Overcoming the Challenges Associated with Image‐Based Mapping of Small Bodies in Preparation for the OSIRIS‐REx Mission to (101955) Bennu

Abstract

The OSIRIS‐REx Asteroid Sample Return Mission is the third mission in National Aeronautics and Space Administration (NASA)'s New Frontiers Program and is the first U.S. mission to return samples from an asteroid to Earth. The most important decision ahead of the OSIRIS‐REx team is the selection of a prime sample‐site on the surface of asteroid (101955) Bennu. Mission success hinges on identifying a site that is safe and has regolith that can readily be ingested by the spacecraft's sampling mechanism. To inform this mission‐critical decision, the surface of Bennu is mapped using the OSIRIS‐REx Camera Suite and the images are used to develop several foundational data products. Acquiring the necessary inputs to these data products requires observational strategies that are defined specifically to overcome the challenges associated with mapping a small irregular body. We present these strategies in the context of assessing candidate sample sites at Bennu according to a framework of decisions regarding the relative safety, sampleability, and scientific value across the asteroid's surface. To create data products that aid these assessments, we describe the best practices developed by the OSIRIS‐REx team for image‐based mapping of irregular small bodies. We emphasize the importance of using 3‐D shape models and the ability to work in body‐fixed rectangular coordinates when dealing with planetary surfaces that cannot be uniquely addressed by body‐fixed latitude and longitude.

In situ evidence of thermally induced rock breakdown widespread on Bennu’s surface

“…We used radiometrically calibrated images from the OCAMS PolyCam camera, designed to collect high resolution images of the surface 25 . Images were acquired during the Detailed Survey Baseball Diamond and Orbital B campaigns 26,27 , which occurred between March 21 and July 26, 2019. The images included were taken at a spacecraft distance of 0.92-4.92 km above the surface and have pixel scales ranging from 0.9 to 6.3 cm/px (see Table 1 and "Methods" section).…”

“…Image backplanes of photometric angles (emission, incidence, phase), oblique pixel scale (ground sample distance), and body-fixed geographic coordinates were calculated for each image on a per pixel basis. Calculations were performed using ray-tracing methods described by DellaGiustina et al 27 and the v28 tessellated plate model of Bennu's shape 53 . Images were registered to the shape model using reconstructed SPICE kernels, resulting in an image-to-shape registration accuracy that varies from 5 to 20 pixels.…”

Episodes of particle ejection from the surface of the active asteroid (101955) Bennu

“…3 shows polygons of the ejection sites overlaid on a mosaic of images acquired by the PolyCam imager of the OSIRIS-REx Camera Suite (OCAMS) (80). To create the mosaics, we used radiometrically calibrated PolyCam images acquired at low emission angles (<20°) during (81). Images were projected to a tessellated 3D shape model (v20) of Bennu (64) and mosaicked using an equirectangular map projection with the Integrated Software for Imagers and Spectrometers version 3 (ISIS3) (82); the mosaicking techniques and procedures used to create the final product are described in (81).…”

Properties of rubble-pile asteroid (101955) Bennu from OSIRIS-REx imaging and thermal analysis

“…The global mosaic of Bennu in Fig. 2 The global mosaic was constructed using the software and methods described in DellaGiustina et al [56]. No photometric correction was performed prior to mosaicking, however a process similar to the ISIS3 noseam application was applied to minimize the presence of seams and improve the cosmetic quality of the mosaic.…”