OSIRIS-REx: Sampling Bennu and Returning to Earth

OSIRIS-REx, the Origins, Spectral Interpretation, Resource Identification, Security–Regolith Explorer, is a NASA asteroid sample-return mission. Selected in May 2011 as the third mission in the New Frontiers program, it targeted the near-Earth asteroid Bennu. Its scientific requirements included returning at least 60 grams of samples, mapping the body and sampling area, and studying the effect of thermal radiation on its orbit.

Principal investigator Dante Lauretta was based at the University of Arizona, with participating organizations including Goddard Space Flight Center, Lockheed Martin, the Canadian Space Agency and Johnson Space Center. On September 8, 2016, the spacecraft launched from Cape Canaveral on an Atlas V 411 rocket, beginning its journey to Bennu.

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Designed and built by Lockheed Martin, the spacecraft combined a deep-space observing platform with a sample-return capsule. Two solar-array wings supplied power, and a high-gain antenna returned data. Its science platform carried cameras, spectrometers, a laser altimeter and attachment components for the extended sampling arm. The return capsule drew on technological experience from the Stardust mission, but sampling Bennu required a specially developed contact device.

The University of Arizona's three-camera system handled tasks including long-range navigation, high-resolution and wide-angle observations, and recording the sampling process. Goddard supplied the visible and near-infrared spectrometer, Arizona State University the thermal emission spectrometer, and Canada the scanning laser altimeter. An X-ray instrument developed jointly by students at the Massachusetts Institute of Technology and Harvard University was used for elemental observations.

These instruments served connected purposes: images revealed terrain and obstacles, spectra offered clues to minerals and organic matter, and altitude measurements established topographic models. Information about spatial distribution and local environments helped select the sampling site and preserved the samples' context for later analysis in Earth-based laboratories.

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The spacecraft arrived near Bennu in 2018 and entered orbit on December 31. The team then undertook global mapping and local reconnaissance. Burns and Lauretta's retrospective noted that the actual surface was strewn with boulders and safe sampling areas were much smaller than anticipated during design. The team therefore adopted navigation based on recognition of natural terrain features and selected the Nightingale sampling site after several rounds of comparison.

An early-observation paper published in 2019 measured Bennu's mean diameter at about 490 meters and bulk density at about 1,190 kilograms per cubic meter, supporting its interpretation as a loose aggregate of fragments. A 2.7-micrometer absorption band in its spectra was consistent with hydrated silicates. Cameras also revealed numerous boulders larger than ten meters, revising the earlier picture of a smooth surface inferred from thermal and radar data.

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The Touch-and-Go Sample Acquisition Mechanism was known as TAGSAM. An approximately three-meter-long robotic arm extended the sampling head toward the surface. Upon contact, high-purity nitrogen gas blew loose material into a collection chamber, where a mesh allowed gas to escape while retaining particles. Twenty-four metallic contact pads on the head's underside could also collect fine material from the uppermost surface. The design avoided the additional operations required for a prolonged landing, fixed attitude and anchoring on a low-gravity body.

During development, engineers tested the device on the ground and in low-gravity conditions produced by aircraft parabolic flights, varying particle sizes, materials and obstacles. Three independent gas bottles provided capacity for up to three attempts. The scientific minimum of 60 grams and the device's higher engineering collection target served different purposes; the latter also had to allow for uncertainty in estimating mass in orbit.

Sampling took place on October 20, 2020. Subsequent images showed that a rock had wedged a flap open and some particles were escaping. The mission team canceled the planned spin-based mass measurement, reduced arm movements and prioritized stowing the sample to prevent the measurement procedure from increasing the loss.

References: [2] [4] [5]

The spacecraft left Bennu on May 10, 2021. On September 24, 2023, the return capsule landed at the Utah Test and Training Range, while the main spacecraft changed course for the OSIRIS-APEX extended mission. Ground personnel transported the sample to Johnson Space Center, where it was handled under high-purity nitrogen. Following further disassembly in early 2024, the returned Bennu material was confirmed to total at least 121.6 grams.

Contamination control extended through development, flight and storage. Ground and flight witness plates recorded particles and chemical substances that might have accumulated at different stages, and material samples processed in the same way as flight hardware were retained as controls. These measures allowed researchers to distinguish the asteroid's own constituents from contamination potentially introduced during manufacturing and transport on Earth. A “pristine sample” was therefore a quality objective requiring evidential verification.

References: [2] [5]

Preliminary analysis in 2024 found abundant magnesium-rich phyllosilicates, together with sulfides, magnetite, carbonates and organic matter, supporting the conclusion that the parent body had undergone alteration by water. Magnesium-sodium-rich phosphates not previously identified through remote sensing were also detected. These findings provided evidence of mineral and chemical evolution, rather than a discovery of life.

Sample return also enabled observing methods to test one another. The mission connected orbital images, spectra from the sampling area, records of the sampling head's contact and laboratory measurements, rather than handing researchers a collection of material detached from its geological setting. The sampler's surface contact pads and main collection chamber targeted different material layers, while witness plates supplied controls for later compositional interpretations. Landing the capsule completed the transport phase; sorting, allocation and research on the samples still had to continue.

References: [2] [5]