Near-Earth asteroid (3200) Phaethon. Characterization of its orbit, spin state, and thermophysical parameters

Abstract: The near-Earth asteroid (3200) Phaethon is an intriguing object: its perihelion is at only 0.14 au and is associated with the Geminid meteor stream. We aim to use all available disk-integrated optical data to derive a reliable convex shape model of Phaethon. By interpreting the available space- and ground-based thermal infrared data and Spitzer spectra using a thermophysical model, we also aim to further constrain its size, thermal inertia, and visible geometric albedo. We applied the convex inversion method to the new optical data obtained by six instruments and to previous observations. The convex shape model was then used as input for the thermophysical modeling. We also studied the long-term stability of Phaethon's orbit and spin axis with a numerical orbital and rotation-state integrator. We present a new convex shape model and rotational state of Phaethon: a sidereal rotation period of 3.603958(2) h and ecliptic coordinates of the preferred pole orientation of (319$^{\circ}$, $-$39$^{\circ}$) with a 5$^{\circ}$ uncertainty. Moreover, we derive its size ($D$=5.1$\pm$0.2 km), thermal inertia ($\Gamma$=600$\pm$200 J m$^{-2}$ s$^{-1/2}$ K$^{-1}$), geometric visible albedo ($p_{\mathrm{V}}$=0.122$\pm$0.008), and estimate the macroscopic surface roughness. We also find that the Sun illumination at the perihelion passage during the past several thousand years is not connected to a specific area on the surface, which implies non-preferential heating.

Characterization of Near-Earth Asteroid (3200) Phaethon

The study of asteroid (3200) Phaethon presents vital insights into its physical and orbital characteristics, revealing complexities relevant to both its own nature as a celestial body and its connection to the Geminid meteor stream. This paper meticulously characterizes Phaethon's orbit, spin state, and thermophysical parameters using a comprehensive approach that combines both optical and infrared observational data.

Orbital and Rotational Dynamics

Phaethon’s orbit, with a perihelion distance of merely 0.14 au, frequently subjects it to extreme solar heating, reaching surface temperatures exceeding 1,000 K. The rotational dynamics, defined by a sidereal rotation period of approximately 3.603958 hours, are crucial to understanding its thermal behavior and surface properties. The inferred pole orientation of (319°, -39°) carries implications for the thermal inertia and surface heating patterns, which are essential in modeling the Yarkovsky effect and understanding the body's orbital evolution in relation to its physical characteristics.

Thermophysical Model and Results

The application of a thermophysical model yields a nuanced perspective on Phaethon’s physical properties. Noteworthy conclusions include the asteroid's estimated diameter of 5.1 ± 0.2 kilometers and a thermal inertia of 600 ± 200 J m⁻² s⁻¹/² K⁻¹. These values hint at a significant surface roughness and geometric visible albedo of 0.122 ± 0.008. The reported thermal inertia is critical in positing that Phaethon's lack of fine regolith may be attributed to high solar wind fluxes which actively alter its surface material distribution.

Implications for the Geminid Stream

One intriguing aspect of Phaethon's study is its relationship with the Geminid meteor stream. While past studies align Phaethon as the progenitor of the Geminids, the mechanisms of mass loss necessary to support such a stream are not conclusively understood. The paper postulates thermal disintegration as a plausible process but acknowledges the need for deeper investigations into thermal fracture mechanisms correlating with Phaethon’s specific physical context and its orbital dynamics over extensive periods.

Future Directions

The paper suggests that further high-resolution studies, particularly during Phaethon’s imminent close approaches to Earth, could refine existing models, especially concerning the Yarkovsky effect. Such encounters offer an opportunity to employ radar astrometry and enhanced photometric analyses, potentially shedding light on the asteroid's bulk density and enhancing understanding of its interior structure.

In summary, this comprehensive analysis of asteroid (3200) Phaethon provides indispensable data that aid in unraveling its unique characteristics. Such studies lay foundational groundwork for future explorations and theoretical developments, particularly within asteroid thermophysics and the broader implications for near-Earth objects.