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On December 3 NASA’s OSIRIS-REx probe completed its long space journey to arrive at Bennu, a very tiny asteroid thought to be composed of organic compounds and wet clays and really no larger than a shopping mall. Sometime in 2020 the probe will descend in orbit and, using a specially constructed arm, attempt to grab some material from its rocky surface. After doing so the probe will pack up and return to Earth, arriving sometime late September 2023 hopefully bringing the 60 grams of samples with it.

If the craft is successful, scientists will have a wonderful chance to examine pristine carbonaceous material that may show organic molecules necessary for life compounded well before the formation of the Earth. The spacecraft was built at Lockheed Martin Space Systems and the principal science operations are being provided by the University of Arizona’s Lunar and Planetary Laboratory.

The accompanying photo shows that Bennu‘s orbit around the Sun is quite eccentric and spans from inside Earth’s path to almost Mars and this gives the asteroid a dangerous chance of hitting our planet some day in the future. In fact, it has the second-highest cumulative rating on the Palermo Technical Impact Hazard Scale (only asteroid Apophis is higher).

While a tiny asteroid can be expected to impact Earth about every 130,000 years or so, a dynamical study completed several years ago by Italian mathematician Andrea Milani (who sadly died last week) predicted a series of eight potential Earth impacts by Bennu between 2169 and 2199. Still, Bennu will pass somewhat uncomfortably close (460,000 miles) to Earth 42 years from now on September 23, 2060.

Because Bennu has a mean diameter of only 1,500 feet, its surface gravity is very small indeed and calculates to only 1/100,000 that of Earth. A fork falling off your table on Bennu will take about three minutes to touch the floor – you could easily grab it before it hits. Having such a low gravitational pull can be hazardous as well. If you jumped up faster than a half a foot per second you would surpass the escape velocity and leave the tiny world, never to return. Analysis of gravitational effects has given a bulk density slightly greater than water, suggesting Bennu has an interior filled as a rubble pile structure.

For some reason Bennu isn’t in the normal Asteroid Belt — that region of space between Mars and Jupiter and home to millions of asteroids of all sizes and shapes. Scientists think that Bennu probably came from there a long time ago and perhaps was once part of a much larger asteroid, maybe the size of Connecticut. Somewhere along the line between 700 million and 2 billion years ago the larger piece met its end in a huge collision with another large object, sending Bennu and other pieces flying in different directions in space. Since the impact, Bennu seems to have gradually spiraled closer to the Sun, winding up near the orbit of Earth.

Last Monday the spacecraft executed a maneuver that transitioned it from flying toward Bennu to orbiting around the asteroid at a distance of about 12 miles. Over the next few weeks OSIRIS-REx will perform closer passes of the Bennu surface, initially at about 4 miles to further refine the shape and on New Year’s Eve it will enter a low orbit around the asteroid at about 4,600 feet to start its extensive remote sensing campaign for the selection of a sample site. Sometime next year they expect to perform several rehearsals for the final robotic arm sampling event.

Since Bennu is as far away from the Sun as Earth its surface temperature is not as cold as you would envision for a barren rock in outer space. But lacking an atmosphere hurts and most of the heat it receives from the Sun is radiated away, thus keeping the surface temperature at a cool negative 34 degrees Fahrenheit – something approaching the nighttime temperature of Mars.

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According to online sources the name Bennu was selected from more than eight thousand student entries from dozens of countries around the world who entered a “Name That Asteroid!” contest run by the University of Arizona, The Planetary Society, and the LINEAR Project. Third-grade student Michael Puzio from North Carolina proposed the name in reference to the Egyptian mythological bird Bennu because the robotic scooping arm resembled the Egyptian deity.

Looking at the online video of a spinning Bennu one can see an object shaped almost like a tiny house complete with windows and chimney. It is fascinating to watch.

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Gary Hanington is Professor Emeritus of physical science at Great Basin College and chief scientist at AHV. He can be reached at garyh@ahv.com or gary.hanington@gbcnv.edu.

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