Cometary outgassing can produce torques that change the spin state of the cometary nucleus, which in turn influences the evolution and lifetime of the comet1,2. If these torques increase the rate of rotation to the extent that centripetal forces exceed the material strength of the nucleus, the comet can fragment3. Torques that slow down the rotation can cause the spin state to become unstable, but if the torques persist the nucleus can eventually reorient itself and the rotation rate can increase again4. Simulations predict that most comets go through a short phase of rapid changes in spin state, after which changes occur gradually over longer times5. Here we report observations of comet 41P/Tuttle–Giacobini–Kresák during its close approach to Earth (0.142 astronomical units, approximately 21 million kilometres, on 1 April 2017) that reveal a rapid decrease in rotation rate. Between March and May 2017, the apparent rotation period of the nucleus increased from 20 hours to more than 46 hours—a rate of change of more than an order of magnitude larger than has hitherto been measured. This phenomenon must have been caused by the gas emission from the comet aligning in such a way that it produced an anomalously strong torque that slowed the spin rate of the nucleus. The behaviour of comet 41P/Tuttle–Giacobini–Kresák suggests that it is in a distinct evolutionary state and that its rotation may be approaching the point of instability.
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We thank M. Siegel and the Swift team for planning the observations of 41P. This research was supported by Swift Guest Investigator Program grant 1316125. We thank A. Thirouin, C. Trujillo and N. Moskovitz for observing and/or donating telescope time to acquire images used to determine rotation periods from morphology. We thank N. Eisner and D. Schleicher for sharing their preliminary results with us. We thank N. Samarasinha for calculating the ζ parameter for 41P and 67P. This work made use of the Discovery Channel Telescope at Lowell Observatory. Lowell is a private, non-profit institution dedicated to astrophysical research and public appreciation of astronomy and operates the DCT in partnership with Boston University, the University of Maryland, the University of Toledo, Northern Arizona University and Yale University. The Large Monolithic Imager was built by Lowell Observatory using funds provided by the National Science Foundation (AST-1005313). This work also made use of NASA’s Astrophysics Data System and of the JPL/Horizons ephemerides service, maintained by the JPL Solar System Dynamics group.