The story of Comet 41P/Tuttle-Giacobini-Kresák is an unusual one. It took nearly a century to identify, has been known to produce bright flares, and when it passed by Earth on April Fools Day in 2017, astronomers realized its spin had slowed down significantly.
It’s not unusual for the speed and direction of a comet’s spin to change, but this was the most dramatic slowdown ever seen. Now, an astronomer has taken a closer look at Hubble Space Telescope images of 41P taken in December 2017, after it made its closest approach to the Sun. This new analysis suggests that the comet’s spin actually completely reversed direction.
“We’ve seen changes in spin,” David Jewitt, author of the study posted to the preprint server arXiv on February 6, told the New York Times. “But not this big and so quick.”
Little comet, big spin change
Astronomers around the world observed 41P when it flew by Earth nine years ago. One team from the University of Maryland later determined that the rate of its spin plummeted from one rotation every 20 hours in March to one every 46 hours by May.
“The previous record for a comet spindown went to 103P/Hartley 2, which slowed its rotation from 17 to 19 hours over 90 days,” Dennis Bodewits, an associate research scientist in the UMD Department of Astronomy and lead author of the 2018 study, said at the time. “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.”
More recently, Jewitt’s Hubble data analysis sought to measure the size of 41P’s nucleus—its solid-ice core—and study how the comet’s lightcurve changed after it made its closest approach to the Sun.
Based on measurements of the comet’s brightness and non-gravitational acceleration, Jewitt determined that 41P’s nucleus is tiny, with a radius of roughly 0.3 miles (0.5 kilometers). Jewitt’s lightcurve analysis further revealed that the spin of the nucleus likely reversed between the comet’s closest approach to the Sun in April 2017 and December 2017 as a result of “outgassing torque.”
When a comet approaches the Sun, solar heating causes volatile ice within its nucleus to sublimate, or transition directly from a solid to a gaseous state. Outgassing—the release of this gas—does not occur evenly across the surface of the nucleus, and research published by Jewitt in 2021 suggested that areas of heightened venting could exert torque strong enough to change a comet’s spin. But 41P marks the first time this phenomenon has been directly observed.
A potential answer to a cometary mystery
When observing the solar system, astronomers see fewer 41P-sized comets than models suggest they should, and Jewitt’s findings could help explain why.
Sub-kilometer comets, or those less than 0.6 miles wide, can have their spins dramatically altered by outgassing, according to Jewitt’s 2021 study. 41P demonstrated this in 2017, when its rotation reversed within a few months of its closest approach to the Sun. This has strengthened Jewitt’s belief that comets of this size can produce outgassing jets that trigger rapid increases in their rate of rotation and cause them to be “blown to bits by their own spin,” he told the NYT.
“The evidence is that comets just don’t live that long,” he said. “There’s some other process that destroys the comets, and I think it’s rotation.”
Astronomers will get another chance to observe 41P when it approaches the Sun again in 2028. It will be fascinating to see whether this comet has undergone another major change, offering a rare glimpse into how tiny comets evolve over time.
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