We present new lightcurve measurements of Comet 10P/Tempel 2 carried out with
Lowell Observatory's Discovery Channel Telescope in early 2013 when the comet
was at aphelion. These data represent some of the first science obtained with
this new 4.3-m facility. With Tempel 2 having been observed to exhibit a small
but ongoing spin-down in its rotation period for over two decades, our primary
goals at this time were two-fold. First, to determine its current rotation
period and compare it to that measured shortly after its most recent perihelion
passage in 2010, and second, to disentangle the spin-down from synodic effects
due to the solar day and the Earth's orbital motion and to determine the sense
of rotation, i.e. prograde or retrograde. At our midpoint of 2013 Feb 24, the
observed synodic period is 8.948+/-0.001 hr, exactly matching the predicted
prograde rotation solution based on 2010 results, and yields a sidereal period
of the identical value due to the solar and Earth synodic components just
canceling out during the interval of the 2013 observations. The retrograde
solution is ruled out because the associated sidereal periods in 2010 and 2013
are quite different even though we know that extremely little outgassing,
needed to produce torques, occurred in this interval. With a definitive sense
of rotation, the specific amounts of spin-down to the sidereal period could be
assessed. The nominal values imply that the rate of spin-down has decreased
over time, consistent with the secular drop in water production since 1988. Our
data also exhibited an unexpectedly small lightcurve amplitude which appears to
be associated with viewing from a large, negative sub-Earth latitude, and a
lightcurve shape deviating from a simple sinusoid implying a highly irregularly
shaped nucleus.Comment: Accepted by AJ; 12 pages of text (pre-print style), 3 tables, 2
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