The story of Comet 41P/Tuttle-Giacobini-Kresák is remarkable. It took nearly a century to be identified, known to produce bright flames, and when it PAST by Earth on April Fools Day in 2017, astronomers realized its rotation slowed considerably.

It’s not unusual for a comet’s speed and direction to change, but this is the most dramatic slowdown ever seen. Now, an astronomer is taking a closer look at Hubble Space Telescope images of 41P taken in December 2017, after it made its closest approach to the Sun. This new analysis suggests that the comet’s rotation is actually completely reversed.

“We see changes in the spin,” David Jewitt, author of the study posted on the preprint server arXiv on February 6, SPOKE the New York Times. “But not as big and quick.”

Small comet, big spin change

Astronomers around the world observed 41P when it flew by Earth nine years ago. A team from the University of Maryland later determined with its rotation rate dropping from one rotation every 20 hours in March to one every 46 hours in May.

“The previous record for a comet spindown went to 103P/Hartley 2, which slowed its rotation from 17 to 19 hours over 90 days,” said Dennis Bodewits, an associate research scientist in the UMD Department of Astronomy and lead author of the 2018 study. SAYS at that time. “In contrast, 41P rotates more than 10 times in just 60 days, so the scale and speed of this change is something we’ve never seen before.”

Recently, Jewitt’s Hubble data analysis sought to measure the size of 41P’s nucleus—its solid-ice core—and study how the comet’s lightcurve changed after it made its closest approach to the Sun.

Based on measurements of the comet’s light and non-gravitational acceleration, Jewitt determined that 41P’s nucleus is small, with a radius of about 0.3 miles (0.5 kilometers). Jewitt’s lightcurve analysis further revealed that the rotation of the nucleus likely reversed between the comet’s closest approach to the Sun in April 2017 and December 2017 as a result of “outgassing torque.”

When a comet approaches the Sun, solar heating causes the volatile ice in its nucleus to sublimate, or directly transition from a solid to a gaseous state. Outgassing—the release of this gas—doesn’t happen uniformly at the surface of the nucleus, and Jewitt’s research published in 2021 proposed that high-emission areas can have a strong enough torque to change a comet’s spin. But 41P marks the first time this phenomenon has been observed directly.

A potential answer to a comet mystery

When observing the solar system, astronomers find far fewer 41P-sized comets than models suggest they should, and Jewitt’s findings help explain why.

Sub-kilometer comets, or those less than 0.6 miles across, can be significantly modified by outgassing, according to Jewitt’s 2021 study. This was demonstrated by 41P in 2017, when its rotation reversed several months at closest approach to the Sun. This reinforces Jewitt’s belief that comets of this size can produce outgassing jets that cause a rapid increase in their rotation speed and cause them to be “blown away by their own rotation,” he told the NYT.

“The evidence is that comets don’t last very long,” he said. “There is another process that destroys comets, and I think it is rotation.”

Astronomers will get another chance to observe 41P when it does came the Sun again in 2028. It will be interesting to see if this comet undergoes another major transformation, offering a unique view of how small comets evolve over time.