These two Hubble Space Telescope images (ATLAS) of comet C/2019Y4 (ATLAS) show the disintegration of the comet's solid nucleus. They were taken on April 20, 2020 and April 23, 2019. The Hubble photos can identify up to 30 fragments. The images were taken at a distance of approximately 91,000,000 miles from Earth. It is possible that the comet is a fragment of a larger comet, which swung by Sun 5,000 years back. This view has been artificially colored to increase detail for analysis. Credit: NASA, ESA, Quanzhi Ye (UMD), Alyssa Pagan (STScI)
A comet could have swept 23 million miles from the Sun around 5,000 years ago, which is closer than Mercury's innermost planet. It may have been an amazing sight for civilizations in Eurasia and North Africa during the Stone Age.
This nameless space visitor, however, is not documented in any historical accounts. How can astronomers possibly know that such an interplanetary visitor existed?
Enter comet ATLAS, C/2019 Y4, which first appeared at the beginning of 2020.
Comet ATLAS was first detected in the Asteroid Terrestrial Impact Last Alert System (ATLAS) operated by the University of Hawaii. It quickly died, disintegrating into a series of small, icy pieces around mid-2020.
Quanzhi Ye, an astronomer at the University of Maryland, College Park, has discovered that ATLAS, an ancient visitor, was a fragment of a NASA Hubble Space Telescope observation. Why? Why? Because ATLAS follows the exact same orbital track as a comet that was seen in 1844. These two comets could be siblings of a parent comet that was broken apart centuries ago. Amateur astronomer Maik Meyer first noticed the link between the two comets.
These comet families are quite common. One of the most striking visual examples was when Shoemaker-Levy 9 was (SL9) pulled into a string by Jupiter's gravitational pulling. The "comet train", however, was only temporary. It was eventually swept into Jupiter in July 1994.
Ye says that ATLAS is only "weird" and that he observed it with Hubble around the time of its breakup. ATLAS, unlike its parent comet, disintegrated at a distance over 100 million miles from the Sun. This was much further than the distance its parent traveled to the Sun. Ye said, "This emphasises its strangeness."
"If it split up so far from the Sun how could it have survived the last passage around it 5,000 years ago?" Ye said that this is the key question. It's unusual, because it's not what we would expect. This is the first time that a long-period family member of a comet was seen breaking apart before moving closer to the Sun.
The fragmentation of the parent comet can be seen as clues by observing the breaks up. According to the conventional wisdom, comets are delicate agglomerations made of dust and water. They may also be lumpy, akin to raisin pudding.
Ye and his co-investigators reported that after a year of analysis, one ATLAS fragment was destroyed in just days. A second piece of ATLAS lasted for several weeks. He said that this showed that one part of the nucleus was more powerful than the other.
One possibility is that streams of ejected material could have spun up the comet at such a fast rate that centrifugal forces ripped it apart. Another possibility is that the comet has super-volatile, or exploding, ices. It is complex because we can see the hierarchies and evolutions of comet fragmentation. Comet ATLAS's behavior can be fascinating, but it is difficult to explain."
Comet ATLAS's sibling will not return to Earth until the 50th Century.
Hubble captures the breakup of ATLAS comet by Hubble
Quanzhi Ye and colleagues, Disintegration Long-period Comet C/2019Y4 (ATLAS). I. Hubble Space Telescope Observations. Astronomical Journal (2021). Information from the Astronomical Journal Quanzhi Ye et al., Disintegration Long-period Comet C/2019 Y4(ATLAS). I. Hubble Space Telescope Observations (2021). DOI: 10.3847/1538-3881/abfec3
