Comet 41P
© Chris Schur/Schurs Astrophotography
Comet 41P/Tuttle-Giacobini-Kresák glides beneath the galaxy NGC 3198 on March 14, 2017, two weeks before the object's closest approach to Earth.
National Harbor, Md. - A small comet broke a rotation-speed record in a big way: New work reveals that an icy rock known as 41P dramatically slowed its spin at an unprecedented rate in 2017, spinning down at about 10 times the pace of the next-ranked comet.

This comet, whose full name is 41P/Tuttle-Giacobini-Kresák, experienced "the largest but also the fastest change that has ever been seen in a comet rotation," said Dennis Bodewits, an associate research scientist at the University of Maryland (UMD) in College Park.

Bodewits presented his team's findings Wednesday (Jan. 10) during a press conference held here at the 231st meeting of the American Astronomical Society.

This slowdown could cause the comet to break up or change direction, Bodewits said, and because it circles the sun often - every 5.4 years - our celestial neighbor may offer tremendous insight into the evolution of these icy rocks. The comet's next approach to Earth will happen in 2022.

Scientists took advantage of the comet's closest-known flyby of Earth, which occurred on April 1, 2017, to study this icy visitor, which was first recorded in 1858. The Swift spacecraft, which was publicly renamed the Neil Gehrels Swift Observatory a few short moments after Bodewits' presentation, played a crucial role in observing the slowdown of Comet 41P.

The comet is thought to have entered the inner solar system from the Kuiper Belt, a stretch of icy objects circling the solar system just beyond the orbit of Neptune. Comet 41P is small, estimated to be less than 0.9 miles (1.4 kilometers) across, and is one of the 20 smallest rocks in a family of comets whose orbits are controlled by Jupiter, according to a statement by NASA.

Like anything made of ice, the surfaces of comets like 41P begin to evaporate as the objects approach something very warm - in this case, the sun. The evaporating material forms a cloud of gas, causing more material, like dust, to jettison off the comet's surface. By observing these jets, scientists can track how quickly - or how slowly, in this case - a comet is spinning around its axis, according to Bodewits.

Fortunately, the Neil Gehrels Swift Observatory is ideally suited to watch these jets, NASA officials said in the statement. The observatory can use its Ultraviolet/Optical Telescope (UVOT) instrument to view the ultraviolet light emitted by molecules called hydroxyls, which are created as sunlight interacts with the gas.
Comet 41P
© Dennis Bodewits et al./Nature
The rotating jets in the gas cloud of Comet 41P/Tuttle-Giacobini-Kresák.
Before investigating with Swift, the team used the Discovery Channel Telescope at Lowell Observatory in Arizona to determine that Comet 41P rotated completely around its axis in about 20 hours.

This helped the team make the most of its Swift observation time, Bodewits said. Keeping an eye on a comet that has a 20-hour rotation speed is tricky unless you come up with a clever technique, because Earth takes about the same time to rotate, he said. To get a full sense of the comet's rotation within schedule constraints, and to avoid seeing the same sides of the comet experience day and night, observations had to be stopped and started at uneven times.

"As you may know, you can't just ask for two weeks of uninterrupted space telescope time, so you need to come up with a plan," Bodewits said during the conference.

The researchers began with a 12-hour observation, stopped for 6 hours, then watched the comet for 12 hours again, then stopped for 9 hours to break that repetition, and then again looked at comet 41P for 12 hours. By doing this for a few days - from May 6-8, 2017 - the researchers figured they would get at least some repetition of the comet's rotation cycles, and so, they would get as accurate a read as possible.

What they found was surprising: In a few short weeks, Comet 41P had gone from a 20-hour rotation speed to one in the range of 46-60 hours. "If the torques [rotational forces] continued acting after the May observations, 41P's rotation period could have slowed to 100 hours or more by now," said Tony Farnham, a principal research scientist at UMD.

"This comet cannot be in a stable state," Bodewits said during the conference. "If you slow down the rotation of it a lot, it becomes easier and easier to completely change the rotation of the comet. Think of a top. At the end [of a spin], when the top no longer has a gyroscoping effect, or it's rotating very slowly, it starts to wobble because other effects can easily change it. That's what we think is going to happen to this comet."

In addition to offering valuable data for this research, the Swift spacecraft got recognition during the same press conference for making some other important discoveries as well as its new name, which came from its late principal investigator.

"The Swift observations were critical in understanding the progenitors of gamma-ray bursts and the two-known classes of gamma-ray bursts," Paul Hertz, Director of Astrophysics in the Science Mission Directorate at NASA, said during the conference. "Swift confirmed that the long gamma-ray bursts represented the birth cries of a black hole from the collapse of a massive star. And Swift pinpointed, for the first time, the location of short gamma-ray bursts which were recently confirmed to be merging neutron-stars through the detection of the gravitational waves."

The findings on Comet 41P/Tuttle-Giacobini-Kresák were published on Thursday (Jan. 11) in the journal Nature.