We’ve caught a comet switching its spin direction for the first time

A small comet seems to have switched the direction in which it is rotating – the first time astronomers have seen evidence of such behaviour. Changes like this may help us learn about the insides 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 kilometre across and takes around 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 that year, it was rotating at a rate of about one full spin every 20 hours. When astronomers observed it just two months later, it had slowed down dramatically to one spin every 46 to 60 hours. Now, David Jewitt at the University of California, Los Angeles, has reanalysed observations from the Hubble Space Telescope taken in December 2017 and found that the comet had sped up again to one spin every 14 hours or so.

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

“It is the first detected ‘fast’ change of the rotation direction for a celestial body,” says Dmitrii 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.

“Following 41P during its next apparition in late 2027/early 2028 will be quite interesting,” says John Noonan at Auburn University in Alabama. “I’d be keen to see if these comets are more likely to fracture as well, due to the stress.” If 41P spins too fast, its main body, or nucleus, will simply fall apart.

“I expect this nucleus will very quickly self-destruct,” said Jewitt in a statement. In fact, it may have already happened. If so, it could present an excellent opportunity to observe the insides of a comet that froze while the solar system was forming. Studying the composition of such ancient ice could not only give us precious insights into the chemical make-up of the early solar system, but also act as a benchmark to work out how that chemistry changed as the solar system matured.