Bennu's Sample Meets NASA's Own Test for a Clean Sample, With One Exception

The team that analyzed NASA's OSIRIS-REx asteroid sample reports that the material returned from Bennu meets the mission's definition of a clean sample, apart from an isolated 1.24% by mass that escaped the sample container and picked up particles shed by the spacecraft.

The mission's own standard is material free of foreign matter that could affect measurements; that is the sense in which the paper uses the word "pristine." Tracing each suspected contaminant back to its origin is what kept real asteroid material from being written off: several particles that had been under suspicion, among them phosphate and sodium fluoride, were identified as native to Bennu.

OSIRIS-REx delivered 121.6 grams of regolith, the loose rock and dust on an asteroid's surface, in September 2023. In a preprint posted to arXiv on Sept. 10 and not yet peer-reviewed, Jason P. Dworkin and 83 colleagues go through every suspected contaminant encountered during the sample analysis phase. Most came from neither the spacecraft nor the handling of the sample on the ground. Some were introduced later, during laboratory analysis itself.

That last group carries the practical lesson the authors draw. Laboratories measuring different things need procedural blanks (control runs containing no sample), and the material has to be shared among them with that in mind, or the analysis starts adding what it is looking for.

The paper sets out the mission's contamination control and knowledge program as a template for future sample-return missions.