Six new studies in Science and Science Advances present results from the OSIRIS-REx spacecraft and reveal insights about the near-Earth asteroid Bennu. The primary goal of NASA's OSIRIS-Rex (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) mission is to collect a sample from the surface of Bennu - a "rubble pile" of carbon-rich material ejected from a parent asteroid - and return it to Earth for analysis. Since its rendezvous with Bennu in late 2018, OSIRIS-REx has conducted detailed orbital surveys and reconnaissance of Bennu's surface, collecting data on the asteroid's composition and structure as well as identifying suitable locations for sample collection, due on October 20, 2020. The findings presented in these studies provide information on Bennu's history and context for the samples returned to Earth, due in 2023.
In the first of three studies in Science, Daniella DellaGiustina and colleagues present multispectral images that map the optical color and reflectance of Bennu's surface. By comparing the differences in color and albedo between the boulders and craters, DellaGiustina et al. infer how Bennu's surface has undergone complex evolution due to space weathering processes. In a second study, Amy Simon and colleagues use infrared spectroscopy to show that carbon-bearing materials, such as organic molecules and/or carbonate minerals, are widespread across most of Bennu's surface and particularly concentrated on individual boulders. In Science's third study, Hannah Kaplan and colleagues present high-resolution images and spectra of OSIRIS-REx's primary sample site - a crater nicknamed Nightingale. Kaplan et al. identify bright veins in some of the area's boulders, with distinct infrared absorptions, suggesting that they are carbonate minerals. The veins would have formed by reactions with flowing water on Bennu's parent asteroid, during the early Solar System.
In a Science Advances study, Michael Daly and colleagues observed Bennu using OSIRIS-REx's Laser Altimeter (OLA). Daly et al. used the OLA data to produce a 20-cm-resolution 3D model of Bennu and measure the asteroid's rocky structure. They find that Bennu's southern hemisphere is rounder and smoother, while its northern hemisphere has higher slopes and a more irregular shape. The second study in Science Advances explores the physical characteristics of boulders that make up the asteroid's rubble-pile structure. Ben Rozitis and colleagues used thermal infrared data to determine the surface roughness and thermal inertia of Bennu's boulders. They find that Bennu likely consists of two different types of boulders with similar mineral compositions but different colors and albedos, which may also have distinct structural properties. In the last Science Advances study in the package, Daniel Scheeres and colleagues tracked the motion of the OSIRIS-REx spacecraft in Bennu's weak gravitational field, and the orbits of pebble-sized particles ejected from Bennu's surface. Modelling those motions allowed the authors to determine the distribution of the asteroid's gravitational field. The findings suggest that the rubble-pile's density is unevenly distributed, with lower-density regions at the equator and center. Scheeres et al. conclude that Bennu's characteristic "spinning-top" shape resulted from either a rapid spin rate in its past or a previous disruption of its surface.