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We present results from our 47-night imaging campaign of Comet41P/Tuttle-Giacobini-Kresak conducted from Lowell Observatory between 2017February 16 and July 2. Coma morphology revealed gas jets, whose appearance andmotion as a function of time yielded the rotation period and other properties.All narrowband CN images exhibited either one or two jets; one jet appeared asa partial face-on spiral with clockwise rotation while the second jet evolvedfrom a side-on corkscrew, through face-on, and finally corkscrew again, withonly a slow evolution throughout the apparition due to progressive viewinggeometry changes. A total of 78 period determinations were made over a 7-weekinterval, yielding a smooth and accelerating rotation period starting at 24 hr(March 21&22) and passing 48 hr on April 28. While this is by far the fastestrate of change ever measured for a comet nucleus, the torque required isreadily within what can exist given likely properties of the nucleus. If thetorque remained constant, we estimate that the nucleus could have stoppedrotating and/or began to tumble as soon as only two months followingperihelion, and will certainly reach this stage by early in the nextapparition. Working backwards in time, Tuttle-Giacobini-Kresak would have beenrotating near its rotational break-up velocity 3-4 orbits earlier, suggestingthat its extreme 7-magnitude outburst observed in 2001 might have been causedby a partial fragmentation at that time, as might the pair of 1973 8-magnitudeoutbursts if there had been an earlier spin-down and spin-up cycle.

Gas Jet Morphology and the Very Rapidly Increasing Rotation Period of Comet 41P/Tuttle-Giacobini-Kresak