These images of the northern hemisphere of the Bennu asteroid indicate that it is covered with stones. NASA
NASA OSIRIS-REx arrived at the Bennou asteroid at the beginning of December last year, traveling more than 2 million kilometers across the space for 27 months. Upon arrival, the five spacecraft's scientific instruments explore the 490 meter wide asteroid to better understand its properties and find a safe landing location to collect samples for return to the Earth.
Scientific inquiries were published in seven papers appearing in Nature and several of his scientific journals. The seven reports are collected on this website.
In some ways, Bennu is related to what scientists were expecting ̵
1; a "pile of gravel" of stony meteorites that gather under the influence of microgravity. Scientists have found that the density of the asteroid is about 1190 kg per cubic meter. For comparison, potatoes have a density of about 700 kg per cubic meter and dry gravel is about 1500 kg per cubic meter.
At a mean albedo of 4.4% this asteroid is one of the darkest objects in the solar system. What surprises scientists, however, is the overall diversity of the albedo or the coverage of Bennu's surface. Albedo varies from extremely dark – like freshly poured asphalt, at 3.3% to almost 15%. In one of the studies, mission scientists have expressed concern that the unexpected dispersion of Bennu's albedo could confuse the spacecraft's lobaridar navigation system as it approaches the asteroid to collect a sample.
Another potential problem in the conditions for finding a safe site for Benunu's regolith is the predominance of the stones on the surface. So far, the research team has identified more than 200 stones at least 10 meters in diameter and mapped more than 3,000 to 1 meter in diameter. Before the spacecraft arrived, scientists expected Bennu's surface to be relatively smooth and to design the sensor sampling device for such conditions.
"As the OSIRIS-REx mission gathers more data, we will be able to better determine the relationship between thermal inertia, regolith and stones distribution, site selection targeting, and future asteroid astronomical studies," the main study, published in Nature, is also
There are other puzzles, because of the discovery of particles in the vicinity of Bennu, scientists believe that the asteroid must periodically erupt material from its surface, but it is not clear how this happens or why. the mission claims that this "unexpectedly appeared is under investigation and that they will fully investigate the issue before forcing OSIRIS-REx to surrender for sampling operations.
Fortunately, planning missions have left much time for such studies, as sampling maneuver will not happen until July 2020. Until then, OSIRIS-REx will explore the asteroid in more detail – mapping the higher-resolution space rock topology than the surface of the Earth. Bennu's map will have the size of 5 cm spatial and a few cm vertical resolution. This should help NASA solve some of the scientific mysteries raised by the initial observations and find a safe place for sampling.