Measurement of Yarkovsky Drift Rates for 247 Near-Earth Asteroids

The Yarkovsky effect is a non-radiative force that can modify the orbits of small celestial bodies, resulting in a slow expansion or contraction of the orbits over time. The effect is subtle (da/dt ~ 10^-4 au/My for a 1 km diameter object) and is thus generally difficult to measure. We analyzed both optical and radar astrometry for 600 near-Earth asteroids (NEAs) for the purpose of detecting and quantifying the Yarkovsky effect. We present measured drift rates for 247 NEAs, which is the largest published set of Yarkovsky detections [Greenberg et al., 2020]. We examine the efficiency with which solar energy is converted into orbital energy and find a median efficiency in our sample of 12%. We interpret this efficiency in terms of NEA spin and thermal properties. Our large sample size provides an opportunity to examine the Yarkovsky effect in a statistical manner. In particular, we describe two independent population-based tests that confirm the hypothesis of Yarkovsky orbital drift. First, we provide observational confirmation for the Yarkovsky effect's theoretical size dependence of 1/D, where D is diameter, refuting an earlier claim that suggested otherwise. Second, we find that the observed ratio of negative to positive drift rates in our sample is 2.34, which, accounting for sampling bias, implies an actual ratio of approximately 2.7. This ratio has a vanishingly small probability of occurring due to chance or statistical noise. It also highlights a serious, unresolved discrepancy with predictions: the observed ratio is two times lower than the ratio expected from numerical predictions from NEA population studies and traditional assumptions about the sense of rotation of NEAs originating from various main belt escape routes. Finally, we find that each additional apparition with radar ranging observations decreases uncertainties on the estimate of the Yarkovsky drift rate by a factor of approximately two, emphasizing the critical importance of radar astrometry in the NEA trajectory prediction and hazard mitigation problems.