Observations with NASA’s Swift Space Telescope have revealed an abrupt, unprecedented slowdown in the rotation of a comet.
Images taken in May of 2017 revealed that comet 41P / Tuttle-Giacobini-Kresák—41P for short—revolved three times slower than in March when it was observed by the Discovery Channel Telescope at the Lowell Observatory in Arizona.
This deceleration is the most spectacular change in the rotation of a comet ever seen, and scientists aren’t really sure what is going on.
According to astronomers, comet 41P orbits our Sun every 5.4 years.
When a comet approaches the Sun, the increase in heating causes the ice on its surface to change directly to a gas, producing jets that throw particles of dust and frozen grains into space.
“The previous record for a comet spindown went to 103P/Hartley 2, which slowed its rotation from 17 to 19 hours over 90 days,” said Dennis Bodewits, an associate research scientist at the University of Maryland (UMD) in College Park who presented the findings Wednesday, Jan. 10, at the American Astronomical Society (AAS) meeting in Washington. “By contrast, 41P spun down by more than 10 times as much in just 60 days, so both the extent and the rate of this change is something we’ve never seen before.”
Scientists note that this material forms an extended atmosphere, called a coma. Ground-based observations established the initial rotation period of 41P in approximately 20 hours at the beginning of March 2017 and detected its deceleration later in the same month.
The comet zipped at a distance of 21.2 million kilometers from Earth on April 1, and eight days later it made its closest approach to the Sun.
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Swift’s Ultraviolet / optical telescope monitored the comet from May 7 to May 9, revealing variations in brightness associated with material recently expelled to coma.
These slow changes indicated that the 41P rotation period had more than doubled, between 46 and 60 hours.
Estimates of 41P water production, along with small body size, suggest that more than half of its surface contains jets activated by sunlight.
That’s a much larger fraction of active jets than in most comets.
Astronomers suspect that these active areas are favorably oriented to produce pairs that slow down the 41P spin.
“We suspect that the jets from the active areas are oriented in a favorable way to produce the torques that slowed 41P’s spin,” said Tony Farnham, a principal research scientist at UMD. “If the torques continued acting after the May observations, 41P’s rotation period could have slowed to 100 hours or more by now.”
A slow turn can make the comet’s rotation unstable, allowing it to start falling without a fixed axis of rotation.
This would produce a dramatic change in the seasonal warming of the comet and could lead to future outbreaks of activity, NASA reports.