It is impossible to sterilize human missions to this level, as humans are host to typically a hundred trillion microorganisms of thousands of species of the human microbiome, and these cannot be removed while preserving the life of the human. Containment seems the only option, but it is a major challenge in the event of a hard landing (i.e. crash).[96] There have been several planetary workshops on this issue, but with no final guidelines for a way forward yet.[97] Human explorers would also be vulnerable to back contamination to Earth if they become carriers of microorganisms.[98]