It took humans a long time to realize we live in a constantly changing universe. The inability to see the day-to-day changes in most objects using just our eyes led to the false notion that the stars and planets were immutable, essentially glowing lights on rotating spheres. When things did change, it was seen as a portent of either great or evil things to come. 

Comets, for instance, were seen as great harbingers of whatever was useful to those in power at the moment. Now, we know that comets are giant iceballs that are orbiting the Sun on highly elliptical paths. As they approach the Sun and heat up, some of their material will transform from solid to gas, creating the dramatic tails we see in books but rarely with our eyes. As they pass near the Sun, like a snowball on a clear day, they get heated up and can fall apart.

In the spring of 2020, as we all desperately looked for some small piece of happiness, Comet ATLAS C/2019 Y4 promised us an amazing light show and one of those naked-eye visible tails. We didn’t really get that. Instead, C/2019 Y4 just crumbled apart while still far from the Sun.

Why a perfectly healthy-looking comet would self-destruct was a bit of a mystery and in the past year, researchers have looked at a lot of different factors to try and figure out what went wrong. 

In looking at its orbit, it was realized the comet’s path was identical to that of an early comet that passed through in 1844. This kind of “two comets, one orbit” situation can arise when a large object shatters, and the remaining pieces get spread out along the path. We saw this in great detail in 1994 when Comet Shoemaker/Levy 9 flew into Jupiter, and there are other families of comets believed to all be remains of a single object that broke apart. If C/2019 Y4 was just a fragment of some other body, that fragment might have inconsistent strength, causing it to fragment. 

It’s also possible it had an area of super-volatile ices that exploded like fireworks, blasting it apart, or maybe even just setting it catastrophically spinning. It’s unclear. In 5000 years, its sibling comet will be back, and we’re just going to have to see what we can learn from it. That, however, is one story I don’t think we’ll bring you here.

More Information

NASA Goddard press release

“Disintegration of Long-period Comet C/2019 Y4 (ATLAS). I. Hubble Space Telescope Observations,” Quanzhi Ye et al., 2021 July 21, The Astronomical Journal