Not since December 2018 when Comet 46P/Wirtanen passed near the Pleiades star cluster has a naked-eye comet graced the night sky. It reached 5th magnitude at the time and looked like a small wad of glowing fuzz from a dark sky site. Wirtanen never developed a bright tail, one of the most distinguishing and beautiful aspects of a comet. Since then plenty of comets have passed by, but only a few have been visible in binoculars and most have required a telescope.
I have hopeful news. On Dec. 28, 2019, astronomers with the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey discovered a faint comet named C/2019 Y4 ATLAS. Back in mid-late February I glimpsed it my 15-inch telescope as a dim, hazy patch of light at the edge of visibility in the bowl of the Big Dipper. A couple weeks later it had brightened to magnitude 11 — still dim but much easier to see than in February. Now in mid-March, skywatchers with big binoculars have spotted it at around 9th magnitude! That’s a remarkable uptick in so short a time.
According to NASA’s Horizon’s website the comet could exceed Venus in brightness when it passes just 23 million miles (37 million km) from the sun on May 31st. We won’t see it then because it will be lost in the solar glare only about a fist (10°) from the sun days before and days after. But at least we should get a decent show before and after closest approach.
There’s reason for optimism. ATLAS follows the same orbit as the Great Comet of 1844 (C/1844 Y1) which grew into a beautiful 2nd magnitude object with a 10° tail in January 1845. Both C/2019 Y4 and C/1884 Y1 appear to be fragments of a single much larger comet that broke apart about 5,000 years ago. Given that they’re siblings we’re hoping for a similar performance this time around. There may even be more fragments around slowly working their way sunward in the coming decades.
Because of its rapidly increasing brightness hopes are high, but if you’ve had any experience with comets before you know they’re little devils. Some live up to predictions, some exceed them and others flop. Comets are prone to “outbursts” and fragmentation which can produce lots of fresh dust and ice, causing them to brighten temporarily. Other times, those pieces quickly go poof and the comet suddenly fades.
For the moment it appears the comet could reach 2nd magnitude by mid-May when it will be visible during evening twilight low in the northwestern sky in the constellation Perseus. Its northerly location in the sky at that time will mean that observers in the northern U.S. will have the best views. If the comet is especially dusty, viewing circumstances are such that we would see an attractive tail instead of a simple fuzzball. Be hopeful but as always, temper your expectations.
A couple weeks before perihelion the comet will likely disappear in the solar glare and then reappear at dawn in late June in Orion for southern hemisphere skywatchers. ATLAS will be very low in the eastern sky at the time and glow between 3rd and 5th magnitude.
Now through late March, you should spot it in a 6-inch or larger telescope from a dark sky. The comet is circumpolar for skywatchers at mid-northern latitudes and visible all night long. I’ve provided a map showing its track across Ursa Major near the Bowl of the Big Dipper. If ATLAS continues to brighten apace it will soon be visible in ordinary binoculars. When that happens I promise to return with more news, observations and photos!