Why do asteroids slowly drift in their orbits?
Sunlight warms asteroids, and the heat they radiate gives a tiny push, called the Yarkovsky effect, that slowly changes their orbits over millions of years.
You might think an asteroid's orbit is fixed, determined only by gravity. But small asteroids slowly drift, pushed by sunlight itself.
The Yarkovsky effect. An asteroid absorbs sunlight on its daytime side and warms up. Because it rotates, the warmest part of its surface is carried a little way into the afternoon before it radiates heat away as infrared light. That radiation carries a tiny amount of momentum, giving the asteroid a small push in one direction. Depending on which way the asteroid spins, the push either speeds it up in its orbit, moving it slowly outwards, or slows it down, moving it slowly inwards.
Named after. The effect is named after Ivan Yarkovsky, a Polish-Russian engineer who proposed the idea around 1900.
How big? The force is extremely small. For an asteroid a few hundred metres across, it can shift the orbit by roughly a few hundred metres per year. Over millions of years, this adds up to large changes.
Measured. Precise tracking of asteroids such as Bennu and Apophis has measured the Yarkovsky effect directly. For Bennu, it was essential for calculating the small chance of a future impact with Earth.
Delivering meteorites. The Yarkovsky effect explains how asteroid fragments in the main belt drift slowly into resonances with Jupiter and Saturn, which then fling them onto Earth-crossing orbits. Without it, far fewer meteorites would reach us.
Related effect. The YORP effect, also caused by sunlight, changes the spin of asteroids.