Hubble Space Telescope watches doomed comet reverse its spin

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The Hubble Space Telescope observed a spinning comet slowing its own rotation and then beginning to spin in the opposite direction. This is the first observation of its kind demonstrating that comets may be even more dynamic than we thought.

Comet 41P/Tuttle–Giacobini–Kresák is a Jupiter-family comet, that is to say it is a short-period comet (in orbit around the sun every 5.4 years) from Kuiper Belt before being caught by Jupiter’s gravity.

41P’s last close approach to the sun – known as perihelion – took place in September 2022, but it was the previous close approach in 2017 that was observed by the Hubble Space Telescopeas well as several other telescopes, including NASA’s Space Telescope Neil Gehrels Swift Observatory and the four-meter (13-foot) Lowell Discovery Telescope in Arizona.

However, the Hubble observations were not analyzed until David Jewitt, a planetary scientist at the University of California, Los Angeles, found the data in the Mikulski Archive for Space Telescopes, named after former Democratic U.S. Senator Barbara Mikulski, who has been a strong supporter of NASA.

Hubble data, combined with data from Swift and the Lowell Discovery Telescope, revealed something very strange about the comet. When Swift observed the comet in May 2017, it was rotating every 46 to 60 hours, about three times slower than in March 2017, when the Lowell Discovery Telescope observed it. This in itself was intriguing, but Hubble observations deepened the intrigue by showing that by December 2017 the comet’s rotation had accelerated again and now had a period of about 14 hours. What happened to restart the comet’s dizzying rotation?

Jewitt thinks that the degassing of the comet’s surface, which heated up during its passage at perihelion, brings it as close as possible to the sun. Earthis the cause. This heating caused volatile gases near the surface to expand and burst into jets, carrying comet dust with them.

“Gas jets escaping from the surface can act as small thrusters,” Jewitt said in a statement. statement. “If these jets are unevenly distributed, they can dramatically change the way a comet, especially a small one, spins.”

The comet’s core is only a kilometer across, which is too small for even Hubble to resolve, but its rotation speed can be measured from its light curve: how the light from the comet’s elongated core changes as it spins and alternates between showing us its longest and shortest sides. Because the comet’s core is relatively small, it is vulnerable to the torques, or twisting forces, produced by the jets. However, it was not possible to infer the direction of this rotation, whether clockwise or counterclockwise, from the observations.

Jewitt was further able to deduce that the rotation, whatever the initial direction, had reversed. The jets countered the comet’s initial rotation, causing the initial slowdown seen between the Lowell Discovery and Swift observations. These jets then continued to work against the rotation and eventually reversed it and caused the comet to spin rapidly in the other direction, which explains the Hubble observations.

“It’s like pushing a merry-go-round,” Jewitt said. “If it turns in one direction and you push against that direction, you can slow it down and reverse it.”

It’s rare to see a comet change so abruptly, and if we look back to Hubble’s observations of the comet in 2001, we can see that its overall perihelion activity has decreased since then by about an order of magnitude. Perhaps repeated perihelions – the comet is thought to have been in its current orbit for around 1,500 years – could begin to deplete its reserves of volatile ices. Or perhaps the dust released by the jets falls back onto the comet, covering these ices with an insulating layer that prevents the ices from being heated by the sun and sublimating as quickly.

However, Jewitt is skeptical that the 41P/Tuttle-Giacobini-Kresák project will last much longer. If changes in its rotation occur quickly, this will gradually make the comet unstable and the rapid rotation will result in centrifugal forces that will rotate the comet.

“I expect this core to self-destruct very quickly,” Jewitt said.

The results were published on March 26 in The astronomical journal.