Abstract
Abstract The Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission collected a sample from the rubble-pile asteroid (101955) Bennu for return to Earth. For the successful Touch And Go sample acquisition maneuver, the shape and mass of the asteroid needed to be known precisely. Here we use a combination of radiometric, image landmark, and laser altimetry data to determine Bennu’s mass, shape, and orientation simultaneously and to verify existing models thereof. Our shape determination consists of estimating a scale factor and three frame rotation angles that apply to both the global digital terrain model (GDTM) and the landmark coordinates. We use a data type called image constraints, where we take the difference of the observation of the same landmark in images taken at two different times. We analyze data from two phases of the OSIRIS-REx mission, Orbital B and Recon B, and show that interphase image constraints greatly reduce interdependencies between estimated parameters for mass, GDTM scale, and biases on the altimetry data. This results in an improved solution for the mass and shape relative to considering a single mission phase. We find Bennu’s gravitational parameter GM to be 4.89256 ± 0.00035 m3 s−2, and we find a scale factor of 1.000896 ± 0.00036 for the altimetry-based GDTM. Using the scaled volume, this results in a bulk density of 1191.57 ± 1.74 kg m−3 , which is within the uncertainties of previous analyses but more precise.
Highlights
The instruments most relevant to the work presented here include the cameras of the OSIRISREx Camera Suite (OCAMS; Rizk et al 2018; Golish et al 2020); the cameras of the Touch And Go Camera System (TAGCAMS; Bos et al 2018), including two fully redundant cameras to support optical navigation and natural feature
Global digital terrain models (GDTMs) produced from OCAMS data mapped Bennu’s top-like shape, confirming that it is a rubble pile formed from reaccumulated fragments of a larger progenitor and has undergone past periods of fast spin (Barnouin et al 2019)
We used direct altimetry, where the tracks are compared with the OSIRIS-REx Laser Altimeter (OLA) GDTM of Bennu (Daly et al 2020)
Summary
For almost 2 yr after arrival, OSIRIS-REx collected measurements to characterize Bennu and its microgravity environment using a suite of scientific instruments and spacecraft tracking data (Lauretta et al 2017, 2021). When OSIRIS-REx arrived at Bennu, it became clear that the asteroid is very different in certain respects from expectations based on pre-encounter knowledge, with surface properties such as albedo variation and roughness being beyond the spacecraft design specifications for sample acquisition (Lauretta et al 2019a, 2021). Bennu’s rotational acceleration, which had been detected from lightcurve analysis (Nolan et al 2019), was shown to be most plausibly due to the Yarkovsky–O’Keefe–Radzievskii–Paddack effect (Hergenrother et al 2019), that is, the absorption of sunlight that is reradiated thermally on an asteroid of irregular shape (Rubincam 2000)
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