Bennu: How a Little Asteroid Became a Rock Star

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Carl Hergenrother fingers a smoke-stack of white, 8½- by 11-inch paper that sits before him on his desk. As he flips by a inch-thick dossier, lines of form are clearly visible.

“This is all we know about Bennu,” says Hergenrother, glancing down during a raise of paper.

Simulated cratering and topography are overlaid on radar imagery of asteroid 1999 RQ36. Image credit: NASA/GSFC/UA/Mike Nolan-Arecibo Observatory/Bob Gaskell-Planetary Science Institute

Simulated cratering and topography are overlaid on radar imagery of asteroid 1999 RQ36. Image credit: NASA/GSFC/UA/Mike Nolan-Arecibo Observatory/Bob Gaskell-Planetary Science Institute

Hergenrother is a staff scientist during a University of Arizona’s Lunar and Planetary Laboratory. Bennu is an asteroid and a little though graphic proprietor of a solar complement — so graphic that it was selected as a end for a OSIRIS-REx mission.

The OSIRIS-REx booster is set to launch in Sep and in 2020 collect adult a representation of a asteroid’s regolith, a lax dirt and hilly element found on a surface. The representation afterwards will be returned to Earth for analysis.

Scientists detected Bennu on Sept. 11, 1999, by regulating electro-optical telescopes operated by a Massachusetts Institute of Technology Lincoln Near-Earth Asteroid Research, or LINEAR, program. The asteroid creatively was famous as 1999 RQ36.

Hergenrother played a pivotal purpose in selecting Bennu for a OSIRIS-REx mission. The initial time he laid eyes on a asteroid, it was 2005, and 1999 RQ36 was cruising tighten to a Earth as it does each 6 years. Through a Kuiper telescope on Mount Bigelow, Bennu seemed as no some-more than a dot of light, “like a star that moves,” Hergenrother says.

Pop radar! Doppler images, presented in a character of Andy Warhol, uncover views of asteroid 1999 RQ36 from Earth. Image credit: NASA/JPL/Goldstone

Pop radar! Doppler images, presented in a character of Andy Warhol, uncover views of asteroid 1999 RQ36 from Earth. Image credit: NASA/JPL/Goldstone

After all, an asteroid is simply a “rock in space,” he says.

Not Your Average Space Rock

But in many ways, Bennu is a special rock.

It was 2004 when a late Michael Drake, executive of a Lunar and Planetary Lab, due securing a representation of a near-Earth asteroid. But behind then, no one knew many about Bennu. In fact, researchers were deliberation an collection of other asteroids, privately ones abounding in carbon.

“We wish to go to a sold form of asteroid, a carbonaceous asteroid,” Hergenrother says. Researchers also wish that asteroid to be abounding in volatiles — substances that boil off easily, like water.

“The meditative is that these are a kinds of objects that in a early days of a solar complement seeded a Earth with a organics that life shaped from, as good as a H2O needed,” says Hergenrother, cautioning that there is still some discuss about a matter.

“But we wish that Bennu is what we would call a time capsule, that it hasn’t altered many over a story of a solar complement and will uncover what materials looked like in a really early days of solar system.”

Hergenrother became Bennu’s champion after years of information collection and research — and a good bargain of deductive reasoning. He served as coordinator for a mission, though many of a regard and reduction fell to other goal scientists.

“We’re now coming a million famous asteroids,” Hergenrother says. “That wasn’t a box when we started doing this 12 years ago. But we were still articulate hundreds of thousands of asteroids during that time.”

However, a researchers didn’t have to investigate scarcely so many. Most of them could be quick separated by their sheer faults, such as staying too distant from Earth. Closer asteroids concede for a execution of a goal within a reasonable volume of time. That pattern alone left usually a integrate thousand asteroids that remained viable possibilities for exploration, Hergenrother says.

And of those, many have orbits that are reduction than stellar.

“They’re too distant from a intent or too tighten to a sun,” Hergenrother says. “Or they’re during a pointy angle to a Earth, that would take a some-more absolute rocket to get into that kind of orbit.”

Looking for ‘Favorable Orbits’

Which, in turn, left usually a few hundred objects that have auspicious orbits — that is, ones staying nearby a Earth and remaining a auspicious stretch from a sun. However, many of those objects are too tiny.

“Some are as little as a desk,” Hergenrother says. “The ones smaller than about 200 meters have a robe of rotating really fast. The fastest ones we know about stagger about each 16 seconds.”

That’s too quick to accommodate a booster looking for a representation of regolith.

But Bennu rotates usually once each 4.3 hours, delayed adequate to representation a regolith.

“So, once we discharge a little guys, we demeanour during a bigger ones that have really Earth-like orbits,” Hergenrother says. “Now you’re articulate usually dozens of candidates, and we wanted to go to one that’s carbonaceous.”

Which Bennu seems to be. Its carbonaceous inlet sent it to a tip of a list of asteroids, says Hergenrother, who emailed Drake and Dante Lauretta, saying, “Guys, this is carbonaceous. It’s looking good. And there’s radar data.”

Lauretta, highbrow of heavenly scholarship and cosmochemistry during a Lunar and Planetary Laboratory, is Drake’s inheritor as a principal questioner for a OSIRIS-REx mission.

The information showed Bennu carrying a frigid hole of 508 meters, a meant hole of 492 meters and an orbital quickness of some-more than 28 km per second. It also showed that a asteroid has a spinning-top shape; that is, it sports a gush along a equator, a common underline among near-Earth asteroids.

“What we consider this means is that this is a rubble-pile object,” Lauretta says, definition that Bennu substantially is done out of many boulders tens to hundreds of meters across.

The boulders “are loose, and they’re responding to a army of a asteroid spinning, and element is migrating from a stick of a asteroid and accumulating during a equator and building adult a ridge,” Lauretta says.

Life Expectancy: 10 Million More Years

For improved or worse, Bennu has an inconstant orbit. That means it substantially won’t final some-more than 10 million years before it collides with Earth or another planet, or falls into a sun, according to Lauretta.

This is a regard to scientists. In fact, Bennu is deliberate a potentially dangerous intent and has a comparatively high probability of impacting a Earth. So scientists are meddlesome in bargain how asteroids’ orbits evolve.

Key to that expansion is something famous as a Yarkovsky effect.

“Which is simply that an asteroid receives appetite from a sun, turns that appetite into heat, and as it rotates into a afternoon throws that appetite behind into space as thermal energy, and that acts like a thruster and changes a circuit of a asteroid solemnly though certainly over time,” Lauretta says. “If we wish to know where an asteroid is going to be in a future, quite in a future, afterwards we wish to know about a Yarkovsky effect.”

Looking behind on his query to assistance name an asteroid suitable for sampling, Hergenrother says he sometimes thinks about what if there were no Bennu.

“There are usually 3 carbonaceous objects that are easy to get to,” he says. “They all seem to be a same shape. They all seem to be comparatively protected targets. They’re scientifically interesting, and they’re easy to get to. But that’s usually 3 out of scarcely a million.”

Source: University of Arizona