China’s robotic rover on Mars is thought to have remained stuck in hibernation for months due to a build-up of sand and dust, its mission designer said. The Zhurong rover entered a planned sleep mode in May last year and was expected to wake up again in December—but images from a NASA probe orbiting Mars show the rover hasn’t moved since at least September. Chinese state TV on Tuesday quoted Zhang Rongqiao, chief designer of China’s Mars exploration program, as saying that an unexpected accumulation of dust most likely inhibited Zhurong’s power generation and prevented it from waking up. He added that the rover had already explored the Martian surface for 358 days—much longer than its originally planned three-month mission.