Comet Borisov has been dominating the comet headlines recently, with good reason – it is the first interstellar comet ever detected in the Solar System. However, not to be outdone, the “doomed” comet C/2019 Y4 (ATLAS) has put on a spectacular display for us as it met its fateful end, disintegrating into as many as 30 pieces, each roughly the size of a house.
Luckily, NASA’s Hubble Space Telescope was on hand to capture this event when the comet was approximately 146 million kilometers (91 million miles) from Earth, inside the orbit of Mars.
“This is really exciting — both because such events are super cool to watch and because they do not happen very often,” Quanzhi Ye, leader of one of the Hubble observing teams from the University of Maryland, said in a statement. “Most comets that fragment are too dim to see. Events at such scale only happen once or twice a decade.”
Discovered right at the end of last year, ATLAS was expected to be just another “dirty snowball” visit to the inner Solar System, too faint to see with the human eye. However, astronomer’s interests were peaked when its magnitude suddenly jumped 100-fold in a few days in March. This put it on course to be one of the most spectacular comets seen in the last 20 years, visible to the naked eye.
Sadly, after abruptly dimming, fragments of the comet were spotted on April 11 by amateur astronomer Jose de Queiroz, whilst ATLAS journeyed around the Sun. A silver lining of the situation was that now on Hubble’s radar, ATLAS’ demise could be captured in crisp resolution. Around 30 fragments were identified on April 20, and 25 pieces on April 23.
“Their appearance changes substantially between the two days, so much so that it's quite difficult to connect the dots,” David Jewitt, professor of planetary science and astronomy at UCLA, California, and leader of the other team who photographed the comet with Hubble, explained. “I don't know whether this is because the individual pieces are flashing on and off as they reflect sunlight, acting like twinkling lights on a Christmas tree, or because different fragments appear on different days.”
The team’s observations of ATLAS suggest that comet fragmentation may well be the primary reason behind the death of a comet’s solid, icy nucleus. But why this happens still remains somewhat of a mystery. One suggestion is that the Sun’s warming influence “unglues” the comet as it enters the inner Solar System. An alternative proposal, is that the comet’s nucleus spun itself apart, as jets of warming gases produced from sublimating ices broke it apart.
“Further analysis of the Hubble data might be able to show whether or not this mechanism is responsible,” Jewitt continued. “Regardless, it's quite special to get a look with Hubble at this dying comet.”
Although our dreams of witnessing ATLAS with the human eye were literally shattered, there's a new comet on the block reviving our comet-hunting hopes. Watch this space.