> For one, the spacecraft (or the empty capsules) could have been pre-contaminated before their launch.

I had a really interesting discussion with someone who oversaw decontamination protocols and preparations for various satellites, including the JWST. The protocols they use to prep satellites for these types of missions are beyond stringent, and the chemicals they use are exotic as all hell.

All this to say, contamination is absolutely possible, but there is so much insane behind the scenes work to control for it that it shouldn’t be thought of as something they might “just forget” or casually missed.