comet C/2019 Y4 (ATLAS)
© Science: NASA, ESA, Quanzhi Ye (UMD); Image Processing: Alyssa Pagan (STScI)
This pair of Hubble Space Telescope images of comet C/2019 Y4 (ATLAS), taken on April 20 and April 23, 2020, reveal the breakup of the solid nucleus of the comet. Hubble photos identify as many as 30 separate fragments. The comet was approximately 91 million miles from Earth when the images were taken. The comet has been artificially colored in this view to enhance details for analysis.
Researchers in the US have wound back the cosmic clock to determine that a spectacular comet whizzed past the Earth 5,000 years ago.

While the event isn't recorded in any historical account, the team were able to gather clues from more recently sighted comets.

In a paper published in the Astronomical Journal, they examined observations made by the Hubble Space Telescope of the comet ATLAS (C/2019 Y4), which Hubble watched break into pieces last year.

This comet is thought to be a fragment of a larger one that passed by the Earth in 1844, shining as brightly as Sirius, the brightest star in the sky.

By tracing the two comets' motions back through time, the researchers figured out that they are likely both parts of an even bigger one that last zipped through the inner Solar System 5,000 years ago, around the time that Ancient Egyptians were first settling into the Nile valley.

The comet is like a Russian nesting doll - breaking into smaller and smaller pieces as it whizzes in huge orbits through the Solar System.

On its trip 5,000 years ago, it would have come within 37 million kilometres of the Sun, closer even than the orbit of Mercury.

But a mystery remains. The comet ATLAS observed by Hubble last year disintegrated when it was still a long way away from the Sun - 160 million kilometres distant. This is a bit "weird", notes lead author Quanzhi Ye of the University of Maryland in College Park, US.

"If it broke up this far from the Sun, how did it survive the last passage around the Sun 5,000 years ago?" says Ye. "It's very unusual because we wouldn't expect it. This is the first time a long-period comet family member was seen breaking up before passing closer to the Sun."

Ye's new paper describes how one bit of ATLAS disintegrated in just days, while another lasted for weeks. One possible explanation for this is that centrifugal forces (driven by its ejected material) tore the comet apart, or perhaps it contained super-volatile ices that blew it apart like a firework.

"It is complicated because we start to see these hierarchies and evolution of comet fragmentation," Ye says. "Comet ATLAS's behaviour is interesting but hard to explain."

The last surviving sibling of this comet family (the bit that passed in 1844) won't visit Earth again for nearly 3,000 years.