Spaceflight Now's ULA’s Atlas 5 is NASA’s go-to rocket for nuclear-powered space probes quotes Tory Bruno, president and CEO of United Launch Alliance:
Bruno said the mobile clean room set up inside ULA’s vertical rocket hangar allowed ground crews to mount the MMRTG ont he spacecraft in a pristine environment to meet stringent “planetary protection” protocols.
“Planetary protection makes us want to be very very careful that we don’t contaminate the lander with biological (material) because we’re off looking for life on Mars, and if you bring life with you, that kind of messes up that experiment,” Bruno said.
That the RTG was added to Perseverance is discussed in Now that Perseverance is "hot" (RTG in place) and before it gets to deep space, how will it stay cool? but here I see that this appears to have happened in ULA's "mobile clean room set up inside ULA’s vertical rocket hangar".
Question: At what "altitude" did this happen, and what happened next? Did Perseverance get put into the fairing and the fair get closed up on the ground level, and then a crane lifted the sealed fairing up to the top of the rocket, or is the mobile clean room "at altitude" near the top of the rocket and everything happens up there? How did the logistics work in this case?