Hubble Observes First-Ever Spin Reversal in Small Comet 41P

In December 2017, astronomers examining images from the Hubble Space Telescope noticed something that had never been documented before: a comet spinning backward. The object was comet 41P, a small body that had passed close to the sun months earlier, and the reversal of its rotation offered researchers an unexpected window into how the violent forces of the inner solar system reshape small bodies in real time.

Comet 41P originates from the Kuiper Belt and was pulled into its current path by Jupiter's gravity. It swings through the inner solar system every 5.4 years, making it a regular visitor to the neighborhood of the sun. During its 2017 approach, something dramatic happened to its spin. Observations from NASA's Neil Gehrels Swift Observatory in May 2017 showed the comet rotating three times more slowly than it had been in March, when the Discovery Channel Telescope at Lowell Observatory in Arizona had tracked it. The slowdown was striking. But the Hubble images from later that year revealed something stranger still: by December, the comet was spinning much faster again, with a rotation period of roughly 14 hours—far quicker than the 46 to 60 hours Swift had measured just months before.

The explanation, researchers concluded, lay in the comet's surface. As 41P approached the sun, heat caused frozen ices embedded in its nucleus to sublimate, releasing gas and dust into space. These jets of material did not shoot out evenly from all directions. Instead, they erupted unevenly across the comet's surface, and because they were asymmetrically distributed, they acted like small thrusters pushing against the comet's rotation. David Jewitt of UCLA, one of the paper's authors, described the mechanism simply: the jets pushed against the comet's original direction of spin, gradually slowing it down. Because they kept pushing, they eventually forced it to rotate in the opposite direction entirely. "It's like pushing a merry-go-round," Jewitt said. "If it's turning in one direction, and then you push against that, you can slow it and reverse it."

What made this reversal possible was the comet's extreme smallness. Hubble's measurements showed the nucleus to be roughly 0.6 miles across—about a kilometer, or three times the height of the Eiffel Tower. A body this small has almost no gravitational hold on itself. The torque from uneven gas jets, which would be negligible on a larger comet, was enough to completely alter its spin. The finding, published in The Astronomical Journal, marked the first time astronomers had observed direct evidence of a comet reversing its direction of rotation.

But the spin reversal was not the only sign of rapid change. The comet's overall activity had declined sharply since its last major passage. During its 2001 perihelion, 41P had been unusually active for an object its size. By 2017, its gas production had dropped by roughly a factor of ten. This suggested that the comet's surface was evolving quickly, possibly as near-surface volatile materials became depleted or buried under layers of insulating dust. Most structural changes in comets unfold over centuries or longer. The rotational shifts in 41P were happening on a human timescale, offering astronomers a rare chance to watch evolutionary processes in the solar system unfold in real time.

The implications were sobering. Modeling based on the measured torques and mass loss rates suggested that if the comet's rotation continued to accelerate, centrifugal forces could eventually overcome its weak gravity and structural strength. A comet spinning too fast can tear itself apart. "I expect this nucleus will very quickly self-destruct," Jewitt said. Yet 41P has occupied its current orbit for roughly 1,500 years, suggesting it has survived many previous passages through the inner solar system. Whether it will survive the next one remains an open question.

The discovery itself came by accident. Jewitt was browsing the Mikulski Archive for Space Telescopes, a central repository holding decades of observations from Hubble and other missions, when he found the December 2017 images. They had been collected but never analyzed. By making NASA's data openly available, observations gathered years or even decades ago can be revisited to answer new questions. In this case, archival images from a space telescope launched in 1990 revealed a phenomenon that had never been seen before.