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At Bennu, NASA finds a mysterious boulder-strewn asteroid

  These images of the asteroid Bennus northern hemisphere show that they are covered with stones.
Enlarge / These images of the asteroid Bennus northern hemisphere show that it is covered with stones.


After NASA's OSIRIS-REx probe, which flew more than 2 million kilometers through space over the course of 27 months, arrived at the Bennu asteroid in early December last year. Since arriving, the spacecraft's five scientific instruments have studied the 490-meter-wide asteroid to better understand its characteristics and to find a safe landing pad from which to collect samples for a return to Earth.

On Tuesday the first results of these scientific investigations were published in seven publications that appeared in Nature and a handful of their research journals. The seven publications are summarized on this website.

In a way, Bennu is concerned with what the scientists expect – a "pile of rubble" of stony meteorites that have accumulated under the influence of weightlessness. The scientists found that the density of the asteroid is about 1,190 kg per cubic meter. For comparison, a potato has a density of about 700 kg per cubic meter and dry gravel about 1,500 kg per cubic meter.

With an average albedo of 4.4 percent, this asteroid is one of the darkest objects in the solar system. What surprised the scientists, however, is the overall diversity of albedo or the reflectivity of the surface of Bennu. The albedo ranges from extremely dark – like freshly poured asphalt – with 3.3 percent to almost 15 percent. In one of the research sessions, mission scientists expressed their concern that the unexpected departure of Bennus Albedo might confuse the spacecraft's lidar guidance system as it approaches the asteroid to collect a sample.

Sampling Another Potential Issue in 2020

To find a safe place to sample Bennus regolith is the overweight of boulders on the surface. The scientific team has so far identified more than 200 boulders with a diameter of at least 10 meters and mapped more than 3,000 to 1 meter in diameter. Before the arrival of the spacecraft, the scientists at Bennu's surface had expected a relatively smooth surface and developed the probe's "touch-and-go" scanner for such conditions.

"As the OSIRIS REx mission gathers more data, we will be able to better define the relationship between thermal inertia, regolith and boulder distribution, direct selection of sample sites, and future astronomical studies on asteroids There are other secrets as well: Due to the detection of particles near Bennu, scientists believe that the asteroid must periodically erupt material from its surface, but it is not clear how this happens and why mission scientists say that this "unexpected phenomenon" is still under investigation and they will investigate the matter thoroughly before OSIRIS-REx goes to the surface for sampling. [LuckilythemissionplannersallowedplentyoftimeforsuchstudiesasThesamplingmaneuverwilltakeplaceinJuly2020UntilthenOSIRIS-RExwilldetailtheasteroidexamineThetopologyofspacerockisrepresentedwithahigherresolutionthantheearth'ssurfaceThemapofBennuhasaspatialresolutionof5cmandaverticalresolutionofafewcmThisshouldhelpNASAsolvesomeofthescientificpuzzlesraisedbyinitialobservationsandfindasafesamplingsite

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