On the capture of small stony asteroids into Earth orbit by atmospheric grazing

An Earth-grazing asteroid can be captured into a gravitational bound orbit around Earth during its transitory atmospheric journey. Otherwise, it will either escape back to space or plunge to Earth directly. With fragmentation taken into account, we subdivide the captured and direct impact modes, expanding the above three modes into five – escaping, captured impact with and without fragmentation, direct impact with and without fragmentation. We then investigate the conditions of those various impact modes of shallow-angle impacts of small stony asteroids no larger than 100 m in diameter. Moreover, the atmospheric entry processes of captured stony asteroids are further studied. Results show that asteroids with larger diameters are easier to fragment for less deceleration due to the smaller area-mass ratio, narrowing the corridor for capture. A captured asteroid can enter the atmosphere many times, highlighting itself by a series of explosive phenomena due to the shock wave it produced during every passage. The number of revolutions before its final entry increases as the theoretical perigee altitude rises. The multi-entry phenomenon of captured impact reduces the velocity and mass of the impactor and raises the possibility of an intact landing of the object via atmospheric dissipation. The time and space intervals between each entry make it difficult to identify whether the scattered impacts come from one captured impact event or just a series of different fireballs. The long path before its final hit also increases the difficulty of predicting the exact airburst position or landing site.