How to Catch a Ghost … Comet That Is

How Comet C/2015 D1 (SOHO) survived a near-death experience long enough to haunt the evening sky — with maps to help you find it before it vanishes into the night. 

Survived the worst ... or did it?

C/2015 D1 (SOHO) seen in the Solar and Heliospheric Observatory's C3 coronagraph on Feb. 20. The coronagraph uses an opaque disk to block the Sun, allowing astronomers to study the solar corona and near-solar space.

A comet that probably shouldn’t have survived its close encounter with the Sun two weeks ago made it out alive. Barely. C/2015 D1 (SOHO), discovered on Feb. 18th by Thai amateur Worachate Boonplod, strayed within 2.6 million miles of the solar furnace at perihelion the very next day.

After its "insanely close" pass, Karl Battams, an astronomer with the U.S. Naval Research Laboratory who maintains the Sungrazer Project website, didn't have much hope for the small object's survival.

Toasted by the Sun's fierce heat and kneaded by its powerful gravity, the comet should have vaporized in a cloud of expanding dust and organics. Instead, it passed perihelion with head and tail still intact.

When it finally exited the C3 coronagraph's field of view, SOHO still glowed around magnitude +4.5. Comet observers took note. Within days, it entered the evening sky in Pisces, running fast and hard into the northern sky along its steeply inclined (70°) orbit.

Headless comet rides again

Edge-on galaxy? Weird aurora? Nope. This is what remains of C/2015 D1 (SOHO) now gliding across Pegasus and Pisces at nightfall in early March. The comet is the 2,875th comet found with Solar and Heliospheric Observatory (SOHO). Photo taken on February 27, 2015.
Michael Jaeger

But it emerged like a chicken without its head — all feathers and feet. To be fair, few expected the comet to remain intact for very long. There was much speculation it might get "isonized" or disintegrate into a formless cloud of dust as a predecessor, Comet ISON, did prior to perihelion in late 2013.

Indeed, something similar happened to SOHO but after perihelion rather than before. Hidden from ground-based cameras by the Sun's glare, the nucleus secretly crumbled apart and vaporized into billions of bits of dust. Blown into a long streak by the solar wind and set aglow by reflected sunlight, the apparition more resembles a feather or Mona Lisa's inscrutable smile than a typical comet.

First image, confirmation soon followed

Justin Cowart of Illinois captured the first definitive ground-based image of D1 SOHO on Feb. 25th. Details: Stack of 31 8-second-long exposures at ISO 1600, shot with a Nikon D5100 + 70-300mm telephoto at 135mm f/4.7.
Justin Cowart, annotations by Karl Battams

Not to be deterred by the rumors of its vaporization, Justin Cowart, a geologist and amateur astronomer from Alto Pass, Illinois, figured he’d have a crack at it with his camera. On Feb. 25th, after stacking 31 8-second exposures, Cowart turned up a candidate:

“I was able to see stars down to about 6th magnitude in the raw frames, but no comet,” wrote Cowart.  “I decided to stack my frames and see if I could do some heavy processing to bring out a faint fuzzy. To my surprise, when DeepSkyStacker spit out the final image I could see a faint cloud near Theta Picsium, right about where the comet was expected to be!”


Confirming photo of D1 SOHO's remnant taken on Feb. 27 from Amrum, Germany. Jost Jahn stacked 59 15-second exposures (ISO 1600, f/2.4) and used an 85-mm telescope.
Jost Jahn

Now only a streak, the comet had clearly evolved since its sojourn under SOHO's eye. Not long after, on Feb. 27th, comet observer Jost Jahn of Amrum, Germany, confirmed Cowart's photo/observation.

Be fruitful and multiply

Successive fragmentation of the Kreutz parent comet birthed hundreds of smaller comets. Most pass so close to the Sun they're vaporized.

Most sungrazing comets captured by SOHO are members of the Kreutz family, a multitude of icy bodies left over after a much larger comet broke apart orbiting the Sun thousands of years ago. Because their orbits are so similar, we know they belong to the same family. SOHO stands apart as a “non-group” comet, unrelated to the Kreutz family — or any other comet group for that matter. According to Battams, several of these relative loners show up in the SOHO coronagraphs each year.

Like breath on glass

Comet D1 SOHO on March 2nd is a faint remnant of its former self.
Alfons Diepvens

That brings us to the present and the big question: Will we see C/2015 D1 in our telescopes? First the bad news. The Moon's full. But come March 7, it begins rising after the end of evening twilight.

Up, up and away to the north

Map showing the sky facing west around 7 p.m. (CST). Nightly positions are ticked off for SOHO's ghostly remnant. Stars to magnitude +6.5. Click to enlarge.
Source: Chris Marriott's SkyMap software

Meanwhile, the comet has been climbing steadily. On the 7th it will set more than 2 hours after twilight's end for observers at mid-northern latitudes. Michael Jaeger of Austria, who specializes in comet photography, estimated the comet ghost at magnitude +8 on Feb. 28th. At the time, it measured 50′ long by 5′ thick and was elongated approximately east-west.

Tracking a cometary ghost

Reversed, detailed chart with stars to magnitude +9 showing the path of Comet SOHO nightly from March 7–15. Time shown is 8 p.m. CST for the 7th and 9 p.m. (CDT) after. Click for larger version.
Source: Chris Marriott's SkyMap software

SOHO has been fading as it recedes from both Sun and Earth. On March 2nd, the remnant measured about 23′ in length, while on the same date, UK observer Neil Norman used a 4.5-inch (11.4-cm) reflector to estimate a total magnitude of 11 and length of 1°.

I've been biting my lip to get out and give it a try. You, too? Above are a couple maps to help you find it. Please let us know if you have success.

As S&T Senior Editor Alan MacRobert says, the Pocket Sky Atlas is "an absolute must-have for every astronomer." Order yours today!

Explore the Night with Bob King, Observing
Bob King

About Bob King

Amateur astronomer since childhood and long-time member of the American Association of Variable Star Observers (AAVSO), Bob King also teaches community education astronomy and writes the blog Astro Bob. The universe invites us on an adventure every single night. All we need do is look up. My new book, "Wonders of the Night Sky You Must See Before You Die", a bucket list of essential sky sights, just published and available at Amazon and BN.

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