Illustration: NASA, ESA, CSA, Ralf Crawford (STScI)
A Cosmic First: Comet Reverses its Spin
Astronomers have long known that comets are unpredictable, but now they are beginning to understand just how dramatically these dirty iceballs can change. New research shows that Comet 41P/Tuttle-Giacobini-Kresák not only slowed its rotation during its 2017 visit to the inner solar system but eventually reversed its spin direction. This marks the first time scientists have ever documented a comet flipping the direction of its rotation, and observations made using several telescopes at Lowell Observatory were essential to uncovering the phenomenon.
The story begins in March 2017, when Lowell Observatory scientist Dave Schleicher led a study using Lowell’s 4.3-meter Discovery Channel Telescope (now known as the Lowell Discovery Telescope [LDT]), 1.1-meter John S. Hall Telescope, and a 0.8 robotic telescope, to measure the comet’s rotation period as it approached the Sun. These observations provided the baseline for understanding how the comet was behaving before solar heating intensified its activity. Without this early dataset, scientists would not have recognized how dramatically the comet’s spin was about to change.
Just two months later, scientists using the Neil Gehrels Swift Observatory observed the comet again and found that it was spinning three times more slowly than when measured with the Lowell Observatory telescopes. This dramatic slowdown over such a short period immediately suggested that powerful forces were acting on the comet’s tiny nucleus.
Then came observations using the Hubble Space Telescope in December 2017. Instead of continuing to slow, the comet was spinning rapidly again—but in the opposite direction. Its rotation period had sped up to about 14 hours, compared to the 46–60 hours measured earlier that year.
The simplest explanation is that jets of gas erupting from the comet’s surface acted like natural thrusters. As sunlight warmed the nucleus, frozen ices vaporized and escaped as jets. Because 41P’s nucleus is extremely small—only about 0.6 miles (1 km) across—these jets were powerful enough to slow the comet nearly to a stop and then push it into reverse.
“The stopping and reversing of the rotation of T-G-K was predicted, and we are gratified to having our prediction proved correct,” says Schleicher.
“Most comets of this size probably change their rotation on comparable or shorter timescales,” adds Lowell Observatory Percival Lowell Postdoctoral Fellow Qicheng Zhang. “They just tend to not pass close enough to Earth for these changes to be observable. In many cases, they’re just destroyed before we ever get a second look at the rotation.”
Comet 41P is changing in other ways as well. Its activity level in 2017 was ten times lower than during its 2001 passage, suggesting that its surface is evolving quickly as volatile ices deplete or become buried beneath dust.
Models indicate that if its rotation continues to shift, the comet could eventually become unstable and break apart. This makes 41P a valuable natural laboratory for studying how small bodies evolve, shed material, and sometimes destroy themselves.
The breakthrough was made possible by the combined strengths of ground-based and space-based observatories. The early 2017 measurements at Lowell Observatory captured the comet before its behavior changed, Swift documented the dramatic slowdown, and Hubble revealed the reversal. And archival data preserved the evidence for researchers to uncover years later. Together, these observations offer a rare look at a comet undergoing rapid evolution.