Comet spins backward near sun, leaving scientists baffled

Comet 41P/Tuttle-Giacobini-Kresák surprised astronomers by doing something few small solar system bodies have been observed to do: it essentially stopped spinning and then began rotating in the opposite direction. The dramatic twist in its behavior was tracked by multiple space telescopes and has opened fresh questions about how comet activity—jets of vapor and dust driven by sunlight—can alter an object’s rotation so dramatically.

Scientists pieced together the event from observations taken during and after the comet’s 2017 close approach to the Sun. What began as a gradual slowdown accelerated into a near-complete halt and then a reversal, a sequence that challenges expectations about rotational changes in these icy relics of the early solar system.

What telescopes revealed about 41P’s unusual spin

NASA’s Swift spacecraft first flagged oddities in the comet’s rotation, and follow-up imaging by the Hubble Space Telescope provided the detail needed to measure how much and how fast the motion changed. Researchers tracked the comet’s rotation through light variations and direct imaging of the active nucleus.

• Early 2017: Observations recorded a spin period on the order of tens of hours.
• March–May 2017: The rotation slowed dramatically, increasing from roughly 20 hours to about 46 hours.
• By December 2017: Data indicated the nucleus was rotating again but in the opposite direction, with a period near 14 hours.

UCLA astronomer David Jewitt and colleagues analyzed Hubble’s images to reconstruct the sequence. The reversal wasn’t a small wobble—it was a large-scale change in angular momentum that required significant torque on the nucleus.

How comet jets can act like rocket thrusters

Comets are mixtures of rock, dust and volatile ices. When they approach the Sun, heat causes surface ices to sublimate and escape as gas, sometimes carrying dust with them. Those escaping materials can produce localized jets that push back on the nucleus.

Mechanics behind rotation changes

• Asymmetric outgassing produces uneven forces on the nucleus.
• If jets are concentrated in particular regions, they can apply sustained torque over weeks or months.
• Small nuclei are especially susceptible because less mass makes it easier to change spin rate or axis.

Jets don’t need to be continuous or uniform—sporadic bursts from freshly exposed ice or seasonal illumination can produce the cumulative effect needed to first slow a rotation, then halt it, and finally reverse direction. Most cometary spin variations are measured in minutes or hours; 41P’s change by tens of hours is exceptional.

Why the 41P flip matters for comet science and population studies

Large rotational jumps like 41P’s offer a rare window into the interplay between comet structure, activity, and longevity. Astronomers suspect such processes could influence how comets evolve or break apart.

• Spin-up from jets may fragment weak nuclei, creating more small comet fragments than expected.
• Conversely, spin-down episodes could temporarily stabilize an object, altering patterns of surface erosion.
• Understanding these dynamics helps refine models for comet lifetimes and for predicting when a comet might shed material or break up entirely.

One emerging idea is that many small comets observed today might be the products of prior objects that were spun to pieces by their own outgassing. If true, rotational dynamics become a key factor in the lifecycle of cometary bodies.

What astronomers will be watching in coming years

41P will return to the inner solar system in 2028, offering another opportunity to monitor its rotation and activity with improved instruments and broader surveys. Facilities coming online—most notably the Vera C. Rubin Observatory—will provide continuous, deep sky monitoring capable of catching rotational shifts and fragmentation events across many comets.

• Planned follow-ups will combine space telescopes and ground-based networks to measure nucleus rotation and jet behavior.
• Large sky surveys will help build a statistical picture: how common are extreme spin changes, and how often do they lead to breakup?
• Detailed studies of 41P and similar objects can help calibrate models of torques from asymmetric outgassing.

Keeping an eye on returning comets will let astronomers observe whether 41P repeats its dramatic behavior, settles into a new rotational state, or shows signs of progressive weakening or fragmentation. Continuous monitoring also increases the chance of catching other comets in the act of spinning up or down, which is essential for understanding how these small worlds die—or transform—under the Sun’s influence.

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William Anderson

William Anderson is a multimedia producer specializing in videos, podcasts, and interactive galleries. With five years of immersive content creation, he turns information into a rich audio‑visual experience. His storytelling skills draw you directly into the heart of every story, on any platform.