Mass and Shape Determination of (101955) Bennu Using Inter-Orbital Phase Tracking Data

The Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission will collect and return a sample from the rubble-pile asteroid (101955) Bennu. To successfully execute the touch-and-go sampling attempt, the shape and mass of the asteroid need to be known precisely. Here, we use a combination of radiometric data from the Deep Space Network (DSN) together with image data from the NavCam camera and altimetric data from the OSIRIS-REx Laser Altimeter (OLA) to determine the mass and shape of the asteroid. Landmarks identified in images and OLA altimetric ranges are directly compared to the prior estimate of the landmark coordinates and of the asteroid shape (in the form of a digital elevation model, or DEM). Geophysical parameters and the spacecraft orbit are adjusted to minimize the misfits between computed and observed radiometric data, landmark coordinates, and altimetric radii, using the NASA Goddard Space Flight Center's GEODYN orbit determination and geodetic parameter estimation software. Our shape determination consists of estimating a scale factor and three frame rotation angles that apply to both the DEM and the landmark coordinates. Within a single orbital phase, the mass and DEM scale are highly correlated. By using off-nadir altimetry and data from independent orbital phases of the mission, correlations can be reduced, but they remain high. We use an additional data type, called image constraints, that differences landmark observation pairs. This differential data type is independent of the shape. We show that, by using image constraint observations linking the two orbital phases Orbital B and Recon, in addition to intra-phase image constraint observations, the correlations between mass, DEM scale, and altimetry biases are greatly reduced. This results in a greatly improved solution for the mass and shape of the asteroid.