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.

What a difference a year makes

Rosetta arrived at Comet 67P/Churyumov–Gerasimenko on August 6, 2014, when the nucleus showed little activity (left). The image on the right was taken on August 6, 2015, exactly one year after Rosetta's historic rendezvous. The comet's distance from the Sun shrank from about 335 million miles last year to 116 million miles this week. The intensity of the sunlight has increased by more than sevenfold, heating and boosting the comet's activity and making it bright enough for amateur astronomers to follow in a telescope.
ESA / Rosetta / NavCam – CC BY-SA IGO 3.0

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.

Follow that  Tapole

Comet 67P/Churyumov-Gerasimenko photographed on August 9, 2015. The comet is a small, 12th magnitude, moderately condensed fuzzball in amateur telescopes. Photographs show a short tail trailing to the west.
Efrain Morales

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.

En route to your eyes

Comet 67P/Churyumov-Gerasimenko's path parallels the ecliptic. This map shows its location from mid-July to late November as seen from the northern hemisphere. The comet remains a morning sky object throughout.
Source: Chris Marriott's SkyMap

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.

The rise before the fall

Light curve and prediction of Comet 67P's brightness from Seiichi Yoshida's Weekly Comet Information site. In his updated analysis, the comet will peak around magnitude +12 later this month. The horizontal axis shows the date; the vertical the magnitude.
Seiichi Yoshida

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.

A run through Gemini

Use this detailed finder map to find 67P/C-G in western Gemini this week. Stars are shown to about magnitude +14 with the comet's position marked for each day at 4 a.m. CDT. Click for a large version you can print out and use at the telescope.
Source: Chris Marriott's SkyMap

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.

Fuzzy visitor to a rich star field

This chart is the same as the one above but takes you through August 23rd. 
Source: Chris Marriott's SkyMap</>

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.

Chance meeting in the night

Back on August 6th, the comet joined Gemini's bright star cluster M35.
Rolando Ligustri

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.

Comet on a Comet

Rosetta’s scientific camera OSIRIS shows the sudden onset of a brilliant jet-like feature emerging from the side of the comet’s neck in the Anuket region on July 29, 2015.
Copyright: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

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.


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

2 thoughts on “Rosetta’s Comet Beckons At Dawn

  1. KevinKevin

    Wow! Seeing that one crystal-clear jet on 67/P is really amazing! Way back in 1986 everyone was thrilled that ESA’s Giotto gave us a close-up look at Halley’s nucleus. Now we’re going to be able to watch a comet sublimate from the vantage point of the nucleus itself and it “could” be incredible. (After the enormous disappointment of comet ISON nobody likes to use the word “will” when referring to comets anymore. It’s safer to use “might” or “could.”)

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