High-accuracy tracking almost rules out asteroid Bennu

Written story for CBS News & used with permission

This Bennu mosaic was created using observations from NASA’s OSIRIS-REx spacecraft that has been near the asteroid for more than two years. Source: NASA / Goddard / University of Arizona

In September 2135, the 1,600-foot-wide asteroid Bennu will pass between Earth and the Moon, and while on Wednesday scientists said there’s no chance of a collision, Earth’s gravity will alter Earth’s path. from collision for a later period. Close encounter.

It all depends on how Bennu’s path is affected by Earth’s gravity, the long-term gravitational influence of plants and other asteroids, and disturbances caused by more subtle factors, including the effects of solar heating.

The latter is known as the Yarkovsky effect, which is a small acceleration produced when heat absorbed by the Sun is emitted into space as an asteroid moves from daylight to darkness and rocks collide.

“The Yarkovsky effect on Bennu is equal to the weight of three grapes,” said David Farnocchia, a researcher at JPL’s Center for Near-Earth Object Studies and lead author of an article describing Bennu’s trajectory in the journal Icarus.

“Think about it. Just three grapes, and that really drives Benno’s movement into the future, because that acceleration is continuous, and its effect builds up over time and gets pretty big by the time we get to 2135.”

Thanks to NASA’s OSIRIS-REx spacecraft, which spent two years orbiting the sun in coordination with Bennu and is now on its way home with a treasure trove of collected rock samples, researchers have been able to more accurately model these forces to determine Bennu’s future path. . .

The very precise tracking of the spacecraft and its movement around Bennu, which is reflected in subtle changes in the probe’s radio signals, allowed the researchers to fix the parameters of the asteroid’s orbit by about 6 feet.

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“We measured the distance between Earth and Bennu, which was sometimes equal to the distance between Earth and the Sun, with an accuracy of two metres,” Varnokia said. “It’s the height of a basketball player.”

Prior to the OSIRIS-REx mission, analysts identified 26 gravitational holes half a mile wide along Bennu’s trajectory during the 2135 encounter. If Bennu’s trajectory was affected by the Yarkovsky effect or other factors, through one of these key holes, Earth’s gravity could put the asteroid in its path.

Thanks to the exceptional tracking accuracy, the researchers were able to exclude all but two of the keyholes to come up with the most accurate assessment of the possibility of the Benno effect to date.

And the odds remain comfortably low: a mere 0.06% chance of a collision on September 24, 2182, the closest date for the near-term showdown, means Benno will miss 99.94%.

“There is no particular cause for concern,” Varnokia said. “We know Bennu is still a potentially dangerous asteroid, but the probability is slim, and we have time to continue tracking the asteroid and eventually find a definitive answer.”

Bennu has been classified as a “potentially dangerous” object because it periodically crosses Earth’s orbit. And while a collision of 1,600-foot-wide asteroids like Bennu wouldn’t trigger a mass extinction like the 6-mile-wide object that wiped out the dinosaurs 66 million years ago, it still caused a massive extinction and widespread devastation.

The impact crater is typically 10 to 20 times the size of the affected object, said Lindley Johnson, a planetary defense officer for NASA’s Planetary Defense Coordination Office.

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“So a half kilometer long object (like Bennu) would create a crater with a diameter of at least five kilometers and a diameter of up to 10 kilometers (6 miles),” he said. But the area of ​​destruction would be much larger than that, up to 100 times the size of the crater.

“The effect of Benno’s body size on the east coast would pretty much destroy things along the coast.”

But as the researchers pointed out, the chances of such an effect occurring are low.

“It must be remembered that the risk to Bennu as a single asteroid is actually less than that of an undiscovered group of similarly sized objects,” Varnokia said. That’s why NASA goes to great lengths to detect more than 90% of near-Earth objects higher than 140 meters (460 feet). “

So far, about 60% of the suspected residents have been identified, Lindley said.

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