No one gets closer to the Sun: Thermophysical properties of Atira object 2021 PH27

Introduction
The near-Earth asteroid (NEA) 2021 PH₂₇ was discovered in August 2021. It belongs to the very rare group of Atira objects which stay always within the Earth’s perihelion [1]. Its orbit has a semi-major axis of 0.46 au, an eccentricity of 0.71, and an inclination of 31.9^◦ , with an orbital period of less than 115 days. At perihelion (at 0.13 au) it reaches temperatures above 1000 K and it moves with about 106 km/s with a relativistic perihelion shift of 42.9"/century, 1.6 times that of Mercury. It is therefore an important reference object to understand the interplay between non-gravitational forces, general relativity effects, orbital changes due to the von Zeipel-Lidov-Kozai mechanism [2]. A recent study [3] claims that 2021 PH₂₇ might be an active asteroid which could then produce observable meteor showers on Venus.
The object has an estimated absolute magnitude of 17.8 mag, corresponding to a size range between 0.7 km (high albedo of 0.25) and 2.6 km (radar-based albedo of 0.02 for Atira). Currently, there are 28 Atira (or Apohele) objects known (https://en.wikipedia.org/wiki/Atira_asteroid). Atira objects never cross the Earth’s orbit and there is no impact risk right now. However, they have frequent close encounters with Mercury and Venus which could eventually push an Atira-orbit into an Earth-crossing orbit [2].
The Atira objects represent a part of our Solar System which is poorly known. The few known ones are considered as the ”tip of the iceberg” of a likely numerous population of similar objects which are still undiscovered due to the difficulty to observe so close to the Sun [4]. There are indications that their orbits are the result of gravitational interactions with the Earth-Moon system, Jupiter and Venus [4], but their origin remains an open question [5]. Also, non-gravitational forces, like Yarkovsky and YORP, are expected to play a strong role for long-term orbital evolution [6], for the spin behaviour [7], and possibly also for the shape of these bodies [8].

Observations

In preparation for the challenging VLT-VISIR observations (fast-moving NEA, at high airmass, in twilight), we made astrometric measurements to improve the orbit solution. At the beginning of April 2022, the positional uncertainty reached values below one arcsecond (plane-of-sky 3-σ error).
We observed the thermal emission of 2021 PH₂₇ with VISIR before opposition in April 2022 (leading the Sun), after-opposition measurements (trailing the Sun) are planned for July 2022. In both cases, the object is in a ”half-Moon” configuration (about -90^◦ and +70^◦ phase angle, respectively). Such measurements guarantee that we see once a warm terminator, and once a cold terminator (see Fig. 1). This allows us to determine the asteroid’s sense of rotation, its size and albedo, and to constrain the thermal properties of its surface. This is especially interesting for an asteroid which is frequently heated up to more than 1000 K (e.g. during perihelion in late May 2022 inbetween our VISIR observations). It is expected that the high temperatures have an influence on the surface properties (hence, the thermal inertia) by thermally breaking the solid rock due to very steep temperature gradients between illuminated and shadowed parts of the surface [9].This could also lead to sporadic activity [3].

Fig. 1: Temperature and flux prediction for 2021 PH₂₇ for our thermal observations in early April and mid July 2022. The calculations assume an object albedo of 0.1 (corresponding to a size of about 1.1 km) and a thermal inertia of 500 Jm⁻² K⁻¹s^−0.5.

Our measurements are the first thermal detections of an Atira family member. We also conducted visible lightcurve measurements in Feb, Mar, and Apr 2022, to establish the object’s rotation period and shape properties, and to determine the H-magnitude. The VISIR measurements can be rotationally phased to derive a good-quality size and albedo solution (see e.g., [10,11], and references therein), and to put constraints on its physical shape via well-established thermophysical model techniques [12].

Results

The pair of before/after opposition measurements allows us to determine the objects sense of rotation (prograde or retrograde) [13] which is important to calculate the non-gravitational Yarkovsky force [6], and to look into possible orbital evolution processes which might slowly push this object into the category of potentially hazardous objects [14]. Finding a good model solution for the measurements taken in both VISIR runs also means to constrain the thermal properties of the surface. A high thermal inertia (connected to a rocky or bare-rock surface) will carry substantial heat from the illuminated into the non-illuminated part of the surface (large before/after opposition asymmetry, see Fig. 1). A low thermal inertia (like for our Moon) leads to almost instantaneous thermal equilibrium and would point to a fine-grain regolith with very low conductivity.
The VISIR measurements of 2021 PH₂₇ are the foundation for the characterization of the entire Atira family, for the study of repeated extremely high surface temperatures, and for exploring the largely unknown terrains of poorly detectable NEAs inside the Earth’s orbit. The physical properties of 2021 PH₂₇ are also relevant in the context of disentangling non-gravitational from relativistic orbit effects.

References

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