When NASA’s OSIRIS-REx spacecraft arrived at asteroid Bennu on Dec. 3, 2018, it found a tiny world that in many respects was as the mission team had expected based on ground-based observations — but the team also found that Bennu held quite a few surprises, too. On March 19, 2019, Nature­ published a special collection of eight papers from the OSIRIS-REx mission team providing a first look at asteroid Bennu. The following provides the key findings published in those scientific papers. The full Nature collection may be accessed directly here.

Lauretta and DellaGiustina et al. “The unexpected surface of asteroid (101955) Bennu.” Nature.

NASA’s OSIRIS-REx spacecraft recently arrived at Bennu, a primitive body that represents the objects that may have brought prebiotic molecules and volatiles such as water to Earth. Bennu is a low-albedo B-type asteroid that has been linked to organic-rich hydrated carbonaceous chondrites. Such meteorites are altered by ejection from their parent body and contaminated by atmospheric entry and terrestrial microbes. Therefore, the primary mission objective is to return a sample of Bennu to Earth that is pristine—that is, not affected by these processes. The OSIRIS-REx spacecraft carries a sophisticated suite of instruments to characterize Bennu’s global properties, support the selection of a sampling site and document that site at a sub-centimeter scale. Here we consider early OSIRIS-REx observations of Bennu to understand how the asteroid’s properties compare to pre-encounter expectations and to assess the prospects for sample return. The bulk composition of Bennu appears to be hydrated and volatile-rich, as expected. However, in contrast to pre-encounter modelling of Bennu’s thermal inertia and radar polarization ratios—which indicated a generally smooth surface covered by centimeter-scale particles—resolved imaging reveals an unexpected surficial diversity. The albedo, texture, particle size and roughness are beyond the spacecraft design specifications. On the basis of our pre-encounter knowledge, we developed a sampling strategy to target 50-meter-diameter patches of loose regolith with grain sizes smaller than two centimeters. We observe only a small number of apparently hazard-free regions, of the order of 5 to 20 meters in extent, the sampling of which poses a substantial challenge to mission success.

Barnouin et al. “Shape of (101955) Bennu indicative of a rubble pile with internal stiffness.” Nature Geoscience.

The shapes of asteroids reflect interplay between their interior properties and the processes responsible for their formation and evolution as they journey through the Solar System. Prior to the OSIRIS-REx mission, Earth-based radar imaging gave an overview of Bennu’s shape. Here we construct a high-resolution shape model from OSIRIS-REx images. We find that Bennu’s top-like shape, considerable macroporosity and prominent surface boulders suggest that it is a rubble pile. High-standing, north–south ridges that extend from pole to pole, many long grooves and surface mass wasting indicate some low levels of internal friction and/or cohesion. Our shape model indicates that, similar to other top-shaped asteroids, Bennu formed by reaccumulation and underwent past periods of fast spin, which led to its current shape. Today, Bennu might follow a different evolutionary pathway, with an interior stiffness that permits surface cracking and mass wasting.

DellaGiustina and Emery et al. “Properties of rubble-pile asteroid (101955) Bennu from OSIRIS-REx imaging and thermal analysis.” Nature Astronomy.

Establishing the abundance and physical properties of regolith and boulders on asteroids is crucial for understanding the formation and degradation mechanisms at work on their surfaces. Using images and thermal data from NASA’s OSIRIS-REx spacecraft, we show that asteroid Bennu’s surface is globally rough, dense with boulders, and low in albedo. The number of boulders is surprising given Bennu’s moderate thermal inertia, suggesting that simple models linking thermal inertia to particle size do not adequately capture the complexity relating these properties. At the same time, we find evidence for a wide range of particle sizes with distinct albedo characteristics. Our findings imply that ages of Bennu’s surface particles span from the disruption of the asteroid’s parent body (boulders) to recent in situ production (micrometer-scale particles).

Hamilton et al. “Evidence for widespread hydrated minerals on asteroid (101955) Bennu.” Nature Astronomy.

Early spectral data from the OSIRIS-REx mission reveal evidence for abundant hydrated minerals on the surface of near-Earth asteroid Bennu in the form of a near-infrared absorption near 2.7 µm and thermal infrared spectral features that are most similar to those of aqueously altered CM-type carbonaceous chondrites. We observe these spectral features across the surface of Bennu, and there is no evidence of substantial rotational variability at the spatial scales of tens to hundreds of metres observed to date. In the visible and near-infrared (0.4 to 2.4 µm) Bennu’s spectrum appears featureless and with a blue (negative) slope, confirming previous ground-based observations. Bennu may represent a class of objects that could have brought volatiles and organic chemistry to Earth.

Hergenrother et al. “Operational environment and rotational acceleration of asteroid (101955) Bennu from OSIRIS-REx observations.” Nature Communications [open access].

During its approach to asteroid Bennu, NASA’s OSIRIS-REx spacecraft surveyed Bennu’s immediate environment, photometric properties, and rotation state. Discovery of a dusty environment, a natural satellite, or unexpected asteroid characteristics would have had consequences for the mission’s safety and observation strategy. Here we show that spacecraft observations during this period were highly sensitive to satellites (sub-meter scale) but reveal none, although later navigational images indicate that further investigation is needed. We constrain average dust production in September 2018 from Bennu’s surface to an upper limit of 150 g s–1 averaged over 34 min. Bennu’s disk-integrated photometric phase function validates measurements from the pre-encounter astronomical campaign. We demonstrate that Bennu’s rotation rate is accelerating continuously at 3.63 ± 0.52 × 10–6 degrees day–2, likely due to the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect, with evolutionary implications.

Scheeres et al. “The dynamic geophysical environment of (101955) Bennu based on OSIRIS-REx measurements.” Nature Astronomy.

The top-shaped morphology characteristic of asteroid Bennu, often found among fast-spinning asteroids and binary asteroid primaries, may have contributed substantially to binary asteroid formation. Yet a detailed geophysical analysis of this morphology for a fast-spinning asteroid has not been possible prior to the OSIRIS-REx mission. Combining the measured Bennu mass and shape obtained during the Preliminary Survey phase of the OSIRIS-REx mission, we find a notable transition in Bennu’s surface slopes within its rotational Roche lobe, defined as the region where material is energetically trapped to the surface. As the intersection of the rotational Roche lobe with Bennu’s surface has been most recently migrating towards its equator (given Bennu’s increasing spin rate), we infer that Bennu’s surface slopes have been changing across its surface within the last million years. We also find evidence for substantial density heterogeneity within this body, suggesting that its interior is a mixture of voids and boulders. The presence of such heterogeneity and Bennu’s top shape are consistent with spin-induced failure at some point in its past, although the manner of its failure cannot yet be determined. Future measurements by the OSIRIS-REx spacecraft will provide insight into and may resolve questions regarding the formation and evolution of Bennu’s top-shape morphology and its link to the formation of binary asteroids.

Walsh et al. “Craters, boulders and regolith of (101955) Bennu indicative of an old and dynamic surface.” Nature Geoscience.

Small, kilometer-sized near-Earth asteroids are expected to have young and frequently refreshed surfaces for two reasons: collisional disruptions are frequent in the main asteroid belt where they originate, and thermal or tidal processes act on them once they become near-Earth asteroids. Here we present early measurements of numerous large candidate impact craters on near-Earth asteroid Bennu by the OSIRIS-REx mission, which indicate a surface that is between 100 million and 1 billion years old, predating Bennu’s expected duration as a near-Earth asteroid. We also observe many fractured boulders, the morphology of which suggests an influence of impact or thermal processes over a considerable amount of time since the boulders were exposed at the surface. However, the surface also shows signs of more recent mass movement: clusters of boulders at topographic lows, a deficiency of small craters and infill of large craters. The oldest features likely record events from Bennu’s time in the main asteroid belt.

Enos and Lauretta. “A rendezvous with asteroid Bennu.” Nature Astronomy.

The OSIRIS-REx mission has reached its target, asteroid Bennu, and is engaging in reconnaissance and early science observations in preparation for sample collection. Principal investigator team Heather Enos and Dante Lauretta provide an overview.