Formation Mechanism of Contact Binary Asteroid (4179) Toutatis
Abstract
Contact binary asteroids are a kind of single asteroids with bifurcated (or bi-lobed) configurations. Ground-based radar observations have shown that about 14% of near-Earth asteroids larger than 200 meters in diameter may be contact binaries. Specially, the targets of the three minor planet missions, Hayabusa, Chang'e-2, and Rosetta, have contact binary configurations. Contact binaries are an important type of asteroids for completely understanding the formation and evolution of asteroids in the solar system.
(4179) Toutatis is a prominent S-type near-Earth asteroid, and has been observed by many ground-based radar and optical campaigns since it was discovered in 1989. These observations reveal that the asteroid is rotating very slowly around a non-principal axis, and its 3D shape model constructed by radar data indicates that it is a contact binary. Chang'e-2 spacecraft flew by Toutatis on 13 December 2012, with a nearest distance of approximately 770 meters away from its surface. A series of high-resolution optical images taken during the flyby confirm that Toutatis is a distinct contact binary composed of two components. A sharply perpendicular silhouette is observed near the connection area, the big lobe has an obviously elongated shape, and the connection locates at the long axis of the big lobe. From a dynamical point of view, the configuration of Toutatis is in an unstable state. Many researchers suggest that the Toutatis' configuration may come from a low-speed impact between two components, but the detailed formation process that how such an unstable state become realistic is still not well understood. By employing the radar-derived shape model and the optical images obtained by Chang'e-2, the research presented in this dissertation mainly includes the following three parts. First, a new 3D shape model of Toutatis is derived by matching the silhouette between the Chang'e-2's optical images and the existing radar model. With that, we assume the precursor of Toutatis is a doubly synchronous binary asteroid composed of two spherical bodies. The dynamical scenario that the binary precursor closely encounters the Earth is investigated by applying the granular dynamical simulations in a wide parameter space. The results show that the gravitational perturbation of Earth may lead to a m/s-level low-speed impact between the primary and secondary, while the impact has a negligible influence on the shape of the primary. But if we choose an appropriate flyby distance (about 1.4-1.5 Earth radii), the Earth's tide may significantly alter the shape and spin of the components prior to their mutual impact, and a Toutatis-like contact binary asteroid may be reconstructed after the impact coalescence. This mechanism gives a new reasonable interpretation for the formation of Toutatis-like elongated contact binary asteroids. Second, the optical images obtained by Chang'e-2 show that 222 boulders from 10 m to 61 m across, and as well as over 70 craters, are identified from the optical images obtained by Chang'e-2, especially a depression with a diameter of 800 meters locates in the endpoint of the big lobe. We use a scaling-law method to obtain the size, position, and velocity distribution of ejecta particles from the depression under strength regime. Numerical simulations of the particles' orbits are conducted, and the re-impact portions and their position distributions are obtained. The results show that the total number and the volume of re-impact ejecta particles are obviously lower than the results identified by the observations of Chang'e-2, which means that most of the boulders on the surface of Toutatis did not come from the crater, but may originate during formation process of the parent body. Finally, we discuss the constraint of the gravitational field nearby an asteroid on its formation and evolution. In order to study the distribution of re-impact ejecta coming from the depression on Toutatis, we need to compute the orbits of these ejecta under the gravitational acceleration of Toutatis. However, the computation cost will be high if we use the polyhedral method to calculate the gravity. To eliminate this problem, we present a new method to calculate the gravity near an irregularly-shaped asteroid by adopting the 3D Chebyshev polynomial interpolation. We have compared four different gravity interpolation methods, and the best one is recommended for the efficiency, in which the gravitational acceleration will be decomposed along the local horizontal coordinate system, and then interpolated separately. An error-adaptive octree division scheme is also introduced to reduce the interpolation error near the surface of asteroid. We take Toutatis as an example to show that the new method may greatly improve the computation efficiency of near-surface gravitational acceleration, and it may be well used to perform the orbit propagation in case that the precision is not rigorous.- Publication:
-
Acta Astronomica Sinica
- Pub Date:
- September 2019
- Bibcode:
- 2019AcASn..60...46H
- Keywords:
-
- Toutatis;
- contact binary asteroid;
- formation and evolution;
- high-speed impact gravitational field