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In early 2017, comet 41P/Tuttle-Giacobini-Kresák made its closest approach to the Sun, or perihelion, after 5.4 years.
As it did so, its rotation appeared to slow to a complete stop, before possibly starting to spin again in the opposite direction, according to astronomer David Jewitt of the University of California, Los Angeles.
The reversal of spin direction itself is not the surprising part; Changes in the spin of comets sometimes occur when these icy objects approach the Sun. What was surprising was how quickly and dramatically the reversal occurred.
“The previous record for a slowdown in a comet’s spin was held by 103P/Hartley 2, which slowed its rotation from 17 to 19 hours over 90 days,” said astronomer Dennis Bodewits of the University of Maryland, explaining the slowdown phase in 2018.
“In contrast, 41P saw a decline of more than 10-fold in just 60 days, so both the magnitude and pace of this change is something we have never seen before.”
The sequence of events is as follows. Observations made in March 2017 showed that 41P’s rotation period was around 20 hours. In May, rotations slow to more than double that duration, with one rotation every 53 hours or so.
However, as December approached, something very strange had happened. The comet’s rotation period shortened to 14.4 hours– a change, according to Jewitt, that is best explained if its rotation slowed to a complete stop around June 2017 and then changed direction.
