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.
Subscribe to Journal
Get full journal access for 1 year
only $3.90 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Tax calculation will be finalised during checkout.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
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.
Lauretta, D. S. et al. The OSIRIS-REx target asteroid (101955) Bennu: constraints on its physical, geological, and dynamical nature from astronomical observations. Meteorit. Planet. Sci. 50, 834–849 (2015).
Hergenrother, C. W. et al. Lightcurve, color and phase function photometry of the OSIRIS-REx target asteroid (101955) Bennu. Icarus 226, 663–670 (2013).
Clark, B. E. et al. Asteroid (101955) 1999 RQ36: spectroscopy from 0.4 to 2.4 μm and meteorite analogs. Icarus 216, 462–475 (2011).
Lauretta, D. S. et al. OSIRIS-REx: sample return from asteroid (101955) Bennu. Space Sci. Rev. 212, 925–984 (2017).
Daly, M. G. et al. The OSIRIS-REx Laser Altimeter (OLA) investigation and instrument. Space Sci. Rev. 212, 899–924 (2017).
Bierhaus, E. B. et al. The OSIRIS-REx spacecraft and the Touch-and-Go Sample Acquisition Mechanism (TAGSAM). Space Sci. Rev. 214, 107 (2018).
Christensen, P. R. et al. The OSIRIS-REx thermal emission spectrometer (OTES) instrument. Space Sci. Rev. 214, 87 (2018).
Reuter, D. C. et al. The OSIRIS-REx visible and infrared spectrometer (OVIRS): spectral maps of the asteroid Bennu. Space Sci. Rev. 214, 54 (2018).
Rizk, B. et al. OCAMS: the OSIRIS-REx camera suite. Space Sci. Rev. 214, 26 (2018).
Masterson, R. A. et al. Regolith X-Ray Imaging Spectrometer (REXIS) aboard the OSIRIS-REx asteroid sample return mission. Space Sci. Rev. 214, 48 (2018).
McMahon, J. W. et al. The OSIRIS-REx radio science experiment at Bennu. Space Sci. Rev. 214, 43 (2018).
Emery, J. P. et al. Thermal infrared observations and thermophysical characterization of OSIRIS-REx target asteroid (101955) Bennu. Icarus 234, 17–35 (2014).
Nolan, M. C. et al. Shape model and surface properties of the OSIRIS-REx target asteroid (101955) Bennu from radar and lightcurve observations. Icarus 226, 629–640 (2013).
Hergenrother, C. W. et al. The design reference asteroid for the OSIRIS-REx mission target (101955) Bennu. Preprint at https://arxiv.org/abs/1409.4704 (2014).
Hamilton, V. E. et al. Evidence for widespread hydrated minerals on asteroid (101955) Bennu. Nat. Astron. https://doi.org/10.1038/s41550-019-0722-2 (2019).
Barnouin, O. S. et al. Shape of (101955) Bennu indicative of a rubble pile with internal stiffness. Nat. Geosci. https://doi.org/10.1038/s41561-019-0330-x (2019).
Scheeres, D. J. et al. The dynamic geophysical environment of (101955) Bennu based on OSIRIS-REx measurements. Nat. Astron. https://doi.org/10.1038/s41550-019-0721-3 (2019).
Walsh, K. J. et al. Craters, boulders and regolith of (101955) Bennu indicative of an old and dynamic surface. Nat. Geosci. https://doi.org/10.1038/s41561-019-0326-6 (2019).
DellaGiustina, D. N. et al. Properties of rubble-pile asteroid (101955) Bennu from OSIRIS-REx imaging and thermal analysis. Nat. Astron. https://doi.org/10.1038/s41550-019-0731-1 (2019).
Hergenrother, C. W. et al. Operational environment and rotational acceleration of asteroid (101955) Bennu from OSIRIS-REx observations. Nat. Commun. https://doi.org/10.1038/s41467-019-09213-x (2019).
Campins, H. et al. Compositional diversity among primitive asteroids. Prim. Meteor. Aster. 2018, 345–369 (2018).
Izawa, M. R. M. et al. Spectral reflectance properties of magnetites: implications for remote sensing. Icarus 319, 525–539 (2019).
Kerridge, J. F., Mackay, A. L. & Boynton, W. V. Magnetite in CI carbonaceous meteorites: origin by aqueous activity on a planetesimal surface. Science 205, 395–397 (1979).
Rubin, A. E., Trigo-Rodríguez, J. M., Huber, H. & Wasson, J. T. Progressive aqueous alteration of CM carbonaceous chondrites. Geochim. Cosmochim. Acta 71, 2361–2382 (2007).
Brearley, A. J. in Meteorites and the Early Solar System II (eds Lauretta, D. S. & McSween Jr, H. Y.), 587–624 (Univ. Arizona Press, Tucson, 2006).
Lantz, C., Binzel, R. P. & DeMeo, F. E. Space weathering trends on carbonaceous asteroids: a possible explanation for Bennu’s blue slope? Icarus 302, 10–17 (2018).
Thompson, M. S., Loeffler, M. J., Morris, R. V., Keller, L. P. & Christoffersen, R. Spectral and chemical effects of simulated space weathering of the Murchison CM2 carbonaceous chondrite. Icarus 319, 499–511 (2019).
Bischoff, A., Scott, E. R., Metzler, K. & Goodrich, C. A. in Meteorites and the Early Solar System II (eds Lauretta, D. S. & McSween Jr, H. Y.), 679–712 (Univ. Arizona Press, Tucson, 2006).
Delbo, M. et al. Thermal fatigue as the origin of regolith on small asteroids. Nature 508, 233–236 (2014).
Gundlach, B. & Blum, J. A new method to determine the grain size of planetary regolith. Icarus 223, 479–492 (2013).
Scheeres, D. J. et al. The geophysical environment of Bennu. Icarus 276, 116–140 (2016).
Binzel, R. P. et al. Spectral slope variations for OSIRIS-REx target asteroid (101955) Bennu: possible evidence for a fine-grained regolith equatorial ridge. Icarus 256, 22–29 (2015).
OSIRIS-REx Mission Status Update, Feb 11, 2019 https://www.asteroidmission.org/?mission_update=feb-11-2019 (2019).
Li, J.-Y., Helfenstein, P., Buratti, B. J., Takir, D. & Clark, B. E. in Asteroids IV (eds Michel, P. et al.) 129–150 (Univ. Arizona Press, Tucson, 2015).
DellaGiustina, D. N. et al. Overcoming the challenges associated with image-based mapping of small bodies in preparation for the OSIRIS-REx mission to (101955) Bennu. Earth Space Sci. 5, 929–949 (2018).
This material is based on work supported by NASA under contract NNM10AA11C, issued through the New Frontiers Program.
Nature thanks Harry Y. McSween Jr and the other anonymous reviewer(s) for their contribution to the peer review of this work.
The authors declare no competing interests.
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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.
Blue and orange outlines represent dark and bright clasts, respectively.
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.
About this article
Cite this article
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
Journal of Guidance, Control, and Dynamics (2021)
Diurnal temperature variation as the source of the preferential direction of fractures on asteroids: Theoretical model for the case of Bennu
Visible-infrared spectroscopy of ungrouped and rare meteorites brings further constraints on meteorite-asteroid connections
Critical spin periods of sub-km-sized cohesive rubble-pile asteroids: dependences on material parameters
Monthly Notices of the Royal Astronomical Society (2021)