NASA: Asteroid Bennu Has Slightly Higher Chance of Impacting Earth

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The asteroid known as Bennu is one of a precious few that have been explored by human spacecraft. NASA’s OSIRIS-REx mission spent more than two years hanging around Bennu, and that has allowed scientists to more accurately model its orbit. The bad news is it’s more likely to smack into Earth than we initially believed. The good news; it’s still much more likely to miss us. 

Bennu is a carbonaceous asteroid discovered in 1999 and is generally believed to be the second most dangerous near-Earth asteroid. That’s one of the reasons NASA chose it as the target for OSIRIS-REx. Scientists now believe Bennu might have a “rubble pile” structure held together by the modest gravity. It’s only about 500 meters across, but that’s still enough to cause a lot of damage if it were to strike Earth. 

OSIRIS-REx completed its sample acquisition mission last year, and it broke orbit and began its trip back to Earth several months ago. In 2023, scientists hope to have the samples from Bennu back on Earth, but OSIRIS-REx has already taught us a great deal about Bennu. For example, it’s a little more likely to hit us. 

In 2135, Bennu will make a close approach to Earth, but it won’t be a threat at that time. However, our improved understanding of Bennu’s orbit suggests that it has a higher chance of falling into a “gravitational keyhole” during this pass. That means Earth’s gravity could nudge Bennu into an orbit that ensures it will impact on a future orbit in the late 23rd century. Based on the latest computer models, NASA now believes there is a 1 in 1,750 (0.057 percent) chance that Bennu will hit Earth before the year 2300. The team determined September 24, 2182 as the most likely impact date, with a 1 in 2,700 chance (about 0.037 percent).

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Now it’s up to astronomers to narrow down Bennu’s trajectory during that 2135 approach. Even small forces like the heating and cooling of the asteroid’s surface (known as the Yarkovsky effect) could determine whether Bennu falls into a keyhole. The knowledge acquired from the OSIRIS-REx samples could help refine our models, and at the very least will tell us what to expect if Bennu does hit Earth. 

In a worst-case scenario, Bennu wouldn’t wipe out human civilization, but it could cause widespread devastation. A land impact would result in a six-mile crater and immediate devastation for hundreds of miles in every direction. The dust plume from the impact could also affect the climate. A water impact (which is more likely) could produce a tsunami that devastates coastal communities. Even though these possibilities are 150 years in the future, it’s never too early to start working on solutions.

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