Title: Yarkovsky Drift Detections for 159 Near-Earth Asteroids,
Abstract: The Yarkovsky effect is a thermal process acting upon the orbits of small
celestial bodies, which can cause these orbits to slowly expand or contract
with time. The effect is subtle -- typical drift rates lie near $10^{-4}$ au/My
for a $\sim$1 km diameter object -- and is thus generally difficult to measure.
However, objects with long observation intervals, as well as objects with radar
detections, serve as excellent candidates for the observation of this effect.
We analyzed both optical and radar astrometry for all numbered Near-Earth
Asteroids (NEAs), as well as several un-numbered NEAs, for the purpose of
detecting and quantifying the Yarkovsky effect. We present 159 objects with
measured drift rates. Our Yarkovsky sample is the largest published set of such
detections, and presents an opportunity to examine the physical properties of
these NEAs and the Yarkovsky effect in a statistical manner. In particular, we
confirm the Yarkovsky effect's theoretical size dependence of 1/$D$, where $D$
is diameter. We also examine the efficiency with which this effect acts on our
sample objects and find typical efficiencies of around 12%. We interpret this
efficiency with respect to the typical spin and thermal properties of objects
in our sample. We report the ratio of negative to positive drift rates in our
sample as $N_R/N_P = 2.9 \pm 0.7$ and interpret this ratio in terms of
retrograde/prograde rotators and main belt escape routes. The observed ratio
has a probability of 1 in 46 million of occurring by chance, which confirms the
presence of a non-gravitational influence. We examine how the presence of radar
data affects the strength and precision of our detections. We find that, on
average, the precision of radar+optical detections improves by a factor of
approximately 1.6 for each additional apparition with ranging data compared to
that of optical-only solutions.