Tiny Comet’s Spin Reversal Caught on Camera | IFLScience

For the first time, astronomers have observed a comet changing the direction of its spin – a feat previously thought impossible. The small comet, designated 41P/Tuttle-Giacobini-Kresák (or 41P for short), exhibited a dramatic slowdown in rotation followed by a complete reversal, offering a new understanding of how these icy bodies behave as they approach the sun. The discovery, made using NASA’s Hubble Space Telescope, highlights the powerful influence of outgassing on the physical evolution of comets.

How a Comet Can Reverse Its Spin

Comets aren’t solid, inert rocks. They’re often described as “dirty snowballs,” composed of ice, dust, and rock. As a comet nears the sun, solar radiation causes the ices to sublimate – transitioning directly from a solid to a gas. This process, known as outgassing, creates jets of gas and dust that erupt from the comet’s surface. These jets aren’t uniform; they emanate from different points and with varying force. It’s these uneven jets that exert a torque, or twisting force, on the comet, influencing its rotation.

Previously, scientists understood that outgassing could slow a comet’s spin. However, the observation of 41P reversing its spin is a significant departure from that understanding. Researchers believe that the comet slowed to almost a complete stop, and then the asymmetrical outgassing jets effectively “pushed” it back into rotation, but in the opposite direction. This is akin to gently tapping a spinning top to change its direction, but on a cosmic scale. The initial slowdown was first detected in 2017 by NASA’s Neil Gehrels Swift Observatory, which showed the comet spinning three times more slowly than it had been earlier that year, as observed by the Discovery Channel Telescope at Lowell Observatory in Arizona. NASA details the process in a recent article.

41P: A Kuiper Belt Visitor

Comet 41P is a Jupiter-family comet, meaning its orbit is influenced by Jupiter’s gravity. It’s believed to have originated in the Kuiper Belt, a region beyond Neptune populated by icy bodies. Jupiter’s gravitational pull flung 41P into its current orbit, bringing it into the inner solar system every 5.4 years. This relatively short orbital period allows for frequent observation and study. The comet is small, making it a particularly interesting case study for understanding the dynamics of smaller cometary nuclei.

Hubble’s Role and the Data Analysis

While the initial slowdown was observed by Swift, it was Hubble that provided the crucial data confirming the spin reversal. Hubble images taken in December 2017 revealed that 41P was spinning much faster again, with a period of approximately 14 hours, compared to the 46 to 60 hours measured by Swift earlier in the year. This change was unexpected and prompted a deeper analysis of the Hubble observations. The research detailing these findings was published Thursday in The Astronomical Journal.

Implications for Cometary Science

This discovery has significant implications for our understanding of cometary evolution. It demonstrates that the spin of a comet is not a fixed property, but rather a dynamic characteristic that can be significantly altered by outgassing. This understanding is crucial for modeling the behavior of comets and predicting their future trajectories. It also suggests that the internal structure of comets may be more complex than previously thought, with variations in composition and density influencing the location and strength of outgassing jets.

The reversal of spin also impacts how comets shed material. The spin rate influences the distribution of dust and gas released during sublimation, which in turn affects the comet’s appearance and its interaction with the solar wind. Understanding these processes is essential for interpreting observations of comets and for unraveling the history of the solar system.

Limitations and Future Research

While the Hubble data provides strong evidence for the spin reversal, it’s significant to acknowledge the limitations of the observations. Determining the precise rotational period of a comet is challenging, as the nucleus is often obscured by a surrounding coma – a hazy atmosphere created by outgassing. The measurements rely on analyzing the brightness variations of the nucleus as it rotates, and these variations can be subtle and difficult to interpret.

Further observations are needed to confirm these findings and to investigate the underlying mechanisms driving the spin reversal. Future missions to comets, such as the European Space Agency’s Comet Interceptor, will provide valuable data on the composition and structure of cometary nuclei, helping to refine our understanding of these fascinating objects. The Space Telescope Science Institute provides additional details on the research and future observation plans.

What Comes Next: Continued Monitoring and Modeling

The research team plans to continue monitoring 41P as it approaches the sun in future orbits. This will allow them to track any further changes in its spin and to refine their models of the outgassing process. The data collected will also be used to improve our understanding of the internal structure of comets and the factors that influence their evolution. The team will also be working to develop more sophisticated computer simulations that can accurately model the complex interplay between outgassing, rotation, and the comet’s internal structure. These simulations will be crucial for predicting the behavior of comets and for interpreting observations from future missions.