 Comet Encke photographed from theKitt Peak Observatory on 5 January 1994. | |
| Discovery | |
|---|---|
| Discovered by |
|
| Discovery date | 17 January 1786 |
| Designations | |
| |
| |
| Orbital characteristics[4][5] | |
| Epoch | 5 May 2025 (JD 2460800.5) |
| Observation arc | 239.45 years |
| Number of observations | 6,016 |
| Aphelion | 4.097 AU |
| Perihelion | 0.339 AU |
| Semi-major axis | 2.218 AU |
| Eccentricity | 0.84711 |
| Orbital period | 3.304 years |
| Max.orbital speed | 69.5 km/s (250,000 km/h)[1] |
| Inclination | 11.344° |
| 334.01° | |
| Argument of periapsis | 187.28° |
| Mean anomaly | 167.19° |
| Last perihelion | 22 October 2023[2] |
| Next perihelion | 10 February 2027[2][3] |
| TJupiter | 3.022 |
| EarthMOID | 0.168 AU |
| JupiterMOID | 0.907 AU |
| Physical characteristics | |
Mean diameter | 4.86 km (3.02 mi)[6][7] |
| 11.083±0.003 hours[8] | |
| 0.046 | |
| (V–R) =0.44±0.06[8] | |
| Comet total magnitude (M1) | 15.6 |
Comet Encke/ˈɛŋki/, orEncke's Comet (official designation:2P/Encke), is aperiodic comet that completes an orbit of theSun once every 3.3 years. (This is the shortest period of a reasonably bright comet; the faintmain-belt comet311P/PanSTARRS has a period of 3.2 years.) Encke was first recorded byPierre Méchain on 17 January 1786,[9] but it was not recognized as a periodic comet until 1819 when its orbit was computed byJohann Franz Encke. LikeHalley's Comet, it is unusual in its being named after the calculator of its orbit rather than its discoverer. Like most comets, it has a very lowalbedo, reflecting only 4.6% of the light itsnucleus receives, although comets generate a largecoma and tail that can make them much more visible during theirperihelion (closest approach to the Sun). The diameter of the nucleus of Encke's Comet is 4.86 km (3.02 mi).[6]
As its official designation implies, Encke's Comet was the first periodic comet discovered afterHalley's Comet (designated 1P/Halley). It was independently observed by several astronomers, the first two beingPierre Méchain andCharles Messier in 1786.[9] It was next observed byCaroline Herschel in 1795[10] and was "discovered" for a third time byJean-Louis Pons in 1818.[11] Its orbit was calculated byJohann 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 journalCorrespondance astronomique, and predicted correctly its return in 1822 (2P/1822 L1). It was recovered byCarl Ludwig Christian Rümker atParramatta Observatory on 2 June 1822.[12]
Comets are in unstable orbits that evolve over time due toperturbations andoutgassing. Given Encke's loworbital inclination near theecliptic and brieforbital period of 3 years, the orbit of Encke is frequently perturbed by the inner planets.[4] Encke is currently close to a 7:2mean motion resonance withJupiter, 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.[13]
Encke's orbit gets as close as 0.173 AU (25.9 million km; 16.1 million mi) to Earth (minimum orbit intersection distance).[4] On 4 July 1997, Encke passed 0.19 AU (28 million km; 18 million mi) from Earth, and on 29 June 2172, it will make a close approach of roughly 0.1735 AU (25.96 million km; 16.13 million mi).[4] On 18 November 2013, it passed 0.02496 AU (3.734 million km; 2.320 million mi) from Mercury.[4] Close approaches to Earth usually occur every 33 years.
Comet Encke has aperihelion (closest approach to the Sun) of 0.34 AU (51 million km; 32 million mi), and at perihelion Comet Enckepasses the Sun at 69.5 km/s (250,000 km/h).[1] Between 1769 and 2103, Comet Encke's perihelion distance only varies from 0.330 AU (in 2050) and 0.347 AU (in 1782).[14] Of thenumbered comets less than 321P, only96P/Machholz gets closer to the Sun.
The comet has been observed at every perihelion since 1818 except 1944.[9][15]
An attempt to photograph the comet close to aphelion was made on 2 July 1913 using theMount Wilson 60-inch telescope but the resulting photographic plate was lost in the mail.[16] A second attempt using the same telescope was made on 1 September 1913 and this showed an object in about the right position (1.5arcminutes from its then predicted position) but orbital uncertainties made it impossible to be sure of its identity.[16][9] 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.[9]
In March 1918 theGreenwich 28-inch aperture telescope took observations of Encke (1917c).[17]
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."[17]
A number of attempts were made to image the comet around the aphelion of 3 September 1972.[18][19]Elizabeth Roemer and G. McCorkle photographed the comet on 15 August.[18] 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.[18]
In 1980, Encke became the first comet to be detected by radar, which was conducted from theArecibo Observatory to measure the size of its nucleus.[20][21]
In April 1984 thePioneer Venus Orbiter observed the comet in ultra-violet and made measurements of its rate of water loss.[22]
On 20 April 2007,STEREO-A observed the tail of Comet Encke to be temporarily torn off by magnetic field disturbances caused by acoronal mass ejection (a blast of solar particles from the Sun).[23] The tail grew back due to the continuous shedding of dust and gas by the comet.[24]
NASA’s failedComet 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.[25]
In the 1970s, theGoddard Space Flight Centre andJet Propulsion Laboratory proposed to NASA probes that would fly past the comet in 1980.[25] 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,[26] or aPioneer-class spacecraft.[27] NASA did not attempt either mission.[25]
Across the globe, the GermanDLR also proposed a mission towards Encke using a spacecraft derived from theHelios probes.[28] However, like the NASA proposals, this mission was also not attempted.
Comet Encke is believed to be the originator of several relatedmeteor showers known as theTaurids (which are encountered as the Northern and Southern Taurids across November, and theBeta Taurids in late June and early July).[29] A shower has similarly been reported affecting Mercury.[30]
The near-Earth asteroid(828534) 2004 TG10 is likely an inactive fragment of Encke that broke off the comet about 4,000–4,500 years ago.[31]
Measurements on board the NASA satelliteMESSENGER 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.[32]
More than one theory has associated Encke's Comet with impacts of cometary material on Earth, and with cultural significance.
TheTunguska event of 1908 may have been caused by the impact of a cometary body and has also been postulated by Slovak astronomerĽubor Kresák as possibly caused by a fragment of Comet Encke.[33]
A theory holds that the ancient symbol of theswastika 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 (seeComets and the swastika motif). Comet Encke has sometimes been identified as the comet in question. In their 1982 bookCosmic Serpent (page 155)Victor Clube andBill Napier reproduce an ancient Chinese catalogue of cometary shapes from theMawangdui 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 theTaurid meteoroid stream.Fred Whipple in hisThe 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.[34] There were no reports of discoveries of any such objects.
Comet Encke (andBiela's Comet) had a role inscientific history in the generally discredited concept ofluminiferous aether. As its orbit was perturbed and shortened, the shortening could only be ascribed to thedrag of an "ether" through which it orbited inouter space. A 1860 school manual for example reads:
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".
![A MESSENGER image of Comet Encke at its closest approach to Mercury, 17/11/2013[36] (NASA/JHUAPL/Carnegie Institution of Washington)](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aComet_Encke_from_MESSENGER.png)
http://www.itc.nl/library/Papers_2004/tech_rep/woldai_umm.pdf (1.56 MB)
| Numbered comets | ||
|---|---|---|
| Previous 1P/Halley | 2P/Encke | Next 3D/Biela |
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