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On December 31, NASA's OSIRIS-REx (Origin, Interpretation, Spectral Interpretation, Resource Identification) has an appointment with the asteroid 101955 Bennu. As part of a mission back samples of asteroids, NASA hopes that the materials of this near-Earth asteroid (NEA) will reveal information about the history of the solar system , the formation of its planets and the origins of life on Earth.
Since the spacecraft has orbited the asteroid, he has witnessed interesting phenomena. This includes the earliest close observations of particle plumes emerging from the surface of an asteroid. Since then, the mission team has monitored these eruptions, which has allowed them to observe a total of 11 "ejection events" since the arrival of the first probe.
Like all NEA, Bennu is composed of material from the formation of the solar system. The study of this asteroid should therefore learn a lot about this process, as well as about how water and organic molecules were distributed in the solar system billions of years ago.
Since the rendezvous of the spacecraft with the asteroid three months ago, the science team has been surprised by many surprises. To begin, the team discovered that the surface of the asteroid was much rougher than originally thought, which forced it to adjust its plan to land on the asteroid and collect samples.
In addition, the first ejection event (observed on January 6) was also unexpected, not to mention the ten eruptions that occurred thereafter. Dante Lauretta, the principal investigator of the OSIRIS-REx mission from the University of Arizona, said in a recent press release:
"The discovery of plumes is one of the biggest surprises of my scientific career. And the rough terrain went against all our forecasts. Bennu already surprises us and our exciting journey is just beginning. "
The first ejection event was discovered after the OSIRIS-REx team noticed the presence of particles in one of the images taken by the probe's NavCam 1 imager as it gravitated around Bennu at a distance of about 1.6 km. After evaluating the event, the team concluded that the particles posed no risk to the spacecraft and chose to continue monitoring ejection events.
Although most of the material ejected by the plume was projected away from Bennu, the team found that some of the particles had fallen into orbit in the form of satellites before returning to the surface. l & # 39; asteroid. This could mean that some of the material (which is ejected from the inside) could be recovered once the spacecraft has collected samples from the surface.
Lori Glaze, acting director of the Planetary Science Division at NASA headquarters in Washington, summarized these developments as follows:
"The first three months of Bennu's thorough investigation by OSIRIS-REx reminded us of the purpose of the discovery: surprises, quick thinking and flexibility. We study asteroids like Bennu to learn more about the origin of the solar system. The OSIRIS-REx sample will help us answer some of the most important questions about our origin. "
Nevertheless, Bennu's biggest surprise is his rugged surface, strewn with blocks. Based on observations obtained on Earth, the team expects to find a relatively smooth surface with few large rocks. It is based on observations of Bennu's thermal inertia (his ability to conduct and store heat) and from radar measurements of his surface.
However, when the spacecraft arrived, the team found that the density of the surface area was much higher than expected. The fact that their initial results proved to be incorrect led the team to revise the models used to interpret the asteroid data, as they clearly do not predict the nature of the small areas of rocky asteroids.
The Bennu rock surface also means that the team's sample collection plan – a Touch-and-Go (TAG) procedure – needs to be adjusted. Originally, it was planned to land on a cleared site about 50 meters (164 feet) in diameter. But because of the density of blocks, the team was not able to find such a big place and started looking for smaller candidate sites.
This will require much more precision from the satellite as it descends to the surface, which is why the mission team is developing an updated approach (called Bullseye TAG). As Rich Burns – the OSIRIS-REx Project Manager at NASA's Goddard Space Flight Center – explained, the mission team was put to the test:
"Throughout OSIRIS-REx's operations near Bennu, our spacecraft and operations team has demonstrated that we can achieve higher system performance than design requirements. Bennu challenged us to manage his rugged terrain and we are confident that OSIRIS-REx is up to it.
Other notable discoveries include the fact that Bennu's rotational speed seems to be slowly slowing down. This is the result of the Yarkovsky-O'Keefe-Radzievskii-Paddack effect (YORP), in which uneven heating and cooling causes a decrease in the rotational speed. As a result, Bennu's rotational period slows by about one second per century.
Another interesting discovery is the presence of magnetite on the Bennu surface, which has been detected by the spacecraft's MapCam color imager and the thermal emission spectrometer (OTES). The presence of this mineral reinforces previous discoveries that indicate interactions between the rocky material and liquid water on Bennu's parent body.
These and other results were presented at the 50th Lunar and Global Conference, held March 18-23 in Houston, Texas. The results were also part of a special collection of articles published by the scientific journal. Nature.
Further reading: OSIRIS-REx, Nature
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