Comet 29P/Schwassmann-Wachmann 1 Blows Its Top . . . Again!

Comet 29P/Schwassmann-Wachmann 1 has gone into outburst again, wracked by yet another eruption of gas and dust. See it while you can! 

A Constant Comet

Comet 29P/Schwassmann-Wachmann 1 exhibits its classic lopsided coma on July 31 during its most recent outburst. Discovered photographically in 1927 by German astronomers Arnold Schwassmann and Arno Arthur Wachmann, 29P is the most active periodic comet known.
Pepe Manteca

Most comets come and go with a regular period or appear unannounced from the Solar System's cryogenic freezer, the Oort Cloud. Not 29P/Schwassmann-Wachmann 1. This strange and remote object with the throat-clearing name is normally visible for at least a few weeks every year in amateur telescopes when it undergoes an outburst and skyrockets in brightness. This happens not just once a year but 6-7 times on average!

On July 28th, French comet observer Jean-Francois Soulier photographed yet another 29P eruption. Prior to his discovery, the comet had slumbered in eastern Sagittarius around magnitude 15 with a dilute coma and faint nucleus. But overnight it surged to magnitude 12.8, the coma thickened and its false nucleus intensified. From a faint, weak fuzz, the comet briefly assumed a near-stellar appearance.

Through my 15-inch reflector one night later (July 28th-29th), I saw a dense fuzzy spot not unlike a planetary nebula or a star smeared by atmospheric turbulence. Upping the magnification to 224x to get a better look, I estimated the coma to be a mere 15″ across with a  bright but indistinct nucleus.

Comet Loops West

29P slowly ambles west in Sagittarius at –25.5° declination over the next month. The comet's position is shown every 5 days at 11 p.m. CDT with stars plotted to about magnitude +13. North is up. Click to enlarge, save and print out for use at the telescope.
Chris Marriott's SkyMap

Though the comet has continued to expand and slowly fade, 29P still glows at 13th magnitude and remains a fine target for telescopes 10 inches or larger. You'll find it using the included map about 1° south of Chi-1 and Chi-3 Sagittarii, a pair of 5th magnitude stars east of the Teapot's handle. The comet tracks westward in retrograde motion through much of August. Despite its low declination I wouldn't rate it as too difficult a catch even from the northern U.S., the Canadian border and central Europe,  as long as you're observing under a dark sky.

Both Comet and Centaur

Periodic comet 29P orbits just beyond Jupiter in a nearly circular orbit.

Comet 29P swings around the Sun once every 14.6 years in a nearly circular orbit, its distance ranging from 5.7 to 6.2 a.u., or just beyond the orbit of Jupiter. Even though it's a comet, its orbit also makes it a member of a special class of objects called Centaurs. Only about 80 are known. Centaurs cycle round the Sun between Jupiter and Neptune and have unstable orbits; they're thought to be relatively recent escapees from the Kuiper Belt.

How a Faraway Comet Gets Bright

Rarely do comets beyond Jupiter flare brightly enough to detect in amateur telescopes, but 29P can vault to 9th magnitude during its brightest outbursts — a 10,000-fold increase in light! — making it an easy target for scopes as small as 6-inches. 

Conflagration, Expansion and Fade

This composite photo shows the fascinating evolution of a 29P flare-up from June 16, 2013 (lower left) to July 28th. The comet's coma often displays a characteristic horseshoe or spiral shape early on as we view the outburst side-on across one hemisphere only.
Damian Peach

Comet nuclei are composed primarily of ices including water, carbon dioxide, carbon monoxide, methane and ammonia mixed with fine silicate and carbon-rich dust. When a comet swings within 3 to 4 a.u. of the Sun, water from vaporizing (sublimating) ice is the primarily volatile carrying off dust to form the coma and tail. At 29P's bone-chilling distance, though, water ice is virtually inert. Instead, vaporizing carbon monoxide (CO) ice appears to be the key driver in its regular outbursts. A variety of different radio telescopes have detected the gas in 29P's coma during its frequent flare-ups.

Odiferous emanation

A spectacular jet, photographed by ESA's Rosetta spacecraft, blasts from Comet 67P/Churyumov-Gerasimenko on July 29, 2015. It enriched the coma with carbon dioxide, methane, hydrogen sulfide as well as organic materials likely related to dust released.

Since the freezing point of CO is –337°F (–205°C), it readily vaporizes from solar heating at the comet's distance. Although the cause of comet outbursts remains an inexact science, it's thought that subsurface ice, in this case carbon monoxide, is warmed by the Sun and vaporizes.

Now under pressure, the gas seeks a way to the surface through cracks and vents. When it finds an escape route, it blasts into space like a geyser, carrying dust and debris along for the ride. Pits and collapse cavities on the comet's surface may also provide convenient exits for cometary gases.

Under constant attack by solar heating from above and below, comets can develop cracks and cavities that expose even more fresh ice, leading to continued episodes of sublimation and outburst.

Dr. Richard Miles, Asteroids and Remote Planets section director for the British Astronomical Association, proposed a new outburst mechanism for the comet in the July 2016 volume of the journal Icarus. Based on data obtained largely by amateur astronomers, Miles describes the outbursts as cryovolcanoes that erupt explosively after CO and methane under pressure beneath the comet's crust melt, mix and release energy. Solar heating causes the crust above these spots to give way, whereby the gases explode into space, carrying with them up to a million tons of dust and debris. Think of it as popping the cork on a bottle of champagne. Because 29P is a large comet, its gravity temporarily reseals the "wound" until another cycle begins.

Like Lake Superior in the wintertime, this comet's gotta lotta ice. Recent measurements by NASA's Spitzer Space Telescope give a diameter of 37 miles (60 km) for the nucleus, making 29P one of the largest comets known. Perhaps a bountiful supply of material coupled with its nearly constant distance from the Sun act to keep 29P active throughout its orbit. Whatever the cause, amateurs are grateful for 29P's fitful behavior. Its followers get to witness firsthand the essentially chaotic and unpredictable nature of comets — every year!

Back to Square One

Comet 29P returned to quiescence by mid-August after the summer 2013 outburst. The comet's nucleus rotates once every ~58 days which allows plenty of time for the sun's heat to penetrate below the surface and heat volatile ices there.
Damian Peach

I caught my first flare-up in the early 1980s. Like avid marathon runners who try to hit marathons in every state, I make it a point to catch at least an outburst a year. I haven't missed one yet thanks to the great observers and astrophotographers who share their images and observations on the Comet Mailing List. If you routinely observe the comet every time you're out with the telescope, you might even be the first to report the next eruption.

As described earlier, Comet 29P has an unstable orbit. One day in the distant future, perturbations from Jupiter may nudge it into the inner Solar System, where its cornucopia of sublimating ice and dust should transform it into a cometary spectacle!

See a selection of the brightest comets over the last century in this classic issue of Beautiful Universe.

12 thoughts on “Comet 29P/Schwassmann-Wachmann 1 Blows Its Top . . . Again!

  1. Mike-Edmunds

    Great article about 29P. That potential “cometary spectacle” you mentioned may not come to pass if and when it gets diverted into the inner solar system. With its continuing low level of activity over the millennia, 29P could develop a thick crust of devolatilized material that would prevent any out gassing. In which case it may appear to an observer as an Earth-crossing C-type asteroid. Which are cool and interesting in their own right.

  2. Richard Miles

    Nice article, Bob and it should encourage observers to keep a close watch on the comet now that it is accessible for northern hemisphere observers in the evening time.

    Can I just say that you state “Under constant attack by solar heating from above and below, comets can develop cracks and cavities that expose even more fresh ice, leading to continued episodes of sublimation and outburst.” yet you do not point out that this is highly unlikely to be the process going on within the nucleus of 29P. Are you aware of the three papers on 29P that have been published in the April issue of the journal, Icarus earlier this year?

    1. Bob KingBob King Post author

      Hi Richard,
      Thank you for sharing your recently-published research with me which I’ll share with our readers in a brief update to the blog.

  3. Graham-Wolf

    Hi Bob. Already knew about this.
    Our T.A. Comet team regularly monitor this comet for it’s outbursts, which seem to occur at ~ 48 to 52 day intervals. My first observations of this latest outburst were just just a few hours after Marco Goitao at Brazil… it was already Mv 10.8 to 11, and in a borrowed 12cm f8 Newtonian at 150x, quite visibile! Couldn’t believe my eyes. My team were almost immediately alerted. Followed the comet for several days… it soon faded out well beyond Mv 12, by August 10th. Some data already posted on LIADA. Keep up the vigilance! Regards:- Graham Wolf, NZ

    1. Bob KingBob King Post author

      Will do, Graham. And did you hear the news? 29P just went into outburst again late on Aug. 22. More news on that as it arrives. This comet never disappoints!

  4. Graham-Wolf

    Yep, Bob…. I know about this outburst too!
    Was field testing Chris Marriott’s Finderchart (in your article) in-between Dunedin-based rain-showers around midnight local time, just 2 days ago. Crikey… it had gone and outburst again!! My T.A. Comet Team (based at Basingstoke, UK) are already onto this. I’ve already created customised Comparison Charts via the AAVSO-VSP website. Enter the 2000.0 Epoch co-ordinates for a suitable finder-star nearby, then enter the desired Mv limit, then FOV, and field orientation. Try say:- 5df MLim 12, or 3df MLim 14 (if feeling adventurous). If you’re based in the antipodes like me, just flip the resultant chart. Don’t you just love these international collaborations?. Real “Astronomy-without-borders”. Richard Miles ((BAA) has already done extensive statistical modelling on this famously capricious comet. “The only predictable thing about a comet, is it’s unpredictability”… a famous quote indeed! Eyes upwards, everybody… 29p is doing it AGAIN. Betcha my wonderful Aussie counterpart:- Chris Wyatt at Walcha, NSW, is giving 29p some serious attention. He usually does! Follow his data on the LIADA database website. Regards from Graham W. Wolf, NZ.

    1. Bob KingBob King Post author

      Glad to hear you’re on it, Graham-Wolf. Although I’ve heard no magnitude estimates, the forecast is for clear tonight, so I plan to set up the scope here at home for a look. Thanks for AAVSO site tip – I’ve never tried that charting method. My fave comet-plotting software is Megastar.

  5. Graham-Wolf

    Hello again, Bob.
    More scattered showers down here in NZ, last few days. Only a few brief 10 – 20 min moments of seeing in Mv 6.4 to 6.6 skies at the Fairfield Quarry Darksite this week:- ( -45.8950830 S, +170.4063333 E, 16.0m amsl). Recent Mv estimates by others are still not yet available… most observational websites not up-dated for nearly a fortnight. Expect Prof Luis Mansilla to publish some 29p comet data updates on LIADA next week. So far, Chris Wyatt’s data for Aug 12 seems to the the latest data publicly out there, that I know of. Here at 46 South, the comet transits at 70 deg alt ~ 9:40pm local time… most convenient! My 29p observational data is being submitted early September to T.A. in the UK, to publish in it’s monthly Journal;- “The Astronomer”. By then, I expect other people’s data to come forward. I notice that Venus and Jupiter are currently extremely close in planetary conjunction, and there is a Penumbral Lunar Eclipse next month on the 17th. I’m also closely monitoring C/2013 X1 Panstarrs. Who said the night sky is boring and dull? Keep up the great work, Bob! Graham Wolf:- NZ.

  6. Graham-Wolf

    Hello Bob.

    Have yet to hear of any Mv estimates for 29p’s latest outburst. A corrected transcription error by me… (converting from NZST to UT, sleep deprivation/”brain fade”), so here’s two of mine, to get things started.

    Aug 23.34 UT (8:10pm NZST) 150x Mv 11.6 ZLM 6.4
    Aug 23.45 UT (10:48pm NZST) 150x Mv 11.3 ZLM 6.6

    It appears that in the space of some 2.6 hr, the comet came up another 1/4 Mv or so. Featureless coma in ZLM 6.4 to 6.6 skies at the Fairfield Quarry Darksite (FQD). Lots of scattered showers this week, and only 10 to 20 minutes of clear sky to use:- between them, at times. Deep twilight soon wipes out, after about 7pm local time. What the comet will finally max out to, is frankly, anyone’s guess. It sometimes hugely outbursts in two stages, about 3 weeks apart… this latest episode seems to be a classic (if rare) case of exactly that. I’ve just found a report from Richard Miles (BAA) that suggests the latest outburst actually took place at a calculated Aug 22.48UT +/- 0.27, when the comet commenced an Mv 1.4 to 1.5 or so sudden climb from quiescent to ~ Aug 22.82 UT. Hope this update helps, Bob. Regards from NZ. Graham Wolf.

  7. Graham-Wolf

    Hi Bob

    My data for the latest 29p outburst, from the Fairfield Quarry Darksite (FQD). Apologies for previous date transcription error… sleep deprivation/brain fade, and trying to convert from NZST to UT etc. Got it right, now….
    Aug 23.34 UT (8:10pm NZST) Mv 11.6 150x ZLM +6.4
    Aug 23.45 UT 10:48pm NZST) Mv 11.3 150X ZLM +6.6

    Observations between rain showers at 46 South. Raining all day today. If it’s not rain, then it’s lunar pollution!!

    A brief Internet report from Richard Miles (BAA) mention that he estimates the onset of 29p’s latest outburst as being Aug 22.48UT +/0- 0.27. He adds that by Aug 22.82UT, the comet had already climbed by ~ 1.5 Mv. On my night, it climbed just over 1/4 deg in 2.6 hrs! What it will max out at, we just don’t know. This is an all-too-rare 2 to 3 stage outburst spaced some 3 weeks apart…. i.e. Mv 16 to 13, then 13 to 10.8 then back to 13 then back to mid 11 etc. What a brazen comet!

    Was rained out last night, and all day today. Hope you’re having better luck than me. No posted Mvs on any websites that I can find, so far. Hope this update helps, Bob. Regards from NZ.
    Graham Wolf

    1. Bob KingBob King Post author

      Thanks Graham – that’s bright! I’m struggling with clouds here in N. Minn. I hope others who’d love to see this comet and are reading your comments take a look.

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