Secular Light Curve of 2P/Encke: A Comet Active At Aphelion
  We present the secular light curve of comet 2P/Encke in two phase spaces, the log plot, and the time plot. The main conclusions of this work are: a) The comet shows activity at perihelion and aphelion, caused by two different active areas: Source 1, close to the South pole, active at perihelion, and Source 2, at the North pole, centered at aphelion. b) More than 18 physical parameters are measured from the secular light curves, many of them new, and are listed in the individual plots of the comet. Specifically we find for Source 1 the location of the turn on and turn off points of activity, RON= -1.63+-0.03 AU, ROFF= +1.49+-0.20 AU, TON= -87+-5 d, TOFF= +94+-15 d, the time lag, LAG(q)= 6+-1 d, the total active time, TACTIVITY= 181+-16 d, and the amplitude of the secular light curve, ASEC(1,1) = 4.8+-0.1 mag. c) From this information the photometric age and the time-age defined in Ferrin (Icarus 178, 493-516, 2005a, and Icarus, 185, 523-543, 2006), can be calculated, and we find P-AGE=97+-8 comet years and T-AGE= 103+-9 comet years (cy). Thus comet 2P/Encke is an old comet entering the methuselah stage (100 cy < age). d) The activity at aphelion (Source 2), extends for TACTIVITY = 815+-30 d and the amplitude of the secular light curve is ASEC (1,Q) = 3.0+-0.2 mag. e) From a new phase diagram an absolute magnitude and phase coefficient for the nucleus are determined, and we find RNUC(1,1,0)= 15.05+-0.14, and betha= 0.066+-0.003. From this data we find a nucleus effective diameter DEFFE = 5.12(+2.5;-1.7) km. These values are not much different from previous determinations but exhibit smaller errors. Additional results appear in the full abstract. 
Ignacio Ferrin