                        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.

                                2021 July 24

                              The Edge of Space
                    Image Credit & Copyright: Ralf Rohner

   Explanation: Where does space begin? For purposes of spaceflight some
   would say at the Karman line, currently defined as an altitude of 100
   kilometers (60 miles). Others might place a line 80 kilometers (50
   miles) above Earth's mean sea level. But there is no sharp physical
   boundary that marks the end of atmosphere and the beginning of space.
   In fact, the Karman line itself is near the transition between the
   upper mesophere and lower thermosphere. Night shining or noctilucent
   clouds are high-latitude summer apparitions formed at altitudes near
   the top of the mesophere, up to 80 kilometers or so, also known as
   polar mesopheric clouds. Auroral bands of the northern (and southern)
   lights caused by energetic particles exciting atoms in the thermosphere
   can extend above 80 kilometers to over 600 kilometers altitude. Taken
   from a cockpit while flying at an altitude of 10 kilometers (33,000
   feet) in the realm of stratospheric aeronautics, this snapshot captures
   both noctilucent clouds and aurora borealis under a starry sky, looking
   toward planet Earth's horizon and the edge of space.

                 Tomorrow's picture: crescent father and son
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       Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)
            NASA Official: Phillip Newman Specific rights apply.
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                             & Michigan Tech. U.


