Sunlight carries a tiny push, and over years that push adds up. The Yarkovsky effect is a slow, steady nudge on rotating asteroids caused by the way they absorb sunlight and re-emit it as heat. It is one of the reasons astronomers cannot predict an asteroid's position far into the future from a few observations alone, and it plays a real role in how we assess long-term impact risk.
The effect is named after Ivan Yarkovsky, a Russian civil engineer who pointed out around 1900 that a spinning body heated by the Sun should experience a small force. The idea was rediscovered and developed by astronomers in the twentieth century, and it was measured directly for the first time in the early 2000s using radar observations of the asteroid 6489 Golevka.
An asteroid absorbs sunlight on its day side and warms up. Because it rotates, the hottest part of its surface is not directly under the Sun at noon: the rock takes time to heat, so the warmest region is shifted toward the "afternoon" side. That warm surface radiates heat into space, and thermal radiation carries momentum, so the asteroid receives a very small thrust in the opposite direction.
The force is minuscule, far smaller than the pull of gravity on the same body. But gravity from the Sun does not change an asteroid's orbital energy, while a steady thrust along the direction of motion does. Applied for decades or centuries, it slowly stretches or shrinks the orbit.
The direction depends on how the asteroid spins:
The size of the drift depends on the object's size, shape, surface material, distance from the Sun and spin rate. Small asteroids, a few hundred meters to a few kilometers across, are affected most. Very large ones barely notice it, and tiny dust grains are dominated by other effects.
For near-Earth asteroids the drift is typically measured in meters to hundreds of meters per year in the size of the orbit. NASA's OSIRIS-REx mission measured the effect on asteroid Bennu with great precision. Tracking data showed that Bennu's orbit changes by roughly 284 meters per year because of the Yarkovsky effect, which is a small shift that nevertheless changes where Bennu will be by the late twenty-second century. For Apophis, researchers have estimated a drift on the order of a couple of hundred meters per year.
Predicting where an asteroid will be decades from now requires knowing its orbit very precisely. The Yarkovsky drift is hard to measure for a newly found object, because it depends on properties such as shape, spin and surface that are poorly known. Small unknown drifts grow with time, so the uncertainty in a far-future position grows too.
Apophis is a good example. Its 2029 flyby will change its orbit dramatically, so the Yarkovsky effect before and after the encounter, together with the planet's gravity, determines where it will be decades later. Teams at NASA's Center for Near Earth Object Studies include the effect in their orbit solutions when calculating impact probabilities. It is part of why NASA's statement that Apophis poses no impact risk for at least the next 100 years is based on careful modeling, not on a single measurement. You can read how risk lists are maintained in How NASA Tracks Asteroids.
Related to Yarkovsky is the YORP effect (Yarkovsky-O'Keefe-Radzievskii-Paddack). Instead of changing an asteroid's orbit, the same kind of heat re-emission acts on an irregularly shaped body as a small torque, gradually changing how fast it spins. Over millions of years, YORP can spin small asteroids so fast that they shed material, which is one proposed explanation for the rubble-pile shapes and tiny moons seen on many near-Earth asteroids.
The Yarkovsky effect is not only a tracking headache. Over millions of years it slowly moves fragments of asteroids in the main belt until they reach orbital resonances with Jupiter, which then fling them onto paths that cross Earth's orbit. Scientists think this is one of the main ways meteorites, and many near-Earth asteroids, are delivered to our neighborhood.
In principle, changing an asteroid's surface reflectivity would alter how much sunlight it absorbs and so change its drift, a concept sometimes called "painting" an asteroid. This remains a theoretical idea and is far slower and less certain than methods such as the kinetic impactor tested by NASA's DART mission. It is an interesting illustration that even gentle forces can steer an object if you have enough time. See Planetary Defense and The DART Mission Explained for the methods that have actually been tested.
Facts in this article are drawn from public scientific sources. For primary information, see: