Yarkovsky Drift Detections for 159 Near-Earth Asteroids

Author(s): Greenberg, AH; Margot, J-L; Verma, AK; Taylor, PA; Hodge, SE | 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$1 km diameter object -- and is thus generally difficult to measure. However, objects with long observation intervals, as well as objects with radar detections, serve as excellent candidates for the observation of this effect. We analyzed both optical and radar astrometry for all numbered Near-Earth Asteroids (NEAs), as well as several un-numbered NEAs, for the purpose of detecting and quantifying the Yarkovsky effect. We present 159 objects with measured drift rates. Our Yarkovsky sample is the largest published set of such detections, and presents an opportunity to examine the physical properties of these NEAs and the Yarkovsky effect in a statistical manner. In particular, we confirm the Yarkovsky effectu0027s theoretical size dependence of 1/$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.