An article published in the “Astronomical Journal” reports the results of a study on comet C/2019 Y4 (ATLAS) whose conclusion is that it’s a fragment of a larger comet whose passage could have been seen on Earth about 5,000 years ago. A team of researchers led by astronomer Quanzhi Ye of the University of Maryland in College Park used observations conducted with the Hubble Space Telescope to examine this comet and its orbit. In fact, there are now various fragments and it’s possible that the progenitor gave rise to an entire family of comets. Amateur astronomer Maik Meyer identified a correlation with C/1844 Y1, nicknamed the Great Comet of 1844.
C/2019 Y4 is a long-term comet that was discovered on December 28, 2019, thanks to the ATLAS (Asteroid Terrestrial-impact Last Alert System) operated by the University of Hawaii. In April 2020, as it approached perihelion, it fragmented. The Hubble Space Telescope was used to try to examine the consequences of that event and up to 30 fragments are visible in the images captured. Some fragments are visible in the top images (NASA, ESA, Quanzhi Ye (UMD); Image Processing: Alyssa Pagan (STScI)). However, it may not have been the first fragmentation that involved this comet.
A few days after the discovery of comet C/2019 Y4 (ATLAS), German amateur astronomer Maik Meyer noticed similarities with the orbital elements of C/1844 Y1, nicknamed the Great Comet of 1844 because it was visible even to the naked eye for several days between late 1844 and early 1845. Meyer isn’t a professional astronomer but he’s a very active comet researcher, so his theories are taken seriously and he seems to be right again.
Quanzhi Ye’s team used data collected by the Hubble Space Telescope on Comet C/2019 Y4 to confirm Maik Meyer’s theory. There’s a possibility that this fragmentation is just the latest of various events that began with a much larger comet that could have been seen from Earth 5,000 years ago.
Comet families generated by the fragmentation of a larger body are nothing new at all but in this case, there are some anomalies that are difficult to explain. We’re now seeing fragments of a comet that fell apart at a distance of over 160 million kilometers from the Sun but how did it survive the Sun flyby 5,000 years ago?
The study conducted by Quanzhi Ye’s team offers some hypotheses based on the analysis of the collected data. One of the fragments of comet C/2019 Y4 disintegrated within a few days while another lasted for weeks. This indicates that one part of the nucleus was stronger than the other. It’s possible that the jets of materials generated such a rapid spin of the nucleus that centrifugal forces tore it apart. Another possibility is that the frozen super volatile materials had a violent sublimation like a firework that shattered the nucleus.
In June 2021, the Solar Orbiter space probe passed through the tail of comet C/2019 Y4 by chance. Its instruments were designed to study solar activity but the study of the detections conducted on that occasion could still offer interesting information. For example, the instruments detected the second tail of C/2019 Y4, the ionic tail generated by the interaction between the comet’s gas and the solar wind.
It’s difficult to have complete answers regarding a comet that was a part of a larger object, not least because the sibling sighted in 1844 will only return to the inner solar system in 5,000 years. However, the information collected offers an interesting picture of the possibilities of comet fragmentation useful for other types of research. For example, comet C/2019 Y4 (ATLAS) shows us how fragmentation can scatter materials that can end up on planets and can include organic materials. In this case, there were no dangers for the Earth but in other cases, one or more fragments of a comet could hit the planet.