Once in orbit around Bennu, OSIRIS-REx studied the near-Earth asteroid for a year, mapped it in 3-D and collected samples to be returned to Earth for laboratory analysis.

OSIRIS-REx (Origins-Spectral Interpretation-Resource Identification-Security-Regolith Explorer) is the third mission in NASA’s New Frontiers Program of medium-class interplanetary missions. Its main objective was to bring back soil samples from an asteroid called Bennu.

This asteroid discovered in 1999 has a diameter of about 575 metres and completes an orbit around the Sun every 1.2 years. Its trajectory intersects the orbit of the Earth; according to available data, it may hit it in 2182. Therefore, the mission also aims to better estimate impact hazards.

OSIRIS-REx was launched in September 2016 and arrived at the asteroid in December 2018.

The instruments (cameras, spectrometers, altimeter) began studying Bennu from late 2018 to October 2020, which included mapping, detecting potential plumes and the presence of natural satellites. They also measured the Yarkovsky effect, which alters the trajectory of the smallest asteroids—usually less than 20 km. The rotation of the object around its centre causes solar energy to be emitted as infrared radiation in a direction that slowly changes its trajectory. This effect is the main source of uncertainty in predicting the trajectories of near-Earth asteroids, and therefore the Earth impact probability. Precise measurement of the Yarkovsky effect on Bennu further refined the calculation of its orbit and helped better assess the risk of a collision.

The instruments also exploited the images of the surface and mapped the asteroid to determine potential regolith sampling sites.

On 20 October 2020, the probe performed a “touch-and-go” manoeuvre of a few seconds to collect a sample (60 g to 2 kg) using a robotic arm. The sample was stored in a capsule similar to that used by the STARDUST spacecraft. OSIRIS-REx then continued to tail the asteroid for two more years before beginning its return trip on 10 May 2021 for a scheduled arrival on 24 September 2023 at 3:42 in the Utah desert.

The capsule containing samples from Bennu was released from the spacecraft on 24 September 2023 four hours before re-entering Earth’s atmosphere at an altitude of 100,000 km, then flying a ballistic trajectory protected by its heatshield and landing in the Utah desert at around 17:00 CET. Mission teams tracked the capsule’s descent to find it and check that none of its contents had spilled out and contaminated Earth’s environment.

Once recovered, the capsule was flown by helicopter to a local curation facility where it was immediately subjected to a nitrogen purge to avoid contamination. It was then sent to the Johnson Space Center, where it was placed in a special clean room for initial non-destructive analysis: before opening the capsule, the gas surrounding the samples will be pumped out and analysed.

The 200-strong international science team behind the mission will then conduct the first analysis campaigns on about one-quarter of the samples’ total mass.

CNES did not provide instruments for this mission but is supporting the work of several co-investigators involved in the project:

• Patrick Michel, from the LAGRANGE laboratory, is involved in the Carbonaceous Meteorite Working Group (CMWG), the Dynamical Evolution Working Group (DEWG), and the Regolith Development Working Group (RDWG).
• Marco Delbo, also from the LAGRANGE laboratory, is involved in the Dynamical Evolution Working Group (DEWG) and the Thermal Analysis Working Group (TAWG) (to assess the Yarkovsky effect).
• Antonella Barucci, from the LESIA space and astrophysics instrumentation research laboratory, is involved in working groups for mineralogy and physical characterization: Spectral Analysis Working Group (SAWG), Thermal Analysis Working Group (TAWG), Imaging Processing Working Group (IPWG), Astronomy and Photometry Working Group (APWG). Several other LESIA members are also collaborating on the OSIRIS-REx mission: Sonia Fornasier, Frederic Merlin, Marcello Fulchignoni, Alice Praet, Prasanna Deshapriya and Pedro Hasselmann.
• Guy Libourel, from the GeoAzur laboratory, is involved in the Regolith Development Working Group (RDWG) and the Carbonaceous Meteorite Working Group (CMWG). He has analysed temperature variations of Bennu to assess their impact on the size of regolith grains and thus determine the most suitable sample sites.
• Bernard Marty’s team from the CRPG Earth and planetary sciences laboratory will be using mass spectroscopy, notably to analyse gases in the sample capsule.