21P/Giacobini-Zinner (0021P)

Type: Periodic Perihelion date: 25 March 2025 Perihelion distance (q): 1.0 Aphelion distance (Q) : 6.0 Period (years): 6.5 Eccentricity (e): 0.71 Inclination (i): 32.1 JPL orbit diagram COBS lightcurve Atlas Michel Giacobini (Nice, France) discovered this comet in Aquarius on 20 December 1900. Ernst Zinner (Bamberg, Germany) discovered a comet while observing variable stars near Beta Scuti on 23 October 1913. He described it as about magnitude 10, with a coma 3 arc minutes across and a tail 30 arc minutes long. The comet was observed for nearly two months during its discovery apparition. During the latter days of December 1900, observers estimated the comet's total magnitude as between 10.5 and 11, while the magnitude of the nuclear condensation was about 12. The coma was near one arc minute in diameter, and no tail was reported. The comet faded as January began and it was last seen on 16 February. The comet was recognized as a short-period comet during the discovery apparition, and the orbital period was ultimately determined as 6.52 years. A prediction was made that it would return to perihelion during 1907 and that it would be very badly placed for observation. No searches appear to have been made. The comet was next expected to return to perihelion in 1914, when conditions would make the comet impossible to recover. As later calculations revealed, the orbital period in 1900 was actually 6.46 years, so that the prediction for the apparent 1914 return was in error by about six months. The comet was missed at the unfavourable 1920 return, but was next recovered on 16 October 1926 at Bergedorf. The precise position indicated the prediction for this return had been in error by only 5 days. The comet reached a maximum brightness of 11 and a maximum tail length of 2 arc minutes. The comet has been seen at every return since discovery, except for the very unfavourable 1953 return. The 1946 apparition was especially noteworthy as the comet passed only 0.26 AU from Earth in late September. The comet was then near magnitude 7, making this the brightest appearance since its discovery. Interestingly, the comet experienced an unexpected outburst in brightness which caused it to reach magnitude 6 during the first days of October. Following the missed return in 1953, the comet returned in 1959. Unexpected brightness outbursts were detected on 31 August, 23 September and 24 October, each increasing the comet's brightness by about one-half magnitude. Ultimately, the comet's maximum brightness reached 7 during 1959 and, at one point, the tail was one degree long. This comet is especially noteworthy as it is one of a small number of comets that can produce very spectacular meteor showers under the right conditions. The meteor display is variously referred to as the Draconids, October Draconids, and the Giacobinids. The meteor shower occurs around 9 October of each year, but is usually unrecognizable; however, meteor storms occurred in 1933 and 1946 which produced several thousand meteors within an hour at maximum. Close approaches to planets: • 0.88 AU from Earth on 15 December 1900 (contributed to comet's first discovery) • 0.51 AU from Earth on 14 November 1913 (contributed to comet's seond discovery) • 0.26 AU from Earth on 20 September 1946 • 0.93 AU from Jupiter on 19 January 1958 (decreased perihelion distance from 0.99 AU to 0.94 AU and decreased orbital period from 6.56 to 6.42 years) • 0.35 AU from Earth on 8 November 1959 • 0.58 AU from Jupiter on 23 September 1969 (increased perihelion distance from • 0.93 AU to 0.99 AU and increased orbital period from 6.41 to 6.52 years) • 0.93 AU from Earth on 24 July 1972 • 0.47 AU from Earth on 6 September 1985 • 0.85 AU from Earth on 27 November 1998 • 0.39 AU from Earth on 11 September 2018 • 0.37 AU from Jupiter on 14 February 2029 (will increase perihelion distance from 1.01 AU to 1.07 AU and increase orbital period from 6.53 to 6.70 years) • 0.53 AU from Earth on 5 September 2031 Observations (VEMag = visual equivalent magnitude) Date 10x10 mag Error VEmag Coma ' 16-Apr-18 17.59 0.02 17.1 0.3 24-Jun-18 14.03 0.01 11.9 1.3 07-Jul-18 13.55 0.09 10.4 2.4 14-Jul-18 13.09 0.01 10.4 4.9 03-Aug-18 11.93 0.01 8.5 7.4 08-Sep-18 11.08 0.01 7.0 9.5 06-Oct-18 12.42 0.03 9.3 3.1 11-Oct-18 12.77 0.02 9.4 2.0 14-Oct-18 13.15 0.02 9.8 1.9 20-Oct-18 13.59 0.02 10.4 2.3 18-Nov-18 15.01 0.01 11.6 1.0 09-Dec-18 15.52 0.03 13.4 0.7 29-Dec-18 15.75 0.06 13.5 0.8 02-Jan-19 15.78 0.02 12.9 0.7 12-Jan-19 16.10 0.03 13.4 0.7