  The near-Earth objects (NEOs) (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, which results
in them having perihelion temperatures of or exceeding 1000 K. The current
activity from Phaethon is relevant to the destruction of NEOs close to the Sun,
which most likely has produced meteor streams linked to asteroids in the past.
We model the past thermal characteristics of Phaethon and 2005 UD using a
combination of a thermophysical model (TPM) and orbital integrations of each
object. 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 of each object. These dynamical results are then combined
with the temperature characteristics to model the past evolution of thermal
characteristics such as maximum (and minimum) surface temperature and thermal
gradient. We find that dwarf planet (2) Pallas is unlikely to be the parent
body for Phaethon and 2005 UD, and it is more likely that the source is in the
inner part of the asteroid belt. The orbital histories of Phaethon and 2005 UD
are characterized by cyclic changes in, resulting in perihelia values
periodically shifting between present-day values and 0.3 au. We find that the
subsurface temperatures are too large over this timescale for water ice to be
stable unless actively supplied somehow. The near-surface thermal gradients
strongly suggest that thermal fracturing may be very effective at breaking down
surface regolith.
