A shortage of memory on board the Spirit Mars rover is what caused it to become unresponsive on the Martian surface on Jan. 22, raising fears that the Martian mission might end almost before it began in earnest. Mike Deliman, a technical staff member at Wind River Systems Inc., which provided the real-time embedded operating system used in the mission, said the problem has been re-created in testing on Earth and appears to be entirely memory-related. “It’s not a software bug, it’s not an application bug, and it’s not a hardware bug,” Deliman said. “It’s a system constraint that we ran up against.”