Orbit and Bulk Density of the OSIRIS-REx Target Asteroid (101955) Bennu

Abstract: The target asteroid of the OSIRIS-REx asteroid sample return mission, (101955) Bennu (formerly 1999 RQ$_{36}$), is a half-kilometer near-Earth asteroid with an extraordinarily well constrained orbit. An extensive data set of optical astrometry from 1999--2013 and high-quality radar delay measurements to Bennu in 1999, 2005, and 2011 reveal the action of the Yarkovsky effect, with a mean semimajor axis drift rate $da/dt = (-19.0 \pm 0.1)\times 10^{-4}$ au/Myr or $284\pm 1.5\;\rm{m/yr}$. The accuracy of this result depends critically on the fidelity of the observational and dynamical model. As an example, neglecting the relativistic perturbations of the Earth during close approaches affects the orbit with $3\sigma$ significance in $da/dt$. The orbital deviations from purely gravitational dynamics allow us to deduce the acceleration of the Yarkovsky effect, while the known physical characterization of Bennu allows us to independently model the force due to thermal emissions. The combination of these two analyses yields a bulk density of $\rho = 1260\pm70\,\rm{kg/m^3}$, which indicates a macroporosity in the range $40\pm10$% for the bulk densities of likely analog meteorites, suggesting a rubble-pile internal structure. The associated mass estimate is $(7.8\pm0.9)\times 10^{10}\, \rm{kg}$ and $GM = 5.2\pm0.6\,\rm{m^3/s^2}$. Bennu's Earth close approaches are deterministic over the interval 1654--2135, beyond which the predictions are statistical in nature. In particular, the 2135 close approach is likely within the lunar distance and leads to strong scattering and therefore numerous potential impacts in subsequent years, from 2175--2196. The highest individual impact probability is $9.5\times 10^{-5}$ in 2196, and the cumulative impact probability is $3.7\times 10^{-4}$, leading to a cumulative Palermo Scale of -1.70.

• The paper presents a detailed analysis of Bennu’s orbit, highlighting a semimajor axis drift rate of (-19.0 ± 0.1)×10⁻⁴ au/Myr due to the Yarkovsky effect.
• It combines thermophysical modeling with observational data to estimate Bennu’s bulk density at 1260 ± 70 kg/m³, indicating a rubble-pile structure with 30-50% porosity.
• These findings refine trajectory models and support improved impact hazard assessments for Bennu's potential Earth encounters.

Analyzing the Dynamics and Composition of Asteroid (101955) Bennu

The research paper by Chesley et al. offers a comprehensive analysis of asteroid (101955) Bennu, focusing on its orbit, bulk density, and the Yarkovsky effect. Bennu, targeted by the OSIRIS-REx mission, is a near-Earth object with significant relevance both scientifically and in terms of planetary defense due to its potential for future Earth impacts.

Orbit Determination and the Yarkovsky Effect

The paper provides an intricate characterization of Bennu's orbit, leveraging an extensive dataset comprising optical astrometry and radar observations. Precise tracking from 1999 to 2013 has revealed the influence of the Yarkovsky effect, a critical nongravitational force, on Bennu's orbit with unprecedented precision. The study determines a semimajor axis drift rate of $(-19.0 \pm 0.1) \times 10^{-4}$ au/Myr, indicating a transverse acceleration that profoundly affects orbital predictions. The analysis confirms that relative errors in observing dynamics, such as neglecting Earth's relativistic perturbations during close encounters, can have significant impacts ($3\sigma$ changes) on this rate.

The Yarkovsky effect's role is particularly enlightening: it connects thermal emissions from Bennu's surface to its trajectory. The accuracy achieved in this measurement exceeds previous attempts, with implications for long-term impact risk assessment covering several hundred years into the future.

Bulk Density Estimation and Structural Insights

By combining the Yarkovsky effect with thermophysical modeling, Chesley et al. infer Bennu's bulk density at $1260 \pm 70\,\rm{kg/m^3}$, suggesting a macroporosity range of 30-50% when cross-referenced with analog meteorites. Such a degree of porosity strongly implies a rubble-pile structure, where Bennu exists not as a monolithic body but as an aggregate of smaller rocks held together by mutual gravitation and interstitial voids.

Impact Hazard Assessment

The paper's systematic impact hazard evaluation is rooted in the astrometric precision achieved for Bennu. Future encounters with Earth, particularly the one predicted in 2135 within the lunar distance, are shown to determine Bennu's risky trajectories due to resulting gravitational scattering. The cumulative impact probability over the next couple of centuries is quantified at $3.7\times 10^{-4}$, with the associated Palermo Scale calculated to be -1.70, indicating a non-negligible hazard potential.

Implications and Future Directions

The research underscores the importance of physical characterization in orbit dynamics and impact risk assessment for near-Earth objects. The application of Yarkovsky effect analysis has broadened our understanding of asteroid bulk densities based on indirect measurements—a method particularly useful for unvisited asteroids.

With the ongoing OSIRIS-REx mission, which will enhance the fidelity of Bennu’s dynamics through direct observations and sample analysis, the expectation is that trajectory models will be further refined, improving predictions and potentially helping calibrate models for other asteroids. This paper sets a precedent in melding observational strategies with theoretical models to achieve high-precision results relevant for both science and planetary defense.

Conclusion

Chesley et al.’s work exemplifies how sophisticated modeling, combined with extensive observational datasets, can yield new insights into the dynamic and physical characteristics of near-Earth asteroids. The adoption of such methodologies is likely to advance our understanding of asteroid behavior, inform mission planning, and enhance our preparedness for potential impact threats.