NASA scientists said on Thursday they had lost contact with the robot rover Spirit on Mars and were unsure what had caused the problem. Spirit project manager Pete Theisinger told a news briefing that there was a “very serious anomaly” in communications with the six-wheeled craft, which landed on Mars on Jan. 3 on a planned three-month mission to explore the geologic history of the planet. Theisinger said scientists had been unable to communicate with Spirit for about 24 hours and had so far been unable to explain the source of the problem.