When scientists first spotted Comet ATLAS last year, they hoped it would be the brightest comet of the decade. Then, the icy hunk unexpectedly fell to pieces.
Now, astronomers think they may have figured out two mysteries surrounding why the comet suffered such an untimely demise. The nucleus, or heart, of Comet C/2019 Y4 (ATLAS) broke up between April 20 and April 23, 2020, when the comet was at 91 million miles (146 million kilometers) away from Earth, about the same distance as the sun is. Even more weirdly, not all pieces broke up at the same rate.
Strangely, scientists believe that ATLAS broke off from an ancestor comet that made a relatively close pass to Earth roughly 5,000 years ago. This huge comet came within only 23 million miles (37 million km) to the sun, closer than Mercury's orbit, and might have been noticed by humans at the time.
"The comet might have been a spectacular sight to civilizations across Eurasia and North Africa at the end of the Stone Age," NASA officials wrote in a statement. But coming to this conclusion — absent any written or pictorial records of the celestial visitor — took some scientific sleuthing.
A new study suggests ATLAS follows the same orbital track as another comet seen in 1844 that survived its journey around the sun — a probable sibling of 2020's visitor. (The link was first noted by amateur astronomer Maik Meyer and confirmed by scientists.) Comet families are not unusual in science, with a famous example being the short-lived fragments of Shoemaker-Levy 9 that slammed into Jupiter in 1994.
ATLAS, however, looks "weird" according to study lead author, astronomer Quanzhi Ye of the University of Maryland in College Park. He couldn't figure out why ATLAS disintegrated when its parent comet had stayed together while passing the sun at a much closer distance.
"If it broke up this far from the sun, how did it survive the last passage around the sun 5,000 years ago? This is the big question," Ye said in the NASA statement. "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."
Observing the fragments suggest that parts of ATLAS featured different structures or compositions, especially because one fragment of the comet dissolved in days while another lasted for several weeks. "This tells us that part of the nucleus was stronger than the other part," Ye said.
The authors have a couple of theories as to why the fragments had different lifespans, which in turn may related to the unknown composition of the ancient parent comet. One theory is that "streamers of ejected material" tore apart the comet with centrifugal forces, NASA said. Or perhaps volatile ices blew apart the weaker piece like "an exploding aerial framework," the agency continued.
While ATLAS is no more, its sibling should still be visible from Earth eventually — but only in the distant future. Astronomers' best guess is that it will swing by our planet in the 50th century.
A paper based on the research was published July 21 in The Astronomical Journal.
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