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The unexpected surface of asteroid (101955) Bennu

Abstract

NASA’S Origins, Spectral Interpretation, Resource Identification and Security-Regolith Explorer (OSIRIS-REx) spacecraft recently arrived at the near-Earth asteroid (101955) Bennu, a primitive body that represents the objects that may have brought prebiotic molecules and volatiles such as water to Earth1. Bennu is a low-albedo B-type asteroid2 that has been linked to organic-rich hydrated carbonaceous chondrites3. 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 processes4. 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-centimetre scale5,6,7,8,9,10,11. 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 inertia12 and radar polarization ratios13—which indicated a generally smooth surface covered by centimetre-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-metre-diameter patches of loose regolith with grain sizes smaller than two centimetres4. We observe only a small number of apparently hazard-free regions, of the order of 5 to 20 metres in extent, the sampling of which poses a substantial challenge to mission success.

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Fig. 1: Range of albedo on the surface of Bennu.
Fig. 2: OCAMS imaging data elucidate Bennu’s diverse surface reflectance and composition.
Fig. 3: OCAMS global mosaic overlain with elevation data and four regions of interest for sampling.

Data availability

Data used in the plots in Figs. 1, 2 are available with this manuscript as Source Data. Raw and calibrated datasets will be available via the Planetary Data System (PDS) (https://sbn.psi.edu/pds/resource/orex/). Data are delivered to the PDS according to the OSIRIS-REx Data Management Plan, available in the OSIRIS-REx PDS archive. Higher-level products—for example, global mosaics and elevation maps—will be available in the Planetary Data System PDS one year after departure from the asteroid.

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Acknowledgements

This material is based on work supported by NASA under contract NNM10AA11C, issued through the New Frontiers Program.

Reviewer information

Nature thanks Harry Y. McSween Jr and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Author information

Affiliations

Authors

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Contributions

D.S.L. led the OSIRIS-REx mission, analysis and writing of the paper. D.N.D. leads the Image Processing Working Group (IPWG), which includes C.A.B., D.R.G., K.J.B., T.L.B., H.C., E.S.H. and P.H.S. The IPWG developed the image calibration pipeline, produced the global mosaic, analysed the surface for albedo variations and calculated the relative reflectance in the different MapCam filters. O.S.B. led the altimetry investigation and produced the elevation data. W.F.B. performed dynamical analysis linking Bennu to dark asteroids in the main asteroid belt. S.S.B.-K., W.V.B., B.E.C., C.Y.D.d’A., H.L.E., C.W.H., M.C.N. and B.R. designed the observation profiles and OCAMS operation plans for mission design and data acquisition. C.W.H. also led the astronomical characterization. H.C.C. Jr, J.P.D. and C.W.V.W. contributed to the content and writing of the manuscript. J.P.E. led the thermal analysis. V.E.H. led the spectral analysis, and M.R.M.I. and H.H.K. led the characterization and interpretation of the magnetite visible spectral properties. H.L.R. led the graphic design and figure development. D.J.S. led the radio science analysis and K.J.W. led the geological investigation of Bennu. The entire OSIRIS-REx Team made the encounter with Bennu possible.

Corresponding author

Correspondence to D. S. Lauretta.

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Extended data

Extended Data Fig. 1 The global mosaic of Bennu, projected onto a sinusoidal map that preserves area.

The PolyCam images were photometrically corrected to mimic imaging conditions with phase, emission and incidence angles of 0°. The map has a pixel scale of 1.2 m per pixel. Images were taken on 25 November 2018.

Extended Data Fig. 2 Areas used for the calculation of the albedo variation in Fig. 1d.

Blue and orange outlines represent dark and bright clasts, respectively.

Extended Data Fig. 3 Timeline of the various observations made during the Approach phase.

The figure shows the key parameters affecting imaging conditions as a function of range to the asteroid and calendar date.

Extended Data Fig. 4 Schematic of Preliminary Survey, showing passes over the north pole, equator, and south pole.

Each trajectory leg lasts two days. The observations consist of MapCam mosaics made far from Bennu, both on the inbound and outbound legs from the closest approach, OLA observations made near the closest approach, both inbound and outbound, and additional MapCam mosaics made soon after the OLA observations but on the outbound legs of the polar flybys only. The time of closest approach to the pole was set at a nominal 17:00 utc for all flybys.

Extended Data Table 1 Observation parameters for early PolyCam images
Extended Data Table 2 Observation parameters for late PolyCam images
Extended Data Table 3 Observation parameters for Preliminary Survey distant MapCam activities
Extended Data Table 4 Observation parameters for close MapCam activities

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Lauretta, D.S., DellaGiustina, D.N., Bennett, C.A. et al. The unexpected surface of asteroid (101955) Bennu. Nature 568, 55–60 (2019). https://doi.org/10.1038/s41586-019-1033-6

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