NASA officials are scaling back the agency's Mars missions, including the Curiosity and Opportunity rovers, citing interference from the sun for most of April that would make it difficult for engineers to communicate with the machines. The Mars solar conjunction occurs every two years when the sun aligns itself between Earth and Mars. "We've been through this before," said Rich Zurek, a NASA Mars scientist. "We're not expecting to have any problems during this period. Let's hope it stays that way."