If you like mixing comets with the quietude of observing in the small hours, September opens with five fuzzy vagabonds just waiting for a visit.
Sometimes I herd comets. Recently, the number of observable objects has grown, with four to five comets visible before dawn and another at nightfall. Despite the fact that none are what you'd call bright, there are too many to ignore. Ever since 67P/Churyumov-Gerasimenko hooked me on comet-watching back in 1982 I try not to let a single one of these celestial sliders slip by. Their movements, beautiful forms, and surprise outbursts and breakups make them irresistible.
I've seen around 350 unique comets since my youth. Many of these are periodic, returning again and again at regular intervals. By far, 29P/Schwassmann-Wachmann tops the list of most frequently observed. With an average distance of 6 a.u. it's ordinarily a 16th-magnitude smudge but undergoes multiple outbursts every year, when it can surge to easy visibility in an 8-inch.
I mention it here because of its recent outburst in late August, when Comet 29P suddenly rose from magnitude 16 to 14. Keep an eye on this uneasy fella. Bright flare-ups that could kick the comet into view in a 6- to 8-inch telescope are inevitable. This season it's easy to keep track of because the comet remains within 2.5° of 2.7-magnitude Iota (ι) Aurigae now through mid-November. Most of the time you'll see nothing at its position, but then one night, a small knot of bright fuzz will alert you to a brand new eruption. If you'd like to participate in the ongoing observing campaign to study the nature and frequency of 29P's outbursts, click here.
** Update Sept. 5 — 29P underwent yet another outburst sometime around September 3-4 to ~12.5 magnitude, and is now fading.
Our other predawn comets include the aforementioned 67P/Churyumov-Gerasimenko, ATLAS (C/2019 L3), 4P/Faye, and 15P/Finlay. Clicking the name will take you to individual high-resolution maps. Before wildfire smoke and moonlight put the kibosh on deep-sky observing here in northern Minnesota I got great views of all four on the mornings of August 13–14. For good measure, let's also include one evening comet in the mix — PanSTARRS (C/2017 K2).
The easiest of the predawn gang is Comet ATLAS. It's small, very compact, and relatively bright at around magnitude 11.5. A comet's degree of condensation, or DC, plays a crucial factor in its visibility. Fainter comets with compressed cores are frequently much easier to see than large, diffuse objects with brighter apparent magnitudes.
Through my 15-inch Dob (used for all these observations) on August 13.4 UT I saw it as a tiny — just 45″ across — but bright and compact patch at magnitude 11.5 with a DC of 7. The comet will gradually swell to magnitude 10 in early 2022, making it a great fall and winter target. During September it slowly crosses from Lynx into Auriga, ideally placed in the northeastern sky before dawn.
What a joy to spot my old pal 67P again! In August it beckoned from Cetus, but it's now in Aries and will cross into Taurus on September 7th. Predictions suggest the comet currently glows at magnitude 12, but I estimated its brightness at 11.6 two weeks ago with a 1.5′-diameter, moderately compact coma (DC 5), and a dim 13.5-magnitude pseudo-nucleus.
"Chury" has brightened since then, so it may actually be closer to 11 at the moment. Take a look and let us know! Perihelion occurs on November 2nd with closest approach to Earth 10 days later. Expect the comet to peak at magnitude 9 around then as it tracks from Gemini into Cancer.
I thought 4P/Faye and 15P/Finlay would be easy catches. Faye swam into view at magnitude 11.2 with a coma diameter of 2′, but because it wasn't as compressed (DC 4) as either ATLAS or 67P, it was less apparent. The comet should wax to magnitude 10 as it crosses from northern Orion into Gemini during the first half of October. Perihelion occurs on September 8th at 1.6 a.u., with closest approach to the Earth on December 6th (0.9 a.u.).
Comet 15P/Finlay took more effort yet, in part because of its low altitude at the time. At 142× I saw a 2′-wide smear of weakly condensed, 12th-magnitude haze (DC 2). Other observers estimated its brightness around magnitude 11 in mid-August. Although Finlay is now better placed as it crawls from Gemini into Cancer this month it will also slowly fade.
During its previous apparition in 2014–15 the comet experienced two bright outbursts that rocketed its magnitude to 9 in December 2014 and 8 in January 2015, so be on the lookout. Finlay next reaches perihelion in 2028 — catch it now or kiss it goodbye.
All four of these scope-worthy subjects stand highest in the sky just before the start of morning twilight. How fortuitous that dawn begins so much later than it did back in June. The moon will be out of the picture from September 3rd through the 19th, and the sky will be dark until around 5 a.m. local daylight time.
I'd be remiss if I didn't mention Comet PanSTARRS (C/2017 K2), which appears high in the southwestern sky in the Keystone of Hercules at nightfall. It inches south-southeast this month while gaining a couple tenths of a magnitude. On August 30.1 UT I easily spotted this dainty mini-marshmallow at magnitude 12 at 64×, my lowest magnification. Its moderately condensed (DC 5) coma measured 1′ across.
Comet PanSTARRS has a very long fuse — not until midsummer 2022 will it crest to a peak magnitude of 6 and shoot across Ophiuchus like a flitting firefly. You have through September 14 to view it before the Moon interferes with observation.