There’s a comet in the sky with the potential to become naked-eye visible in mid-October. Comet C/2023 A3 Tsuchinshan-ATLAS (hereafter referred to as A3) was discovered in 2023 by two observatories, thus accounting for its complex name.
Comets of A3’s type originate in the Oort Cloud, a spherical collection of small icy bodies on the outskirts of our solar system.
Occasionally, gravitational encounters with other Oort-Cloud objects disturb their locations. Some get tossed toward the sun, where they become gravitationally attracted by our day star and become comets.
If their angles of approach are just right, they increase in speed and are slingshotted around the sun and back out toward the Oort Cloud.
Some comets have parabolic or hyperbolic trajectories that carry them out of the solar system, never to return. Others go into stretched-out orbits around the sun.
Those orbits can last for thousands, even millions, of years before they return to the sun’s vicinity. It is unclear whether A3 will be ejected or is a repeater. A3 likely took millions of years to journey from the Oort Cloud to the inner solar system where we live.
As comets get close to the sun, their ices, mainly water and methane ice, turn directly to glowing gasses, a process called sublimation, with a bit of dust mixed in. That material is blown away from the comet to create two tails — a dust tail and a glowing gas tail.
If the comet is large enough and its approach to the sun close enough, it becomes visible to the unaided eye as a fuzzy ball with a glowing gas tail pointed away from the sun.
Some comets break apart or evaporate from the sun’s energy. If A3 survives its close encounter with the sun, we might see it in evening twilight from Oct. 13-16 as it makes its closest approach to Earth. Go out early and wait. The comet sets just after the sun.
The comet will be close to the sun, so you’ll need a western horizon free of trees, buildings, and other obstructions. Also, try to get to the darkest, most rural sky possible, although the comet may be bright enough to see from suburban skies.
How bright will it be? No one knows. Because of the varying size, shape, and composition of the hunk of ice that sublimates, cometary brightness is inherently unpredictable.
That fact doesn’t stop astronomers from trying. They predict that the comet may get as bright as magnitude 1.0, about as bright as the star Antares in the constellation Scorpius. However, such comparisons are a bit deceiving. Antares is a point source, and A3’s brightness spreads over its larger surface area.
It will almost certainly not be as bright as Comet Hale-Bopp, which achieved a maximum brightness of magnitude -1.8 in April 1997. (The lower the number, the brighter the object.)
If the sky is clear on Oct. 13, look toward the western horizon during evening twilight, just after sunset. Binoculars may or may not be necessary, but I always start with them by sweeping the horizon.
On that date, the comet may be too close to the horizon to see. A lot depends on the presence or absence of obstructions at your observing location.
You’ll find the comet hovering between dazzlingly bright Venus on the left and the bright star Arcturus in the constellation Boötes to the right.
As the days pass, A3 will rise higher in the sky, but unless there is an unexpected flare-up of its brightness, it will get a tad dimmer each night. Still, it may become more visible as it rises because you can view it in complete darkness by the end of October.
For a finder chart of the comet, check out my Facebook page at https://www.facebook.com/tomburns123/. The chart was produced by Bryan Simpson of the Cincinnati Astronomical Society using the planetarium application Stellarium. It shows the comet’s position at 8 p.m. every day until Nov. 1.
We haven’t seen a truly spectacular comet since Hale-Bopp in 1997. We’re due. Will A3 be that comet, or will it be a dud? I’m guessing it will be somewhere in between. The only way to find out is to go outside and look.
Tom Burns is the former director of the Perkins Observatory in Delaware.
