In a Scientific First, Hubble Captures a Doomed Comet Reversing Its Spin

A collection of photos had been sitting unexamined since 2017 somewhere in the enormous Mikulski Archive for Space Telescopes, a digital repository containing over thirty years of observations from Hubble and more than a dozen other missions. Photographs of a small, unremarkable comet traveling through the inner solar system were taken by one of the most advanced instruments ever created by humans. The images were then stored away while the scientific community moved on to other projects. While looking through the archive, UCLA planetary scientist David Jewitt discovered them. He gave them a close inspection. Then he became aware of what he was staring at.

For those who study small solar system bodies, the comet is referred to as 41P/Tuttle-Giacobini-Kresák, or just 41P. It is a Jupiter-family comet, which means that every 5.4 years, Jupiter’s gravity pulls it into a tighter orbit around the Sun from its original location in the Kuiper Belt, that far-off, frozen reservoir beyond Neptune. The nucleus is about three times the height of the Eiffel Tower and is only about 0.6 miles across. That’s particularly tiny for a comet, which is crucial to the plot.

Several telescopes tracked 41P during its closest approach to the Sun in 2017. In March 2017, it was observed and its rotation was recorded by the Lowell Discovery Telescope in Arizona. When NASA’s Neil Gehrels Swift Observatory observed it once more in May, a significant change had occurred: in just sixty days, the comet’s rotation had slowed by about three times. According to a 2018 NASA report, it was the comet’s most dramatic rotational slowdown to date. Another comet, 103P/Hartley 2, had previously been seen to slow down, but by a much smaller margin and over a period of ninety days rather than sixty. 41P had broken the previous record. It then seemed to pick up speed again. The rotation was once again rapid, lasting roughly 14 hours, according to Hubble photos taken in December 2017.

Before Jewitt put those three data points together into a cohesive picture, no one had completely figured out what they meant. The comet had not just decelerated and come back. After nearly coming to a stop, it began spinning in the exact opposite direction. As it was heated by the Sun, gas jets that emerged from its surface acted as uneven thrusters, pushing against the initial direction of rotation until the motion completely reversed. “It’s like pushing a merry-go-round,” Jewitt remarked. “If it’s turning in one direction, and then you push against that, you can slow it and reverse it.” The comet was spinning quickly in its new direction when the December Hubble photos were taken. Nearly nine years after the events it describes, the complete discovery was published in The Astronomical Journal on March 26, 2026.

Cometary scientists have hypothesized the mechanism underlying this, but they have never made direct observations. Heat from 41P’s approach to the Sun causes volatile ices close to the surface to sublimate, changing straight from solid to gas and escaping in jets that carry material and dust. The distribution of those jets within the nucleus is not uniform. Because some areas of the surface are more active than others, the spinning body experiences an uneven push that can alter its rotation over time. The torque needed to change 41P’s spin is well within what those jets can generate because its nucleus is so tiny and low in mass. “Jets of gas streaming off the surface can act like small thrusters,” Jewitt clarified. “If those jets are unevenly distributed, they can dramatically change how a comet, especially a small one, rotates.”

Additionally, the study discovered that 41P’s overall activity has drastically dropped since previous observations. It was exceptionally productive for its size when it passed the Sun in 2001. Gas production had decreased by about an order of magnitude by 2017. The near-surface volatile ices may have been depleted by repeated perihelion passages during its estimated 1,500 years in this orbit, or the sublimation process may have been slowed by dust released by the jets falling back and forming an insulating layer over them. Usually, comets change over centuries. According to Jewitt’s framing, witnessing 41P change so quickly is an uncommon window—the kind of thing that typically occurs too slowly for anyone to see in a single observational campaign.

The comet’s future looks less promising. According to modeling based on measured torques and mass loss rates, 41P may become structurally unstable if rotational changes continue. The weak gravity and internal strength holding the nucleus together will be overcome by centrifugal forces if it spins quickly enough, possibly causing fragmentation or total disintegration. “I expect this nucleus will very quickly self-destruct,” Jewitt stated. The timeline is ambiguous because “quickly” in cometary terms can still refer to decades or more. However, the course seems predetermined.

It’s difficult to ignore this discovery’s unique quality: it was derived from old data that had been patiently waiting for the right question to be asked, rather than from fresh observations. For decades, the Mikulski Archive has been gathering astronomical observations, and now is one of those times when mining what already exists proves to be just as fruitful as starting something new. The pictures were present. The narrative was finished. All that was needed was someone to look.