Yarkovsky effect

Sunlight warms the surface of a small body. Because the body rotates and its material takes time to heat and cool, the warmest region does not necessarily point directly toward the Sun. The body emits thermal infrared radiation from this uneven temperature pattern. Photons carry momentum, so their directional emission produces a very small reaction force on the body. Accumulated over many revolutions, that force can cause a measurable change in the orbit.

The diurnal component is associated with daily rotation: surface material tends to emit heat after the local midday heating as the body turns. For a simplified model, its effect on average orbital distance is outward for prograde rotation and inward for retrograde rotation. The seasonal component is associated with heating and cooling over the orbit; its average contribution to semimajor-axis drift is inward in the models discussed in the literature. Their relative importance depends on properties including spin-axis orientation, rotation rate, thermal inertia, shape and orbit.

The force is extremely weak, so it is usually inferred from precise astrometric observations accumulated over time and incorporated into a dynamical fit, rather than noticed as a short-term deflection. Researchers must distinguish it from gravitational perturbations and other non-gravitational forces such as direct solar-radiation pressure. A 2018 survey reported 87 reliable Yarkovsky-drift detections among near-Earth asteroids after applying signal and physical-consistency checks. Later modelling has explored how departures from spherical shape affect predictions; numerical work on irregular shapes found that a commonly used linear model can overestimate the effect for real shapes in some cases.

A gradual orbital drift can alter where an asteroid is found after long periods, influence the delivery of fragments from the main belt toward resonances and near-Earth space, and affect the precision of future orbit predictions. For objects with exceptionally well-measured orbits, omitting a small non-gravitational force can matter when evaluating distant future trajectories. The effect is one input to orbital analysis, not by itself evidence that an asteroid poses a hazard.