How To Observe The Best Comets of September

This month we focus on a comet that's unexpectedly produced two major outbursts recently, and there's another you'll want to keep an eye on, because it may be brightening much faster than expected. Let's start with our outburst comet, 220P Mignot. It's a short-period Jupiter family comet with an orbit of 5.5 years, carrying it from out near Jupiter at the most distant point in its orbit to just past the orbit of Mars at its closest. It passed perihelion, or closest approach to the Sun, in mid-June. It will next return in 2031. 220P Mignot has had an unusual year. On June 1st, it went from an 18th magnitude object that was essentially below the radar of amateur observers to magnitude 8.5 in just a couple of days, then produced a second major outburst in August. Not only does an outburst make a comet brighter, but expanded gas and dust makes it larger, and they can develop a longer, more interesting tail. Because it's a transient event, the appearance of the comet can change from week to week, or even night to night. Astronomers call events like this outbursts. The exact mechanism is unknown. The most likely possibility is the sudden exposure of fresh ice, with proposed explanations including the collapse of material after repeated solar heating, fragmentation, or even a collision with a small comet fragment. The June 1st outburst came as a surprise because it was the first recorded outburst for 220P Mignot. It also may have been the largest such event observed in a comet since 17P Holmes in 2007. The behavior of other comets suggested that a second, probably smaller, outburst could occur, and we suggested that comet chasers keep an eye on it for this reason. Just a few days after the release of our August video, on August 6th, 220P Mignot produced the second outburst, and it was another major one. Afterward, 220P took on an amazing appearance, which began as an expanding cloud of dust and fluorescing gas that became shaped by radiation pressure and solar wind as the cloud was slowly pushed away from the sun. As of now, we're seeing a very regular fade, at about the same rate it did following the June outburst. As September begins, 220P Mignot is now a morning comet in Cetus at about magnitude 10.9. The coma is three and a half arc minutes across with a fused condensation at the center. The tail extends about 19 arc minutes at a position angle of 262 degrees. Currently, the standard predictions without the outburst are mostly flat, so the brightness of the comet is still entirely due to the outburst. It is currently fading by about one magnitude every six days, likely to settle at magnitude 12.5 at mid-month. As a result, we suggest getting it as early in the month as possible. By mid-month, it will likely be much fainter. In the northern hemisphere, a four-inch telescope at a dark site will show the comet as perceptible before morning twilight beginning on September 5th. Near the equator, the observing window begins a little earlier. A four-inch telescope at a dark site should show the comet as perceptible starting on September 4th. At 30 degrees south, a four-inch telescope shows 220P Mignot as perceptible at a dark site from September 3rd before morning twilight. Next, let's catch up with our old friend 10P Tempel. 10P Tempel is still observable in September, but the situation has changed quite a bit since last month. It's now an evening comet, which is good, but fading rapidly. For observers at a dark site, it remains within reach of binoculars in the southern hemisphere and is a telescopic target farther north. 10P Tempel is another Jupiter family comet with a very similar orbit to that of 220P. Its orbital period is 5.4 years, passing just inside the orbit of Mars when closest to the Sun, Perihelion, and extending out close to the orbit of Jupiter, Aphelion. It passed Perihelion on August 2nd and is now heading back out towards the outer solar system. 10P Tempel begins September in Pisces Astrinus at about magnitude 9.1. The coma is about five arcminutes across with the center that is much wider than the edges, although it remains diffuse. The tail position angle is 26 degrees with a length of about 44 arcminutes. There is no visibility this month from 55 degrees north. At 40 degrees north, the comet is perceptible in a 4-inch telescope from a dark site around midnight from September 1st through the 14th. With a 6-inch telescope, it remains perceptible through September 19th. Near the equator, 10P Tempel is perceptible in binoculars from a dark site near midnight from September 1st through the 7th. It should be perceptible in a 4-inch telescope from September 1st through the 21st. At 30 degrees south, binocular observers at a dark site can find it before midnight from September 1st through the 9th. A 4-inch telescope shows it easily from September 1st through the 6th, then perceptibly through the 21st, with another brief opportunity on the 28th and 29th. So, like 220P, 10P Tempel is still worth checking in early September, particularly from the southern hemisphere, but this is a comet that is also fading rapidly. If you want to catch it, don't wait too long. There's one more comet to keep an eye on this month, 161P Hartley-Iras. This is a Halley-type comet with an orbital period of 21 years. Its orbit reaches out to 14 astronomical units, beyond the orbit of Saturn. It will pass closest to the Sun on November 27th, at a distance of just 1.3 astronomical units, just beyond the orbit of the Earth. We don't have many recent observations of this comet, and the observations we do have show a lot of scatter. It's not obvious yet what the actual brightening trend is. One interpretation suggests that 161P may have turned on as it approaches the Sun, and is brightening rapidly. Another suggests that much of that apparent trend is simply scatter in the observations. We've adopted a more conservative brightening rate that's somewhere in the middle, but even that prediction suggests that 161P could brighten dramatically during September, which makes it worth checking on if you're already out observing. This SkyTools Atlas view shows its path across the sky during September. This image of the comet shows what you'd expect from a classic telescopic comet, a small, fuzzy star. If 161P does brighten as rapidly as our more optimistic interpretation suggests, that won't turn it into a naked-eye comet. It would simply make that fuzzy star easier to detect in smaller telescopes, perhaps as small as 6 inches, although an 8-inch telescope is more likely. So this is one to watch. If you're already out observing, give 161P a look. Even if you're using a smaller scope, it may turn out to be visible. It never hurts to have a look. This may also make a good imaging target as it approaches the Sun, perhaps developing a tail. Lastly, we're updating the comet chasing page. Not only does it look better, but we think the information is presented in a more useful way for observers. We're still ironing out some of the kinks, so it may take a couple of days to appear after the first. Let us know in the comments what you think, and especially if you have any suggestions. Well, that's it for September. We'll see you next month, or if something interesting happens before then. Clear skies, and thanks for watching.