You gotta love comets. OK, except when they fizzle. But outside of that, they can be tremendous fun to watch. That's so true of Comet PanSTARRS (C/2017 S3), which is now undergoing a second outburst just two weeks after its first. This latest explosion has pumped enough life into the vagabond to see it in a pair of binoculars. No kidding. I spotted with ease in my 10×50s on July 16.20 UT. Nothing fancy, just a small blob, but at magnitude ~8.2 it was obvious.
With a Frankensteinian jolt, C/2017 S3 first came to life on or about June 30th, sitting straight up at magnitude 9.5 after a three magnitude outburst. The explosion released a significant amount of C2 or diatomic carbon gas, which gives comet comas their characteristic aqua-green hue. Swan Band filters capitalize on this fact by transmitting only a few narrow slivers of light, including C2emissions, to provide clearer and contrastier views of "gassy" comets.
Days later, C/2017 S3's core began to fade as the explosion played itself out. But just when we suspected the comet had hit the self-destruct button — as some predicted it would and still might — it burst back to life on or about July 14th, bigger and brighter than before. Austrian comet observer and astrophotographer Michael Jäger may have been first to record the new blast. His photo, taken July 1.04 UT, shows a gorgeous green coma 4′ in diameter trailed by a narrow, 25′-long ion tail to the northwest.
This photo of C/2017 S3 was taken on July 16.18 UT with a 180-mm telephoto lens and two-minute time exposure. I then cropped and enlarged the view for the inset photo.
Bob King
In my 15-inch (37-cm) telescope on July 16th and 17th, the blue-green coma color was obvious at 64× magnification. Without the Swan filter I estimated the coma diameter at 3′ with a DC (degree of condensation) of 6, i.e., strongly condensed. With the filter in place the comet's head expanded to 4′. No tail was seen despite dark Bortle Class 3 skies.
C/2017 S3 could brighten to magnitude 3 or 4 as it beelines from Camelopardalis into Gemini now through about August 8th, when morning twilight gets in the way. Perihelion occurs on August 15th with the comet only 0.2 a.u. from the Sun. Closest approach to Earth happens a week earlier on August 7th at 0.76 a.u. Since the comet is fresh from the Oort Cloud, it's a bit of an unknown. It may explode again and cash in its chips before perihelion or power through. We'll know very soon.
The orbit of C/2017 S3 is steeply inclined at 99° and passes close to the Sun at perihelion.
JPL / HORIZONS
Because of its northerly declination (currently +55°N), C/2017 S3 is visible all night long for the northern third of the U.S. and Europe. You can look for it low in the northern sky at nightfall; by the start of dawn, it's ~30° high in the northeast. For the southern third of the U.S., it's at or below the northern horizon at nightfall and best seen just before dawn about 30° up in the northeastern sky. This green phantom is slipping south and will slowly lose altitude over the next few weeks. Take a look soon because, well, it's a comet, and you just never know.
This basic map shows the position of Comet PanSTARRS every 3 days at 8:30 UT (4:30 a.m. EDT, 3:30 a.m. CDT, etc.) in the morning sky. Stars are plotted to magnitude 6.5. Click here for a more detailed chart.
Stellarium
Another fuzzball, 21P/Giacobini-Zinner, is fattening up nicely. Now at magnitude ~10, I caught it crawling past Delta (δ) Cephei on the night of July 15–16. It was still small with a 2′ coma and short 4′ tail pointing southwest, but the coma was bright and dense with a false nucleus — not only pretty but a good omen for continued brightening. According to predictions, 21P/G-Z will peak around magnitude 7 in August and September. For the moment, an 8-inch should give a good view, but it won't be long before small scopes scoop it up.
Easy comets like these make it a little easier to tolerate the pesky mosquitoes that descend this time of year. To find our targets, use this chart for 21P/Giacobini-Zinner. It plots stars to magnitude 9.5 with positions marked at 0h UT every three days. For Comet PanSTARRS (C/2017 S3) use the one above or try this more detailed map. You can also visit The Sky Live where current comets' daily positions are plotted on a photographic sky.