Astronomy Picture of the Day

   [1]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
    [2]See Explanation. Clicking on the picture will download the highest
                        resolution version available.

                              The Edge of Space
                 Image Credit & [3]Copyright: [4]Ralf Rohner

   Explanation: [5]Where does space begin? For purposes of spaceflight
   some would say at the [6]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
   [7]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
   [8]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. [9]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 [10]stratospheric aeronautics,
   this snapshot captures both noctilucent clouds and aurora borealis
   under a starry sky, [11]looking toward planet Earth's horizon and the
   edge of space.

                 Tomorrow's picture: crescent father and son
     __________________________________________________________________

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    Authors & editors: [23]Robert Nemiroff ([24]MTU) & [25]Jerry Bonnell
                                 ([26]UMCP)
          NASA Official: Phillip Newman [27]Specific rights apply.
              [28]NASA Web Privacy Policy and Important Notices
                A service of: [29]ASD at [30]NASA / [31]GSFC
                           & [32]Michigan Tech. U.

References

   1. https://apod.nasa.gov/apod/archivepix.html
   2. https://apod.nasa.gov/apod/image/2107/AuroraNoctilucent33k_rohner.jpg
   3. https://apod.nasa.gov/apod/lib/about_apod.html#srapply
   4. https://www.instagram.com/skypointer2000/
   5. https://earthsky.org/human-world/the-billionaire-space-race-and-the-karman-line/
   6. https://en.wikipedia.org/wiki/Kármán_line
   7. https://www.nasa.gov/mission_pages/sunearth/science/atmosphere-layers2.html
   8. https://apod.nasa.gov/apod/ap200619.html
   9. https://apod.nasa.gov/apod/ap210114.html
  10. https://www.nasa.gov/feature/sofia-offers-new-way-to-study-earth-s-atmosphere
  11. https://www.youtube.com/watch?v=zwydBbEVrZQ
  12. https://apod.nasa.gov/apod/ap210723.html
  13. https://apod.nasa.gov/apod/archivepix.html
  14. https://apod.nasa.gov/apod/lib/apsubmit2015.html
  15. https://apod.nasa.gov/apod/lib/aptree.html
  16. https://antwrp.gsfc.nasa.gov/cgi-bin/apod/apod_search
  17. https://apod.nasa.gov/apod/calendar/allyears.html
  18. https://apod.nasa.gov/apod.rss
  19. https://apod.nasa.gov/apod/lib/edlinks.html
  20. https://apod.nasa.gov/apod/lib/about_apod.html
  21. http://asterisk.apod.com/discuss_apod.php?date=210724
  22. https://apod.nasa.gov/apod/ap210725.html
  23. http://www.phy.mtu.edu/faculty/Nemiroff.html
  24. http://www.phy.mtu.edu/
  25. https://antwrp.gsfc.nasa.gov/htmltest/jbonnell/www/bonnell.html
  26. http://www.astro.umd.edu/
  27. https://apod.nasa.gov/apod/lib/about_apod.html#srapply
  28. https://www.nasa.gov/about/highlights/HP_Privacy.html
  29. https://astrophysics.gsfc.nasa.gov/
  30. https://www.nasa.gov/
  31. https://www.nasa.gov/centers/goddard/
  32. http://www.mtu.edu/