For the first time, we have captured a comet that changes its direction of rotation

A small comet appears to have changed its direction of rotation – the first evidence of such behavior for astronomers. Changes like this can help us learn about the interiors of comets, which could reveal information about the composition of the early solar system.

Comet 41P/Tuttle-Giacobini-Kresák, or simply 41P, measures about 1 kilometer in diameter and takes about 5.4 years to orbit the Sun. We can only see it when it visits the inner solar system and its trajectory happens to take it relatively close to Earth. It was last seen in 2017.

In March of that year, it was rotating at a rate of about one complete rotation every 20 hours. When astronomers observed it just two months later, it had slowed dramatically to one revolution every 46 to 60 hours. Now, David Jewitt at the University of California, Los Angeles, reanalyzed observations from the Hubble Space Telescope taken in December 2017 and found that the comet reaccelerated by one revolution every 14 hours.

The simplest explanation is that the comet’s spin rate slowed down until it eventually reached zero, at which point the comet started spinning in the opposite direction, picking up spin speed as it did so. This may be because sunlight caused the ice on the comet’s surface to sublimate into gas, which then acted as a jet. If this jet was fired in the opposite direction to the original direction of the comet’s rotation, it would slow the comet’s rotation rate and eventually send it spinning in the opposite direction.

“It is the first detected ‘rapid’ change in the direction of rotation of a celestial body,” he says Dmitry Vavilov at the University of Washington in Seattle. Most of the time, significant changes in any celestial body, even such a small comet, take decades or centuries.

“It will be quite interesting to watch 41P during its next appearance in late 2027/early 2028,” he says John Noonan at Auburn University in Alabama. “I wonder if these comets are more likely to break as well, due to stress. If 41P spins too fast, its main body or core will simply break apart.

“I expect this core to self-destruct very quickly,” Jewitt said declaration. In fact, it may have already happened. If so, this could present an excellent opportunity to observe the interior of a comet that froze during the formation of the solar system. Studying the composition of such ancient ice could not only provide us with valuable insights into the chemistry of the early solar system, but also serve as a benchmark for how that chemistry changed as the solar system matured.

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