Comet Encke

As its official designation implies, Encke's Comet was the first periodic comet discovered after Halley's Comet (designated 1P/Halley). It was independently observed by several astronomers, the first two being Pierre Méchain and Charles Messier in 1786. It was next observed by Caroline Herschel in 1795 and was "discovered" for a third time by Jean-Louis Pons in 1818. Its orbit was calculated by Johann Franz Encke, who through laborious calculations was able to link observations of comets in 1786 (designated 2P/1786 B1), 1795 (2P/1795 V1), 1805 (2P/1805 U1) and 1818 (2P/1818 W1) to the same object. In 1819 he published his conclusions in the journal Correspondance astronomique, and predicted correctly its return in 1822 (2P/1822 L1). It was recovered by Carl Ludwig Christian Rümker at Parramatta Observatory on 2 June 1822. Although named after Johann Encke, he himself insisted that the comet should be referred to as Pons's Comet instead, however this was not widely adopted. == Orbit ==

Comets are in unstable orbits that evolve over time due to perturbations and outgassing. Given Encke's low orbital inclination near the ecliptic and brief orbital period of 3 years, the orbit of Encke is frequently perturbed by the inner planets. Encke is currently close to a 7:2 mean motion resonance with Jupiter, and it is possible that some of the larger fragments shed by the comet, or released by a larger progenitor of the comet, are trapped in this resonance. Encke's orbit gets as close as to Earth (minimum orbit intersection distance). On 4 July 1997, Encke passed from Earth, and on 30 June 2172, it will make a close approach of roughly . On 18 November 2013, it passed from Mercury. Close approaches to Earth usually occur every 33 years. Comet Encke has a perihelion (closest approach to the Sun) of , and at perihelion Comet Encke passes the Sun at . Between 1769 and 2103, Comet Encke's perihelion distance only varies from 0.330 AU (in 2050) and 0.347 AU (in 1782). Of the numbered comets less than 321P, only 96P/Machholz gets closer to the Sun. == Observations ==

The comet has been observed at every perihelion since 1818 except 1944. An attempt to photograph the comet close to aphelion was made on 2 July 1913 using the Mount Wilson 60-inch telescope but the resulting photographic plate was lost in the mail. A second attempt using the same telescope was made on 1 September 1913 and this showed an object in about the right position (1.5 arcminutes from its then predicted position) but orbital uncertainties made it impossible to be sure of its identity. A recalculation of Encke's orbit in the 1970s resulted in a calculated position only a few arcseconds (2.0 in ascension and 4.6 in declination) from the imaged object meaning the object probably was Encke. In March 1918 the Greenwich 28-inch aperture telescope took observations of Encke (1917c). An observer of Encke's in March 1918 had this to say of the comet on March 12, comparing to the early March 9 observation, "The comet much shaper, brighter, smaller; its diameter was 1 1/2', magnitude 7.7 (B.D. scale). Its magnitude in the 6-inch Corbett was almost stellar, but in the 28 inch no definitive nucleus could be seen." A number of attempts were made to image the comet around the aphelion of 3 September 1972. Elizabeth Roemer and G. McCorkle photographed the comet on 15 August. R.E. McCrosky and C.-Y. Shao photographed it on 5 September and Elizabeth Roemer this time with M.R. Gonzales photographed the comet on 13 September. In 1980, Encke became the first comet to be detected by radar, which was conducted from the Arecibo Observatory to measure the size of its nucleus. In April 1984 the Pioneer Venus Orbiter observed the comet in ultra-violet and made measurements of its rate of water loss. On 20 April 2007, STEREO-A observed the tail of Comet Encke to be temporarily torn off by magnetic field disturbances caused by a coronal mass ejection (a blast of solar particles from the Sun). The tail grew back due to the continuous shedding of dust and gas by the comet. == Exploration ==

'' spacecraft making a flyby of a comet. NASA’s failed Comet Nucleus Tour (CONTOUR) mission was planned to perform a flyby of Encke in November 2003, however it was destroyed shortly after sending itself into an interplanetary trajectory in August 2002. Proposed missions In the 1970s, the Goddard Space Flight Centre and Jet Propulsion Laboratory proposed to NASA probes that would fly past the comet in 1980. The Goddard Space Flight Centre's proposal was for a pair of probes launched directly towards the comet while the Jet Propulsion Laboratory plan was to use solar electric propulsion, or a Pioneer-class spacecraft. NASA did not attempt either mission. Across the globe, the German DLR also proposed a mission towards Encke using a spacecraft derived from the Helios probes. However, like the NASA proposals, this mission was also not attempted. == Meteor showers ==

image of Encke and its debris trail in infrared light Comet Encke is believed to be the originator of several related meteor showers known as the Taurids (which are encountered as the Northern and Southern Taurids across November, and the Beta Taurids in late June and early July). A shower has similarly been reported affecting Mercury. The near-Earth asteroid is likely an inactive fragment of Encke that broke off the comet about 4,000–4,500 years ago. Mercury Measurements on board the NASA satellite MESSENGER have revealed Encke may contribute to seasonal meteor showers on Mercury. The Mercury Atmospheric and Surface Composition Spectrometer (MASCS) instrument discovered seasonal surges of calcium since the probe began orbiting the planet in March 2011. The spikes in calcium levels are thought to originate from small dust particles hitting the planet and knocking calcium-bearing molecules into the atmosphere in a process called impact vaporization. However, the general background of interplanetary dust in the inner Solar System cannot, by itself, account for the periodic spikes in calcium. This suggests a periodic source of additional dust, for example, a cometary debris field. == Effects on Earth ==

More than one theory has associated Encke's Comet with impacts of cometary material on Earth, and with cultural significance. The Tunguska event of 1908 may have been caused by the impact of a cometary body and has also been postulated by Slovak astronomer Ľubor Kresák as possibly caused by a fragment of Comet Encke. A theory holds that the ancient symbol of the swastika appeared in a variety of cultures across the world at a similar time, and could have been inspired by the appearance of a comet from head on, as the curved jets would be reminiscent of the swastika shape (see Comets and the swastika motif). Comet Encke has sometimes been identified as the comet in question. In their 1982 book Cosmic Serpent (page 155) Victor Clube and Bill Napier reproduce an ancient Chinese catalogue of cometary shapes from the Mawangdui Silk Texts, which includes a swastika-shaped comet, and suggest that some comet drawings were related to the breakup of the progenitor of Encke and the Taurid meteoroid stream. Fred Whipple in his The Mystery of Comets (1985, page 163) points out that Comet Encke's polar axis is only 5 degrees from its orbital plane: such an orientation is ideal to have presented a pinwheel like aspect to our ancestors when Encke was more active. Astronomers planned a 2019 search campaign for fragments of comet Encke which would have been visible from Earth as the Taurid swarm passed between July 5–11, and July 21 – August 10. There were no reports of discoveries of any such objects. == Importance in the scientific history of luminiferous aether ==

Comet Encke (and Biela's Comet) had a role in scientific history in the generally discredited concept of luminiferous aether. As its orbit was perturbed and shortened, the shortening could only be ascribed to the drag of an "ether" through which it orbited in outer space. A 1860 school manual for example reads: :Astronomy furnishes evidence of the presence in space of a medium resisting the motion of the heavenly bodies. Encke's comet is found to lose about two days in each successive period of 1,200 days. Biela's comet, with twice that length of period, loses about one day. That is, the successive returns of these bodies is found to be accelerated by this amount. No other cause for this irregularity has been found but the agency of the supposed ether. Encke's pole tumbles in an 81-year period, therefore it will accelerate for half that time, and decelerate for the other half of the time (since the orientation of the comets rotation to solar heating determines how its orbit changes due to outgassing forward or aft of the comet's course). The authors of this 1860 textbook of course could not know that the pole of the comet would tumble as it does over such a long period of time, or that outgassing would induce a thrust to change its course. The supposed shortening of the orbit of Encke's Comet demonstrating the existence of ether was mentioned in Edgar Alen Poe's story, "The Unparalleled Adventures Of One Hans Pfaall". == Gallery ==

File:Comet Encke from MESSENGER.png|A MESSENGER image of Comet Encke at its closest approach to Mercury, 17/11/2013 (NASA/JHUAPL/Carnegie Institution of Washington) == References ==