Rosetta's Comet Beckons At Dawn

We've patiently waited for Comet 67P/Churyumov-Gerasimenko to grow bright enough to see in amateur telescopes. That time has finally arrived. Here's how to spot it before dawn.

We've waited a long time for this. Looked at hundreds of close-up photos of Comet 67P/Churyumov-Gerasimenko taken by the Rosetta spacecraft since it arrived at the comet a year ago this month. Now it's time to put real comet photons in our eyes. Right here. On Earth.

During this perihelion week up through early next month, 67P/C-G will crest at around magnitude 12 in the predawn sky. This week, the pesky but ravishing crescent Moon transitions to the evening sky, leaving mornings free to track down the comet under dark skies.

67P/C-G canters across Gemini this month and early next, favoring Northern Hemisphere skywatchers, though not by much! Observers with big Dobs will more than likely have to assume the "praying position" with knees on the ground for a good look. I know I did. But it was worth every bit of strain. More on that in a minute.

Amateur astronomers participating in the Pro-Amateur Collaborative Astronomy (PACA) campaign have been following the comet since March 2014 when it glowed at an anemic magnitude 21. After solar conjunction on February 10,  2015, it was recovered in mid-April low in the morning sky at 17th magnitude. 67P/C-G continued to slowly brighten through the spring and early summer en route to its August 13th perihelion. One of the first visual observations of the comet was made by Chris Wyatt of Walcha, New South Wales, Australia, who called it diffuse, about 30″ across, and magnitude 14.2 on June 23rd.

Since then, the trickle of Earth-based observations has quickly become a flood as amateurs and professionals alike take advantage of the incredible opportunity to coordinate their visual observations, photos and spectra with Rosetta's in situ measurements. If you'd like to join the campaign, there's no time like now. Sign up right here.

From my latitude of +47°, long summer twilights, a small elongation, and extreme faintness kept the comet off-limits until just this week. But from here on out, it's smooth sailing as the famed fuzzy climbs higher and higher up the dark sky ladder during late summer and fall. The moonless periods from August 12-27 and Sept. 10-26 will be your best times to look. By late September, 67P/C-G will still be visible but fading, as its distance from the Sun continues to increase.

With great anticipation I turned my 15-inch (37-cm) Dob toward the star cluster M35 Tuesday morning, August 11th, a half-hour before the start of dawn and used the star map to navigate to the comet's location. Just 10° high at the time, poor seeing — so common at low altitudes — made everything a little mushy, but with patience I suspected an ill-defined, hazy presence at the position using a magnification of 142×. After making a mental note of its location with respect to several field stars, I moved on to less knee-bending targets.

Returning at dawn's first blush, conditions were much improved. With 67P/C-G now a user-friendly 16° high, the view had sharpened with the comet now much more distinct. Upping the magnification to 245×, I saw a moderately condensed, round, milky glow about 0.7′ across with a magnitude of 12.5. That lovely little tadpole tail that shows so well in recent photos was sadly beyond my scope and these eyes.

Comet observers often describe how dense or bright a comet appears using a degree of condensation (DC) scale numbered from 0 to 9, with a DC of 0 being a flat profile coma with no central brightening and a DC of 9 being a completely stellar object. 67P/C-G's current DC is 4 or 5, depending on the observer and equipment.

Realizing that the Rosetta spacecraft was buried somewhere within the comet's coma gave me the greatest thrill. That and watching it crawl eastward among the stars like some fuzzy caterpillar looking for a meal. To my surprise, high power revealed motion in just 20 minutes. Watching an object like a comet or asteroid move against the stars adds excitement to an observation and makes it that much more real.

I love checking in on old comet friends. This is my third go-round with 67P/Churyumov-Gerasimenko, a comet I first saw in 1982 and that inspired me on a long journey to find and follow each and every comet bright enough to show from my house. Maybe it will inspire you, too.

An 8-inch scope under dark skies will be up to the task of making acquaintance with 67P/C-G, but as always with the deep sky, the bigger the scope, the better. Go out at least a half hour before the start of local dawn, so you have to time to dark adapt your eyes and zero in on the comet's location without stressing. Try a range a magnifications to find the one that offers a pleasing compromise of sharpness and detail. Note the comet's position and then return for another look.

As we're all aware, comets are the most fickle of astronomical objects, subject to sudden outbursts that jolt them into brilliance or crumble them to bits. On July 29th, the Rosetta spacecraft recorded the most dramatic outburst of 67P/C-G this apparition in the form of a brilliant jet. While it's not clear whether this particular event caused much brightening as seen from the ground, it serves to remind us that anything can happen at the comet. I wouldn't be surprised if 67P/C-G undergoes something even more explosive, big enough to vault it to binocular brightness. We'll just have to wait and see.

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Curious to know more about Rosetta? Read our comprehensive mission overview in the August 2014 issue of Sky & Telescope.