Credit: NASA, ESA, STScI, D. Jewitt (UCLA)

About 5,000 years ago a comet swept within 23 million miles of the sun, closer than the innermost planet Mercury. The comet must have been a spectacular sight to those young civilizations across Eurasia and North Africa that were arising at the end of the Stone Age.

However, this nameless space visitor is not recorded in any known historical account. So how do astronomers know that there was such an interplanetary intruder?

Enter comet ATLAS (C/2019 Y4), which first appeared near the beginning of 2020.

ATLAS quickly met an untimely death in mid-2020 when it disintegrated into a cascade of small icy pieces. Such a comet’s self-destruction happens once or twice a decade.

Astronomer Quanzhi Ye of the University of Maryland reports that ATLAS is a broken-off piece of that ancient visitor from 5,000 years ago. Why? Because ATLAS follows the same orbital “railroad track” as that of a comet seen in 1844. This means the two comets are siblings from the parent comet that broke apart very long ago. The link between the two comets was first noted by amateur astronomer Maik Meyer.

Such comet families are common. The most dramatic visual example was in 1994 when the doomed comet Shoemaker-Levy 9 (SL9) was pulled into a string of pieces by Jupiter’s gravitational pull. This “comet train” was short-lived. It fell piece by piece into Jupiter in July 1994.

But comet ATLAS is just “weird,” says Ye, who observed it with Hubble about the time of the breakup. Unlike its hypothesized parent comet, ATLAS disintegrated while it was farther from the Sun than Earth, at a distance of over 100 million miles. This was much farther than the distance where its parent passed the Sun. “This emphasizes its strangeness,” said Ye.

“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,” said Ye. “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 closest to the sun.”

Observing the breakup of the fragments offers clues to how the parent comet was put together. The conventional wisdom is that comets are fragile agglomerations of dust and ice. And, they may be lumpy, like raisin pudding.

In a new paper to be published in the July 21 Astronomical Journal after one year of analysis, Ye and co-investigators report that one fragment of ATLAS disintegrated in a matter of days, while another piece lasted for weeks. “This tells us that part of the nucleus was stronger than the other part,” he said.

Two possibilities are that because of the action of jets, the short-lived piece may have spun up so fast that centrifugal forces tore it apart. An alternative explanation is that it has so-called super-volatile ices that just blew the piece apart like an exploding aerial firework. “It is complicated because we start to see these hierarchies and evolution of comet fragmentation. Comet ATLAS’s behavior is interesting but hard to explain.”

Comet ATLAS’ surviving sibling won’t return until the 50th century. These latest images from NASA's Hubble Space Telescope of the doomed comet C/2019 Y4 (ATLAS), taken on April 20 and 23, 2020, provide the sharpest views yet that the comet's solid icy nucleus is breaking apart into as many as 30 pieces that are each roughly the size of a house. So, despite the name, ATLAS doesn't look like anything to be afraid of. The comet was discovered on December 29, 2019 by the ATLAS (Asteroid Terrestrial-impact Last Alert System) robotic astronomical survey system based in Hawaii. ATLAS' fragmentation was confirmed by amateur astronomer Jose de Queiroz, who was able to photograph around three pieces of the comet on April 11. Hubble has a front row seat, with its crisp resolution, to go looking for more pieces. And, astronomers weren't disappointed. Planetary experts know that the solid comet nucleus – the fountainhead of the glamorous tail – is a fragile agglomeration of ices and dust. However, astronomers don't know why some comets break apart like exploding aerial fireworks shells. Could it be due to the warming influence of the Sun as a comet enters the inner solar system, causing it to become unglued? Or could the icy nucleus spin up as it shoots out jets of warming gasses? This could cause it to fly apart. Though classified as "minor bodies" in our solar system family, comets and Earth's fate go back billions of years. A shower of comets may have irrigated the dry newborn Earth, bringing enough water for oceans. They may have seeded Earth with organic compounds, the precursors to life as we know it. A wayward comet may have struck the Earth 65 million years ago, creating such an environmental disaster that the dinosaurs became extinct. This was good news for small mammals, our earliest ancestors, to take over the blue planet.

Provider: Space Telescope Science Institute

Image Source: https://hubblesite.org/contents/news-releases/2021/news-2021-031

Curator: STScI, Baltimore, MD, USA

Image Use Policy: http://hubblesite.org/copyright/