Secular light curve of comet 103P/Hartley 2, target of the EPOXI mission

In support of the EPOXI mission, we have compiled and updated the secular light curve (SLC) of comet 103P/Hartley 2, a Jupiter Family comet, the next target of the Deep Impact extended mission. We have collected 845 observations from 1997, 121 from 1991 and 25 from 2004, and have added our own observations in 2005-2006. The main results of this investigation are: (a) of the order of 30 photometric parameters are measured and listed in the plots of this comet, over 20 of them new. Paper I (2005, Icarus 178, 493-516) gives the definitions of the parameters measured from the SLCs, although a brief description is included here. The turn on point of activity is −4.2±0.1 AU from the Sun, which corresponds to −400±40 d before perihelion. The total active time is TACTIVE=1904±43 d. For comparison the active time of comets 1P/Halley and 9P/Tempel 1 are TACTIVE=1992 and 1069 d. (b) 103P is a young dwarf comet, young because it has a photometric age P-AGE(1,1)=13±2 cy (comet years), and dwarf because its diameter is DEFFE=1.14±0.16 km. For comparison comets 1P/Halley and 9P/Tempel 1 have photometric ages P-AGE=7.1 cy and P-AGE=22 cy (Paper V) and diameters 9.8 and 5.5 km. (c) The nucleus is very active as can be deduced from the fact that the amplitude of the SLC is ASEC=10.8±0.1 mag in 1997. For comparison comets 1P and 9P have ASEC=10.8 and 9.0 mag. Thus this comet is as active as 1P/Halley, but much smaller. (d) This comet belongs to the class of spill-over comets defined in Paper VI. After remaining active up to aphelion, the comet spills-over its activity into the next orbit. (e) 103P also belongs to the family of comets that exhibit a break point in their SLC. Two linear laws change slope at a break point located at RBP=−1.20±0.1 AU before perihelion, and magnitude mBP=9.4±0.1. The slope of the first linear law after turn on is n=+9.44. This value can be compared with the slope of comets Hale-Bopp, 1P/Halley and 9P/Tempel 1: +10.3, +8.92 and +7.7, respectively. Since the SLC after turn on does not exhibit curvature in any of these comets, it is safe to conclude that sublimation is not controlled by water ice. The controlling substance may be CO or CO2 ice. CO2 has been detected spectroscopically. (f) The water budget of this comet is calculated. The comet expends 1.88×1010 kg of water per apparition vs 5.5×1011 kg for comet 1P/Halley and 1.28×1010 kg for comet 9P/Tempel 1. A new water-budget age is defined and it is found that WB-AGE=19 cy vs WB-AGE=0.65 cy for 1P/Halley and 28 cy for 9P. (g) The values of Afρ are calculated on two dates and we find dust production rates of 106 and 37 kg/s, decreasing with solar distance as expected. (h) The thickness of the layer lost per apparition is calculated, and we find Δr=34 m where r is the radius. Since the radius of this comet is r=570 m, it is found that at the present rate the comet may disappear in only 17 revolutions (∼109 yr). (i) By chance all comets visited by spacecraft have small photometric ages and thus are young objects. 103P follows the same trend. Thus it is expected that the surface morphology that will be found in future EPOXI images should be very similar to that of previous comets. It is suggested that any new mission to a comet should be made to a methuselah object.