Hubble Catches a Comet Spinning Backward for the First Time

Somewhere out past Mars, a lump of rock and ice barely a kilometre wide is quietly spinning itself to death. Comet 41P/Tuttle-Giacobini-Kresák — 41P to astronomers — did something no comet has done before: it slowed almost to a complete stop, then reversed direction, driven around by its own escaping gases. The finding, published March 26 in The Astronomical Journal, came from a re-analysis of archival data from NASA's Hubble Space Telescope by astronomer David Jewitt of the University of California, Los Angeles.

A Comet That Surprised Everyone

Comet 41P is what planetary scientists call a Jupiter-family comet — a Kuiper Belt refugee whose orbit was captured and reshaped by Jupiter's gravity. It swings through the inner solar system every 5.4 years. Nothing unusual about that. What happened during its 2017 pass around the Sun, though, was another matter. Observations in March, made at Lowell Observatory's Discovery Channel Telescope in Arizona, showed it spinning with a period of around 20 hours. By May, data from NASA's Neil Gehrels Swift Observatory told a different story: the spin had slowed to somewhere between 46 and 60 hours — a factor-of-three drop in two months. Scientists noted it, filed it away, and moved on. Then Jewitt went looking through the Hubble archive.

How Scientists Tracked the Slowdown

Jewitt found Hubble images of 41P taken in December 2017, months after the Swift observations. Nobody had analysed them with the spin question in mind. When he did, the result was startling: the comet was spinning again — fast — with a rotation period of roughly 14 hours. That's quicker than the original March rate, and in the opposite direction. The most straightforward explanation is that the comet kept decelerating after the Swift data was collected, crept close to a standstill, and was then wound back up the other way by persistent gas jets pushing against its former spin. Call it a celestial merry-go-round getting pushed from the wrong side until it reverses.

Why Did a Comet Suddenly Spin the Other Way?

Size is almost everything here. At roughly a kilometre across — about three times the height of the Eiffel Tower — 41P's nucleus is unusually small, and small bodies are easy to torque. Gas jets venting unevenly from the surface act like tiny rocket thrusters, and on a body this light, they don't need to be powerful to have a dramatic effect. The jets that slowed 41P's original spin kept firing after it nearly stopped, and because angular momentum doesn't care about intention, they wound it back the other direction. Jewitt's analogy is blunt and effective: push a merry-go-round against its spin long enough and you'll eventually reverse it. What makes 41P the first confirmed case isn't exotic physics — it's that catching a comet in this precise moment of near-zero rotation requires both luck and the right archive.

What This Tells Us About Comet Lifespans

The spin reversal is striking on its own. What it implies about 41P's future is darker. The study found that the comet's overall activity has dropped sharply since its 2001 perihelion — gas production fell by roughly an order of magnitude, suggesting near-surface ice is running out or being buried under insulating dust. Comets are supposed to evolve over centuries. Comet 41P appears to be changing on a timescale humans can actually watch. If outgassing continues to torque the nucleus, the spin rate could rise beyond what its weak gravity can handle, pulling the body apart — a process called rotational breakup. Jewitt put it plainly: he expects the nucleus to self-destruct. That said, 41P has held its current orbit for around 1,500 years, so "quickly" here may still mean decades to come.

An Archive Discovery — and What Comes Next

Jewitt didn't get new telescope time to make this finding. He browsed the Mikulski Archive for Space Telescopes — the public repository holding over three decades of Hubble data — and found images sitting unanalysed. The spin reversal of 41P was hiding in plain sight for years. Open data made the discovery possible; one researcher with the right question made it happen. What remains uncertain is how common spin reversals are among small comets. The event requires catching a body at the precise moment it nearly stops, and most short-period comets aren't monitored continuously enough to catch that window. Dedicated monitoring campaigns targeting small Jupiter-family comets could reveal whether 41P is an outlier or just the first one anyone noticed mid-reversal.

• Gas jets steer comets — outgassing isn't just a visual spectacle; on small nuclei, it exerts enough force to reverse the direction of rotation entirely.
• Archive data still surprises — this discovery required no new observations, only a researcher willing to revisit Hubble images collected eight years ago with a fresh question in mind.
• 41P may be near its end — declining gas output and accelerating rotational changes point toward eventual breakup, making this comet a rare live case study in small-body destruction.

"I expect this nucleus will very quickly self-destruct." — David Jewitt, University of California Los Angeles · The Astronomical Journal, 2026.