The low surface thermal inertia of the rapidly rotating near-Earth asteroid 2016 GE1

Abstract

Context. Asteroids smaller than about 100 m in diameter are observed to rotate very fast, with periods often much shorter than the critical spin limit of 2.2 h. Some of these super-fast rotators can also achieve a very large semimajor axis drift induced by the Yarkovsky effect, which, in turn, is determined by internal and surface physical properties. Aims. We consider here a small super-fast-rotating near-Earth asteroid, designated as 2016 GE1. This object rotates in just about 34 s, and a large Yarkovsky effect has been determined from astrometry. By using these results, we aim to constrain the thermal inertia of the surface of this extreme object. Methods. We used a recently developed statistical method to determine the thermal properties of near-Earth asteroids. The method is based on the comparison between the observed and the modeled Yarkovsky effect, and the thermal conductivity (inertia) is determined via a Monte Carlo approach. Parameters of the Yarkovsky effect model are fixed if their uncertainty is negligible, modeled with a Gaussian distribution of the errors if they are measured, or deduced from general properties of the population of near-Earth asteroids when they are unknown. Results. Using a well-established orbit determination procedure, we determined the Yarkovsky effect on 2016 GE1 and confirm a significant semimajor axis drift rate. Using a statistical method, we show that this semimajor axis drift rate can only be explained by low thermal inertia values below 100 J m−2 K−1 s−1/2. We benchmarked our statistical method using the well-characterized asteroid Bennu and find that only knowing the semimajor axis drift rate and the rotation period is generally insufficient for determining the thermal inertia. However, when the statistical method is applied to super-fast rotators, we find that the measured Yarkovsky effect can be achieved only for very low values of thermal inertia: namely, 90% of the probability density function of the model outcomes is contained at values smaller than 100 J m−2 K−1 s−1/2. Conclusions. We propose two possible interpretations for the extremely low thermal inertia of 2016 GE1: a high porosity or a cracked surface, or a thin layer of fine regolith on the surface. Though both possibilities seem somewhat unexpected, this opens up the possibility of a subclass of low-inertia, super-fast-rotating asteroids.

ASTERIA—Asteroid Thermal Inertia Analyzer

“…A preliminary model and ASTERIA code testing on asteroid Bennu have already been performed by Fenucci et al (2023a). affect the final TI estimates and showed that the model can reproduce reasonably well Bennu's TI.…”

“…Building on these findings, here we extended this analysis, investigating the other relevant factors that may affect the match between our estimate and the reference value(s). In particular, the factors that were not considered by Fenucci et al (2023a) are the role of the nonspherical shape and that of the spatial variations of Bennu's surface TI. Referent TI estimates for asteroid Bennu, used to benchmark our model, are listed in Table 4.…”

“…Recently, Fenucci et al (2021) introduced a statistical method for the thermal estimation of NEAs based mainly on ground-based observations. The method was successfully applied to determine the TI of two rapidly rotating NEAs, asteroids 2011 PT and 2016 GE1 (Fenucci et al 2021(Fenucci et al , 2023a. The method requires, in fact, a determination of the semimajoraxis drift produced by the Yarkovsky effect (see, e.g., Bottke et al 2006;Vokrouhlický et al 2015, for a review), a thermal effect caused by sunlight that strongly depends on the surface properties.…”

“…Motivated by these facts, one of the aims of the "Demystifying Near-Earth Asteroids" (D-NEAs) project funded by the Planetary Society 8 is to develop further methods for the characterization of NEAs based on ground-based observations. In this paper, we propose extensions of the concept introduced in Fenucci et al (2021Fenucci et al ( , 2023a in order to make the TI estimation more reliable and accurate. In particular, we implemented different Yarkovsky effect models suitable for different dynamical and physical characteristic scenarios.…”

Multicolor Photometry of Tiny Near-Earth Asteroid 2015 RN35 across a Wide Range of Phase Angles: Possible Mission-accessible A-type Asteroid

“…According to these observations, asteroid 2009 BD has an inconclusive surface nature and could be either covered by fine regolith or composed of a collection of bare rocks, while the bulk density of 2011 MD is estimated to be 1.1 10 kg m 0.5 0.7 3 3 -+ -, indicating a rubble-pile origin. Recently, Fenucci et al (2021Fenucci et al ( , 2023 found that the tiny superfast rotators (499998) 2011 PT (D ∼ 35 m and rotation period P ∼ 10 minutes) and 2016 GE 1 (D ∼ 12 m and P ∼ 34 s) have small thermal conductivities of K 0.1 W m −1 K −1 and K 100 W m −1 K −1 , respectively. Such small conductivities imply that these two tiny asteroids are covered with fine regolith or highly porous rocks (Avdellidou et al 2020;Cambioni et al 2021).…”