2P/Encke

Image of Encke taken by the Spitzer Space Telescope, and its slight trace of infrared light.

Comet Encke or Encke's comet (official designation: 2P/Encke) is a periodic comet that completes one orbit around the Sun. every 3.3 years. It is known to have the shortest period of a reasonably bright comet (the fainter main-belt comet 311P/PANSTARRS has a period of 3.2 years).

Encke was first recorded by Pierre Méchain in 1786, but it was not recognized as a periodic comet until 1819, when Johann Franz Encke calculated its orbit. Like Halley's Comet, it was unusually named after the astronomer who calculated its orbit, rather than its discoverer as usual. Like most comets, it has a very low albedo, reflecting only 4.6% of the light it receives. The diameter of Encke's core is 4.8 km.

Discovery

As indicated by the number 2 of its official designation, Encke's Comet was the second periodic comet discovered, after Halley's Comet (designated 1 P/Halley). It was independently observed by several astronomers. Thus, it is known that Caroline Herschel spotted it on a second visit (in 1795) and Jean-Louis Pons on a third occasion (in 1818). Its orbit was calculated by Johann Franz Encke, who through laborious calculations was able to to link comet observations 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 correctly predicted his return in 1822 (2P / 1822 L1). It was sighted again by Karl Rümker from the Sydney Observatory on June 2, 1822, confirming Encke's calculations.

Detailed history of discovery

Comet Encke was first seen in Paris on January 17, 1786 by Pierre Méchain (1744-1804), close to the star Beta Aquarii (Sadalsuud). The next day, he reported his discovery to Charles Messier (1730-1817), who observed the comet on the 19th, the date on which it was also sighted by Jean-Dominique, Count of Cassini (1748-1845). It had a bright core, but no tail.

On November 17, 1795, Caroline Herschel (1750-1848) discovered a faint comet, without a nucleus, more with a slight central condensation of light, near the star Gamma Cygni (Sadr); it was barely visible to the naked eye. That same comet was observed during the month of November by the German astronomer Johann Elert Bode (1747-1826), by the Frenchman Alexis Bouvard (1767-1834) and by the German physician and amateur astronomer Heinrich Wilhelm Olbers (1758-1840). The orbit of this star perplexed the calculators, since they could not translate the observations with the help of a parabola.

One of the most industrious observers and one of those who most devoted himself to the discovery of comets was Jean Louis Pons (1761-1831). At thirty-nine, Pons was a goalkeeper at the Marseille Observatory; he located his first kite in 1801; by 1813 he had discovered a dozen of them, rising from porter to assistant astronomer. In 1819 he left the Marseille Observatory to become director of the Royal Observatory of Lucca.

On November 26, 1818, Pons discovered a telescopic comet in the constellation Pegasus, and since it remained visible for about seven weeks, until January 12, 1819, a long series of observations could be obtained and whose elements parabolic, compared to those of the comets cataloged at that time, led to the suspicion that it had been seen in 1805, and that Pons himself had already observed it. The suspicion that it could be the same comet occurred to Dominique Arago (1786-1853), director of the Paris Observatory, when Bouvard presented it to the Bureau des Longitudes, since the orbital elements of the two stars were very similar.

In 1819, the German astronomer and director of the Gotha Observatory, Johann Encke (1791-1865), demonstrated that a parabolic orbit could not suit this comet in any way and he undertook the task of rigorously determining the elements using the method of Gauss, thanks to which it had been possible to recover the first asteroid, Ceres, lost shortly after its discovery. And he found that the orbit was elliptical with a period of three and a half years. When consulting a catalog, he was struck by the similarities between the elements he had calculated and those of the comets of 1786, 1795 and 1805, and he thought that they could be the same star. To demonstrate this, one had to calculate, going back in time, the effects of planetary perturbations; which he did successfully, with extraordinary effort, in six weeks. Between 1786 and 1818 the comet had passed perihelion seven times without being seen. This extraordinary work of calculation has been rewarded by posterity linking the name of Encke to the comet; although Encke himself modestly always called it Pons's comet. In 1824 Encke was awarded the gold medal of the Royal Astronomical Society and on this same occasion the silver medal was sent to Pons, who had discovered it.

The comet passed next to Mercury in 1838, indicating Encke that the mass of this planet had to be much less than that indicated by Pierre Simon Laplace (1749-1827), which has subsequently been confirmed.

Orbit

Comets evolve into unstable orbits over time due to perturbations and outgassing. Given Encke's low orbital inclination relative to the ecliptic and its short orbital period of 3 years, its orbit is frequently perturbed by the inner planets.

Encke's trajectory is only 0.17309 AU (about 26 million km) from Earth (minimum orbital intersection distance). On July 4, 1997, Encke passed within 0.19 AU of Earth, and on June 29, 2172 it will make an approach to about 0.1735 AU. On November 18, 2013, it passed within 0.02496 AU (3.7 million km) of Mercury. The closest approaches to Earth generally occur every 33 years.

Observations

A coronal mass ejection tears the tail of the Encke comet

The failed CONTOUR mission was launched to study comets Encke and 73P/Schwassmann-Wachmann.

On April 20, 2007, the Stereo mission observed the tail of Comet Encke temporarily formed by a magnetic storm triggered by a coronal mass ejection (a burst of particles emitted from the Sun). The tail grew behind due to to the continued release of dust and gas by the comet.

The near-Earth object 2004 TG10 is thought to be an Encke fragment.

Meteor Showers

Meteor shower on Earth

Comet Encke is thought to be responsible for several phenomena associated with meteor showers known as the Taurids (appearing as the Northern and Southern Taurids in November, and the Beta Taurids in late June and early July).

Meteor Shower on Mercury

Similarly, another Encke-linked meteor shower has been reported to affect Mercury. Measurements from NASA's MESSENGER satellite have revealed that Encke may contribute to the meteor shower season on Mercury. The Instrument for the Spectroscopic Analysis of the Composition of Mercury's Atmosphere and Surface (MASCS) has discovered seasonal spikes in calcium since the probe began orbiting the planet in March 2011.

The spikes in calcium levels are thought to be caused by small dust particles picked up by the planet. These calcium molecules are dispersed in the atmosphere in a process called impact vaporization. However, the general background of interplanetary dust in the interior of the Solar System alone cannot explain the periodic calcium spikes. This suggests that there is a periodic source of additional dust, for example a cometary debris field.

Cultural reminiscences

An astronomical atlas on Han Dynasty silk fabric, with comet drawings possibly related to Encke

Different theories have associated Comet Encke with cometary impacts on Earth on the one hand and with its cultural importance on the other.

Regarding the Tunguska bolide of 1908, probably caused by the impact of a cometary body, the Czechoslovakian astronomer Ľubor Kresák postulated that it may have been a fragment of Encke's comet.

One theory holds that the ancient symbol of the swastika appeared in a variety of cultures around the world at a similar time, and that it might have been inspired by the appearance of a four-tailed comet with a similar shape, the swastika shape reminiscent of this apparition (see Comets and the origin of the swastika). Comet Encke has sometimes been identified as the comet in question. In their 1982 book Cosmic Serpent (page 155) Victor Clube and Bill Napier reproduced an ancient Chinese catalog of cometary forms from the Mawangdui Texts, including a swastika-shaped comet, and suggest that some drawings of the comet were related to the breakup of a possible progenitor of Encke and the Taurid meteoroid stream. Fred Whipple in his work Mystery of Comets (1985, page 163) points out that the polar axis of Comet Encke is only 5 degrees with respect to its orbital plane: such an orientation is ideal for the formation of a whirlpool with the appearance that would be so striking in antiquity, when Encke could have been more active.

Importance in the scientific history of the luminiferous ether

The behavior of the periods of comets Encke and Biela was used as an argument during the long controversy whose evolution led to the finally discarding of the luminiferous ether theory. During the 19th century, the observed shortening of their orbits was initially put forward as an effect of "ether" through which they were supposed to orbit. A reference from the time, stated that:

Encke's comet loses about two days in each successive period of 1200 days. Biela's comet, with a two-fold period, loses about a day. That is, the successive returns of these bodies are accelerated in these quantities. No other cause of this irregularity has been found except the alleged effect of ether.

Since Encke's pole rotates over a period of 81 years, the comet will accelerate half the time and decelerate the other half (since the orientation of the comets' rotation relative to the relative position from which they receive solar radiation, determines how its orbit changes due to the emission of gases forwards or backwards with respect to its trajectory). Of course, the authors of this 1860 textbook could not have known that the comet's pole would return to the same position over such a long period of time, or that outgassing would cause a push to change its course.

Image gallery

Picture of the comet Encke in its maximum approach to Mercury (Photo MESSENGER, 17/11/2013).

Comet Encke represented by the Celestia application, using data acquired from Earth.

Another image of Encke and its tail (Celestia application).

Current state

In 2017, this comet has a cometary age of 109 years (one year for each of its cycles, which last about 3.3 Earth years), which is why it is classified as a Methuselah comet. It is estimated to have a life until the year 2050-2060: from then on, it will become an asteroid like (3552) Don Quixote.

Its disproportionate loss of mass (it will only last about 120 comet years) is due to the fact that this comet sublimates during its entire orbit, from aphelion to perihelion. Currently, in its closest approximations, it barely reaches magnitude 7, when it once reached magnitudes 3 and 4. At its current passage through perihelion, it raises a small coma of dust and, on rare occasions, a small tail, symptoms of age advance of this comet, which in 2050 is estimated to have exhausted all its fuel. When this comet reaches its aphelion, it is practically indistinguishable from the surrounding stars, then reaching magnitudes close to +17, making it almost impossible to distinguish it from an asteroid.

Comet 2P/Encke has been observed in almost all of its perihelion passes, which are:

• [NOTE: These data come from the Seiichi Yoshida website, called (comet@aerith.net) (Encke 2p)]

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