Sunlight can move an asteroid
A rotating asteroid absorbs sunlight and later releases that energy as infrared radiation. Because the heat is emitted unevenly, the escaping photons produce a minute recoil force that can shift the asteroid’s orbit by kilometres over years and much farther over geological time.
The effect is called Yarkovsky drift. Afternoon-facing terrain is usually warmer than morning-facing terrain, so thermal photons are not emitted symmetrically around a rotating body. Each photon carries momentum, and the imbalance supplies a tiny continuous thrust. Radar ranging of asteroid 6489 Golevka measured a displacement of about 15 kilometres from the orbit predicted without the effect over twelve years. The direction and size depend on rotation, shape, thermal inertia, surface roughness and orientation.
Long-term celestial mechanics depends on the thermal properties of a rock’s surface, not only on gravity. The force is tiny in any moment but decisive across many revolutions, so planetary-defence calculations must know whether an asteroid stores and releases heat quickly or slowly.
Thermal radiation and direct sunlight pressure are related but distinct forces. Radiation pressure pushes mainly away from the Sun when light arrives. Yarkovsky recoil depends on when and where absorbed energy leaves as heat. A prograde and a retrograde rotator can therefore drift in opposite orbital directions even under the same sunlight.
Once precise tracking reveals unexplained drift, researchers can infer combinations of density, thermal inertia and spin state. The orbit becomes an indirect experiment on the asteroid’s surface. The inference is not unique, which is why radar, infrared observations, shape models and spacecraft encounters are combined rather than treating one trajectory fit as a complete physical description.
Isn’t ordinary sunlight pressure enough to explain the orbital shift?
Direct radiation pressure is modelled separately and acts with a different geometry. The measured drift matches a delayed thermal recoil whose sign and magnitude depend on rotation and heat retention. Removing the thermal term leaves systematic orbit residuals for well-observed asteroids such as Golevka.
Repeated precision tracking that matched gravity and direct radiation pressure while showing no rotation-dependent thermal residual would undermine the current account.