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Cohesive forces prevent the rotational breakup of rubble-pile asteroid (29075) 1950 DA

Abstract

Space missions1 and ground-based observations2 have shown that some asteroids are loose collections of rubble rather than solid bodies. The physical behaviour of such ‘rubble-pile’ asteroids has been traditionally described using only gravitational and frictional forces within a granular material3. Cohesive forces in the form of small van der Waals forces between constituent grains have recently been predicted to be important for small rubble piles (ten kilometres across or less), and could potentially explain fast rotation rates in the small-asteroid population4,5,6. The strongest evidence so far has come from an analysis of the rotational breakup of the main-belt comet P/2013 R3 (ref. 7), although that was indirect and poorly constrained by observations. Here we report that the kilometre-sized asteroid (29075) 1950 DA (ref. 8) is a rubble pile that is rotating faster than is allowed by gravity and friction. We find that cohesive forces are required to prevent surface mass shedding and structural failure, and that the strengths of the forces are comparable to, though somewhat less than, the forces found between the grains of lunar regolith.

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Figure 1: Physical property distributions of (29075) 1950 DA.
Figure 2: Degree of negative ambient gravity for (29075) 1950 DA.
Figure 3: Gravitational slopes of (29075) 1950 DA.
Figure 4: Minimum internal cohesive strength of (29075) 1950 DA.

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Acknowledgements

This publication uses data products from NEOWISE, a project of the Jet Propulsion Laboratory/California Institute of Technology, funded by the Planetary Science Division of NASA. We made use of the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory/California Institute of Technology under a contract with NASA. This work was supported by NASA contract NNM10AA11C (Principal Investigator D. S. Lauretta) through the New Frontiers programme, and by NASA contract NNX12AP32G (Principal Investigator J. P. Emery) through the Near-Earth Object Observing programme.

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Authors and Affiliations

Authors

Contributions

B.R. performed the thermophysical and cohesive force analyses, E.M. retrieved the WISE data and helped with its analysis, and J.P.E. helped with the scientific interpretation of the results. B.R. wrote the manuscript with all co-authors contributing to its final form.

Corresponding author

Correspondence to Ben Rozitis.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Example ATPM fit to the WISE thermal-infrared observations.

This fit (lines for WISE infrared bands W3 and W4) was made for a thermal inertia of 24 J m−2 K−1 s−1/2 and a surface roughness of 50%. The error bars correspond to the 1σ uncertainties on the measured data points.

Extended Data Figure 2 WISE thermal-infrared images of (29075) 1950 DA.

The image scale is 2.75 arcsec per pixel for the W1, W2 and W3 bands and 5.53 arcsec per pixel for the W4 band. White pixels are ‘bad’ pixels that do not contain data. The object seen to the upper left of (29075) 1950 DA (red circle) in the W1 (3.4 μm) and W2 (4.6 μm) bands is a faint background star (green circle).

Extended Data Figure 3 Physical properties derived for (29075) 1950 DA as a function of thermal inertia.

a, Diameter; b, bulk density. The dashed lines represent the 1σ uncertainty for the average solid lines. The red horizontal lines represent the radar diameter constraint8 of 1.30 ± 0.13 km, and the red vertical lines represent the corresponding thermal inertia constraint of ≤82 J m−2 K−1 s−1/2.

Extended Data Figure 4 Enhancement of Yarkovsky orbital drift by surface roughness for (29075) 1950 DA.

Extended Data Table 1 WISE thermal-infrared observations of (29075) 1950 DA
Extended Data Table 2 Physical properties of (29075) 1950 DA

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Rozitis, B., MacLennan, E. & Emery, J. Cohesive forces prevent the rotational breakup of rubble-pile asteroid (29075) 1950 DA. Nature 512, 174–176 (2014). https://doi.org/10.1038/nature13632

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