Central void may be tearing asteroid Bennu apart

Central void may be tearing asteroid Bennu apart
A mosaic image of the asteroid Bennu, snapped by OSIRIS-REx from a distance of 24 km (15 mi)
A mosaic image of the asteroid Bennu, snapped by OSIRIS-REx from a distance of 24 km (15 mi)
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A mosaic image of the asteroid Bennu, snapped by OSIRIS-REx from a distance of 24 km (15 mi)
A mosaic image of the asteroid Bennu, snapped by OSIRIS-REx from a distance of 24 km (15 mi)

NASA’s OSIRIS-REx probe has been in orbit around asteroid Bennu for almost two years now, providing us with an unparalleled look at the surface of an asteroid. But what lurks below? New findings from its gravity field suggest Bennu’s core is less dense than the outside shell, and possibly contains a large void – which could spell its doom in the not-too-distant future.

OSIRIS-REx first rendezvoused with Bennu in December 2018, and it’s been mapping the surface ever since. But its instruments have been studying the unseen as well – namely, measuring how the strength of its gravity field varies in different areas. That can reveal much about how dense the material is in different regions, giving a glimpse of what the center of the asteroid is like.

“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,” says Andrew French, co-author of the study.

The team measured how Bennu’s gravity tugged on the OSIRIS-Rex spacecraft in orbit, and the results revealed some unexpected things about the asteroid’s core.

Bennu is considered a fairly “loose” asteroid, but that’s no judgement on its character – it just means that it’s essentially a pile of rocks bound together by gravity. Previously it had been assumed that it should be basically the same composition all the way through, but the new observations suggested that wasn’t the case.

Instead, the results indicate that some parts of Bennu’s interior are more densely packed together than other parts. And interestingly, some of the least dense areas appear to be around the bulge at its equator, and in the core itself.

“It’s as if there is a void at its center, within which you could fit a couple of football fields,” says Daniel Scheeres, co-lead author of the study.

So how did that void get there? Other studies have shown that Bennu’s spin is accelerating, and the team suggests that this could be flinging material away from the core towards the surface. If that’s the case, the whole thing might crumble in the (relatively) near future, of a million years or less.

It’s a fascinating find, but the best is likely yet to come. OSIRIS-REx is due to swoop down to the surface of Bennu next week to snatch a sample, before bringing it back to Earth in 2023 for study.

The research was published in the journal Science Advances.

Source: University of Colorado Boulder

Cue the conspiracy theories? It does make sense, in an asteroid that's loose enough to have deep internal voids at all, that spin would generate a centrifugal force (ahem) on items inside those voids.
Capturing an asteroid and hollowing it out has been proposed as a relatively economical means to create a space station. A near-Earth orbit asteroid with a pre-existing two football-field sized void that's only about half a kilometre in diameter sounds like an ideal candidate for such a venture.

That Bennu already travels to within less than half the distance from Earth to Mars every few years seems like it might make a practical delivery system for supplies to a future Mars exploration/colonization efforts, perhaps utilizing a solar sail to "improve" its orbit towards that end in addition to avoiding "keyholes" in 2135 and stabilize its rotation.
Would slowing the spin then decrease its surface area? If so, by how much?
Ralf Biernacki
Primitive burrowing lifeforms are excavating the asteroid, dumping the spoil on the surface---hence the "loose rock" layer on the outside. Parts of the burrow-riddled rock have collapsed into larger voids, the centrifugal inertia depositing the material on void walls. These larger spaces have made possible the evolution of larger, predatory lifeforms living in the voids and feeding on the burrowers. Remain calm---its still a long, long time until sentience rears its ugly head. We will be able to tell that sapient lifeforms have evolved, because they will deliberately tweak the asteroid's interaction with the solar wind to slowly increase its spin, an unnatural phenomenon (migration of material from the inside to the outside has the opposite effect). The purpose of spin increase is to eventually overcome gravity at the surface, enabling launch into space and colonization of nearby bodies.