This artist’s concept depicts the tiny comet 41P, a Jupiter-family comet. As it approached the sun, frozen gases began to sublimate and shoot material off into space. Scientists analyzing observations from NASA’s Hubble Space Telescope found that these jets slowed the comet’s spin and then caused it to rotate in the opposite direction. Image via NASA/ ESA/ CSA/ Ralf Crawford (STScI).
Comet 41P slowed its spinning and then reversed to spin in the opposite direction. It’s the first time this behavior has ever been observed in a comet.
As the comet neared the sun, frozen material warmed and turned to gas. This in turn created jets that slowed and then steered the spin in the opposite direction.
The tiny comet will likely fragment or disintegrate soon, according to the author of the study. Currently, the comet orbits the sun every 5.4 years.
NASA published this original story on March 26, 2026. Edits by EarthSky.
Tiny comet shows first-ever spin reversal
Astronomers using NASA’s Hubble Space Telescope have found evidence that the spinning of a small comet slowed and then reversed its direction of rotation. This offers a dramatic example of how volatile activity can affect the spin and physical evolution of small bodies in the solar system. It’s the first time researchers have observed evidence of a comet reversing its spin.
The object, comet 41P/Tuttle-Giacobini-Kresák, or 41P for short, likely originated in the Kuiper Belt. And it was flung into its current trajectory by Jupiter’s gravity. Now the comet visits 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 Lowell Discovery Telescope 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. That’s compared to the 46 to 60 hours measured by Swift. The simplest explanation, researchers said, is that the comet continued slowing until it almost stopped. Then, outgassing jets on its surface forced it to spin in the near-opposite direction.
On March 26, 2026, The Astronomical Journal published the peer-reviewed findings.
Small, temperamental nucleus
Hubble also constrains the size of the comet’s nucleus, measuring it at around 0.6 miles across (about a kilometer). That’s 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. Paper author David Jewitt of the University of California at Los Angeles said:
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.
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. Jewitt said:
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.
The tiny comet shows evidence of rapid evolution
The study also shows the comet’s overall activity has declined significantly since earlier returns. During its 2001 perihelion passage, 41P was unusually active for its size. By 2017, its gas production had decreased by roughly an order of magnitude.
This change suggests the comet’s surface may be evolving quickly, possibly as near-surface volatile materials become depleted or covered by insulating dust layers.
Most changes in comet structure occur over centuries or longer. The rapid rotational shifts observed in comet 41P provide a rare opportunity to witness evolutionary processes unfolding on a human timescale.
Modeling based on the measured torques and mass loss rates suggest that continued rotational changes could eventually lead to structural instability for comet 41P. If a comet spins too rapidly, centrifugal forces can overcome its weak gravity and strength. And that can potentially cause it to fragment or even disintegrate. Jewitt said:
I expect this nucleus will very quickly self-destruct.
Yet, comet 41P has likely occupied its present orbit for roughly 1,500 years.
Archival find
Hubble has been collecting imaging and spectroscopic data from across the cosmos for more than 35 years. And all of those observations are available in the Mikulski Archive for Space Telescopes. This archive is a central repository for data from more than a dozen astronomical missions, including Hubble.
Jewitt found these observations while browsing the archive, and realized they were yet-to-be analyzed.
By making NASA’s science data open to all, observations made years, or even decades ago, can be revisited to answer new scientific questions. In many cases, scientists continue to make discoveries not just with new observations, but by mining the archive built over decades of space exploration.
Bottom line: For the first time, Hubble observations reveal that a tiny comet has stopped spinning in one direction and has reversed to spin in the opposite direction.
Source: Reversal of Spin: Comet 41P/Tuttle–Giacobini–Kresak
Read more: When will this sungrazing comet get bright enough to see?
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