Comet 21P (Giacobini-Zinner)

Comet Giacobini-Zinner is catalogued officially as 21P/Giacobini-Zinner. The "21P" means that it's the 21st short-period comet to have its orbit calculated. The very first comet to have its orbit calculated ("1P") was the famous  Halley's Comet. Comet Giacobini-Zinner  was discovered in December 1900 by the French astronomer Michel Giacobini at the Nice Observatory in France. Initially, Giacobini calculated that this newfound comet had a relatively short orbital period of just under 7 years, but no one saw the object during its anticipated 1907 return. Comet 21P's orbital period varies between 6.4 and 6.6 years, so its perihelion — or its closest approach to the sun — can happen in any month. This happens because the comet's orbit may shift a little every time it passes by Jupiter. This year (2018), Giacobini-Zinner will arrive at perihelion on Monday (Sept. 10). This image was taken one day before perihelion, and on one of the last days this comet will be high enough above the horizon to image from this location.