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We present new lightcurve measurements of Comet 10P/Tempel 2 carried out withLowell Observatory's Discovery Channel Telescope in early 2013 when the cometwas at aphelion. These data represent some of the first science obtained withthis new 4.3-m facility. With Tempel 2 having been observed to exhibit a smallbut ongoing spin-down in its rotation period for over two decades, our primarygoals at this time were two-fold. First, to determine its current rotationperiod and compare it to that measured shortly after its most recent perihelionpassage in 2010, and second, to disentangle the spin-down from synodic effectsdue to the solar day and the Earth's orbital motion and to determine the senseof rotation, i.e. prograde or retrograde. At our midpoint of 2013 Feb 24, theobserved synodic period is 8.948+/-0.001 hr, exactly matching the predictedprograde rotation solution based on 2010 results, and yields a sidereal periodof the identical value due to the solar and Earth synodic components justcanceling out during the interval of the 2013 observations. The retrogradesolution is ruled out because the associated sidereal periods in 2010 and 2013are quite different even though we know that extremely little outgassing,needed to produce torques, occurred in this interval. With a definitive senseof rotation, the specific amounts of spin-down to the sidereal period could beassessed. The nominal values imply that the rate of spin-down has decreasedover time, consistent with the secular drop in water production since 1988. Ourdata also exhibited an unexpectedly small lightcurve amplitude which appears tobe associated with viewing from a large, negative sub-Earth latitude, and alightcurve shape deviating from a simple sinusoid implying a highly irregularlyshaped nucleus.

The Nucleus of Comet 10P/Tempel 2 in 2013 and Consequences Regarding Its Rotational State: Early Science from the Discovery Channel Telescope