Comet Holmes – even more a mystery

Remember Comet Holmes? If you are an amateur astronomer you undoubtedly do.

Comet Holmes among the stars of Perseus, Nov 11, 2007 - from Driftway Observatory, Westport.

It was putting on a great show through late October and most of November of last year. What’s more, it was really a freakish one that surprised astronomers. Holmes is one of those “regulars” in the comet world, a familiar periodic comet that comes around every six years or so and puts on a pretty hum-drum display. It has been known for well over a century. But twice – once in November of 1892 and once last year – it suddenly exploded – no one knows why – and brightened a million fold. Now that get’s your attention.

I was thinking about it today when I stumbled across some of my pictures of it, and I wondered whether anyone had figured out yet just what caused the sudden eruption. What I found was a fairly recent (October) NASA press release that explains that it is still a puzzle – in fact, in some ways it’s now something more of a puzzle then it was last year. (Next two photos are from NASA.)

Spitzer’s infrared picture at left reveals fine dust particles that make up the outer shell, or coma, of the comet. The nucleus of the comet is within the bright whitish spot in the center, while the yellow area shows solid particles that were blown from the comet in the explosion. The comet is headed away from the sun, which lies beyond the right-hand side of the picture.(From NASA press release.)

The contrast-enhanced picture . . . shows the comet’s outer shell, and strange filaments, or streamers, of dust. The streamers and shell are a yet another mystery surrounding comet Holmes. Scientists had initially suspected that the streamers were small dust particles ejected from fragments of the nucleus, or from hyerpactive jets on the nucleus, during the October 2007 explosion. If so, both the streamers and the shell should have shifted their orientation as the comet followed its orbit around the sun. Radiation pressure from the sun should have swept the material back and away from it. But pictures of comet Holmes taken by Spitzer over time show the streamers and shell in the same configuration, and not pointing away from the sun. The observations have left astronomers stumped.(From NASA press release.)

Here's how Comet Holmes appeared to me on November 11, 2007. Like so many amateurs, I observed it many nights that fall with naked eye and various telescopes. This is simply a snapshot of a video screen taken when using the Color Hyper Malincam to observe it.

Here are some more details from the NASA release:

Comet Holmes not only has unusual dusty components, it also does not look like a typical comet. According to Jeremie Vaubaillon, a colleague of Reach’s at Caltech, pictures snapped from the ground shortly after the outburst revealed streamers in the shell of dust surrounding the comet. Scientists suspect they were produced after the explosion by fragments escaping the comet’s nucleus.

In November 2007, the streamers pointed away from the sun, which seemed natural because scientists believed that radiation from the sun was pushing these fragments straight back. However, when Spitzer imaged the same streamers in March 2008, they were surprised to find them still pointing in the same direction as five months before, even though the comet had moved and sunlight was arriving from a different location. “We have never seen anything like this in a comet before. The extended shape still needs to be fully understood,” said Vaubaillon.

He notes that the shell surrounding the comet also acts peculiarly. The shape of the shell did not change as expected from November 2007 to March 2008. Vaubaillon said this is because the dust grains seen in March 2008 are relatively large, approximately one millimeter in size, and thus harder to move.

“If the shell was comprised of smaller dust grains, it would have changed as the orientation of the sun changes with time,” said Vaubaillon. “This Spitzer image is very unique. No other telescope has seen comet Holmes in this much detail, five months after the explosion.”

“Like people, all comets are a little different. We’ve been studying comets for hundreds of years — 116 years in the case of comet Holmes — but still do not really understand them,” said Reach. “However, with the Spitzer observations and data from other telescopes, we are getting closer.”