NASA is preparing to send four astronauts on a roughly 10-day journey around the moon as early as Wednesday evening, a mission that represents the agency’s most significant human spaceflight effort in decades and a key milestone in its plan to return astronauts to the lunar surface ahead of China’s anticipated crewed landing.

Astronauts, from left, Canadian Space Agency astronaut Jeremy Hansen, pilot Victor Glover, commander Reid Wiseman and mission specialist, Christina Koch leave the Operations and Checkout building on t...

Hansen is the first non-U.S. citizen to fly to the moon.