“What you’re doing today never happens,” says NASA’s David Seidel as the van rumbles along. We are headed to the Spacecraft Assembly Facility at NASA’s Jet Propulsion Laboratory in California, where a group of journalists will be allowed to enter the clean room, a place members of the public are almost never allowed to go.

We are here to see the Mars 2020 rover, which is scheduled to launch to the Red Planet in July 2020. But before we can get up close and personal with …