Despite early expectations Comet SWAN appears to be fizzling, providing yet another opportunity to appreciate what makes these objects so unique.

Comet SWAN
Comet SWAN (C/2020 F8) makes an appearance at the start of dawn north of Duluth, Minn. on May 19. The photo conveys the comet's small apparent size and blue-green color easily visible in a telescope at the time. The trees are motion-blurred because a tracking mount was used to follow the stars. Details: 200mm telephoto at f/2.8, ISO 400, 50-second exposure.
Bob King

Like you, I waited patiently for Comet SWAN (C/2020 F8). Watched it brighten and develop a beautiful gas tail in April and early May, excited that it was on track to become an easy naked-eye object by mid-May. Southern Hemisphere observers recorded a steady increase in brightness through April, including an outburst at month's end that boosted its magnitude to 5.2. Observers with dark skies reported seeing the comet without optical aid. Even the May 2nd electronic telegram from the International Astronomical Union (IAU) predicted a peak magnitude of 2.8 for SWAN around May 21st. The world was poised with anticipation.

Comet SWAN light curve
This light curve of Comet SWAN from the Comet Observation Database (COBS) shows SWAN's rapid increase in brightness in late April followed by stagnation and then a downturn in recent days.
COBS Comet Observation Database / CC BY-NA-SA 4.0

Then it all came to a screeching halt. The comet's brightness stalled and then reversed itself just days before its transition from southern to northern skies. The visual feast we'd expected became a modest morsel instead. SWAN may still have an outburst in the cards, but what I saw at dawn on May 19 — my first swing at the comet — didn't give cause for optimism.

Through 10×50 binoculars it was a small, fuzzy patch of light glowing wanly at magnitude 6.2. While I couldn't detect a tail in the glass, it was obvious in short time-exposure photos. Needless to say, I couldn't see the comet with the naked eye. Through my 15-inch (38-cm), the head shined a beautiful Mediterranean blue with a delicate, broad tail streaming to the west.

Comet SWAN
Comet SWAN has lost its oomph! On May 21 at 3:27 CDT very little condensation was evident in the coma.
Bob King

Clear skies prevailed again on May 21st. I set the alarm half-hoping SWAN would turn around, but when I trained the binoculars on it for the second time I quickly discovered it had faded by another half magnitude. Standing only 7° high at twilight's start it appeared quite faint in binoculars but a 10-inch scope showed a 10′ wide coma and faint tail. The core, which appeared condensed on May 19, looked diluted and washed out, resembling the post-breakup appearance of Comet ATLAS (C/2019 Y4).

Comet SWAN chart
Map showing the comet's position at 10:15 p.m. CDT every other night from May 22 to June 3. Stars plotted to magnitude 7.5. Click for a large version.
Stellarium

As you read this, SWAN is transitioning from the morning to the evening sky, making it easier for many more of us to keep an eye on it. The comet will stand just 5–10° high above the north-northwestern horizon at the end of evening twilight when it's best visible, so be sure to find a spot with a wide-open view in that direction. While it may prove difficult or impossible to see in ordinary binoculars, a telescope will still provide a good view.

Comet SWAN among the constellations
Comet SWAN travels from Perseus to Auriga in the next two weeks. This photo, taken from Duluth, Minn. at 47° N, gives you an idea of its low path across the northern sky.
Bob King

It's always possible that a cache of fresh ice could lead to another outburst or the nucleus could fragment as it approaches perihelion on May 27th, but don't count on it. Just the same, I encourage you to have a look if only to see what all the fuss is about. If clouds or an obstructed horizon make this impossible, our friend Gianluca Masi will feature live views of the comet on his Virtual Telescope site on May 27th starting at 19:00 UT (3 p.m. EDT).

Before the current fading episode, SWAN was expected to slowly dim during the final week of May while en route toward the bright star Capella — a most helpful sky guide.

Fizzle and Fade

So why did the comet stall and fade? SWAN is a newcomer to the inner solar system, having arrived after a multi-million year journey from the Oort Cloud. Nascent ices of nitrogen, carbon monoxide and carbon dioxide in its nucleus rapidly vaporized on approach to the Sun, causing the comet to brighten rapidly and leading us to think the trend would continue. Once those ices were used up, the light curve flattened.

Apparent double nucleus in Comet SWAN
An apparent double nucleus in Comet SWAN photographed on May 8 with a 40-cm f/4.5 reflecting telescope. Victor Buso

While that's the most likely explanation, it's also possible the nucleus underwent fragmentation. Claudio Martinez of the Fundación AZARA de Historia Natural in Argentina ,along with Argentine amateur Victor Buso, took close-ups of the comet's core on May 8th that appear to show a double nucleus. They're still waiting for their results to be confirmed.

Over the years I've learned not to complain about comets even when they "underperform." They are what they are — volatile, unpredictable, fragile — and refreshingly innocent of expectations.

Comments


Image of Anthony Barreiro

Anthony Barreiro

May 22, 2020 at 5:14 pm

The light curve graph conveys information about more than just the brightness of the comet. Interestingly, the visual observations have a smaller error bar than the CCD observations. If a CCD camera counts photons more accurately than an eyeball does, what would account for the greater consistency of visual observations? I would hypothesize that visual observers are aware of, and influenced by, one another's estimates. Alternatively, a visual observer may take in more of an overall gestalt and, consciously or unconsciously, adjust their estimate to minimize the effect of extraneous error factors like atmospheric transparency and differential atmospheric extinction caused by the comet appearing lower or higher in the sky.
But at the moment of peak brightness, something happened to the visual observers that didn't happen to the CCD's. Up to that point both the CCD and visual trend lines look roughly parallel. The CCD brightness measurements drop off quickly after the peak, while the visual estimates decline more slowly. I'm guessing visual observers are influenced by their desire to see a bright comet.

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Image of richard szweda

richard szweda

May 22, 2020 at 7:14 pm

Just guessing, but another factor may be the brain's propensity to find a pattern in a sensory stimulus. Just the way we are "hard wired".

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Image of Yaron Sheffer

Yaron Sheffer

May 23, 2020 at 3:38 pm

Well, if you ask me, these "CCD data" are a disaster! They have a huge spread of up to 2 magnitudes on the same (or adjacent) days, while surely we expect CCD photometry to perform much better than this. I suspect that these CCD observations are very heterogenous, suffering from differences in calibration, field of view, pointing, comparison stars, etc. etc. On the other hand, eye balling is very simple and cosistent: unfocus the comet and compare with a nearby star. This yields the total magnitude from the entire comet, which should be steadier than the limited and inconsistent apertures used with CCD photometry. Besides, the latter tend to show the nuclear magnitude, which is always fainter than the total brightness of the comet. My conclusion: the visual observers were not hallucinating 🙂

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Image of Anthony Barreiro

Anthony Barreiro

May 24, 2020 at 12:36 pm

Thanks Yaron. I appreciate you sharing your expertise.

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Image of Zubenelgenubi 61

Zubenelgenubi 61

May 24, 2020 at 12:42 pm

The more experienced visual observers are very reliable. All methods have potential issues. There is often a spike in reported magnitude estimates when a comet becomes bright enough to be seen in binoculars as opposed to just telescopes. If a comet is very diffuse, it is hard to get a good estimate by refocusing it further. For example, when Comet 17P/Holmes had its huge outburst, it briefly was easily visible to the naked eye at second magnitude. But it quickly became so huge and diffuse that even though it's total brightness was still high, it was hard to see.

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Image of Bob King

Bob King

May 24, 2020 at 3:55 pm

Hi Yaron and all,
I also wanted to comment on the CCD vs. visual difference. CCD magnitudes are typically based on the nuclear region (and often involve different filtration methods) and fail to take in the full coma which necessarily bumps up the magnitude for visual observers.

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Image of Zubenelgenubi 61

Zubenelgenubi 61

May 22, 2020 at 8:35 pm

There was a big morphology change as SWAN once had a long gas tail instead of the stubby little dust tail it has now. I also think people's expectations were raised too high, maybe by the showy gas tail which was only visible to imagers.
The peak predicted magnitude of 2.8 would have been similar to Halley in 1986, which put on a good but not great show and disappointed the general public. But the circumstances of SWAN's apparition are far worse for the northern hemisphere even than Halley's was.

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Image of Zubenelgenubi 61

Zubenelgenubi 61

May 27, 2020 at 1:04 pm

Images today show no central condensation remaining. ATLAS also, just before disappearing into twilight last week. It must be COmet Vanishing Into Dust syndrome.

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Image of KHartnett

KHartnett

May 29, 2020 at 4:59 pm

I took some images with a C8 SCT on Monday evening, May 25, from Maryland and saw only a diffuse smudge - nothing that looked like a nucleus was evident.

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