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In June 2018, the Hayabsua-2 spacecraft and the Japan Aerospace Exploration Agency (JAXA) sample return mission arrived on the near-Earth asteroid 162173 Ryugu . The first scientific results of the mission, published this week in a trio of newspaper articles Science, portray Ryugu as a porous "pile of rubble".
In one of the three studies, Dr. Sei-Ichiro Watanabe of JAXA and Nagoya University and co-authors presented the first close observations of Ryugu's mass, shape, and density.
"The low density of the asteroid suggests a very porous interior and a" pile of rubble "of roughly aggregated rocks, which took the form of a spinning top during a period of rapid rotation," they said. .
"From the asteroid remote sensing, we identified potential landing sites for the spacecraft best suited for collecting samples, which could further inform the way Ryugu has got its shape.
In a second study, Dr. Kohei Kitazato from Aizu University and his colleagues used the Hayabusa-2 Near-Infrared Spectrometer (NIRS3) to analyze the composition of the Ryugu surface.
They discovered that hydrated minerals are ubiquitous on the surface of the black asteroid.
NIRS3 spectral data are similar to known meteorites of carbonaceous chondrite metamorphosed thermally and / or by shock.
"Only a few months after receiving the first data, we have already made tantalizing discoveries," said Dr. Seiji Sugita, a researcher at the University of Tokyo and at the Chiba Institute of Technology and co-authored author of the three studies.
"The main one being the amount of water or its lack, Ryugu seems to possess. It is much drier than expected, and since Ryugu is quite young, according to asteroid standards, it would be about 100 million years old, which suggests that the body of his society was also largely devoid of 'water.
By combining the results of the two previous studies and observing the geological characteristics of Ryugu, Sugita and his colleagues – in a third study – attempted to limit the origin of Ryugu.
"The small asteroids, such as Ryugu, are thought to have been born from much older parent organs, as a result of catastrophic disturbances and re-accumulation of fragments during the evolution of the solar system," the scientists said.
"Ryugu's probably formed as rubble, ejected by the impact of a larger parent asteroid."
"The preponderance of materials on the asteroid with a weak water signature suggests that a dominant part of its original parent body was also" poor in water "."
They think that Ryugu comes from a parent asteroid several tens of kilometers wide, most likely families of Polana or Eulalia asteroids.
"Thanks to the parallel missions of Hayabusa-2 and OSIRIS-REx, we can finally address the question of how these two asteroids appeared," said Dr. Sugita.
"The fact that Bennu and Ryugu are brothers and sisters, while presenting very different traits, implies that there must be many exciting and mysterious astronomical processes that we have yet to explore."
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S. Watanabe et al. Hayabusa2 arrives on the carbon asteroid 162173 Ryugu – A pile of rubble in the shape of a top. Science, published online March 19, 2019; doi: 10.1126 / science.aav8032
K. Kitazato et al. The surface composition of the 162173 Ryugu asteroid of near infrared spectroscopy of Hayabusa2. Science, published online March 19, 2019; doi: 10.1126 / science.aav7432
S. Sugita et al. The geomorphological, thermal and thermal properties of Ryugu: implications for parent-body processes. Science, published online March 19, 2019; doi: 10.1126 / science.aaw0422
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