Detected: First Ever Spin Reversal of Comet 41P
In a first, astronomers have found evidence of a comet reversing its spin
Washington: In a first, astronomers have found evidence of a comet reversing its spin.
The NASA astronomers using Hubble Space Telescope detected that the spinning of a small comet slowed and then reversed its direction of rotation.
The finding offers an example of how volatile activity can affect the spin and physical evolution of small bodies in the solar system.
The object, comet 41P/Tuttle-Giacobini-Kresák, or 41P for short, likely originated in the Kuiper Belt, and was flung into its current trajectory by Jupiter’s gravity, now visiting the inner solar system every 5.4 years.
After its 2017 close passage around the Sun, scientists found that comet 41P experienced a dramatic slowdown in its rotation. Data from NASA’s Neil Gehrels Swift Observatory in May 2017 showed the object was spinning three times more slowly than it had in March 2017 when it was observed by the Discovery Channel Telescope at Lowell Observatory in Arizona.
A new analysis of follow-up Hubble observations has shown the spin of this comet took an even more unusual turn. 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, NASA said in a blog post.
The comet continued slowing until it almost stopped, and was then forced to spin in the near-opposite direction by outgassing jets on its surface, the researchers explained in a detailed paper published in The Astronomical Journal Thursday March 26, 2026.
Why size matters?
Hubble also constrains the size of the comet’s nucleus, measuring it at around 0.6 miles across (about a kilometer), or about three times the height of the Eiffel Tower.
This is especially small for a comet, making it easy to torque, or twist.
As a comet approaches the Sun, heat causes frozen ices to sublimate, venting material into space.
“Jets of gas streaming off the surface can act like small thrusters. If those jets are unevenly distributed, they can dramatically change how a comet, especially a small one, rotates”, said paper author David Jewitt of the University of California at Los Angeles.
The comet was originally spinning in one direction, but gas jets pushing against that motion gradually slowed it down. Because the jets kept pushing, they ultimately caused the comet to start rotating in the opposite direction.
“It’s like pushing a merry-go-round. If it’s turning in one direction, and then you push against that, you can slow it and reverse it”, Jewitt said.
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