The Thermal and Dynamical History of (3200) Phaethon and (155140) 2005 UD: Relationship to Geminid Meteors
Authors: MacLennan Eric , Granvik Mikael , Toliou Athanasia 
Published: October 2020

The near-Earth asteroids (NEAs) (3200) Phaethon and (155140) 2005 UD are thought to share a common origin, with the former exhibiting dust activity at perihelion that is thought to directly supply the Geminid meteor stream. Both of these objects currently have very small perihelion distances (0.140 and 0.163 au for Phaethon and 2005 UD, respectively), which results in them having perihelion temperatures of or exceeding 1000 K. NEA population models compared to observation suggest that low-perihelion objects are destroyed over time by a temperature-dependent mechanism that becomes relevant at heliocentric distances < 0.3 au. Thus, the current activity from Phaethon is relevant to the destruction of NEAs close to the Sun, which most likely has produced meteor streams linked to asteroids in the past.In this work, we model the past thermal characteristics of Phaethon and 2005 UD using a detailed thermophysical model (TPM) and orbital integrations of each object. Our aim is to investigate and inform a temperature-dependent mechanism responsible for Phaethon's dust activity and the destruction of NEAs at small heliocentric distances. We consider volatile sublimation and thermal fracturing as potential candidate processes. First, a TPM is used to calculate temperatures (surface and subsurface) along an entire orbit for a spherical object, given its semimajor axis and eccentricity (a and e). Temperature characteristics such as maximum daily temperature, maximum thermal gradient, and temperature at varying depths are extracted from the model, which is run for a predefined set of a and e. Next, dynamical integrations of orbital clones of Phaethon and 2005 UD are used to estimate the past orbital elements&#160;of each object. These dynamical results are then combined with the temperature characteristics to model the past evolution of thermal characteristics.We find that predictions of the orbital history for these objects is reasonably accurate up to ~100,000 yr in the past, and is characterized by cyclic changes in&#160;e&#160;resulting in perihelia values periodically shifting between present-day values and 0.3 au. The thermal history of the maximum surface temperatures, for example, thus follows a pattern of extreme heating (up to 1000 K) every 20,000 yr. Currently, Phaethon is experiencing relatively large degrees of heating compared to the recent 20,000 yr. We find that even temperatures at-depth are too large over these timescales for water ice to be stable-unless actively supplied somehow and that thermal fracturing may be extremely effective at breaking down surface regolith. Observations of dust activity from the DESTINY+ flyby mission will provide important constraints on the mechanics of dust-loss.Past estimates of Phaethon's dust tail and mass-loss rate assume particle size of &#8776;1 micron and are insufficient to explain the entire mass of the Geminid stream of its ~1,000 year lifetime. However, observations of Geminid meteors show that it consists of a wide range of particle sizes (from micron-sized up to a few centimeters). Assuming a similar particle size distribution as the Geminids for Phaethon's dust tail we re-evaluate the mass-loss rate. We find that the annual dust activity from Phaethon may be sufficient to actively supply the Geminid stream in steady-state.