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.
The authors declare no competing financial interests.
Reviewer Information Nature thanks B. E. A. Mueller and the other anonymous reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
Production rates were derived from hydrogen Lyman-α emission observed by the SWAN instrument on board the SOHO spacecraft15 in 2001 (black circles) and 2006 (red triangles). For the SWAN data, 1σ stochastic errors are shown; systematic uncertainties are at the 30% level15. We used Swift/UVOT observations of hydroxyl (OH) emission to determine the water production rate in 2017 (blue diamond). For the Swift data, the error bars represent the systematic uncertainty. The comet had two 4-mag outbursts in optical wavelengths just before its perihelion in 200122; these are evident as peaks at approximately 35 and 15 days before perihelion.
Absolute magnitudes based on Swift/UVOT photometry (black circles) are corrected for different relationships (characterized by A) between the activity of the comet and its distance to the Sun (see Methods). An increase in A corresponds to an increase in the rotation period that is needed to phase the overlapping sine curve segment (red triangles). Top, A = 0, period = 46 h; middle, A = 28, period = 57 h; bottom, A = 35, period = 60 h. Error bars indicate 1σ stochastic uncertainties.
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Bodewits, D., Farnham, T., Kelley, M. et al. A rapid decrease in the rotation rate of comet 41P/Tuttle–Giacobini–Kresák. Nature 553, 186–188 (2018). https://doi.org/10.1038/nature25150