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EI2GYB > ASTRO 28.08.21 11:26l 91 Lines 4158 Bytes #999 (0) @ WW
BID : 13815_EI2GYB
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Subj: Comet ATLAS may have been a blast from the past
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Comet ATLAS may have been a blast from the past
It's suspected that about 5,000 years ago a comet may swept within
23 million miles of the Sun, closer than the innermost planet Mercury.
The comet might have been a spectacular sight to civilizations across
Eurasia and North Africa 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.
Comet ATLAS, first detected by the Asteroid Terrestrial-impact Last
Alert System (ATLAS), operated by the University of Hawaii, quickly met
an untimely death in mid-2020 when it disintegrated into a cascade of
small icy pieces.
In a new study using observations from NASA's Hubble Space Telescope,
astronomer Quanzhi Ye of the University of Maryland in College Park, r
eports 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 probably siblings from a parent comet
that broke apart many centuries earlier. 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 J
upiter'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 closer 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 published in the 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.
One possibility is that streamers of ejected material may have spun
up the comet 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's surviving sibling won't return until the 50th century.
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