Astronomy Picture of the Day

    Discover the cosmos! Each day a different image or photograph of our
      fascinating universe is featured, along with a brief explanation
                    written by a professional astronomer.

                              2020 September 16

                     Gravel Ejected from Asteroid Bennu
      Image Credit: NASA's GSFC, U. Arizona, OSIRIS-REx Lockheed Martin

   Explanation: Why does asteroid Bennu eject gravel into space? No one is
   sure. The discovery, occurring during several episodes by NASA's
   visiting ORISIS-REx spacecraft, was unexpected. Leading ejection
   hypotheses include impacts by Sun-orbiting meteoroids, sudden thermal
   fractures of internal structures, and the sudden release of a water
   vapor jet. The featured two-image composite shows an ejection event
   that occurred in early 2019, with sun-reflecting ejecta seen on the
   right. Data and simulations show that large gravel typically falls
   right back to the rotating 500-meter asteroid, while smaller rocks skip
   around the surface, and the smallest rocks completely escape the low
   gravity of the Earth approaching, diamond-shaped asteroid. Jets and
   surface ejection events were thought to be predominantly the domain of
   comets, responsible for their tails, comas, and later meteor showers on
   Earth. Robotic OSIRIS-REx arrived at 101955 Bennu in late 2018, and is
   planned to touchdown to collect a surface sample in October 2020. If
   all goes well, this sample will then be returned to Earth for a
   detailed analysis during 2023. Bennu was chosen as the destination for
   OSIRIS-REx in part because its surface shows potential to reveal
   organic compounds from the early days of our Solar System, compounds
   that could have been the building blocks for life on Earth.

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                        Tomorrow's picture: cycle 25
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       Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)
            NASA Official: Phillip Newman Specific rights apply.
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                      A service of: ASD at NASA / GSFC
                             & Michigan Tech. U.


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