OS Tiger Team (OSTT) Recommendation on OS Cleanliness

The NASA-ESA Mars Sample Return (MSR) campaign involves the collection of samples on Mars by the Perseverance (Mars 2020) rover and their planned return to Earth. To accomplish this, the ‘Orbiting Sample’ (OS) container would be sent to Mars to accommodate the collected samples, then launched from Mars and returned to Earth, from which the samples would be removed to be examined in a Sample Return Facility (SRF). Crucial to this whole sequence is to decide what level of cleanliness is required inside the OS. In February 2021, the NASA Headquarters Science Mission Directorate and the Office of Planetary Protection assembled an MSR OS “Tiger Team” (OSTT) to debate and determine the appropriate cleanliness level of the interior of the OS. The team’s remit was primarily focused on making a decision on the trade-offs between Planetary Protection cleanliness level 4a and 4b. These cleanliness levels are determined by COSPAR planetary protection regulations, where 4a requires < 300 bacterial spores/m2 and < 3 x 105 bacterial spores on the spacecraft (in this case, the interior of the OS), and 4b mandates the more stringent requirement of < 30 bacterial spores. This report documents the decision of the OSTT to recommend that the interior of the OSTT be cleaned to a 4a requirement with any feasible added effort toward 4b, and it provides the rationale for that decision.