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As the days go by until the Touch-And-Go asteroid sample collection attempt from NASA’s OSIRIS-REx spacecraft, scientists from the Southwest Research Institute have helped determine what the spacecraft can expect to return from. the surface of the near-Earth asteroid Bennu. Three articles published online by Sciences on October 8, they discuss the color, reflectivity, age, composition, origin and distribution of the materials that make up the asteroid’s rough surface.
On October 20, the spacecraft will descend to the rock-strewn surface of the asteroid, touch the ground with its robotic arm for a few seconds, and collect a sample of rocks and dust, marking the first time NASA has taken pieces of a asteroid to return. to the earth. SwRI scientists played a role in the selection of the sampling sites. The first attempt will be made in Nightingale, a rocky area 20 meters in diameter in the northern hemisphere of Bennu. If this historic attempt is unsuccessful, the spacecraft will retry at a secondary site.
Since the spacecraft arrived at Bennu in 2018, scientists have been characterizing the composition of the asteroid and comparing it to other asteroids and meteorites. The mission discovered carbon-containing compounds on the surface of Bennu, a novelty for a near-Earth asteroid, as well as minerals that contain or form water. The scientists also studied the distribution of these materials, globally and at sampling sites.
“Our recent studies show that organic and mineral compounds associated with the presence of water are widely dispersed around Bennu’s surface, so any sample returned to Earth must contain these compounds and minerals,” said Dr. Vicky Hamilton from SwRI, co-author of the three articles. “We will compare the relative abundances of organic compounds, carbonates, silicates and other minerals in the sample with those of the meteorites to help determine the scenarios that best explain the composition of Bennu’s surface.”
Asteroid Bennu is a dark pile of debris held together by gravity and is believed to be the collision remnant of a much larger main belt object. Its debris-pile nature and many-cratered surface indicate that it has had an eventful life since it broke free from its much larger parent asteroid millions or even billions of years ago.
“The scattered boulders near the Nightingale site have bright carbonate streaks,” said Hamilton. “Bennu shares this compositional trait with water-altered meteorites. This correlation suggests that at least some carbonaceous asteroids were altered by water seepage in the early Solar System.”
Bennu boulders have various textures and colors, which can provide information on their varying exposure to micrometeorite bombardment and solar wind over time. The study of color and reflectance data provides information on the geological history of planetary surfaces.
“Bennu’s diverse surface includes abundant primitive material potentially from different depths in its main body plus a small proportion of foreign materials from another family of asteroids scattered across its surface,” said Dr. Kevin Walsh of SwRI, co-author of one of the articles. “In addition, the primary and backup sampling sites, Nightingale and Osprey, are located within small spectrally reddish craters that are believed to be more pristine, having experienced less spatial weathering than most of Bennu’s bluish surface.”
The OSIRIS-REx team is also comparing Bennu to Ryugu, another near-Earth asteroid. Both asteroids are believed to have originated from primitive asteroid families in the inner main belt. The Japan Aerospace Exploration Agency launched Hayabusa2 in 2014 and reunited with the near-Earth asteroid Ryugu in 2018. After examining the asteroid for a year and a half, the spacecraft collected samples and is expected to return to Earth on December 6, 2020.
The sample returned by OSIRIS-REx, combined with the surface context maps that OSIRIS-REx has collected, will improve interpretations of the data available from ground and space telescopes for other early dark asteroids. Comparing the returned Bennu samples to those of Ryugu will be critical to understanding the diversity and history of asteroid families and the entire asteroid belt.
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