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NASA/Goddard/University of Arizona
The asteroid Bennu as seen from the OSIRIS-REx spacecraft.

Editor's note: A version of this was previously published on the author's website.

The OSIRIS-REx spacecraft arrived at the asteroid called Bennu on Dec. 3, and already has made scientific discoveries: Bennu has a few craters, boulders litter the surface like chocolate in a chocolate chip cookie, and the asteroid shows signs of the presence of water at some time in the past.

NASA’s ludicrously named spacecraft — the acronym stands for Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer — cruised past the asteroid, having traveling 1.2 billion miles since its launch on Sept. 16, 2016. It’s supposed to go into orbit around the tiny conglomeration of rocks and dust on the last day of 2018, then map the surface in detail, scoop up a sample and, in 2023, return to Earth.

The one-third-mile diameter asteroid is a rubble heap of material that clumped together while the solar system was forming, starting 4.6 billion years ago; it probably broke off from a larger mass 700 million to 2 billion years ago; and its density is only 30 percent greater than water, NASA estimates. The name Bennu was suggested by Mike Puzio, 9, identified as a North Carolina resident, city not mentioned. Bennu was an ancient Egyptian deity depicted as a bird, a linguistic tie-in with the OSIRIS spacecraft. Osiris was the mythological lord of the underworld.

A Dec. 10 Twitter entry that NASA filed in the name of the probe shows the agency’s reaction to the first views: "My close-range images of Bennu are beginning to reveal geological features on the asteroid’s surface, such as boulders and craters. Bennu has some rugged terrain to explore." Actually, when one examines the photos, it is obvious Bennu has plenty of big rocks, but craters are rare, especially compared with another airless object, the moon.

University of Arizona, NASA and NASA Goddard
A close-up of the asteroid Bennu shows an abundance of boulders and a lack of sharp-rimmed craters.

Those boulders mark Bennu as similar to a couple of other asteroids, Ryugu and Itokawa, according to a study uploaded this month to the internet site Research Gate. The paper, available as a preprint, is “Boulder Standing in Ejecta Launched by an Impact Generated Seismic Pulse,” by Esteban Wright and Alice C. Quillen of the University of Rochester in New York, Erik Asphaug of Arizona State University and six other scientists.

Boulders rise to the surface in a phenomenon like the “Brazil nut effect,” the group writes, an action that "brings the largest nuts in a shaken bowl of mixed nuts to the top." Using high-speed video to record the process, researchers hit the bottom of a collection of different-sized dry gravel (color-coded by size) "with a strong impact" and watched particles fly up and settle.

"We find that initially buried larger particles are often left on the surface after impact. … Collisions primarily take place upon landing and small particles scatter off of larger ones, uncovering larger particles on the surface.

"(W)e propose that ballistic sorting of ejecta launched by an impact generated seismic pulse can strand boulders on an asteroid surface. Our experiments show that a single pulse can strand a previously buried large particle. We have found that multiple pulses (but separated in time) continue to unearth larger particles and once a larger particle is on the surface it tends to stay there."

In another significant discovery, two spectrometers aboard OSIRIS-REx found indications of water on Bennu. Water is "bound up in hydrated clay minerals over a large part of the asteroid’s surface," the "spacecraft" tweeted.

"While Bennu is too small to have free-flowing water or chunks of ice, our data suggest the asteroid’s larger parent body hosted liquid water in the distant past. It also means that mining water from asteroids like Bennu may be possible on future missions," NASA adds.

As worthwhile as these findings are, scientists hope that examining Bennu will have the far more valuable effect of learning how to better protect Earth from a disastrous asteroid strike. A NASA site, headlined "Planetary Defense: The Bennu Experiment," says, "If it impacted Earth, Bennu would cause widespread damage." The chance it will hit Earth in the next couple of hundred years are miniscule but not zero — the odds are 2,700 to 1 against. However, the farther into the future the calculations stretch, the greater the uncertainty becomes.

Joe Bauman
Mount Vesuvius hunches over the ruins of Pompeii, in this picture taken March 24, 2006.

Looking at the Bennu photos, I get a creepy sensation, something like what I felt with my first good look at Mount Vesuvius. During our visit to Pompeii and Naples in 2006, Cory and I were surprised at the extent Vesuvius dominated the region; it had a grim and threatening appearance. In the year A.D. 79, an eruption of Vesuvius destroyed the Roman towns of Pompeii and Herculaneum, as well as other sites. Thousands of people perished.

Residents had no notion that Vesuvius was an active volcano, as it remains today. The destruction of so many lives was a tragedy that is still visceral, with the empty shops and bakeries, small destroyed apartments, stone streets with their chariot ruts, the ruins of temples and grand houses and taverns. Vesuvius glowers over all.

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Most depressing were casts of people who were struck down. Volcanic ash had piled around and over bodies, packed in solidly, and formed hollows that remained after the corpses decayed. Excavators poured liquid plaster into these spaces, and now the hardened plaster preserves details of the persons' clothing, limbs, features and even their dying grimaces.

We need to learn all we can about Bennu. Unless we as a civilization keep our wits about ourselves, unless we learn what we can about asteroids and comets and how to deflect them, unless we retain our high technology, this pile of rubble Bennu, or some sibling, could become a mass murderer.