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(CNN) - NASA's OSIRIS-REx mission reached the near-Earth asteroid Bennu a year ago and the rotating space rock has been full of surprises. The latest findings now classify it as an active asteroid with observable events that occur on the surface.
OSIRIS-REx and Bennu met face to face on December 3 of last year. OSIRIS-REx has been orbiting the asteroid, which is 112 million kilometers from Earth, since December 31, 2018. It is an asteroid "pile of rubble", a grouping of rocks joined by gravity instead of a single object.
The biggest surprise of the mission happened only a few days later, when an unexpected observation occurred that pointed to activity in Bennu, the mission researchers said. They detected columns of particles that were ejected from the surface on January 6, followed by smaller columns on January 19 and February 19.
Before the close approach of the spacecraft a year ago, they could not be sure if there was any activity in Bennu. Now they know for sure, which makes Bennu an active asteroid that regularly ejects material into space. This is not only rare, but it is also the first time scientists have had close-up observations of columns of particles erupting from the surface of an asteroid, NASA said.
Particles can be seen being ejected from the asteroid.
The particles range from single-digit centimeters to tens of centimeters in length, and some of them move slowly while others move rapidly through interstellar space. Some fall back on the surface of the asteroid, while others enter orbit around it, like small satellites.
Around one hundred particles were expelled during each event in January and February. The three events occurred in different specific places of the asteroid. And everything happened during what could be called the end of the afternoon in Bennu. None of them represented a risk to the spacecraft. The researchers noted that they cannot rule out possible ejection events that occurred before the spacecraft reached the asteroid in December 2018.
They also observed persistent particles that orbit around the asteroid, six of which they will review in future observations.
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"The discovery of particle columns is one of the biggest surprises of my scientific career," said Dante Lauretta, principal investigator for OSIRIS-REx at the University of Arizona, Tucson.
Active asteroids can generally lose mass and expel particles through decay caused by unstable rapid rotations or ice sublimation, but that was not the case here.
The surface of Bennu is covered with stones.
Instead, they believe that ejections are caused by micrometeorites that impact the surface; the loss of water molecules that are inside the surface hydrated minerals; or thermal stress fracture of the asteroid rocks, according to a study published in the journal Science on Thursday. The three causes could work together to also cause the expulsion of particles, the authors said.
Bennu is covered with rocks instead of the large areas of fine-grained material that scientists expected. And OSIRIS-REx instruments have confirmed that hydrated minerals, including magnetite, are abundant and widespread in the asteroid.
Bennu's surface experiences extreme temperature variations during its 4.3-hour retrograde rotation period, sometimes up to 100 Kelvin, according to the study. When two of the events that occurred in the middle latitudes of the asteroid were observed, the temperature varied from 250 Kelvin to 400 Kelvin. Since the afternoon time is consistent for all three events, researchers believe that temperature changes are causing thermal fracture.
Some of the particles that escape the asteroid, only to fall back on it, could reach Earth.
Since arriving at the asteroid Bennu, OSIRIS-REx has been mapping its surface to determine the best place to collect samples in 2020. This will be NASA's first mission to return with samples collected from an asteroid, and samples are expected to arrive to Earth in 2023.
The asteroid is full of valuable materials that may even contain clues about how life began. Asteroids could even serve as fuel stations for robotic and human missions if we can unlock the hydrogen and oxygen inside them, NASA said.
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It is also older than expected, is between 100 and 1 billion years old, and probably originated in the main asteroid belt. Bennu probably got rid of a larger asteroid in the belt between Mars and Jupiter a couple of billion years ago. This knocked him down through space until an orbit close to Earth blocked him instead.
Bennu is the smallest body that has ever been orbited by a spaceship, just slightly wider than the height of the Empire State Building, according to NASA.
The mission, which stands for Origins, Spectral Interpretation, Resource Identification, Security Regolith Explorer, was launched in September 2016 and has been near Bennu for two years.
Bennu's sample could help scientists understand not only more about the asteroids that could affect the Earth, but also about how the planets formed and how life began.
The asteroid could pass closer to Earth than the Moon in 2135, with possible approximations even closer in 2175 and 2195. A direct impact is unlikely, but data collected during this mission can help determine the best ways to divert asteroids near Earth.