Primitive asteroids are small bodies that have evolved relatively little since their formation. Their study is essential to understand the chemical composition of the early solar system and its evolution over the past 4.57 billion years. The understanding of the evolutionary processes of primitive asteroids is all the more important as these objects could have brought water and organic matter into the inner solar system.

In the 2010s, two space missions, OSIRIS-Rex (NASA) and Hayabusa2 (JAXA), embarked on a journey to two primitive asteroids in order to collect samples from their surface. The carbonaceous-type asteroid (162173) Ryugu was the target of the Hayabusa2 mission. The spacecraft performed two samplings, collecting surface and subsurface materials excavated by an artificial impactor, because the subsurface is more likely preserved from the alteration by space weathering processes. In December 2020, the sealed capsule containing 5.4 of precious samples returned to Earth. The capsule was opened in the Curation Facility (Sagamihara, Japan), a complex of clean chambers, in order to carry out a first analysis of the grains without exposing them to the terrestrial atmosphere.

A non-destructive spectral analysis of the whole collection was conducted in the Curation Facility. Then, some of the grains, up to several mm in size, were extracted from the Facility and analysed with other techniques, by international teams, in order to precisely characterise their mineralogy and their chemical composition. These analyses revealed that the samples are mainly composed of minerals formed by the aqueous alteration at low temperature (~40°C) of Ryugu’s parent body. The detection of rare anhydrous minerals reveals that some regions in the parent body were preserved from extensive aqueous alteration. Moreover, the analyses of the samples show that space weathering modified the physical and chemical properties of the particles exposed to space environment at the surface of the asteroid. The closest meteoritical analogs of Ryugu samples are CI chondrites, a class of meteorites probably originating from carbonaceous asteroids. However, some of the spectral and compositional differences between them suggest that CI chondrites could have been partly contaminated by terrestrial contamination. Thus, the asteroid samples are the most pristine primitive materials in our collections.

The capsule of the OSIRIS-REx spacecraft, containing the samples from (101955) Bennu, landed in the Utah desert in September 2023, and was then transferred to the Johnson Space Center at Houston. Analysis of Bennu samples is underway, and initial results suggest that Bennu, like Ryugu, contains hydrated minerals and organic matter. The laboratory characterization of these two asteroids will represent a major advancement in understanding the composition of our primitive solar system and its evolution.