50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 2608.pdf

OSIRIS-REX ARRIVES AT ASTEROID (101955) BENNU: EXPLORATION OF A HYDRATED PRIMITIVE NEAR-EARTH ASTEROID. D. S. Lauretta1, M. M. Al Asad2, R.-L. Ballouz1, O. S. Barnouin3, E. B. Bierhaus4, W. V. Boynton1, L. B. Breitenfeld5, M. J. Calaway6, M. Chojnacki1, P. R. Christensen7, B. E. Clark8, H. C. Connolly Jr.9,1, C. Drouet d’Aubigny1, M. G. Daly10, R. T. Daly3, M. Delbó11, D. N. DellaGiustina1, J. P. Dworkin12, J. P. Emery13, H. L. Enos1, D. Farnocchia14, D. R. Golish1, C. W. Haberle7, V. E. Hamilton15, C. W. Hergenrother1, E. R. Jawin16, H. H. Kaplan15, L. Le Corre17, T. J. McCoy16, J. W. McMahon18, P. Michel11, J. L. Molaro17, M. C. Nolan1, M. Pajola19, E. Palmer17, M. E. Perry3, D. C. Reuter12, B. Rizk1, J. H. Roberts3, A. Ryan11, D. J. Scheeres18, S. R. Schwartz1,11, A. A. Simon12, H. C. M. Susorney2, K. J. Walsh15, X.-D. Zou8, and the OSIRIS-REx Team. 1Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA (lau- [email protected]), 2Dept. of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Van- couver, BC, Canada, 3The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA, 4Lockheed Martin Space Systems, USA, 5Dept. of Geosciences, Stony Brook University, Stony Brook, NY, USA. 6NASA Johnson Space Center, Houston, TX, USA, 7School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA, 8Dept. of Physics and Astronomy, Ithaca College, Ithaca, NY, USA, 9Dept. of Geology, Rowan University, Glassboro, NJ, USA,10Dept. of Earth and Space Science and Engineering, York University, Toronto, ON, Canada, 11UCA–CNRS–Observatoire de la Côte d’Azur, Nice, France, 12NASA Goddard Space Flight Center, Greenbelt, MD, USA, 13Dept. of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN, USA, 14Jet Propulsion Laboratory, California Institute of Technology, Pasadena CA, USA, 15Southwest Research Insti- tute, Boulder, CO, USA, 16Smithsonian National Museum of Natural History, Washington, DC, USA, 17Planetary Science Institute, Tucson, AZ, USA, 18Smead Dept. of Aerospace Engineering Sciences, University of Colorado, Boulder, CO, USA, 19INAF–Astronomical Observatory of Padova, Padova, Italy.

Introduction: In May 2011, NASA selected the Survey passes provided the final data set to complete Origins, Spectral Interpretation, Resource Identifica- the 75-cm-resolution global shape model and corre- tion, and Security–Regolith Explorer (OSIRIS-REx) sponding rotation state data. asteroid sample return mission as the third mission in With the asteroid mass, shape, and rotation state the New Frontiers program [1]. The spacecraft de- constrained, the navigation team had all the infor- parted Earth on September 8, 2016, on a seven-year mation needed to finalize the design of the orbital journey to study and return a sample from near-Earth insertion maneuver sequence. The spacecraft was asteroid (101955) Bennu [2]. placed into orbit on December 31, 2018, beginning Mission Phases: The outbound-cruise trajectory the Orbital A phase. For stability, relative to solar resulted in optical acquisition of Bennu on August 18, radiation pressure, these orbits reside in the terminator 2018, when the asteroid was just bright enough for plane. The orbital distance varies between 1.6 (peri- detection by the PolyCam imager. These first science apse) and 2.1 (apoapse) km. The orbit has a 61.2-hour observations marked the beginning of asteroid prox- period and a ~5-cm/s orbital velocity. imity operations and the Approach phase of the mis- Results: Hazard Search. A primary objective of sion. This phase afforded adequate time to optically the Approach phase was to survey the asteroid opera- acquire the asteroid using the spacecraft’s onboard tional environment for potential spacecraft hazards. cameras, and also to survey the vicinity of the aster- Discovery of either a dusty environment or a natural oid for any hazards that may be present. The PolyCam satellite would have triggered a safety assessment and and MapCam cameras and the OVIRS and OTES review of the approach strategy. Spacecraft observa- spectrometers characterized the asteroid’s photomet- tions during these phases show no evidence for dust ric and spectroscopic properties and enabled compari- emission from Bennu, and we did not detect dust as son to ground-based telescopic data. an extended source along Bennu’s orbital track. The The Approach phase was followed by Preliminary spacecraft performed a series of natural satellite Survey, which consisted of five hyperbolic trajecto- searches, and no satellites were detected. ries that crossed over the north and south poles and Color and Spectral Properties. To provide the equator at a range of ~7 km. The team obtained ground-truth data for telescopic characterization, the laser altimeter data from the OLA scanning lidar in- team obtained “disk-integrated” photometric and strument for the first time, along with additional im- spectral data of Bennu. These data supply an im- agery. These passes permitted radio science meas- portant link between the telescopic data and the spa- urements to determine Bennu’s mass, a prerequisite to tially resolved data that will be obtained later in the planning the maneuvers that placed our spacecraft mission. In addition, they provided important first- into orbit. The imaging data from the Preliminary look information, constraining the rotation period and 50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 2608.pdf

photometric properties, identifying important spectral evidence for mechanical weathering, suggesting that features, and allowing for an assessment of the ther- thermal expansion and contraction plays an important mal inertia, a proxy for the average particle size. role in regolith development. We have identified sev- Starting in mid-November 2018, PolyCam and eral areas, ranging from 10 to 20 meters in extent MapCam obtained well-resolved images of Bennu. where apparent fine-grained material has collected. One of the most striking aspects of Bennu is the range In a first-of-its-kind investigation, Chesley et al. of surface reflectance. The largest boulders (> 30 m) [3] applied a Yarkovsky model to develop a mass have geometric albedos near the average terrain. A estimate for Bennu with a corresponding bulk density population of bright decimeter-scale boulders also of 1260±70 kg/m3. This result has been validated by exist with albedos ranging from 7 to 10%. A small our radio science mass determination from three fly- number of very bright features have been detected in bys yielding an asteroid density of 1191±7 kg/m3. The PolyCam data, but are not yet fully resolved. These low density of Bennu is consistent with a rubble-pile features have albedos of at least 17%. structure with 25 to 50% macroporosity. The darkest material on Bennu has ~3.4% geomet- Given the mass, shape, and spin, we determine the ric albedo and includes a large exposed outcrop in the surface and near-surface dynamical environment. We southern hemisphere. Dark boulders and diffuse units calculated the Roche Lobe of Bennu and found that it of dark material are well distributed across the aster- intersects the shape; material within the lobe is ener- oid. Some dark material also appears to show an ab- getically bound to the surface. Observed changes in sorption at 0.55 µm and is redder than the global av- the global slope distribution are evident at this transi- erage terrain, which may be related to the tentative tion. The global acceleration profile across Bennu’s magnetite from OTES. surface ranges from 3 to 8 µG and shows Bennu to be OVIRS disk-integrated data from the Approach a microgravity aggregate. The global geopotential Phase reveal a visible to near-infrared spectrum that across the surface defines the energetically favorable has a blue (negative) slope and no visible features directions for migration of surface material. The above the level of the noise. At longer wavelengths, a global slope structure of Bennu shows a clear transi- 2.7-µm band is present, consistent with the presence tions at specific latitudes across the surface. of hydrated silicates, including those contained in CI Conclusions: We infer that Bennu is an ancient and CM carbonaceous chondrites. object that has witnessed over 4.5 Gyr of Solar Sys- OTES spectra have low contrast (2%) and a spec- tem history. Its chemistry and mineralogy were estab- tral shape that is also consistent with CI and CM car- lished within the first 10 Myr of the Solar System. bonaceous chondrites. A single spectral feature at 440 The bulk material is likely to be similar to CI and CM cm-1 is consistent with phyllosilicates in carbonaceous chondrites. However, there are materials that do not chondrites. The OTES spectra also show features at appear to be related to these meteorites, i.e., the 555 and 346 cm-1 that we tentatively attribute to mag- bright boulders. The clastic rocks likely formed by netite (Fe3O4). impact brecciation, mixing, and reaccretion on the Asteroid Shape and Surface Features. The asteroid surface of a large (~100-km) carbonaceous asteroid. shape model is a mission-critical data product that is Bennu likely originated as a discrete asteroid in the required for successful proximity operations. It also inner main belt as reaccumulated fragments from the provides the framework upon which all other map catastrophic disruption of this larger parent body. The products are overlain. Images acquired during the lack of a distinct equatorial ridge, the large number of final Approach allowed the creation of a shape model well-defined impact craters, and the evidence for ex- needed by both the navigation team and science plan- tensive thermal mechanical weathering suggest that ners using stereophotoclinometry (SPC). Bennu’s surface is old. The extensive mapping and Bennu exhibits a spinning top shape similar to as- site-specific characterization campaigns in 2019 will teroid (162173) Ryugu and (341843) 2008 EV5. Ben- provide additional information in these areas. nu’s surface displays a wide variety of terrain. We References: [1] Lauretta, D. S. et al. Space Sci- find a substantial boulder population, impact craters, ence Reviews 212 (2017): 925-984. [2] Lauretta, D. linear ridges, hemispherical dichotomy, retrograde S., et al. Meteoritics & Planetary Science 50.4 rotation, and a small offset between the center of fig- (2015): 834-849. [3] Chesley, S R. et al. Icarus 235 ure and center of mass. In contrast to the other spin- (2014): 5-22. ning-top asteroids, Bennu lacks a distinct ridge. Acknowledgments: This material is based upon Many of the boulders appear to be clastic rocks work supported by NASA under Contract that are composed of large angular fragments, similar NNM10AA11C issued through the New Frontiers in appearance to impact breccias. There is substantial Program.