Asteroids are easy to push around. The gravity of a planet can shove one into a new orbit. A nudge by another space rock can also give it a big push. And even sunlight can alter its path. That makes it tough to predict an asteroid’s orbit very far into the future.

The change caused by sunlight is known as the Yarkovsky effect. It’s named for an engineer who first published the idea more than a century ago.

As an asteroid orbits the Sun, it spins on its axis, just as Earth does. The side that faces the Sun gets warmer, while the side away from the Sun gets cooler. It cools off by emitting infrared radiation, which gives the asteroid a tiny “push.” Over time, the push builds up, altering the orbit.

It’s especially important to understand this process for asteroids that come close to Earth. A tiny change in orbit can change the odds that an asteroid will hit our planet.

The Yarkovsky effect is hard to predict, though. Scientists have to know the asteroid’s mass, size, and shape. They also need to know the patterns of light and dark on its surface, because lighter colors reflect more sunlight, while darker colors absorb more.

The best measurement of the Yarkovsky effect so far is on an asteroid named Bennu. The effect has altered Bennu’s position by about a hundred miles in just a dozen years. A spacecraft that arrived at Bennu last year is providing an even better measurement. More about that tomorrow.

Script by Damond Benningfield