In a new paper published here, the accomplished astronomer David Jewitt reported about an unprecedented behavior of the Jupiter-family comet 41P/Tuttle-Giacobini-Kresák. The report uses archival data collected by the Hubble Space Telescope on December 11–14, 2017, coincidentally about a month after the discovery of the interstellar object 1I/`Oumuamua. This comet, which likely originated in the Kuiper Belt and was flung into its current trajectory by Jupiter’s gravity, is now visiting the inner solar system every 5.4 years.
It is well known that the spin of cometary nuclei changes as a result of the rocket effect from torques induced by outgassing. The nucleus of 41P/Tuttle-Giacobini-Kresák exhibited dramatic rotational changes when it passed near the Sun in April 2017. Eight months later, the combination of Hubble imaging and measurement of non-gravitational acceleration implies a nucleus diameter of about a kilometer (± 200 meters). Systematic brightness variations are consistent with a rotation period of 0.60 (± 0.01) days, substantially different from periods measured earlier in 2017.
Data from NASA’s Neil Gehrels Swift Observatory in May 2017 implies that the object was spinning three times slower than in March 2017 when it was observed by the Discovery Channel Telescope at Lowell Observatory in Arizona. The Hubble images from December 2017 detected the comet spinning much faster again, with a period of approximately 14 hours, compared to the 46 to 60 hours measured by Swift. The simplest explanation is that the comet continued slowing until it almost stopped, and was then forced to spin in the near-opposite direction by outgassing jets from its surface — induced by solar illumination of surface ice. Jets of gas streaming off the sublimated pockets of ice can act like thrusters and if these jets are unevenly distributed, they can change the spin of the comet.
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