When Wyomingites step outside shortly after sunset over the next few days and look west, the brightest object in the sky could be "the green comet."
Comet Tsuchinshan – ATLAS, aka C/2023 A3 or the green comet, has passed the sun and is making its closest approach to Earth this week.
The next time it could come around and be seen on Earth again could be in 70,000 years — or maybe never.
“This is the last time humanity will get to see this comet,” said Max Gilbraith, planetarium coordinator for the University of Wyoming. “Its furthest distance from the sun is over 90,000 times the distance from Earth to the sun, so its orbital period is thousands of years.”
Wyomingites will want binoculars or a telescope for the best view, but C/2023 A3 will be a “naked-eye comet” for the next few days, Gilbraith said. It could even shine brighter than our nearest planetary neighbors and be one of the brightest comets in the last decade.
“The tail will be pointing up and away from the sun at sunset,” Gilbraith said. “Mercury and Venus will be nearby, so don't mix it up with them. That’s how big and bright it'll appear.”
The Green Comet
Comet Tsuchinshan – ATLAS is a dirty block of ice 30,000 miles in diameter, with a tail stretching more than 18 million miles long. It will make its closest approach to Earth on Saturday, about 44 million miles away.
This is the first time in recorded history this comet has passed by Earth. It’s called the green comet because of the color of its extremely long tail, but it shouldn’t be confused with the green Devil Comet, 12P/Pons-Brooksthat passed by earlier this year.
Gilbraith said the brilliance of a comet from Earth depends on how the sun's heat and energy impact it. The sun sustains life on our planet, but it can very easily rob a comet of its luster.
“The sun can melt comets, so they lose all this material,” he said. “Comets can rapidly decrease in brightness sometimes and won't have quite as much gusto after going near or around the sun because it doesn't have as much material to lose.”
It’s too early to say how much C/2023 A3 will be affected by its trip around the sun. It reached perihelion, its closest point to the sun, on Sept. 27.
The comet has been visible in the Southern Hemisphere for over a month. For residents of the Northern Hemisphere, this weekend will be the best and brightest opportunity to see C/2023 A3.
“Let's hope it doesn't sizzle out after going this close to the sun,” Gilbraith said.
Where And When
C/2023 A3 will be visible shortly after sunset and shortly before sunrise when it makes its closest approach to Earth this weekend. Stargazers will want to look towards the constellation Virgo in the western skies.
Mercury and Venus will be in the same field of view this weekend. Gilbraith provided a handy guide for anyone with binoculars so they could differentiate between the comet and the Roman gods of commerce and love.
“All of these are fun to pick up in binoculars,” he said. “The one with the tail is the comet, the yellowish one is Venus, and the little white, smaller one is Mercury.”
At its peak, C/2023 A3 could have an apparent magnitude of -4.9, its last observed apparent magnitude. Gilbraith said that could make it one of the brightest objects in the night sky.
“When (apparent magnitude) gets into the negatives, that means it's really bright,” he said. “The brightest star in the sky is zero. Mercury and Venus have negative magnitudes because they're brighter than any star can be in the sky. The last observed brightness of the comet should make it one of the brightest things in the sky right now.”
The best moments to see “the green comet” will be 30 minutes after sunset on Oct. 12 and 30 minutes before sunrise on Oct. 13. That’s when it will be closest to Earth and should appear at its brightest.
However, Gilbraith said seeing the comet from the plains will be easier than from the mountains. Because of its trajectory, it will appear close to the western horizon, and Wyomingites living at high elevations might have to search for an unobstructed view.
“Hopefully, folks can catch it right in the evening, and it doesn't set too quickly,” he said. “They should get an unobstructed view to the west so it doesn't set too early, but If you're right at the foot of some mountains, you'll have a disadvantage.”
Witness To Humanity
Gilbraith said this has been a good decade for spotting comets from Earth, even if Wyoming has missed some of the better flybys. Several comets have passed through our celestial neighborhood, putting on spectacular shows as they go by.
“We can go through phases where there's a good showing as we've had in the last decade, but we can also go through a drought of comets,” he said. “Comets don't always orbit in the ecliptic, which means sometimes a certain hemisphere can miss out. So even though we've had a lot of good comets lately, it's mostly for folks in the Southern Hemisphere. But we’ll see this one from the Northern Hemisphere.”
Once C/2023 A3 moves beyond Earth in late October, nobody in either hemisphere will have an opportunity to see it again. Its orbital period is around 70,000 years, which means mammoths and Neanderthals were alive when it last appeared in the night skies.
Halley's Comet will make its next earthly approach on July 28, 2061, and the "Green Devil Comet" will return in another 71 years. C/2-23 A3 could return in another 70,000 years, but there’s also a chance it might never be seen again.
Gilbraith said the sun might have knocked it off course, and permanently out of the solar system.
“The comet’s losing mass as it goes around the sun, but it has the same momentum that could make it escape the solar system entirely,” he said. “Depending on how its interaction with the sun affected it, this could be the last time humanity will see this comet.”
That’s all the more reason for Wyomingites to find an unobstructed view of the western skies on Sunday night, 30 minutes after sunset. Then, they’ll be able to see “the green comet” before it hurls into the infinite void of outer space, never to be seen again by humankind.
“That's the give and take of a comet,” Gilbraith said.
Andrew Rossi can be reached at arossi@cowboystatedaily.com.