Rotational changes of the asteroid 99942 Apophis during the 2029 close encounter with Earth

Abstract

Context. The close approach of the asteroid (99942) Apophis to Earth on April 13, 2029, at a distance of about 38 330 km, is naturally considered a dramatic and fascinating event by the astronomical community, as well as by the public.

Aims. Although the orbital changes of the asteroid (due to the gravitational perturbations of our planet during the encounter) have been abundantly investigated, the simultaneous changes in the rotational characteristics of the asteroid have been poorly studied.

Methods. In this paper, after numerically computing the perturbing rotational potential of Apophis due to Earth, we used a classical way of calculating variations in the precession psi and the obliquity epsilon, which enables us to determine the changes in position of the instantaneous pole of rotation in space during the encounter. By assigning a reasonable value of the unknown dynamical ellipticity of Apophis, deduced from a statistical study of a full sample of small asteroids, it follows that the changes in the precession Lambda psi and in the obliquity Delta epsilon of the asteroid during the 2029 encounter could be of a few degrees, which makes them an interesting challenge of detection by modern ground-based observations.

Results. This study confirms the possible observability of the asteroid rotational change during a close approach. It should be updated and refined in the future, when orbital, physical, and rotational parameters of Apophis become more precise.