Exploration of asteroid (101955) Bennu by the OSIRIS-REx mission has provided an in-depth look at this rubble-pile near-Earth asteroid. In particular, the measured gravity field and the detailed shape model of Bennu indicate significant heterogeneities in its interior structure, compatible with a lower density at its center. Here we combine gravity inversion methods with a statistical rubble-pile model to determine the density and size-frequency distribution (SFD) index of the rubble that constitutes Bennu. The best-fitting models indicate that the SFD of the interior is consistent with that observed on the surface, with a cumulative SFD index of approximately $-2.9$. The rubble bulk density is approximately $1.35$ g/cm$^3$, corresponding to a $12$% macro-porosity. We find the largest rubble particle to be approximately $145$ m, whereas the largest void is approximately $10$ m.

The gravity field of a small body provides insight into its internal mass distribution. We used two approaches to measure the gravity field of the rubble-pile asteroid (101955) Bennu: (i) tracking and modeling the spacecraft in orbit about the asteroid and (ii) tracking and modeling pebble-sized particles naturally ejected from Bennu's surface into sustained orbits. These approaches yield statistically consistent results up to degree and order 3, with the particle-based field being statistically significant up to degree and order 9. Comparisons with a constant-density shape model show that Bennu has a heterogeneous mass distribution. These deviations can be modeled with lower densities at Bennu's equatorial bulge and center. The lower-density equator is consistent with recent migration and redistribution of material. The lower-density center is consistent with a past period of rapid rotation, either from a previous Yarkovsky-O'Keefe-Radzievskii-Paddack cycle or arising during Bennu's accretion following the disruption of its parent body.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

The top-shaped morphology characteristic of asteroid (101955) 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 Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (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.

The Comet Astrobiology Exploration Sample Return (CAESAR) mission was selected by the NASA New Frontiers Program for Phase A study in December 2017. CAESAR will acquire and return to Earth for laboratory analysis a minimum of 80 grams of surface material from the nucleus of comet 67P/Churyumov-Gerasimenko (67P). CAESAR will characterize the surface region sampled, preserve the collected sample in a pristine state, and return evolved volatiles by capturing them in a separate gas reservoir. NASA Goddard Space Flight Center provides project management, systems engineering, safety and mission assurance, contamination control, mission operations, and many other important functions. Northrop Grumman Information Systems (formerly Orbital ATK) will build the spacecraft, based on Dawn mission heritage, which like CAESAR, uses solar electric propulsion.

Scheeres, D. J.; McMahon, J. W.; French, A. S.; Brack, D. N.; Chesley, S. R.; Farnocchia, D.; Takahashi, Y.; Leonard, J. M.; Geeraert, J.; Page, B.; Antreasian, P.; Getzandanner, K.; Rowlands, D.; Mazarico, E. M.; Small, J.; Highsmith, D. E.; Moreau, M.; Emery, J. P.; Rozitis, Benjamin; Hirabayashi, M.; Sánchez, P.; Van wal, S.; Tricarico, P.; Ballouz, R.-L.; Johnson, C. L.; Al Asad, M. M.; Susorney, H. C. M.; Barnouin, O. S.; Daly, M. G.; Seabrook, J. A.; Gaskell, R. W.; Palmer, E. E.; Weirich, J. R.; Walsh, K. J.; Jawin, E. R.; Bierhaus, E. B.; Michel, P.; Bottke, W. F.; Nolan, M. C.; Connolly, H. C. and Lauretta, D. S. (2019). The dynamic geophysical environment of (101955) Bennu based on OSIRIS-REx measurements. Nature Astronomy, 3 pp. 352–361.

The Laser ranging (LR) experiment on the Lunar Reconnaissance Orbiter (LRO) has been in operation for more than 4 years, since shortly after the arrival of the spacecraft in lunar orbit in June 2009. Led by NASA's Next Generation Satellite Laser Ranging (NGSLR) station at Greenbelt, Maryland, ten laser ranging stations over the world have been participating in the experiment and have collected over 3,400 hours of one-way laser ranging data. These range measurements are used to generate precise orbital solutions for LRO and monitor the behavior of the LRO clock. To achieve high-quality range measurements, the NGSLR and four other ground stations are using Hydrogen-maser clocks as a stable and continuous time reference for the orbit solutions. In January 2013, an All-View GPS receiver was installed at NGSLR, to monitor the H-maser time against the master clock at the United States Naval Observatory (USNO) via GPS satellites. With these improvements, NGSLR established nano-second level epoch time accuracy and 10 clock stability. By using the LR tracking data alone, together with a high-resolution gravity model from the GRAIL Discovery mission, the LRO orbit solutions have an average total position error of 10 meters, and show the same quality as those generated using conventional radiometric tracking data. With this approach, 2-week long arcs were used with range biases adjusted once per arc. A timing bias was also adjusted to compensate both the ground and spacecraft clock characteristics for each arc. We present the results from both orbit prediction and orbit reconstruction, and an assessment of the quality of the orbital solutions by comparing the results with those from S-band radiometric tracking data.