Astronomers report comet 41P/Tuttle–Giacobini–Kresák sped up, slowed, and appears to have flipped its rotation direction—revealing how sunlight-driven jets can reshape a comet’s interior.
A small comet has provided a rare new clue about what happens inside these icy leftovers—astronomers say it may have switched the direction it spins.
A rotation that changed twice
Comet 41P/Tuttle–Giacobini–Kresák. often shortened to 41P. is about a kilometer across and takes roughly 5.4 years to orbit the Sun.. Because it only swings close enough to be observed in the inner solar system on particular passes—and its path sometimes comes relatively near Earth—scientists mainly get snapshots during those apparitions.
The latest twist in its story centers on how fast it was rotating.. In March 2017, astronomers measured a rotation rate of about one full spin every 20 hours.. But observations just two months later showed a dramatic slowdown, with the comet taking about one spin every 46 to 60 hours.. Now. a reanalysis of archival Hubble Space Telescope data from December 2017 suggests the comet sped up again. reaching a rotation period of around 14 hours.
Why a comet might flip
The changing rotation does not just look like a gradual wobble; the simplest interpretation offered by the analysis is more extreme. The comet’s spin appears to have slowed toward a stop, and then accelerated in the opposite rotational direction.
In a scenario like that, the comet would effectively “reverse gears.” One plausible engine is sunlight.. As the comet approaches the Sun, ice near its surface warms and sublimes—turning directly from solid to gas.. That gas can escape through narrow active regions, forming jets that act like tiny thrusters.. If one of these jets fires in the direction that opposes the comet’s existing rotation. it can reduce the spin rate.. Continue long enough, and the rotation could stall, then build up again in reverse.
This kind of behavior matters because comets are not just passive snowballs. Their rotation is a diagnostic: it reflects how activity is distributed across the nucleus and how strongly gas production couples to the motion of the entire body.
What it could reveal about comet interiors
Scientists have long used comet spin as a window into their interiors, but this event may sharpen that picture. The best reason is timing. A clear, rapid shift in rotational state implies that the driving forces—most likely jets tied to localized heating—can reorganize quickly.
That opens the door to testing ideas about how fresh ice and dust are arranged inside a comet nucleus.. If jets can reverse rotation, they may also be reshaping the surface and near-surface layers where sublimation begins.. Over repeated passages. that could alter the comet’s shape and activity pattern. effectively changing how it behaves from one orbit to the next.
There is also a broader solar-system angle.. The composition of comets is tied to conditions from the early solar system. when temperatures and chemistry were vastly different than today.. If 41P truly undergoes such dramatic rotational transitions, it may be shedding material from particular regions or exposing deeper layers.. That would turn the next observing window into a chance to sample more of the comet’s “fossil” chemistry.
A comet flurry you can’t see every day also has a practical scientific payoff: it gives astronomers an opportunity to watch rapid dynamical change rather than only slow, decade-spanning evolution.
The next apparition will be a stress test
The key test comes when 41P returns for its next apparition, expected in late 2027 to early 2028. If the comet’s rotation speeds up again—or keeps behaving erratically—researchers will have a chance to connect spin changes with possible structural consequences.
When a comet spins faster, it experiences greater stress.. If the nucleus cannot hold together under that stress, it may fracture or break apart.. That matters not only for understanding the comet’s lifetime. but also for how comets distribute material into space—feeding meteor streams and contributing to the dust and gas populations astronomers track.
Misryoum will be watching for what changes next: whether the rotation direction stabilizes. whether the spin period continues to swing. and whether the nucleus shows signs of fragmentation.. For now. 41P stands out as a rare case where rotation—normally persistent on human timescales—may have turned. giving scientists a moving target and a powerful way to probe what drives comet activity.
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