Distant comet cracks into two halves after being heated by the sun

A fragmentation event has given astronomers a new way to study a comet from the outer reaches of the solar system.

Comet C/2018 F4 split in two and now the halves are moving away from each other
Hui et al.

A comet from the distant reaches of our solar system has been observed splitting in two as it made its way past the sun.

Comet C/2018 F4, initially discovered in 2018, was seen by amateur astronomers breaking into two pieces in September 2020, but what precisely happened was unknown. Man-To Hui at the Macau University of Science and Technology in China and his colleagues have now taken a closer look using several telescopes around the world.

They show that the comet’s solid bulk, or its nucleus, may initially have been similar to that of Comet 67P, famously visited by ESA’s Rosetta spacecraft in 2014, with two “lobes” connected by a neck.

C/2018 F4 appears to have split at its neck as it approached the centre of the solar system, reaching a closest point of about three times Earth’s distance from the sun before heading out again on a so-called long-period orbit, in this case one that takes some 300,000 years.

Heating of the surface during this part of its journey may have ejected material that “caused it to spin up”, says team member Michael Kelley at the University of Maryland. “There’s a point at which it is rotating so quickly, the two main components can’t stick together any more, so they separate.”

The two halves of the comet, each up to about 4 kilometres across, drifted apart at a rate of about 3 metres per second and are continuing to separate. They are now about 400,000 kilometres away from each other, roughly the distance from Earth to the moon. “Future astronomers may not even know they were related,” says Kelley.

More than 40 comet fragmentation events have been observed over the past 150 years. The C/2018 F4 event is somewhat unusual, however, in that the comet involved originated in the very outer part of the solar system, probably in the Oort cloud – a shell of icy objects that orbits the sun far beyond Pluto.

It could give astronomers a rare opportunity to observe the innards of a long-period comet. “It’s a great opportunity to study the physics of the nucleus,” says Kelley. “We can learn about how the nucleus sticks together and its properties. We can’t do this any other way without sending a spacecraft there.”


arXivDOI: https://arxiv.org/abs/2306.03635

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