We look ahead to see what fuzzy visitors, newly-discovered or returning, will brighten the nights ahead. One and possibly two naked-eye comets are on the way.
Comet chasers stayed busy in 2017 with at least 16 comets brighter than magnitude 13.5. These included three brand new discoveries: Lovejoy (C/2017 E4), the sixth find by Australian amateur Terry Lovejoy; ASASSN1 (C/2017 O1 ASASSN) and Heinze (C/2017 T1).
The enigmatic "rock comet" 174P/Echeclus surprised and delighted with a bright outburst in December that put it within range of an 8-inch scoipe under dark skies, while the ever-fitful 29P/Schwassmann-Wachmann provided a half dozen flare-ups that kept us on our toes.
My notes show no naked-eye comets for 2017 — at least from my observing site — but both 41P/Honda-Mrkos-Pajdusakova and PanSTARRS (C/2015 ER61) came close at magnitude 6.2 in January and April respectively. These, along with four others, made for a total of six binocular comets, depending on your instrument and sky conditions.
The new year opens with four modest comets, all in the 10th-magnitude range, schlepping about about the night sky: dim ASASSN1 (C/2017 O1) still at Polaris's side; PanSTARRS (C/2016 R2) with its free-form, ever-evolving tail; Heinze (C/2017 T1) zipping across the northern evening sky; and 62P/Tsuchinshan, on the fade in Virgo.
In the coming year at least one and possibly three comets may reach naked-eye level with five or six bright enough to see in binoculars. Comet 46P/Wirtanen is the most highly anticipated — it's expected to crest at magnitude 4 in a picture-perfect location alongside the Pleiades and Hyades in December. The key word is expected. You know comets as well as I do, and they don't like to be tied down by our expectations. Expect flare-ups, fades, contrary light curves, and even dissolution of these highly volatile objects.
Read on for a guide to comets predicted to peak at magnitude 10.5 or brighter in 2018.
Currently about magnitude 10 with a 4′ coma and faint, south-pointing tail, you'll find Heinze winging from Cassiopeia into Lacerta in the coming week. At 1h UT on January 5th, it was a diffuse puff of magnitude-9.8 light with a degree of condensation (DC) of 3 on a scale from 0 (totally diffuse) to 9 (completely stellar) through my 15-inch. The comet is expected to slowly fade but remains visible in the evening sky through mid-February.
Documenting the corkscrew twists and turns of the tail of PanSTARRS (C/2016 R2) and the comet's unusual blue color has held the attention of astrophotographers in recent weeks. Sign up to the comet mailing list for more details and photos. Visually, the tail is barely detectable, but an 8-inch or larger telescope will show a nice 10.4-magnitude patch of haze about 4′ across with a DC = 3. The comet's quite easy to find with ample naked-eye stars nearby: it crossed the Hyades a week ago and will pass just 2° northwest of the Pleiades at the end of the month. It appears that C/2016 R2 is unusually rich in frozen carbon monoxide (CO), which when ionized by ultraviolet sunlight, fluoresces blue.
I've seen a wide range in magnitude estimates for this one in recent days. Some observers peg it at magnitude 11.7, others a magnitude brighter. Amateur Scott Harrington even caught it in 8×56 binoculars at magnitude 10.3 on December 28th. Either way, it's a tailless, weakly condensed blob. As the Moon wanes and then departs the morning sky later this week, we'll soon be able to see "what's up" with 62P/T. It culminates in central Virgo around 6 a.m. local time and creeps eastward, slowly fading over time.
Let's hope a new comet's discovered soon or it's gonna be a long winter and spring! Comet-quietude descends until May when Comet PanSTARRS (C/2016 M1) comes to the rescue. It's presently in the pre-dawn sky in Ophiuchus at magnitude 13 and won't crack magnitude 10.5 until early May. Thereafter it picks up speed, traveling from Aquila into Sagittarius. By the summer solstice, it's racing across Corona Australis and enters Ara in early July, when it will be out of view for observers in the northern states. The comet shines brightest at magnitude 9–9.5 through much of June and July and may be visible in binoculars under dark skies.
This one's a wild card. It starts its run at magnitude 10.5 in mid-July, well placed in the morning sky in Camelopardalis. As it scurries east through Auriga and Gemini C/2016 S3 quickly becomes a binocular object at magnitude 7. Even as it brightens rapidly, possibly reaching 6th magnitude on August 7th on its way to perihelion (August 16th), dawn encroaches. We'll probably get our last views when it passes Castor and Pollux on August 4–5 before twilight and daylight finish it off. Thereafter the comet remains near the Sun and rapidly fades from view.
The scenario I just described is based on the most recent orbital elements used by several computer programs and comet sites, but I always crosscheck with JPL's HORIZONS database, which provides detailed ephemerides for many comets. Imagine my surprise when HORIZONS offered an entirely brighter picture. It shows C/2017 S3 at magnitude 5.5 in mid-July, –0.1 in the first days of August, and peaking at –4.6 at perihelion. Holy comas, Batman! If this plot version plays out, the comet may briefly be visible in daylight through a properly shielded telescope.
Expect an excellent apparition of this well-known returning comet and parent of the Draconid meteor shower. Watch for 21P/G-Z to reach magnitude 10.5 in the early July evening sky near the Cygnus–Cepheus border. At the time of the August Perseid shower, it will have swelled to magnitude 8 in Cassiopeia. Peak brightness of 7th magnitude occurs in September when the comet crosses Auriga and Taurus and stands high in the pre-dawn sky. This comet will make an ideal target for sidewalk astronomy gatherings in mid-summer. G-Z drops south in fall but remains brighter than 10.5 magnitude through mid-November, when it finally "bows out" in Puppis.
Long time no see! The 38th comet to have its orbit determined also takes 38 years to orbit the Sun. Discovered in 1867 and last seen in 1980, those of us who missed it then (yeah, yours truly) will finally get a second chance. It's predicted to be a fine apparition with the comet entering the stage 5° north of Betelgeuse in Orion at magnitude 10.5 during the first week of October. Come late November, it peaks around magnitude 9 in Gemini near Castor and Pollux and may be visible in binoculars. 38P remains brighter than magnitude 10.5 through New Year's Day 2019.
I'm cheating a little on this one only because I have a soft spot for periodic comets. Originally discovered in 1889 by the American astronomer Lewis Swift, 64P rounds the Sun every 9.2 years. This time around it hangs out in the vicinity of Beta (β) Andromedae from late October through late November when it's brightest at magnitude 11. Not only is it well placed in the evening sky, the bright star will serve as a sweet guidepost.
Dessert comes last so don't spoil your appetite. Comet 46P/Wirtanen begins its apparition (magnitude 10.5) along the Cetus–Fornax border in the late September morning sky. Before heading north, it slips further south, reaching declination –33° on November 1st but pumping up to magnitude 7, within easy binocular range. About mid-November, 46P launches straight north with ever-increasing speed, rocketing in brightness and altitude until it towers in Taurus in all of its 4th-magnitude glory two weeks before Christmas. Comet 46P's exceptionally close approach to Earth of 11.5 million km on December 16th could mean several weeks of naked-eye visibility from dark skies.
Comet Wirtanen is intrinsically bright, comes closest to our planet just four days after perihelion, and remains visible all night, making this an exceptional apparition. That's why it's the focus of the Planetary Science Institute's 4*P Coma Morphology Campaign.
Not that I want to throw comet dust on your parade, but HORIZONS offers a slightly different scenario, showing the comet's brightness peaking at magnitude 7.6. Should 46P only manage binocular brightness, it would only be asserting its individuality, something we've come to expect from these icy characters.
For visual observers and astrophotographers, the late fall and early winter season will be a great time to visit with "old friends" with no fewer than four periodic comets visible simultaneously — 21P, 38P, 64P, and 46P. Happy hunting!
For more information on upcoming comets, check these sites, too: