NASA has its sights set on landing astronauts on Mars in the 2030s, the long-term goal of its Artemis program. As a first step, the space agency is scrambling to meet a White House-imposed deadline of 2024 for returning astronauts to the moon. The Trump administration wants to achieve new space milestones ahead of other countries, including China — but congressional Democrats question whether the 2024 deadline is realistic, especially when NASA's deep-space rocket is years behind schedule and billions of dollars over budget. In addition, some analysts say, China's Mars goals are too far in the future to be competitive with NASA. Private companies such as SpaceX are planning their own Mars missions, even as they work with NASA to solve the technical challenges of deep-space travel, including how to protect astronauts from radiation. Some experts cite such dangers in arguing that exploration of Mars and other planets is best left to robots, but others say human explorers have a unique significance in space exploration.