Comet Encke - An Historical Perspective Discovery 1786 Jan. 17 Pierre Mechain at Paris discovered faint comet but only two nights of observations by Mechain, Messier, and Jean-Dominque Cassini did not allow orbit determination 1795 Nov. 7 Caroline Herschel at Slough England discovered her 6th comet - rough orbit determined 1805 Oct. 20 Comet discovered by Jean Louis Pons at Marseille 1818 Nov. 26 Pons discovered another comet - Conjectures by von Zach that the comets of 1819 and 1805 were the same and by Olbers who suspected that comets discovered in 1818, 1795 and 1786 were one and the same object 1819 - Johann Encke, a student of Gauss, confirmed Olber's conjecture by integrating motion of comet under perturbative influences of all known planets except Uranus. Encke also integrated motion forward in time to successfully predict next perihelion passage time as 1822 May 24 - the 2nd successful comet return prediction (2P/Encke). Encke himself always referred to the comet as that of Pons.... Interplanetary Resisting Medium Encke noted that observed time of perihelion passage in 1822 was a few hours earlier than his prediction and in 1823, he suggested an interplanetary resisting medium to explain this discrepancy. Resisting force supposed proportional to medium density and square of orbital velocity. Resisting medium was envisioned as either an extension of solar atmosphere or debris of cometary or planetary atmospheres left in space Resisting medium thought to vary as Inverse Square of heliocentric distance Using his model of resisting medium to reduce comet's orbital period by about 2.5 hours per return (0.16 m/s), Encke successfully predicted returns between 1825 and 1858 1819 - 1848 Perturbations taken into account fairly rigorously 1848 - 1858 Only Jupiter's effects considered - computers overloaded Friedrich Bessel - best remembered for discovering stellar parallax cut his teeth by computing an orbit for the 1607 return of comet Halley After observing some of the observations of comet Halley in 1835, Bessel noted some sunward emanations that resembled a burning rocket. Argued that a resisting medium was not evident in the motion of the planets, Bessel suggested that the orbital period of comets could be shortened by the rocket-like thrusting of the comet itself. Bessel considered radial thrusting that was asymmetric w/r to perihelion As an example, Bessel calculated how much comet Halley's orbital period would be shortened if its emanations took place before perihelion and lasted 23 days beyond Oct. 2, 1835. If the daily mass loss from the nucleus was assumed to be 1/1000 of the total the result was a shortening of the comet's period by 1,107 days. While a considerable overestimate (Halley's orbit is actually lengthened by 4 days per period), Bessel's rocket effect anticipated the currently accepted explanation for these effects advanced by Fred Whipple in 1950. Bessel clearly considered that the nuclei of comets were solid 1878 Friedrich Emil von Asten published study of Encke's motion over 1818 - 1875 interval trying to test whether Encke's resisting medium or Bessel's rocket-like thrusting was correct. From the computed decrease in the orbital eccentricity, he concluded that Encke's hypothesis was preferred and that since the so-called nongrav. effects were not evident in comets with larger perihelion distances, the resisting medium did not extend much beyond the orbit of Mercury. While Encke's motion over the 1818 - 1868 interval was consistent with resisting medium, the return in 1871 was not the usual 2.5 hours early. Von Asten speculated that the apparent disappearance of the nongravs in 1871 was due to a collision of an asteroid with the comet in June 1869. Jons Oskar Backlund - Swedish astronomer who became director of Pulkovo Obs. (1895) Received RAS-London gold medal for his work in 1909 - yet he only had limited success in explaining the motion of comet Encke. Backlund rejected von Asten's asteroid collision hypothesis He concluded that the comet's orbital period decreased in a uniform fashion over the 1818 - 1858 interval but subsequently, this decrease became less apparent. Backlund's explanation was that the density of the resisting medium which took the form of a meteoric ring near the comet's perihelion, was decreasing with time. Final blow to resisting medium occurred with discovered that some comets showed INCREASE in orbital periods: Michael Kamienski (1933) P/Wolf Albert Recht (1940) P/d'Arrest Nongravitational Effects Bessel's Original Mechanism vs. Whipple Rotation Mechanism Light Curve Asymmetry vs. Nongravitational Effects Asymmetry Kresak (1965) pointed out that comet's brightness excess was 2-3 magnitudes about 3-4 weeks pre-perihelion Whipple and Sekanina (1979) suggested that vaporization takes place from one hemisphere, the pole of which points away from sun about 3 weeks after perihelion A'Hearn et al. (1983) At r = 0.75 AU pre-perih., OH prod. rate decreases by factor of 3-4 Production rate of other species lower post-perihelion Current State of Encke's motion - Nongravs nearly zero currently Orbital evolution is very modest due to relatively low aphelion distance Long. Of Asc. Node + Arg. of Perih = 160.994 in 2000 and 159.369 deg. in 1786. Illustrations Mechain, C. Herschel, J.L. Pons, J. Encke, F. Bessel, O. Backlund, Von Asten Whipple et al at car Comet Halley rocket like emanations (1835) N.G. Force diagrams (Bessel and Whipple) Asymmetric N.G. force equations Sekanina diagrams showing evolution of light curves and N.G. forces Encke images Data Arc A1 A2 1795 1786 - 1805 -2.17 -0.0361 1813 1805 - 1822 +0.69 -0.0396 1825 1818 - 1832 +1.14 -0.0400 1838 1832 - 1845 -0.63 -0.0376 1852 1845 - 1858 +0.38 -0.0350 1865 1858 - 1871 -0.78 -0.0317 1878 1871 - 1885 +0.07 -0.0266 1890 1885 - 1895 -0.14 -0.0226 1906 1901 - 1911 +0.77 -0.0168 1918 1911 - 1924 -0.09 -0.0146 1931 1924 - 1937 -0.09 -0.0114 1944 1937 - 1951 -0.05 -0.0082 1957 1950 - 1964 +0.03 -0.0059 1971 1963 - 1977 0.00 -0.0035 1977 1970 - 1985 -0.03 -0.0030 1990 1984 - 1996 -0.01 -0.0016 1994 1989 - 2000 0.00 -0.0012