There’s one particular date when Bennu is most likely to hit – September 24, 2182. 
“The OSIRIS-REx data give us so much more precise information, we can test the limits of our models and calculate the future trajectory of Bennu to a very high degree of certainty through 2135,” said study lead Davide Farnocchia from NASA’s Center for Near-Earth Object Studies. 
So why are they worried about 2182 then? It greatly depends on a very difficult-to-measure phenomenon called the Yarkovsky effect. Bennu, like most asteroids, is spinning – its ‘day’ is about 4.3 hours long. NASA says the side facing the sun is heated up, then as it turns around that heat is lost, “which generates a small amount of thrust on the asteroid”.
“The effect on Bennu is equivalent to the weight of three grapes constantly acting on the asteroid – tiny, yes, but significant when determining Bennus future impact chances over the decades and centuries to come,” said senior research scientist Steve Chesley, who contributed to the study.