Asteroid Bennu Spews Rock Dust, Makes Sampling Challenging

This photo taken by NASA’s OSIRIS-REx spacecraft shows a plume of particles flying off the asteroid Bennu’s southern hemisphere and into space on Mar. 7. The spacecraft stood about 3 miles (5 km) from the asteroid. Bennu spins once every 4.3 hours. NASA/Goddard/University of Arizona

The NASA OSIRIS-REx spacecraft that will return a sample of a near-Earth asteroid named Bennu to Earth in 2023 made a most exciting discovery recently — particle plumes erupting from an asteroid’s surface. Bennu also revealed itself to be more rugged than expected, challenging the mission team to alter its flight and sample collection plans, due to the rough terrain.

“The discovery of plumes is one of the biggest surprises of my scientific career,” said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson. “And the rugged terrain went against all of our predictions.”

With all its rocks, mission scientists are grappling with exactly where to land on Bennu to safely retrieve a sample. This photo was taken from 3 miles (5 km) away. NASA/ Goddard / University of Arizona

The spacecraft has been orbiting the 0.3-mile-wide (492-meter) asteroid since Dec. 31 and sending back photos of a surface paved in broken boulders as mission controllers look for a safe place to pluck a sample. The plumes were first spotted on Jan. 6 with 11 “ejection events” recorded through early mid-March. Although many of the particles were ejected clear of Bennu, the team tracked some particles that orbited Bennu as mini-satellites before returning to the asteroid’s surface.

One of the smaller, less rocky sites being looked at for sampling. NASA / Goddard / Univ. of Arizona

For now, scientists don’t know what’s causing Bennu to spit rocks but after a safely assessment concluded that the particles didn’t pose a risk to the spacecraft. Certainly the asteroid has very low gravity. Perhaps rocks and dust are spun off into space. Or material that tumbles downhill manages to reach escape velocity and take off on its own.

Bennu is the smallest body ever orbited by a spacecraft and believed to be a fragment of a much larger asteroid that was broken apart in a collision long ago. Some of the shattered remains self-assembled through gravity into Bennu. Rocks rich in carbon and clay-like minerals litter its surface.

Last night, the zodiacal light — the glowing, tilted cone of light at right and center — made a beautiful appearance. The glow is caused by comet and asteroid dust in the plane of the solar system illuminated by sunlight. Could there be a little bit of ejected Bennu dust in there? Mars and the Pleiades are near the center with Orion off to the left. To see the light, look west at twilight’s end from a dark sky. Bob King

Mission team scientists are eager to get their hands on samples of the material that may be identical to the carbon-and-water-rich meteorites thought to have peppered the early Earth without which life could not have evolved. If all goes according to plan bits of Bennu’s bounty will be flown back to Earth in 2023. But gathering those samples will prove challenging. The OSIRIS-REx team didn’t anticipate the number and size of boulders on Bennu’s surface. Observations from Earth indicated a smoother, less hazardous surface.

One hemisphere of Bennu from 15 miles (24 km). NASA / Goddard / Univ. of Arizona

The mission team was looking for a hazard-free zone 164 feet (50 meters) across but there a no “smooth” sites that big anywhere on the asteroid, so they’re looking for a smaller target zone. But the smaller the zone, the more accurate (and demanding) the descent to the surface will be to pick up the rocks.

We’ve also learned that Bennu’s rotation period is decreasing by about a second every 100 years due to the uneven heating and cooling of its surface as it rotates in sunlight. Another reason we’re interested in this pile of rotating rock is that Bennu may slam into our planet sometime in the future.  There’s a 1-in-2,700 chance of a strike between 2175 and 2199. While that’s small, it does make you rub your chin for a moment.