Planetary Sciences News

Asteroid Visited by Mission Spews Rocks into Space

Mission scientists observed 11 separate particle ejection events in a 1-month period. They are still trying to figure out what could be causing the particle plumes.


Asteroid 101955 Bennu regularly ejects plumes or jets of particles from its surface. This recent discovery reveals that Bennu, a near-Earth asteroid currently playing host to an orbiting spacecraft, is one of a rare class of active asteroids, of which only about a dozen are known.

“The discovery of plumes is one of the biggest surprises of my scientific career,” said Dante Lauretta, principal investigator of NASA’s Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer (OSIRIS-REx) mission. OSIRIS-REx has been orbiting Bennu since 31 December 2018.

Mission scientists, including Lauretta, discussed this and other first discoveries from OSIRIS-REx at a press conference on 19 March.

Bennu and the Jets

The OSIRIS-REx team first detected a jet of particles on 6 January. The flight navigators noticed bright spots in the craft’s navigational camera that could not be image artifacts. After consulting with mission scientists, the team realized the spots were, in fact, small rocks. The navigation team quickly created new analysis tools to detect the particles and set up a dedicated monitoring campaign between 11 January and 18 February.

“We have seen about 11 such events over that time period, and more are being discovered as we get better at analyzing and processing the data,” Lauretta said. “Three of those events have been substantial, with dozens or over a hundred particles being ejected clearly into the asteroid environment.”

The ejected particles are centimeters to tens of centimeters in size and leave the surface at speeds ranging from tens of centimeters per second to a few meters per second, according to the team.

“Some of them have been observed to fall back onto the surface,” Lauretta said. “Basically, it looks like Bennu has a continuous population of particles raining down on it from discrete ejection events across its surface.”

Some slower-moving particles “are ending up in orbit around Bennu. It’s creating its own set of natural satellites,” he said. “That has never been seen before in any solar system object in history.”

Unexpected and Exciting

The team said that despite the unexpected discovery of particle jets, there is a very low probability that one of the particles will strike OSIRIS-REx in orbit. The researchers are continuing to monitor Bennu for more ejection events, hoping to discover where the ejections are coming from, when they might happen, and what might be causing them.

“This is incredibly exciting,” Lauretta said. “We don’t know the mechanism that is causing this right now. In fact, we’re still learning how to process the data, analyze the information, and make sense of what’s going on at this asteroid.”

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer

Citation: Cartier, K. M. S. (2019), Asteroid visited by mission spews rocks into space, Eos, 100, Published on 22 March 2019.
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