CAPE CANAVERAL, Fla.—The countdown to the launch of humanity’s first mission around the moon in more than 50 years is underway at Kennedy Space Center in Florida.

Possibly as soon as 6:24 p.m. on April 1, three Americans and one Canadian will become the first astronauts to sit atop the most powerful human-rated rocket NASA’s ever built and lay the foundation of the U.S.-led campaign to go back to the moon to stay.