A 73bn-kg, skyscraper-size chocolate creme egg spinning fast enough to eventually explode – it's asteroid Bennu

And we're about to extract a sample from it

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Scientists have compared asteroid Bennu to a chocolate creme egg – and say it may explode as it continues to spin at an ever-increasing rate.

“You could imagine maybe in a million years or less the whole thing flying apart,” said Daniel Scheeres, coauthor of a study published in Science Advances on Thursday into the cosmic rock. Scheeres and a large team of astronomers studied the asteroid using data gathered from OSIRIS-REx, a spacecraft launched by NASA that is expected to retrieve a sample of the space rock’s surface on October 20.

The trick to probing the hidden interior of Bennu was to measure its gravity field. Scientists mapped this out by measuring the gravitational pull exerted on the OSIRIS-REx as the probe orbited the asteroid, and modelling the tiny pebble-sized bits of dirt being flung from the space boulder's surface and placed into temporary orbits around the thing.

Previous measurements allowed researchers to estimate the size and mass of Bennu; it’s about as tall as the Empire State Building, and a whopping 73 billion kilograms.

This latest analysis looked deeper into the asteroid, and found it was less dense inside compared to outside. Bennu is essentially a giant mound of debris held together in a spinning-top shape. Its exterior is hard and rugged while the rocks in its interior are loose and less tightly packed. What’s most concerning is that it’s softest at its very core, and its equator bulges out.

NASA/Goddard/University of Arizona image of Bennu from 24km

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“If you can measure the gravity field with enough precision, that places hard constraints on where the mass is located, even if you can’t see it directly,” said Andrew French, a coauthor of this latest study and a scientist at NASA’s Jet Propulsion Laboratory. Like all celestial bodies in the Solar System, Bennu’s motion is affected by the Sun. Its surface is warmed by solar rays, and some of that heat is absorbed and re-emitted into space. The photons leaving the asteroid act like a kick, causing it to spin.

The phenomenon is described as the Yarkovsky–O'Keefe–Radzievskii–Paddack effect, or the YORP effect for short. A previous study found that Bennu’s spin rate appeared to be accelerating over time, threatening to tear it apart.

“When you spin this guy up, you create a competition between the gravity that’s holding you down and the centrifugal acceleration, which is trying to throw you off,” said Scheeres. “If its core has a low density, it’s going to be easier to pull the entire asteroid apart.”

But don’t worry, it may take over a million years for Bennu to spin itself into a destructive frenzy. It’ll stick around long enough for NASA’s OSIRIS-REx probe to retrieve a regolith sample, and for many other spacecraft to study it in more detail.

“We were hoping to find out what happened to this asteroid over time, which can give us better insight into how all of these small asteroids are changing over millions, hundreds of millions or even billions of years,” Scheeres said. “Our findings exceeded our expectations.” ®

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