Nasa knows when asteroid Bennu is most likely to crash into Earth – and the devastation it would cause

Tiny forces, such as the Sun heating up the asteroid or the gravitational pull of space debris, could have significant effects on Bennu’s orbit over time

Adam Smith
Thursday 12 August 2021 14:23 BST
Nasa animation shows potentially disastrous impact with Bennu asteroid
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Nasa astronomers now know when asteroid Bennu will make its close approach to Earth, as well as the day that it is most likely to strike the planet.

The space agency’s researchers used precision tracking data from the Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer (OSIRIS-REx) spacecraft to understand the movements of Bennu from now until 2300.

OSIRIS-Rex has spent two years in close proximity to the asteroid, gathering information about its size, shape, mass, and composition.

Bennu will make a close approach to Earth in 2135, and scientists will use that passing to predict how Earth’s gravity will affect the asteroid’s path around the Sun – and a future potential impact.

The scientists examined whether Bennu would pass through a “gravitational keyhole”, which are areas in space with greavitational energy that would pull it towards Earth.

Even the smallest force can significantly deflect Bennu over time, causing it to pass through or completely miss a keyhole. One example is the Sun heating Bennu’s surface, which would then cool and release infrared energy that generates a small amount of thrust, pushing the asteroid in the opposite direction (known as the Yarkovsky effect). While this effect is small, over time it can build up – especially due to the lack of resistance in the vacuum of space.

“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 Steve Chesley, senior research scientist at Nasa’s Jet Propulsion Lab and co-investigator of the study.

Other forces include the gravity of the Sun, planets, moons, over 300 other asteroids, drag caused by interplanetary dust, solar winds, and even the asteroid’s own particle-ejection events, when it sheds tiny amounts of rock into space.

Nasa calculated that the 24 September 2182 as the date in which the 500 meter-wide Bennu is most likely to crash into the Earth. However, the chance of it happening is one in 2,700 (or about 0.037%), and the chance of Bennu crashing at any time between now and 2300 is one in 1,750 (or 0.057%).

That date, coincidentally, is also when OSIRIS-Rex is scheduled to return to Earth with a sample of Bennu for scientists to study – albeit 159 years earlier, in 2023.

“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, of the Center for Near-Earth Object Studies (CNEOS).

“We’ve never modeled an asteroid’s trajectory to this precision before.”

If Bennu did slam into Earth, it wouldn’t wipe out life, dinosaur-style, but rather create a crater roughly 10 to 20 times the size of the asteroid, said Lindley Johnson, Nasa’s planetary defense officer. The area of devastation would be much bigger: as much as 100 times the size of the crater.

If an object Bennu’s size hit the Eastern Seaboard, it “would pretty much devastate things up and down the coast," he told reporters.

Scientists already are ahead of the curve with Bennu, which was discovered in 1999. Finding threatening asteroids in advance increases the chances and options for pushing them out of our way, Johnson said.

“One-hundred years from now, who knows what the technology is going to be?” he said.

In November, NASA plans to launch a mission to knock an asteroid off-course by hitting it. The experimental target will be the moonlet of a bigger space rock.

The study, titled “Ephemeris and hazard assessment for near-Earth asteroid (101955) Bennu based on OSIRIS-REx data,” was published in the journal Icarus.

Additional reporting by Associated Press

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