There has been a lot of talk in recent days on social media regarding the approach of a new comet that could possibly evolve into a spectacular sight in the coming weeks ahead.
The comet has been christened "ATLAS" which is an acronym for Asteroid Terrestrial-Impact Last Alert System ("ATLAS"); a robotic astronomical survey and early warning system based in Hawaii, optimized for detecting smaller near-Earth objects a few weeks to a few days before they impact Earth.
But besides finding asteroids, ATLAS has also found more than half a dozen comets. The one that everyone is currently interested in is classified at C/2019 Y4.
When first sighted on Dec. 28, 2019, the comet was an exceedingly faint object, but in the days and weeks that followed, it brightened at an incredibly rapid pace. In fact, from the time of discovery through March 17, Comet ATLAS increased some 27,500-fold in brightness!
Such an incredible rise in luster, plus the fact that calculations showed that it was moving an orbit virtually identical to the "Great Comet of 1844," suggested that ATLAS was on its way to becoming a dazzling object. Indeed, some calculations suggested that when the comet arrives at perihelion — its closest point to the sun — on May 31 — that it might rival the planet Venus or even the full moon in brilliance!
But based on observations made over the past couple of weeks, that doesn't look very likely to happen.
For all intents and purposes, Comet ATLAS hit a wall on March 17. Ever since that time, the comet's incredible rise in brightness has dramatically slowed. Over the last two weeks, the comet has brightened by only about 0.5 magnitude. (Magnitude is a measurement of brightness used by astronomers, with lower numbers denoting brighter objects.)
As of April 1, a consensus of observations taken the Comet Observations Database showed Comet ATLAS hovering at around magnitude +8.0. That's within range of a small telescope or good binoculars, though many have had difficulty in seeing it because its head, or "coma" is so large — about 10 arc minutes across, or roughly one-third the apparent size of the moon.
Because of this, instead of being concentrated into a tiny dot of light like a star, the comet's brightness is spread out over a large area, giving it a "ghostly" appearance. And unless you have a reasonably dark, non-light polluted night sky, you'll likely have difficulty picking out the comet's pale image against the background of the sky.
Sizzler or fizzler?
The best way to determine if a comet is going to put on a good show is to pin down whether it is a "new" comet moving in a parabolic orbit — that is, one that originated from the Oort Cloud, a giant repository of comets located far beyond the known limits of our solar system.
Comets that have been around the sun before will appear to be traveling around our star in the shape of a stretched ellipse, because gravitational interactions during their journeys through the solar system have reshaped their orbits. "New" comets, on the other hand, travel in parabolic orbits because they literally fall from the depths of space toward the sun in a straight line, swing around the sun and are then flung back out into space.
Such first-timers have never interacted with the sun before and are covered with very volatile materials such as frozen nitrogen, carbon monoxide and carbon dioxide. These ices vaporize far from the sun, giving a distant comet a rapid surge in brightness that can raise unrealistic expectations. But once those ices are gone, the comet's rapid brightening dramatically slows.
But Comet ATLAS doesn't appear to be a newcomer. We already noted that its orbit closely matches that of the Great Comet of 1844. Both appear to be following trajectories which swing out far beyond the outer reaches of the solar system, and taking around 6,000 years to make one complete orbital circuit. In essence, both comets have circled the sun at least once, if not more than a few times.
So, if Comet ATLAS has been here before, why is it acting more like a new comet?
Old is new again
Well-known comet expert John Bortle believes that ATLAS is basically a significant chunk broken off the comet of 1844 (officially designated C/1844 Y1), probably during the sunward bound leg of that comet's journey some centuries ago.
"What is approaching the sun now," Bortle told Space.com, "is a modest-size piece of cometary ice and dust with a large freshly exposed portion of surface; one that's never seen the sun, such that it reacts to sunlight similar in the manner of an Oort Cloud comet like Comet Kohoutek did back in '73. Intrinsically it is probably several magnitudes fainter than we currently assume it to be and may, or may not, be large enough to survive perihelion passage."
In other words, there's a chance that Comet ATLAS might completely disintegrate when it arrives at the sun's vicinity at the end of May.
Where to look and when
From now through April 10, those who try searching out for the comet are going to be handicapped by the presence of the moon, which will turn full on April 7. The moon's dazzling light is going to make a sighting of this dim, wispy object all that more difficult.
On a positive note, its path through the constellations will continue to be very favorable for Northern Hemisphere observers as it will remain circumpolar — always remaining above the horizon. As darkness falls, it will be positioned more than halfway up in the north-northwest sky. The comet currently resides within the boundaries of Camelopardalis, the giraffe — a rather dim, shapeless star pattern. There it will stay right on through April.
For the technically inclined, I have calculated an ephemeris below — a table giving the positions of Comet ATLAS at five-day intervals through the balance of April on into May. Observers can use it to plot the projected path of Comet Atlas on a star chart or atlas. The coordinate positions provided will also be useful for those who own a "smart" or "GoTo" telescope (one that can locate and track objects as they move across the night sky).
As to how bright Comet ATLAS may get, I have followed the predicted light curve of Japanese comet expert Seiichi Yoshida, which is posted on his "Visual Comets in the Future" website. His values suggest that ATLAS might become faintly visible to the unaided eye by the end of April or beginning of May. By mid-May, the comet might reach second magnitude — as bright as Polaris, the North Star. During the final week of May, as the comet disappears into the glare of the setting sun, it possibly could attain first magnitude or brighter.
But we must stress caution: comets are notoriously unpredictable. We can only guess how bright it will get and how long its prospective tail might be. If we are lucky, we might have a conveniently visible comet in the western evening twilight by mid-May. However, I personally don't think it will rank with spectacular comets of the recent past like Comet McNaught (2007) or Hale-Bopp (1997), but hopefully it will evolve into a noteworthy object.
Then again, it could also disappoint. We're just going to have to wait and see.
Editor's note: If you have an amazing comet photo you'd like to share for a possible story or image gallery, you can send images and comments in to firstname.lastname@example.org.
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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications. Follow us on Twitter @Spacedotcom and on Facebook