Another SOHO/SWAN comet discovery!

July 21, 2006 — Another SOHO/SWAN comet discovery!

A slight underachiever by SOHO standards, the SOHO/SWAN instrument recently discovered its eight comet — C/2006 M4 (SWAN). The comet was found and reported independently by Rob Matson and Michael Mattiazzo. As with SWAN’s previous discovery, Rob Matson has been kind enough to share his discovery story, plus some images from Rob McNaught and Sebastian Hoenig.

"I first spotted the comet around 10:30 am exactly a week ago today when the 7/5 SWAN image was first posted. I should have spotted the comet a few days earlier, but as I later explained to Brian Marsden and Tim Spahr at MPC, I missed it because it was hugging one of the superimposed gridlines. By the time of the 7/5 image, though, the comet had enough separation from the grid line to draw attention to itself.

I e-mailed Eric Christensen at Catalina Sky Survey (with whom I’d been working on a different SWAN candidate) that our old phantom had disappeared, but had been replaced by something much more promising, and I would get back to him once I confirmed it wasn’t a known comet.

By 11 am I had checked known bright comets and found no matches, but wrote Eric back that due to the sky location, the southern hemisphere was the only hope for confirmation. I measured pixel positions for six nights from 6/20 to 7/5, converted to ecliptic coordinates and then to equatorial. I checked residuals on a parabolic orbital solution, tweaked a few positions until I was satisfied, and at 1:22 pm sent a discovery report and rough ephemeris to Brian and Tim Spahr at MPC. Ten minutes later I sent the same positions to Eric.

Since the southern hemisphere was the only hope for confirmation, the next step was to contact Rob McNaught and Gordon Garradd at Siding Spring Observatory to see what they could do. Like most observatories, SSO was down for full moon and wasn’t planning their next run until Thursday. But just in case, I sent Rob and Gordon predicted comet positions for Monday and Tuesday, and told them if no additional SWAN images appeared by Wednesday, I’d extend the ephemeris a few more days.

That evening I was able to check my e-mail and confirm that Tim had gotten my message; also, Rob replied that he would try for the comet on Thursday from the Uppsala telescope. There were no new SWAN images Tuesday so I went ahead and extended the ephemeris through Friday for Rob, running some excursions to give him an idea of positional uncertainty. A few hours later, Rob replied that Michael Matiazzo had independently found the same comet and reported it to him. He also indicated that he was thinking about heading up to SSO a day early if the weather cooperated.

Well, late Tuesday night (Wednesday afternoon for Australia) Rob informed me that Terry Lovejoy had located the comet in one of his June 30th survey images from Thornlands, QLD, and forwarded me Terry’s crop of an image stack showing a noticeably green cometary object. The weather had cleared sufficiently for Rob to make the decision to head up to Siding Spring, so I checked Terry’s position against my rough orbit to see if it needed some real-time tweaking before Rob made the recovery attempt. I didn’t have all of my astronomy tools on my laptop, but working from my hotel room I decided that my predicted position needed to shift a bit to the east and to the north. I forwarded the correction to Rob and at 1 am cross my fingers and called it a night.

Wednesday morning I got the blow-by-blow in a series of messages from Rob. The comet wasn’t at the nominal position (no surprise), but it was off by quite a bit more than I would have expected — nearly 3 degrees. Indeed — it is a testament to Rob’s skill and perseverance that he was able to spiral out and locate it before the comet set at his location."

Robert H.McNaught,RSAA/ANU, 2006                            S.Hoenig,J.C.Pelle,N.Teamo, 2006


The image on the left is a stack of four July 12th images taken with SSO’s 0.5-meter Uppsala Schmidt by Rob McNaught at Siding Spring, New South Wales.. 

The image on the right is a stack of 26 15-second exposures taken by Sebastian Hoenig, J.-C. Pelle and N.Teamo at the Hibiscus Obs in Tahiti.

It’s looking likely that C/2006 M4 (SWAN) will cross the LASCO field of view in August. The question, however, is whether it will be bright enough to see… More on this next week!(The original text is from

Another SOHO/SWAN comet discovery!

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