Hubble Witnesses First-Ever Comet Rotation Reversal

Comets are among the most ancient objects in our Solar System, yet they rarely exhibit significant changes over periods as short as months or years. Comet 41P/Tuttle-Giacobini-Kresák, however, has become a unique case: astronomers have observed its rotation not only slow down but completely reverse direction. This marks the first time such a phenomenon has been documented.

This discovery was made possible through various observations, including those from the Hubble Space Telescope. The comet, approximately 1 km wide, belongs to the Jupiter family of comets and completes an orbit around the Sun every 5.4 years. It likely formed in the frigid Kuiper Belt, a distant region at the Solar System's edge, before being gravitationally nudged inward by Jupiter.

During its close approach to the Sun in 2017, astronomers began to notice unusual behavior: the comet's rotation rate was rapidly changing. Initially, it appeared to be merely decelerating, but subsequent observations revealed a more extreme process: its rotation nearly ceased before resuming in the opposite direction. This study, published on March 26, 2026, in The Astronomical Journal, represents a rare example of a comet evolving significantly over a very short timeframe.

Why the Reversal?

The underlying reason for this behavior is linked to the comet's surface activity. As it approaches the Sun, the heat causes ices on its nucleus to evaporate. This process, known as sublimation, creates jets of gas and dust that are expelled into space.

These jets function like small "natural thrusters." If they are not distributed uniformly, they can exert force on the comet and modify its rotation. In the case of 41P, this is precisely what happened. Observations from the Neil Gehrels Swift Observatory had already indicated a rapid deceleration of the comet between March and May 2017, compared to data collected by the Lowell Observatory.

However, images captured by Hubble months later unveiled an even more astonishing change: the comet had resumed rotating more rapidly, but in the opposite direction. Its rotation period had shifted from tens of hours to approximately 14 hours. The simplest hypothesis is that the jets initially slowed the rotation, nearly bringing it to a halt, and then propelled it in the reverse direction. This is feasible because the nucleus is very small, making it more susceptible to such influences.

A Rapidly Changing Comet

Beyond its rotation, the comet's overall activity has also evolved over time. In 2001, during a solar close approach, 41P was exceptionally active for its size. By 2017, however, the amount of gas emitted had decreased by roughly tenfold. This suggests a rapid transformation of the comet's surface, possibly due to the depletion of volatile materials or their concealment beneath layers of dust.

Typically, such changes unfold over very long timescales, often hundreds or thousands of years. In the case of 41P, however, these transformations are occurring within just a few solar passages. This provides astronomers with a unique opportunity to directly observe a comet's evolution.

Models also suggest a potentially unstable future for the comet. Should its rotation accelerate excessively, 41P might lose its structural integrity. Its weak internal forces might prove insufficient to hold it together, potentially leading to fragmentation or complete disintegration.