Lively, Green Comet PanSTARRS Glows Before Dawn

Comet PanSTARRS (C/2017 S3) glowed blue-green and sported a several-degree-long gas tail in this photo taken on July 20. The comet was ~7th magnitude at the time. Michael Jäger

Yesterday, I said that getting up before dawn isn’t easy especially in summer, when the sun rises so early. And while I may not rise with the birds for the space station I will for a comet. Comet PanSTARRS (C/2017 S3) has become active in the past couple weeks after an “outburst” at the end of June when it shot up to magnitude 9.5, putting it within range of a modest-sized telescope. It’s one of many discovered by the Panoramic Survey Telescope and Rapid Response System (PanSTARRS) based atop Mt. Haleakala on Maui, a popular Hawaiian island tourist destination. Although it was discovered on Sept. 23, 2017, it only recently grew bright enough to see in amateur telescopes.

In this animation made from multiple photos on July 20 you can see how dynamic the tail is. Michael Jäger

I didn’t bring this to your attention back then because the comet was too dim to spot in binoculars or a small telescope. Indeed, C/2017 S3 began to fade and become more diffuse just a few days after the explosion. Fresh comets like S3 hail from the Oort Cloud, a vast reservoir of dusty ice balls located between 5,000 and 100,000 times Earth’s distance from the sun. They’re famous for undergoing dramatic changes in brightness — called outbursts — when they drop into the inner solar system and “feel” the heat of the sun for the first time. Ice can vaporize explosively, releasing clouds of gas and dust and causing the comet to brighten up.

That’s what happened to S3. And it exploded again about July 17, when it topped out at magnitude 7, definitely bright enough to see in binoculars from a reasonably dark sky. As I write this, the comet’s since faded back to magnitude 9, right at the binocular limit. But because it could shoot up again at any time, I wanted to alert you to the fact and provide a chart to track it.

Just one night later (July 21), the comet had faded a magnitude or more and lost its tail! Michael Jäger

We’ll get your best chance to see C/2017 S3 just before dawn when it’s highest in the eastern sky. Thankfully, the bright, twinkly star Capella in the constellation Auriga, lies nearby. If you can find Capella you’re halfway there! The chart shows the constellation with the brighter stars, which are labeled with Greek letters. Later this month, notice that the comet passes through a field of dimmer stars (but easily visible in binoculars) that all start with the letter “psi” or ψ.

Use this map to help you find C/2017 S3. Its position is shown every other day with stars plotted to magnitude 7.5 (easy binocular limit). The comet is best visible in a dark sky just before the start of dawn when it stands ~25° high. If you’d prefer an a black stars-on-white-background chart for easier use at the telescope, click here and download. Stellarium with additions by the author

I can’t tell you how bright S3 will be when you wake up to see it tomorrow morning, but if it goes into outburst, I’ll update this blog immediately, so bookmark it and check back. Let’s just say for now (July 22), it’s around magnitude 9 and faintly visible as a round, misty puff of light with a brighter center when viewed in telescopes from 4-inches up. The comet is expected to brighten as it approaches perihelion (closest point to the sun) on August 15, when it will come within 18.6 million miles from the sun, half the distance of Mercury.

If it doesn’t fall to pieces and disintegrate, a habit of small, fresh comets like this one, it could wax to magnitude 3 or 4 and briefly become visible with the naked eye in early August before it’s lost in a bright dawn sky. If possible, try to catch C/2017 S3 during the next few mornings with the moon absent from the sky. Once the moon returns after the 25th, it will be considerably harder to see until about Aug. 4 … unless it blows up again!

** UPDATE July 22: The comet may be as faint as magnitude 9.8 right now. You’ll need a 6-inch telescope and dark skies to see it. As always, that could change any day with the comet either fading further or experiencing another outburst.

** UPDATE July 23: Easily saw it before dawn this morning in my 10-inch scope at magnitude 9.5. And that was looking through haze from Ontario forest fires. It was a big, diffuse puff brighter toward the center. I suspected it in 10×50 binoculars but couldn’t be sure of seeing it.