Should we welcome an icy visitor from the outer Solar System that tweets its position?

Everyone is talking about Comet SWAN, which is right on the cusp of being visible in the northern hemisphere this week.

The arrival of a comet (a “cosmic snowball” of frozen gas, rock and dust that orbits the Sun) is not rare. What is rare is one that’s bright enough to glimpse, which Comet SWAN may—or may not—turn out to be.

For those that want to spend some time trying, here’s how to find Comet SWAN using some up to date sky-charts.


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Just don’t expect instant gratification because as well as being visible only in pre-dawn twilight skies in the northern hemisphere, Comet SWAN is now entering the “danger zone” as it gets closer to the Sun.

“I’m feeling a little less excited about Comet SWAN,” says astrophysicist Jackie Faherty at the American Museum of Natural History. “It swept in just after Comet Atlas fizzled, something comets are notorious for. They tease you into thinking they’re going to put on a beautiful, bright spectacular show, then they fizzle out before they become visible to the naked eye.”

“Some of the pictures of it that has been showing up on the internet are spectacular, and are definitely a tease that’s getting us excited about what it could be, but if you look at the light curves it looks like Comet SWAN might be dimming,” she says. 

For two fabulous images—which are arguably got the world’s attention—see the Astronomy Picture of the Day for April 29 and May 8. Wow!

However, in terms of it being the “comet of the century” there are a few of things that are not in Comet SWAN’s favor. For starters, it’s just not the right type of comet. 

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What kind of comet is Comet SWAN?

When it comes to composition, you really want a dust comet rather than a gas comet. Comet SWAN? It’s gas. “Comets are very fragile astronomical objects composed of ices and volatiles and as they get close to the Sun what they go through is unpredictable,” says Faherty. “We don’t know how much material they have, how they’re going to interact with the solar wind and with the radiation from the Sun as it gets closer to it, and we can’t quite predict how the material—which is already fragile—is going to light-up as it gets close.”

What we do know is that for a comet to be easily seen by the naked eye, it has to brighten significantly as it gets closer to the Sun. “You want it to start to outgas, have a glowing tail and a that beautiful coma, and have enough material for a sustained brightening as it gets closer and closer to the Sun,” says Faherty. “But this is a tiny piece of rock that’s millions of miles away from us, and when we’re asked to say what the brightness trajectory of these fragile objects will be it really stretches the limits of our ability to make predictions in astronomy.”  

Why comet-hunting is so difficult

On a cosmic scale, comets are tiny little pieces of rock and ices. Plus, if you do see Comet SWAN, it will be a one-time event; this is a long-period comet that won’t be back in the solar system for around 12,000 years. It was only noticed for the first time in late March 2020 as it approached the inner solar system. 

Astronomers know of less than 7,000 comets, a tiny fraction of what’s actually out there in the outer—and occasionally inner—Solar System.

Why is it called Comet SWAN? 

SWAN is just a nickname. It’s actually called C/2020 F8 (SWAN), and it’s named after an acronym for the camera that was used to discover it; the Solar Wind ANisotropies (SWAN) instrument on the ESA/NASA Solar Heliospheric Observer (SOHO) space observatory. Working since 1996, SOHO has so far found over 3,000 comets.  

Who found Comet SWAN?

Michael Mattiazzo from Australia spotted it on March 25, 2020 while inspecting images on his computer that had been posted online from SOHO. “On 2020 April 9, I found another comet on low-resolution public website hydrogen Lyman-alpha images obtained with the Solar Wind Anisotropies (SWAN) camera on the Solar and Heliospheric Observer (SOHO) spacecraft,” wrote Mattiazzo, who has now found eight comets. “It appeared to be brightening steadily.” His finding was confirmed on the ground by Martin Masek using a remote telescope in Argentina.

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Eyes on the sky

“It’s a reminder that there are immense discoveries in space because there’s a huge amount of material out there and many of these things could come floating by without us noticing,” says Faherty. “It just happens to be that we’ve got more eyes on the sky right now that are examining such things.” Despite the dominance of technology and remote telescopes, amateur astronomers still dominate comet-hunting. “Almost all of SOHO’s comet discoveries so far have been made by citizen scientists scouring images returned by SOHO’s LASCO instrument,” says Karl Battams, LASCO team comet expert at the US Naval Research Laboratory and lead researcher of the Sungrazer Project.

Could Comet SWAN work as an invitation for more amateur astronomers to get involved in comet-hunting? “The more eyes you have on the on the sky, the better off we are at discovering these kind of things,” says Faherty. That said, most comets are only bright enough for amateur astronomers and astrophotographers to grab images of using remote telescopes. 

Why bright naked-eye comets are so rare

Most comets enter the inner solar system, then they break up and leave without anyone noticing. On his website Mattiazzo has written beside Comet SWAN’s entry this warning: “strong risk of disintegrating, but has potential for outbursting.” 

“It’s very nice that that Comet SWAN was discovered—it could have come and gone without anybody noticing,” says Faherty. “There’s a flurry of comets out there and they’re left undiscovered because they don’t get bright and so no one pays attention to them … comets can brighten and fizzle out or break-up at any time.”

When a comet is discovered, it’s usually before it’s becomes a naked-eye comet. Most don’t brighten enough to see while out stargazing; there are typically one or two a year that get talked-up, but the last really bright comet was Comet Hale-Bopp back in 1997. That was the most widely observed of the 20th century, and it’s been a long wait for another. That’s possibly why Comet SWAN is getting so much attention. 

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Why sky conditions are so important

When a comet becomes bright enough to be seen with the naked-eye it’s an invitation for everybody to step outside and have a look at it. However, what a comet is going to look like depends on what the night sky visibility is like where you comet-gaze from. It’s about light pollution. “We can all see the Moon even in a light-polluted Time Square in New York City, but not everybody can see the faint stars in, for example, the constellation of Delphinius—you need a dark sky,” says Faherty. That’s going to be an issue for many people outside hoping for an easy view of Comet SWAN.

What makes Comet SWAN tricky to see is that it’s only now becoming visible in the northern hemisphere (and it’s right on the cusp of naked-eye visibility—so use binoculars) as it gets close to the Sun. That means it’s only possible to see it about an hour before sunrise. That’s also when the sky is brightening. 

No, comets are not dangerous

Just this week I’ve been asked if Comet SWAN and Comet ATLAS would collide, hit Earth, or somehow cause a shower of asteroids that could bombard us. None of that is going to happen. “These comets are not going to hit Earth,” says Faherty. “They are so far away and they’re orbiting the Sun, not on a track to collide with our planet.”

So why do people get so scared about comets? “People know the story of how we believe the dinosaurs went extinct—that rocks hit the Earth and that such things are very dangerous,” he says.  

Why does everyone get so excited about comets?

Their sheer rarity makes a truly bright, naked eye comet an astronomical event like no other. Comets have been observed and recorded since ancient times, and there’s a simple reason for that. “In old times the nighttime sky was a form of entertainment—the original movie theater,” says Faherty. “The Sun went down, the stars came up and the stories started to flow—and people really knew and understood their sky. The night sky is ingrained in human culture, so if the night sky suddenly changed it was a big thing. “Comets historically were an omen for something different,” says Faherty. “So when a comet suddenly showed up it was a big deal.”

A comet appearing in the night sky to everyone on the planet is still a big deal—it’s just so rare—but finding Comet SWAN, alas, will not be anywhere near as easy as just stepping outside.

How to see Comet SWAN 

Now in the constellation of Perseus, Comet SWAN is likely to be at its brightest and visible low in the northeast just as nautical dawn is starting on Monday, May 18, 2020—and for a few days after.

I’ve written another post on exactly how and when to look for Comet SWAN (spoiler alert: look as soon as you can—preferably on Monday—before it potentially fizzles and fades). “If you’ve never gone comet-hunting before you will have to be extremely patient,” says Faherty. “This is old-school astronomical observing because you’ll have to navigate the sky using a star chart with the up-to-date position of the comet—it’s moving very quickly relative to the background stars.”

Are you up to it? “I would not encourage somebody to get up early and go look for it unless they understand that this isn’t just about looking out the window and there you go,” says Faherty. “You’ll have to learn some astronomy.”

Here’s how to approach a comet-hunt:

  • Be up early.
  • Figure out the brightest stars around where you need to look (the the above sky-chart).
  • Slowly scan the skies with binoculars.
  • Be extremely patient.

However, once you do find it, the smudge-like Comet SWAN will look, though binoculars, like nothing you’ve ever seen before. “It’s extremely gratifying when you do find it because you’ve now learned some astronomical techniques,” says Faherty. “The satisfaction of seeing this object will open up your world.”

Wishing you clear skies and wide eyes.