NASA’s TESS Watched an Outburst from Comet 46P/Wirtanen

Comet 46P/Wirtanen. Image Credit: By Stub Mandrel at English Wikipedia, CC BY-SA 4.0,

TESS, the Transiting Exoplanet Survey Satellite, has imaged an outburst from the comet 46P/Wirtanen. It caught the outburst in what NASA is calling the clearest images yet of a comet outburst from start to finish. A comet outburst is a significant but temporary increase in the comet’s activity, outside of the normal sunlight-driven vaporization of ices that creates a comet’s coma and tail.

Astronomers aren’t certain what causes them, but a new study based on this observation is shedding some light on them.

Continue reading “NASA’s TESS Watched an Outburst from Comet 46P/Wirtanen”

Catch Comet 21/P Giacobini-Zinner at Its Best

Comet 21P/Giacobini-Zinner from August 14th. Image credit and copyright: Rolando Ligustri.

Comet 21/P Giacobini-Zinner from August 14th. Image credit and copyright: Rolando Ligustri.

A periodic comet may put on a fine show for northern hemisphere viewers over the next few months.

Comet 21/P Giacobini-Zinner is currently a fine binocular comet, shining at +8th magnitude as it cruises across the constellation Cassiopeia. This places it above the horizon for the entire night for observers north of the equator in August, transiting the local meridian at dawn. And unlike most comets that get lost in the Sun’s glare (like the current situation with C/2017 S3 PanSTARRS), we’ll be able to track Comet 21/P Giacobini-Zinner right through perihelion on September 10th.

The orbit of comet 21P, showing the comet’s position at closest approach. Credit: NASA-JPL.

This is because the comet is on a short period, 6.6 year orbit around the Sun that takes it from an aphelion of 6 Astronomical Units (AU) exterior to Jupiter’s orbit, to a perihelion of 1.038 AU, just 3.3 million miles (5.2 million kilometers) exterior to Earth’s orbit. The 2018 apparition sees the comet pass 0.392 AU (36.5 million miles/58.3 million kilometers) from the Earth on September 11th.

This is the closest passage of the comet near Earth since September 14th, 1946, and won’t be topped until the perihelion passage of September 18th, 2058. Its next cycle of passes to Earth closer than 0.1 AU aren’t until next century in the years 2119 and 2195, respectively.

Comet 21/P from August 10th. Image credit and copyright: John Purvis.

Discovered by astronomer Michel Giacobini at the Côte d’Azur Observatory in Nice, France on the night of December 20th, 1900 as it was crossing the constellation Aquarius, the 21st periodic comet was recovered two orbits later by Ernest Zinner on October 23rd, 1913 as it passed a series of variable stars near Beta Scuti.

Though the comet generally tops out at +8th magnitude, it has been known to undergo periodic outbursts near perihelion, bringing it up about 3 magnitudes (about 16 times) in brightness. This occurred most notably in 1946.

The light curve for Comet 21/P. The black dots represent actual observations and magnitude observations. Credit: Seiichi Yoshida’s Weekly Information on Bright Comets.

Comet 21/P Giacobini-Zinner is also the source of the Draconid (sometimes referred to as the Giacobinid) meteors, radiating from the constellation Draco the Dragon on and around October 7th and 8th. Feeble on most years, this shower can produce surprises, such as occurred in 1998, 2005 and most recently in 2011, when a Draconid outburst topped a zenithal hourly rate of 400 meteors per hour, flirting with ‘meteor storm’ status. And while we’re not expecting a meteor storm to accompany the 2018 perihelion passage of Comet 21/P Giacobini-Zinner, you just never know… it’s always worth keeping an eye out on early October mornings for the “Tears of the Dragon,” just in case. Note that the Moon reaches New phase on October 9th, just a few days after the meteor shower’s expected annual peak, a fine time to watch for any unheralded Draconid outbursts.

Prospects for Comet 21P

The comet is visible from the northern hemisphere through the remainder of August and all through September as it glides across Auriga, Taurus and Gemini and visits several well known celestial sights. In fact, it actually transits in front of several deep sky objects, including Messier 37 (Sept 10th), and Messier 35 (Sept 15th).

Comet 21P passes in front of open cluster Messier 35 on September 15th. Credit: Starry Night.

The comet will be moving at about two degrees per day when it’s nearest to the Earth, on and around September 11th.

We begin to lose the comet, as it heads southward in late October. Still, the comet is over 50 degrees above the eastern horizon at dawn come October 1st as seen from latitude 30 degrees north, having maintained a similar elevation throughout most of September. Not bad at all.

Here are some upcoming dates with destiny for Comet 21/P Giacobini-Zinner:

August 19: Crosses into the constellation Camelopardalis.

August 29: Crosses into the constellation Perseus.

August 30th: Crosses into the constellation Auriga.

The celestial path of Comet 21P from August 16th through September 15th. Credit: Starry Night.

September 2: Passes one degree from the bright star Capella.

Sept 7-8: Grouped 2 degrees from the open clusters M36 and M38.

Sept 10: Photo-Op: Skirts very near the open cluster M37. Also reaches perihelion on this date, at magnitude +7.

Sept 11: Passes closest to the Earth, at 0.392 AU distant.

Sept 13: Nicks the corner of the constellation Taurus.

Sept 14th : Enters the constellation Gemini.

Sept 15th: Photo-Op: crosses in front of the open cluster M35.

Sept 16: Crosses the ecliptic southward and near the +3.3 magnitude star Propus (Eta Geminorum).

Sept 17: Crosses into Orion.

Sept 21: Crosses into Gemini.

Sept 23: Crosses into Monoceros.

Sept 24: Passes near the Christmas Tree Cluster, NGC 2264.

The celestial path of Comet 21P from September 15th through October 15th. Image credit: Starry Night.

Oct 1: Crosses the galactic plane and the celestial equator southward.

Oct 7: Crosses in front of the open cluster M50.

Oct 10: Crosses into Canis Major.

Oct 31st: Passes near the bright star Aludra and may drop below +10th magnitude.

Binoculars are your best friend when you’re looking for comets brighter than +10th magnitude. With a generous field of view, binoculars allow you to sweep a suspect area until the faint fuzzball of a comet snaps into view. I like to ‘ambush’ a comet as it passes near a bright star, and a good time to spot comet 21/P Giacobini-Zinner is coming right up on September 2nd when it passes less than one degree from the bright +0.1 magnitude star Capella.

Don’t miss this year’s fine apparition of Comet 21/P Giacobini-Zinner, coming to a night sky near you.

New Comet: C/2017 O1 ASAS-SN Takes Earth by Surprise

Getting brighter... Comet O1 ASAS-SN from July 23rd. Image credit and copyright: iTelescope/Rolando Ligustri.

Getting brighter… Comet O1 ASAS-SN from July 23rd. Image credit and copyright: iTelescope/Rolando Ligustri.

A new comet discovery crept up on us this past weekend, one that should be visible for northern hemisphere observers soon.

We’re talking about Comet C/2017 O1 ASAS-SN, a long period comet currently visiting the inner solar system. When it was discovered on July 19th, 2017 by the All Sky Automated Survey for Supernovae (ASAS-SN) system, Comet O1 ASAS-SN was at a faint magnitude +15.3 in the constellation Cetus. In just a few short days, however, the comet jumped up a hundred-fold in brightness to magnitude +10, and should be in range of binoculars now. Hopes are up that the comet will top out around magnitude +8 or so in October, as it transitions from the southern to northern hemisphere.

ASAS-SN North on the hunt. Credit: ASAS-SN

Never heard of ASAS-SN? It’s an automated sky survey hunting for supernovae in both hemispheres, with instruments based at Haleakala in Hawaii and Cerro Tololo in Chile. Though the survey targets supernovae, it does on occasion pick up other interesting astronomical phenomena as well. This is the first comet discovery for the ASAS-SN team, as they join the ranks of PanSTARRS, LINEAR and other prolific robotic comet hunters.

Evoking the very name “ASAS-SN” seems to have sparked a minor controversy as well, as the International Astronomical Union (IAU) declined to name the comet after the survey, listing it simply as “C/2017 O1”. Word is, “ASAS-SN” was to close to the word “Assassin” (this is actually controversial?) For our money, we’ll simply keep referring to the comet as “O1 ASAS-SN” as a recognition of the team’s hard work and their terrific discovery.

The orbit of Comet C/2017 O1 ASAS-Sn through the inner solar system. Credit: NASA/JPL

But what’s in a name, and does an interplanetary iceball really care? On a long term parabolic orbit probably measured in the millions of years, O1 ASAS-SN has an orbit inclined 40 degrees to the ecliptic, and reaches perihelion 1.5 AU from the Sun just outside the orbit of Mars on October 14th. This is most likely Comet C/2017 O1 ASAS-SN’s first passage through the inner solar system.

Currently located in the constellation Eridanus, hopefully comet O1 ASAS-SN’s current outburst holds. Expect it to climb northward through Taurus and Perseus over the next few months as it begins the long climb towards the north celestial pole.

Anatomy of an outburst: Comet ASAS-SN shortly after discovery over the span of a week. Credit ASAS-SN1.

As seen from latitude 30 degrees north, the comet will move almost parallel to the eastern horizon, and clears about 20 degrees altitude around local midnight, very well placed for northern hemisphere observers.

The path of Comet C/2017 O1 ASAS-SN parallel to the eastern horizon through September as seen from latitude 30 degrees north. Credit: Stellarium

At its closest in mid-October, Comet O1 ASAS-SN will be moving a degree a day through the constellation Camelopardalis

Here’s a month-by-month blow by blow for Comet O1 ASAS-SN:


14- Crosses into Cetus.

16- Crosses the celestial equator northward.

20- Crosses into Taurus.

The celestial path of Comet C/2017 O1 ASAS-SN from late July through mid-October (click to enlarge). Credit: Starry Night.


11-The waning gibbous Moon passes two degrees to the south.

17- Crosses the ecliptic northward.

20- Photo op: passes 4 degrees from the Pleiades open star cluster (M45).

28-Crosses into Perseus.

The projected light curve for Comet C/2017 O1 ASAS-SN. Note the outburst from actual observations (black dots). Credit: Seiichi Yoshida’s Weekly Information About Bright Comets.


1-Reaches max brightness?

12-Crosses the galactic equator northward.

14-Reaches perihelion 1.5 AU from the Sun.

17-Crosses into Camelopardalis.

18- Passes closest to Earth at 0.722 AU distant.

29-Passes 10′ from the +4 mag star Alpha Camelopardalis.


17-Crosses into Cepheus


6-Passes 3 degrees from the north celestial pole.

12-Reaches opposition.

31-Drops back down below +10th magnitude

At the eyepiece, a small comet generally looks like a small fuzzy globular cluster that refuses to snap into focus. Seek out dark skies in your cometary quest, as the least bit of light pollution will dim it below visibility. And speaking of which, the Moon is also moving towards Full next week so the time to hunt for the comet is now.

We’ve still got a few weeks left before the August 21st total solar eclipse for a bright “eclipse comet” to show up… unlikely, but it has happened once in 1948.

Comet C/2017 O1 ASAS-SN from July 23rd. Credit: Remanzacco Observatory.

Keep in mind, current magnitude estimates for Comet O1 ASAS-SN are still highly speculative, as we seem to have caught this one in outburst… hey, remember Comet Holmes back about a decade ago in 2007? One can only dream!

-Also check out this recent NEOWISE study suggesting that large long period comets may be more common that generally thought.