How Sunlight Pushes Asteroids

Rotating asteroids have a tough time sticking to their orbits. Their surfaces heat up during the day and cool down at night, giving off radiation that can act as a sort of mini-thruster. This force, called the Yarkovsky effect, can cause rotating asteroids to drift widely over time, making it hard for scientists to predict their long-term risk to Earth.

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From a distance, everything in our solar system appears to be in its place. However, if you take a closer look, sometimes you can find asteroids, like Bennu, leaving their home in the inner asteroid belt and passing very close to Earth. Most other asteroids tend to stay grouped together in a few regions of our solar system, yet some still end up in our backyard. So once these asteroids get close, what makes the difference between a near-miss, and a potential hit?

NASA's OSIRIS-REx mission will help better answer this question when it visits Bennu, but scientists think that a force called the Yarkovsky effect might be an important part of the answer. So how does this effect work?

Well, like Earth, most asteroids rotate slowly as they move through space. During the day, the surface of the asteroid is illuminated by the Sun, so it absorbs heat and grows warmer. During the night, however, the surface cools down, emitting the heat it absorbed as radiation. This radiation exerts a force on the asteroid, acting as a sort of mini-thruster that can slowly change the asteroid's direction over time.

On larger asteroids this doesn't amount to much, but on small ones it can make a pretty large change over time. Because the surface emits the most heat radiation at the end of the day, the direction the asteroid rotates can ultimately determine what happens in the long run. Other factors, such as composition, asteroid shape, and surface features, can modify the magnitude and direction of the Yarkovsky thrust.

By studying the Yarkovsky effect on Bennu with the OSIRIS-REx spacecraft, NASA scientists hope to better predict how an asteroid might move through the solar system, and whether it poses any danger to us here on Earth. So the next time an asteroid starts gradually moving into our neighborhood, we'll have a better idea of exactly where it will end up.

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