The leading theory is air currents from the planet slowly contaminated the station over the decades, not contamination before launch. It's also then still prett

The leading theory is air currents from the planet slowly contaminated the station over the decades, not contamination before launch. It's also then still pretty protected by radiation still being so close to Eath. Our probes go through pretty tight clean room packaging, shed almost all their outside layers before leaving Earth orbit while still being very toasty, and then get blasted by intense radiation for months and are finally separated by some pretty massive distances on the planet.