AI-generated illustration for Galileo Whispers. Single-asteroid thermal-recoil concept with illustrative shape and lighting; not a Bennu observation, temperature map or visible thrust.
The Yarkovsky effect changes an asteroid’s orbit because the asteroid releases absorbed sunlight as heat unevenly. Outgoing thermal photons carry momentum. If their recoil has a persistent component along or against the asteroid’s motion, a tiny force can gradually alter its orbit. No rocket exhaust or collision is required.
Gravity still does most of the work of shaping the orbit. The thermal force is a small correction whose accumulated effect can matter when astronomers compare precise positions over many years.
A warm surface pushes back
Sunlight heats an asteroid’s surface, which then emits infrared radiation. The important detail is the direction and timing of that emission. A perfectly balanced pattern would produce no net recoil; an uneven pattern can leave a small residual force.
This is different from the pressure of incoming sunlight. Radiation pressure acts as sunlight transfers momentum to an object. The Yarkovsky effect concerns the subsequent thermal emission and its dependence on the body’s temperature distribution. Both belong in sufficiently precise orbital models, but they are not interchangeable names for one process.
The 2015 review by David Vokrouhlický and colleagues explains how thermal forces and thermal torques affect small bodies. The distinction between a force that changes an orbit and a torque that changes spin will matter below.
Why the afternoon side matters
A rotating surface does not necessarily reach its highest temperature at local noon. Material takes time to warm and cool, so its strongest emission can be displaced toward the afternoon side. This thermal lag makes the emission pattern asymmetric relative to the Sun.
In the simplified diurnal picture, a prograde rotator tends to gain orbital energy and drift outward in semimajor axis; a retrograde rotator tends to lose energy and drift inward. These are trends for that component, not a rule that spin direction alone determines every asteroid’s total drift.
There is also a seasonal contribution associated with the changing illumination of a tilted spin axis. Shape, thermal properties, rotation and orbital geometry affect the combined result. An irregular asteroid with rough terrain is more complicated than a uniformly heated sphere.
A cool-looking surface is therefore not evidence of a negligible effect. The relevant question is how its emission is distributed through time and direction, not simply whether it is hot by terrestrial standards.
A tiny change in orbital size can build a timing difference
The semimajor axis describes an orbit’s size. Changing it changes the orbital period as well as the path. For a small change around a fixed central mass, Kepler’s third law gives the approximate relation ΔP/P ≈ 1.5 Δa/a.
As an illustrative calculation, suppose an idealized orbit has a semimajor axis of 150 million kilometers and that axis changes by 300 meters. The fractional size change is 2 × 10⁻⁹. The corresponding fractional period change is about 3 × 10⁻⁹, or roughly 0.095 seconds for a one-year period. This is a teaching example, not a new measurement of Bennu.
Continued drift changes the period repeatedly, so a prediction can accumulate a growing difference in orbital phase. A quoted drift in meters per year is a change in orbital size; it is not the asteroid’s speed through space or the distance it moves away from Earth each year.
Bennu supplied a measurable test
In a 2014 orbital analysis of Bennu, Steven Chesley and colleagues used optical and radar observations to constrain a small drift beyond a gravitational-only model. They also modeled thermal emission using physical information about the asteroid. Agreement between those approaches helped test the interpretation.
This is an inference from a dynamical model and observations, not a photograph of a visible thrust. The study explicitly discusses how modeling choices affect the precision of the result. Its historical impact probabilities are not a substitute for an updated risk assessment.
NASA’s 2021 account of later OSIRIS-REx tracking describes how additional measurements improved Bennu’s predicted orbit, including the thermal force. More data can narrow an uncertainty; they do not eliminate every future uncertainty.
Orbital recoil is not rotational breakup
The related YORP effect is a torque produced by uneven reflection and thermal emission. It can change how an asteroid spins. Yarkovsky concerns the net force and orbital evolution. A body can experience both, but an orbital drift is not, by itself, evidence that the body is spinning apart.
The same caution applies to danger. Detecting thermal recoil does not mean an asteroid has suddenly become an impact threat. It means a small, physically explainable force has become relevant to the precision of the prediction. Its importance comes from patient accumulation, not a dramatic burst of propulsion.
