NASA’S Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) spacecraft recently arrived at near-Earth asteroid (101955) Bennu, a primitive body that represents the objects that may have brought prebiotic molecules and volatiles such as water to Earth [ 1 ]. Bennu is a low-albedo B-type asteroid [ 2 ] that has been linked to organic-rich hydrated carbonaceous chondrites [ 3 ]. Such meteorites are altered by ejection from their parent body and contaminated by atmospheric entry and terrestrial microbes. Thus, the primary mission objective is to return a sample of Bennu to Earth that is pristine, i.e., not affected by these processes [ 4 ]. The OSIRIS-REx spacecraft carries a sophisticated suite of instruments to characterize Bennu’s global properties; support selection of a sampling site; and document that site at sub-centimeter scales [ 5 - 11 ]. Here we consider early observations to understand how Bennu’s properties compare to pre-encounter expectations and 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 modeling of Bennu’s thermal inertia [ 12 ] and radar polarization ratios [ 13 ]—which indicated a generally smooth surface covered by centimeter-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-m-diameter patches of loose regolith with grain sizes less than 2 cm [ 4 ]. We observe only a small number of apparently hazard-free regions, on the order of 5 to 20 meters in extent, the sampling of which poses a substantial challenge to mission success.

[1] The objectives of this paper are twofold: first, to report our estimates of the meter-todecameter-scale topography and slopes of candidate landing sites for the Phoenix mission, based on analysis of Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) images with a typical pixel scale of 3 m and Mars Reconnaissance Orbiter (MRO) High Resolution Imaging Science Experiment (HiRISE) images at 0.3 m pixel À1 and, second, to document in detail the geometric calibration, software, and procedures on which the photogrammetric analysis of HiRISE data is based. A combination of optical design modeling, laboratory observations, star images, and Mars images form the basis for software in the U.S. Geological Survey Integrated Software for Imagers and Spectrometers (ISIS) 3 system that corrects the images for a variety of distortions with single-pixel or subpixel accuracy. Corrected images are analyzed in the commercial photogrammetric software SOCET SET ( 1 BAE Systems), yielding digital topographic models (DTMs) with a grid spacing of 1 m (3-4 pixels) that require minimal interactive editing. Photoclinometry yields DTMs with single-pixel grid spacing. Slopes from MOC and HiRISE are comparable throughout the latitude zone of interest and compare favorably with those where past missions have landed successfully; only the Mars Exploration Rover (MER) B site in Meridiani Planum is smoother. MOC results at multiple locations have root-mean-square (RMS) bidirectional slopes of 0.8-4.5°at baselines of 3-10 m. HiRISE stereopairs (one per final candidate site and one in the former site) yield 1.8-2.8°s lopes at 1-m baseline. Slopes at 1 m from photoclinometry are also in the range 2-3°a fter correction for image blur. Slopes exceeding the 16°Phoenix safety limit are extremely rare.

Multispectral images (0.44 to 0.98 μm) of asteroid (4) Vesta obtained by the Dawn Framing Cameras reveal global color variations that uncover and help understand the north-south hemispherical dichotomy. The signature of deep lithologies excavated during the formation of the Rheasilvia basin on the south pole has been preserved on the surface. Color variations (band depth, spectral slope, and eucrite-diogenite abundance) clearly correlate with distinct compositional units. Vesta displays the greatest variation of geometric albedo (0.10 to 0.67) of any asteroid yet observed. Four distinct color units are recognized that chronicle processes--including impact excavation, mass wasting, and space weathering--that shaped the asteroid's surface. Vesta's color and photometric diversity are indicative of its status as a preserved, differentiated protoplanet.