A potentially hazardous asteroid has a much greater chance of hitting the Earth than previously believed, NASA says.
Bennu is about 490m across and has a similar orbit around the sun to our planet. Since its discovery in 1999, astronomers have been concerned it might one day hit us - estimating there was a 1/2700 chance of that happening in the 22nd century.
NASA sent a probe to Bennu a few years ago, OSIRIS-REx, which is currently on its way back with a sample from the asteroid.
Measurements taken while it was there have improved NASA's understanding of its trajectory - and the news isn't good. The odds of a strike anytime in the 22nd century have increased about 35 percent - to 1/1750.
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 Bennu’s future impact chances over the decades and centuries to come," said senior research scientist Steve Chesley, who contributed to the study.
That, combined with the unpredictable push and pull from all sorts of other sources - including solar wind, interplanetary dust, other asteroids and even OSIRIS-REx itself - make it hard to know what will happen beyond 2135, when it's expected to pass Earth closer than the moon.
NASA has identified a number of what it calls 'gravitational keyholes' that year, spots in space where the conditions will be just right for the Earth's gravity to put Bennu on a collision course with Earth. If Bennu passes through one, our descendants will have a major problem.
"We shouldn't be worried about it too much," Dr Farnocchia told the Associated Press (AP), noting not only are the odds still very much in our favour, scientists now know a lot more about Bennu's movements.
Lindley Johnson, NASA's planetary defense officer, told AP if Bennu did hit it wouldn't wipe out human civilization, but "pretty much devastate things" for tens, if not hundreds of kilometres across.
OSIRIS-REx principal investigator Dante Lauretta in 2018 said it could cause "immense suffering and death", depending on where it hit.
Another asteroid, (410777) 2009 FD, was previously feared to be on a collision course just three years later in 2185, but new measurements recently have ruled that out.
The world might not even make it that far - another close call will take place in 2068 with an asteroid called Apophis. Last year scientists feared it would be a collision, but new measurements in March 2021 effectively ruled that out too.
