Astro Bob: Brightest comet of year to appear at dawn
Comets take the name of the discoverer, whether that be a human or an automated telescopic survey that robotically inspects the sky every night. For example, Comet Hale-Bopp is named for Alan Hale and Thomas Bopp, who co-discovered the comet in 1995. Comet PanSTARRS comes from the PanSTARRS survey conducted from Mt. Haleakala in Hawai'i.
Our featured comet is Tsuchinshan-ATLAS. The first of its two names, pronounced , is a romanized version of the Chinese word for "purple mountain," and refers to the Purple Mountain Observatory in China where it was discovered in January 2023. At the time, the comet was so faint and star-like in appearance it was reported as an asteroid.
No one followed up on the Chinese observation so the "asteroid" was considered lost. But a few weeks later, the ATLAS Project ( nabbed it again and officially put it in the books. Further study revealed a tiny, compact cloud of dust and gas surrounding the star, which indicated it was a comet. ATLAS is an asteroid impact early warning system with four telescopes stationed in Chile, South Africa and Hawai'i that scan the skies every clear night looking for moving objects.
Most new comets are found when they're faint and far away and Comet Tsuchinshan-ATLAS was no exception. It took about a year for it to brighten enough to see in amateur telescopes. I first spotted the fuzzy fellow on a cold, calm morning in January this year. The comet gradually brightened through the spring and summer and then disappeared in the glow of evening twilight toward the end of July.
After a sufficient number of observations, astronomers were able to determine its orbit. Turns it originates in a vast repository of comets over a trillion miles away called the Oort Cloud. Although located in interstellar space beyond the reach of the sun's tempests, the Cloud is still gravitationally bound to our star. Its icy inhabitants are the frozen leftovers from the time of the solar system's formation that were never incorporated into the planets. Scientists consider them pristine material from that bygone era.
Occasional encounters with other stars or gas clouds in the Milky Way can ruffle the Cloud and send one or more of its comets on a path toward the sun and inner solar system. The journey is a long one — of years as in the case of Comet Tsuchinshan-ATLAS. When an Oort Cloud comet reaches approximately Jupiter's distance from the sun, solar heating starts to vaporize its dust-laden ice and a fuzzy coma and tail form.
Comet Tsuchinshan-ATLAS is currently about 41 million miles from the sun and will make its closest approach on Sept. 27. About 2 weeks later on Oct. 12 it swings a healthy 44 million miles from Earth.
Currently, it's only visible at dawn for southern hemisphere observers. That will change starting next weekend (Sept. 28-29) when the comet finally climbs into northern skies. Although it could be as bright as first magnitude — a level above the Big Dipper stars — the Oort Cloud visitor will be just a few degrees above the eastern horizon and compete with morning twilight. Although we might see it with the unaided eye from the Duluth region I wouldn't count on it. But I'm confident a pair of binoculars will pick it up provided the sky is free of wildfire smoke.
You can start looking as soon as Sept. 27. Find a location with an unobstructed eastern horizon. The Lake Superior shoreline is perfect. The comet will be visible a few degrees above the eastern horizon about an hour before sunrise or around 6 a.m. We can thank our lucky stars that the waning crescent moon will pass relatively near the comet on the mornings of the Sept. 29 and 30 (see map), lending observers a hand in locating it. After Oct. 1, Tsuchinshan-ATLAS dips back toward the sun and disappears again in the solar glow.
Observers in Australia report that the comet is steadily brightening. Binoculars reveal a dense, luminous coma and streaky tail. The best is yet to come. Starting Oct. 12, Tsuchinshan-ATLAS swings into the evening sky. Provided it doesn't disintegrate under the heat and gravitational stress it will experience during its close approach to the sun on Sept. 27, it should become a beautiful object with a bright head and long tail visible with the naked eye. I'll have more information, photos and maps as we approach that special time.
