Spin Change of Asteroid 2012 TC4 probably by Radiation Torques

TL;DR

This study analyzes the changing spin state of asteroid 2012 TC4 between 2012 and 2017, suggesting that the YORP radiation torque, rather than gravitational effects or impacts, likely caused its spin evolution, marking the first such detection in a tumbling asteroid.

Contribution

The paper provides the first evidence that the YORP effect can alter the spin state of a tumbling asteroid, supported by a self-consistent dynamical model and observational data.

Findings

Spin and precession periods differ between 2012 and 2017 data.

Gravitational torques are negligible in spin change.

YORP torque plausibly explains the observed spin evolution.

Abstract

Asteroid 2012 TC4 is a small ($\sim$10 m) near-Earth object that was observed during its Earth close approaches in 2012 and 2017. Earlier analyses of light curves revealed its excited rotation state. We collected all available photometric data from the two apparitions to reconstruct its rotation state and convex shape model. We show that light curves from 2012 and 2017 cannot be fitted with a single set of model parameters -- the rotation and precession periods are significantly different for these two data sets and they must have changed between or during the two apparitions. Nevertheless, we could fit all light curves with a dynamically self-consistent model assuming that the spin states of 2012 TC4 in 2012 and 2017 were different. To interpret our results, we developed a numerical model of its spin evolution in which we included two potentially relevant perturbations: (i) gravitational torque due to the Sun and Earth, and (ii) radiation torque known as the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect. Despite our model simplicity, we found that the role of gravitational torques is negligible. Instead, we argue that the observed change of its spin state may be plausibly explained as a result of the YORP torque. To strengthen this interpretation we verify that (i) the internal energy dissipation due to material inelasticity, and (ii) an impact with a sufficiently large interplanetary particle are both highly unlikely causes its observed spin state change. If true, this is the first case when the YORP effect has been detected for a tumbling body.