Last modified on Tue 13 Jul 2021 10.20 EDT “This, what you’re doing today, never happens,” Nasa’s David Seidel told us. “This is a rare chance,” agreed the director of the Jet Propulsion Laboratory, Michael Watkins, welcoming us into the lab’s spacecraft assembly facility, located in the hills outside Pasadena, California. The exceedingly unusual adventure awaiting us was a trip into the clean room where Perseverance, Nasa’s latest Mars rover, having been assembled under conditions of exacting sterility, sat awaiting shipment to Cape Canaveral. Our visit, in December 2019, had been prefaced by a long email laying out extremely detailed rules: we were instructed not to wear any perfume, cologne, makeup or dangly earrings; flannel, wool or frayed clothing was not allowed; even our fingernails had to be smooth, rather than jagged. After a quick welcome, our phones and notebooks were confiscated, and a hi-tech doormat vacuum-brushed the soles of our shoes. In the gowning room, we were issued with face wipes, a sterile full-body “bunny suit”, plastic overshoes, hood, gloves and face mask, then offered a mirror in which to admire the final look. Finally, we were sent through the air shower – an elevator-sized chamber studded with nozzles that blasted us with pressurised air from all sides, in order to dust off any final stray particles – before stepping out into a white-floored, white-walled room filled with white-suited engineers.