Four comets, a famous variable star, and sprinkles of dust from Halley's Comet (which create the Orionid meteor shower) are highlights of the October sky.
I hope you had the opportunity to observe C/2018 W2 (Africano), the brightest comet of the summer and fall. It peaked around 9th magnitude earlier this month while speeding through Pisces after making its closest approach to Earth on September 27th. I saw it last on October 3rd before the Moon interfered. In my 15-inch at 64× magnification, the comet's moderately condensed coma measured about 3′ across with a stubby tail pointing to the northeast. Photographs reveal green carbon emission, a bright pseudo-nucleus and smear of a tail.
Comet Africano continues to clip southward. This week and next it slices across Piscis Austrinus before dropping out of view for observers at mid-northern latitudes. The comet will also fade from its current magnitude of 9.5 to the mid-10s by late October, so catch it the next clear night. Moonless skies return as soon as tonight, with best viewing times between 8 and 11 p.m. local time.
Faint Fall Comets
C/2018 N2 proved a pleasant surprise at 142× on October 7th with a strongly condensed (DC=6) coma 1.5′ across. With averted vision and 245× I teased out a faint, 14th-magnitude stellar nucleus and suspected a ~1.5′ tail to the south. Given its high DC and overall magnitude of 11.5, an 8-inch scope under dark skies should nail this one.
Closest approach to Earth occurs on October 20th followed by perihelion on November 10th, so this celestial Q-tip will be easy to follow throughout the fall as it brightens a smidge through late October. If you're looking for the perfect time to find it, C/2018 N2 will pass within 1° of 2nd-magnitude Beta (β) Andromedae on October 19-20 and 3° southwest of the Andromeda Galaxy on November 2nd.
260P/McNaught, another minnow of a comet, spends October wriggling northeastward across Perseus. On October 7th, it glowed at magnitude 12 but was easy to see even at low magnification because of its small size — just 1′ across — and compact (DC=5) coma. You'd think a 12th-magnitude comet wouldn't show a tail, but it was surprisingly easy to spy with averted vision as a 1.5′ long streak pointing southwest. I estimated the false stellar nucleus at magnitude 13.5. Even as 260P slowly fades this month it remains well-placed for viewing from mid-northern latitudes throughout the fall.
C/2017 T2 (PanSTARRS) is expected to be the main act for comets in 2020, as it's expected to brighten to magnitude 8 by next May. For now, it slumbers around magnitude 12.5 while inching northwestward through the open cluster-rich region of central Auriga. On the nights of October 27th and 28th, the comet ambled up alongside the bright open cluster M36, some 20′ east of its core. While the previous three comets are visible at nightfall, C/2017 T2 doesn't climb high enough for a good look until after 11 o'clock local time (10 p.m. by end of month). Be aware that the Moon will interfere until Oct. 20.
On October 7th, PanSTARRS sported a small, dense 45″ coma with a DC=6 at 142×. Upping to 245× I noted a nearly opaque inner coma with a 15th-magnitude flicker of a nucleus. I applied a Swan Band filter on this and the other three comets, but none showed an improvement in contrast or brightness — an indication that the whole lot are relatively rich in dust compared to fluorescing diatomic carbon gas, one of the substances that causes comas to glow green.
Below you'll find detailed maps created with SkyMap to track each of our four visitors over the next few weeks. The wide-field map above will give you an idea of when and where to start looking. North is up in all charts and positions are shown daily at 0 hours UT. Subtract 4 hours for EDT, 5 for CDT, 6 for MDT and 7 for PDT. For example: October 20, 0h UT = October 19, 8 p.m. EST.
You might be wondering what's up with interstellar comet 2I/Borisov. It remains very faint at around magnitude 16. I've heard of no amateur visual sightings, though it's increasingly showing up in astrophotos. We'll check in with this alien visitor again in December when it's closer to perihelion. I've still got my fingers crossed amateurs will spot it in bigger instruments around the time of its December perihelion.
Under Mira's Eye
I like to picture all these comets wending their ways under the watchful eye of Mira the Wonderful, one of the best known and beloved variable stars. The brightness of the red giant star varies over a period of about 333 days and is just now coming into good view, while simultaneously experiencing one of its brighter maxima. With a current magnitude of about 2.9, you can't miss it! Especially when this weekend's full moon departs the scene. Will you be able to detect the star's ruddy hue with the naked eye? I know it's visible in my binoculars.
Mira's peak brightness varies from cycle to cycle, but right now it's at or near maximum. Keep an eye on it and you can watch it slowly return to minimum brightness over the coming months. The chart above will help you estimate the Mira's changing brightness.
Orionid Meteor Shower
Hmm ... what else would spice up late October skygazing? How about a nice little meteor shower? The annual Orionids peak on the night of October 21-22 (that is, Monday night through Tuesday morning), with 15-20 swift meteors visible per hour from a dark, moonless sky. Each crumble of rock or mote of dust flashing before your eyes is a souvenir from Halley's Comet. Although the waning last-quarter Moon (42% illuminated) will interfere, it's not so bright as to call off spending a relaxing hour before dawn watching meteors fly. You might even try your luck from 11:30 to 12:30 before moonrise when Orion first appears in the east.