Sunday, November 22, 2020

let's look into comet C/2020 S3 (Erasmus)

OK. Let's try to get ahead of this one...

Comet C/2020 S3 (Erasmus).

I'm always been sceptical of people promoting an "amazing" or "incredible" new comet that you gotta check out now, you won't believe what's going to happen next!

Bah. Humbug. Click bait.

the crazies

I first saw a new item about this late last night/early this morning with some feed item showing up on my Android phone.

KQED urged us to check out a "Once-in-a-Lifetime Comet Is Visible Now in Pre-Dawn Sky." Please! That's a large dose of selling newspapers with a headline like that. Now the article itself seems fairly balanced. I'm being generous here. The author appears to be well-informed and did some decent research. But I wondered, did the editor write the ridiculous headline?

Some key points. Ben Burress says in the opening paragraph, you "may" be able to see the comet. From the get-go, setting expectations in a good way...

He tells us where to look and cautions the reader that it will be low and faint. Suggests using obvious Venus as a jumping point and using your hand as a measuring stick (though he forgot to say you must extend your arm). He hints, somewhat, that it might be possible to see visually. Er... I dunno about that. But then quickly recommends binoculars (but doesn't suggest stabilising them). He does give a rather fair description of how it might look in binos. "Cotton ball." I like it.

But then Ben trumpets this is a 2500 year period comet which we must rush outside to look at now 'cause it's your last chance. Here we go... That's steering into sensationalism. 

Happily the author gets back to facts including it's likely origin. Oh, look at that. Ben quotes NASA's Solar System Dynamics page. Excellent: a reporter referring to a reputable source (assuming you believe NASA). That's where the 2500 orbital period data came from. And Ben uses this to justify the once-in-a-lifetime soapboxing exclamation.

Goes on to discuss the bad harbinger aspects of comets. Then builds it up a little, the potential for a collision, to conclude with a somewhat calming, "relax." Oh boy.

Were you (my reader) worried about this comet hitting the Earth?! I wasn't. 

Am I to trust you (Ben) about near-Earth objects and whether they are going to kill us or not? Um, respectfully, I will consult the scientists.

Roller coaster. Some good stuff here but a lot of hyperbolic remarks.

Keep in mind during all this, this is a FM radio station.

serious amateurs

Items showed up in the RASC national listserv this morning including an observing report. Good, so it truly is a visible comet. 

Ron M started things off.

Near Venus in morning sky.  According to Space Weather Mag 7,? perhaps going to mag 5.?? Dec 12 perihelion.??  Very long tail.  All we need is clear skies...

(I don't know what Ron did to produce all the non-translatable characters.) It's too bad that we're in the beginning of The Grey Months in the northern hemisphere of the third rock from the Sun but it sounds like if you work at it, you might see it.

Curt N shared.

I spotted it in 15x63 binos and 8" Dob last weekend.  A few days later, after not seeing P1 NEOWISE, I had Erasumus [SIC] in the Dob and noticed a larger coma and hints of extended elongation.

An 8-inch telescope. Good stuff. More good level-setting information.

OK. Now to some hard data.

good comet sites

Off to by Seiichi Yoshida, specifically the current weekly comet page.

Now it is very bright as 7.6 mag (Nov. 17, Carlos Labordena).  It will approach to Sun down to 0.4 a.u. on Dec. 13, and will brighten up to 6 mag.  It will be unobservable soon.

Remember, he's in Japan. Latitude around 36° north.

A few more details... 

Discovered on September 17, 2020 by Nicolas Erasmus with the Asteroid Terrestrial-Impact Last Alert System (ATLAS) search program. Nicolas works at the South African Astronomical Observatory (SAAO) and studies near-Earth asteroids. There's the moniker attached to the comet. (I wonder why ATLAS wasn't used...)

Seiichi's finder charts show it moving quickly eastward, exiting Corvus, skirting Hydra, heading toward Libra and Scorpius. Very low for us in Ontario.

The magnitude graph shows measurements currently in the 10 to 9 range. You will need magnification or long exposures to get this, right now.

The prediction graph shows it peaking around 6. I consider 6 the magic number for borderline visibility with Mark I eyeballs. In dark sky locations free from light pollution, an experienced observer may be able to see down to magnitude 6.5. And to be very clear, that's for POINT SOURCES. Comets have bright cores but the coma and the tail, extended objects, are much fainter...

Over to Comet Chasing at Skyhound (maker of the amazing SkyTools). The comet is listed third. The quick synopsis says:

C/2020 S3 (Erasmus) has brightened rapidly.  It will reach periehlion [SIC] on December 12, when It is predicted to reach maximum brightness of magnitude 6.8.

That's below normal human vision limits.

The detailed section notes:

C/2020 S3 (Erasmus):  A morning comet visible in small telescopes. 

I always like Greg Crinklaw's rapid one-liner. What kind of gear do I need to see this comet? And how early do I have to get up?

The by-line reads: 

This comet begins the month [of November] in Sextans at magnitude 10.0.  Look for a 4' coma.  It should brighten rapidly, moving into Libra by month's end.

As of November 21, for people at a latitude of 55 degrees, S3 is "Very low in the southern sky during morning twilight at ~06:00." And for humans near 40°, "Low in the eastern sky during morning twilight at ~05:30." So that's a bit better news for astronomers at N44.

The finder chart produced by SkyTools 4 echoes what Seiichi showed.

I noted the text on the PDF file. "C/2020 S3 (Erasmus) (Comet). Magnitude: 8.66 Coma Diameter: 3.9' SBr: 20.3 Mag/arcsec² Earth Distance: 1.0 AU." That's a big coma.

Wait! Look at the surface brightness number! That's what comet fans in their easily excitable states often overlook. This is the light spread out for the comet's entire area, nucleus, coma, and tail(s). And mag 20 is super dim. Photographic target.

Remember, Crinklaw updates this page monthly.

Finally, let's visit the COBS web site. This is a relatively new tool in my kit. The great thing about the Comet Observation Database is that recent and current observations, often with measures, show up day by day.

Now, their light curve chart with visual and photographic measures is more optimistic. Recent observations are in the high 6 range with the peak cresting 4. Ah. This might mean we'll get some naked eye viewing opportunities (again, in good conditions, away from bright lights) like we enjoyed for C/2020 F3 (NEOWISE).

Comet C/2020 M3 (ATLAS), by the way, remains a bit brighter, for the moment. And is maybe a touch easier to spot, though dimming, between Orion and Taurus (thus a night-time object).

Ron M referred to Spaceweather. While not primarily a comet web page, they nevertheless showed a lovely image of this comet. See the dedicated page to enjoy a large version of the two-panel shot by Gerald Rhemann. Awesome. Look at that long tail. Note: Gerald used an ASA Astrograph 12" f/3.6 telescope with a FLI ML 16200 camera. "Exposure time: Mosaic of 2 panels LRGB 12/4/4/4 min. each." That's a big fast 'scope with a 24 minute exposure.

showing it in software

I loaded the comet data into Stellarium 0.18.0 for Windows. I emphasised the path for the next 4 weeks. Departing The Crow, running along the border between Virgo and Hydra, passing through The Scales (USA needs those right now)... The plot or trace runs from today to the solstice, Dec 21. Oh. And the green line is the horizon.

comet C/2020/S3 path for next 4 weeks from Stellarium

What do you notice? Yes, very good. It's going down! The sun diver is getting lower in the sky every day. 

That's good on one hand: closer to the Sun means brighter comet. But lower means many bad things too: closer to the Sun in a brightening pre-dawn sky, potentially dangerous to view with magnification as is draws closer to our bright star, lower in your eastern horizon to be potentially blocked by mountains, trees, churches, skyscrapers, rocks, ducks, and clouds. And if you do get a clear sightline (say, over a lake) on a cloud-free day (which won't happen until Wiarton Willie tells us), you'll be punching through a lot of air. More atmosphere, more airmasses means more dimming, aka atmospheric extinction, therefore an effective drop in magnitude or brightness.

Now a chart from SkyTools 4.0j Visual Professional. I show dates.

comet path plotted and labelled with SkyTools

So, again, a consistent path plotted. Good stuff. Good data there. With the dates showing, we can see the comet enters Libra around November 30 and is beside the Sun around December 15.

Oh, and I did see the "very long tail" in SkyTools, at least for today... (not visible in the wide-field image above).

I don't mean to dampen your spirits. Go look, carefully look, while you can, with at least binoculars, but I daresay it will become increasingly difficult in the coming days. Then impossible in mid-December.

There's this delicate balance. Comets need to be close to the Sun to show off their fancy tails and colourful comas but they need to be high enough in a dark sky for us to catch a glimpse.

official sources

You can keep tabs on alerts at the ATLAS web site.

Here is data about the comet from the MPC, the Minor Planet Center web site. The report states "N. Erasmus reports his discovery of a comet on four 30 second exposures taken on Sep 17 UT by ATLAS-MLO (T08), noting a 13" coma."

Visit the SSD web page and browse on "C/2020 S3" for more info. From an official source. Data supported by over 400 observations.

Hope this helps. Happy hunting.


Check out my July blog posts on deciphering comet names and understanding comet magnitudes.


Here's another screen snap from SkyTools. The view tomorrow morning at 5:00 AM EST.

telescope simulation of the comet

Zoomed in on the comet. The ring is the field of view for a 12" telescope with a 25mm eyepiece.

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