Captured by the Kitt Peak 0.9m telescope, Comet 21P/Giacobini-Zinner lights up the sky Oct. 31, 1998. Credit: N.A.Sharp/NOAO/AURA/NSF
On Dec. 20, 1900, astronomer Michel Giacobini spotted a 10th-magnitude comet while working at Nice Observatory, in France. Calculations following his discovery suggested the comet had an orbital period of a bit less than seven years. When German astronomer Ernst Zinner rediscovered the comet on Oct. 23, 1913, happening across it by accident while observing variable stars, the calculations were revised: Comet 21P/Giacobini-Zinner has an orbital period of 6.6 years. It also is responsible for the debris that gives us the Draconid meteor shower each year in October. In 1985, the International Cometary Explorer (ICE) mission flew by Comet 21P/Giacobini-Zinner, in the first spacecraft/comet rendezvous.
