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 .