driving commands last Sunday, although it acknowledged receiving them, and did not record its activities as it usually does. (Spirit and its partner, Opportunity, were originally tapped for three-month missions, but both continue to operate five years later.)

The memory glitch seems to be remedied, as Spirit properly recorded its activities Tuesday. And the rovers' project manager, John Callas of the NASA Jet Propulsion Laboratory, said in a statement that Spirit is now "reporting good health and [is] responsive to commands from the ground."

The rover team is unsure what could have caused the temporary snafu. One possibility is that cosmic rays, which pose a major obstacle to human exploration of the Red Planet, caused a computational error by delivering or producing unwanted electric charges. (Even on Earth, where the magnetic and atmospheric protection from such radiation is more robust, cosmic rays can mess with computers.) So what's the deal? Not sure, said Sharon Laubach, head of the rover's command team: "We don't have a good explanation yet for the way Spirit has been acting for the past few days." A senile moment, perhaps?