# New observational evidence of active asteroid P/2010 A2: Slow rotation of the largest fragment

## TL;DR

This study presents new observational data on active asteroid P/2010 A2, revealing its slow-rotating, elongated largest fragment and detailed ejecta properties, supporting an impact shattering origin.

## Contribution

First rotational light curve of P/2010 A2's largest fragment obtained, revealing slow rotation and elongation, with detailed ejecta size distribution supporting impact shattering.

## Key findings

- Largest fragment has a double-peaked period of 11.36 hours.
- Largest fragment is highly elongated with an axis ratio ≥ 1.94.
- Ejecta size distribution follows a broken power law.

## Abstract

We report new observations of the active asteroid P/2010 A2 taken when it made its closest approach to the Earth (1.06 au in 2017 January) after its first discovery in 2010. Despite a crucial role of the rotational period in clarifying its ejection mechanism, the rotational property of P/2010 A2 has not yet been studied due to the extreme faintness of this tiny object ($\sim$120 m in diameter). Taking advantage of the best observing geometry since the discovery, we succeed in obtaining the rotational light curve of the largest fragment with Gemini/GMOS-N. We find that (1) the largest fragment has a double-peaked period of $11.36\pm0.02$ hr spinning much slower than its critical spin period; (2) the largest fragment is a highly elongated object ($a/b\geqslant 1.94$) with an effective radius of $61.9^{+16.8}_{-9.2}$ m; (3) the size distribution of the ejecta follows a broken power law (the power indices of the cumulative size distributions of the dust and fragments are $2.5\pm0.1$ and $5.2\pm0.1$, respectively); (4) the mass ratio of the largest fragment to the total ejecta is around 0.8; and (5) the dust cloud morphology is in agreement with the anisotropic ejection model in Kim et al. (2017). These new characteristics of the ejecta obtained in this work are favorable to the impact shattering hypothesis.