Great Exploding Comets!

In October 2007 an obscure comet somewhere beyond Mars underwent an astonishing metamorphosis. In one day comet 17/P Holmes brightened literally by a million times, and then over a couple of weeks its coma of dust and ice expanded until it was larger than the sun. There it hung, high in the night sky, visible to the naked eye despite city lights.

Gradually comet Holmes receded as it continued racing away from the sun on its looping orbit, heading out beyond Jupiter. Holmes is on track to return in 2014, and it’s a safe bet that hundreds of astronomers will be watching for a repeat performance.

What caused the outburst? Opinions were divided. Some thought interior material was heated by the sun and burst through the surface, like ice volcanoes. Others speculated it had collided with an asteroid.

Holmes is a repeat offender. In 1892, when it was discovered, it brightened suddenly. After the comet returned to the quiet life it was so hard to see that it was lost until the 1960s. The asteroid argument made sense because Holmes’ orbit is largely within the asteroid belt.

On the early morning of Nov. 6, 2007, I used my 12-inch telescope to photograph Holmes. In retrospect, I wish I’d had access to a smaller scope; mine enlarges fabulously but has a limited field of view. A small telescope would have accommodated the whole comet without trouble, while I had to take many exposures and put them together in a mosaic, which is laborious.

In our front yard, one tripod leg on the sidewalk and the others on the grass, I worked taking photos. By 3 a.m. I had taken 360 images, a third each through red, green and blue filters. Many were not useful because Holmes moved against the background stars; the nucleus would be close to a star in one view and was farther away in others. Only photos taken within a short period could be put together to give a good view.

I noted that day:

“I took 120 three-color photos of this amazing comet, working until after 3 a.m. to finish up in the biting cold and put my gear away. Some cute ladies dropped by to ask me what I was doing and I was able to point out the comet to several people. It’s visible to the naked eye and easy to find, in [the constellation] Perseus. … I’ve had to abandon the idea of using a color photo, as for some strange reason it shows up as green. This is probably because of light pollution in our neighborhood. Now I’m trying to put together an acceptable mosaic.”

[A black-and-white view I took of Comet Holmes early on Nov. 6, 2007]

My finished product, this black and white view, is a compromise, with cometary motion making the nucleus fatter than it was because some of the pictures were taken after the comet had moved slightly. Also, it turned out that some astronomers’ photos show the comet as bluish, greenish or a combination.

On Wednesday the European Planetary Science Congress 2009 released an update on 17/P Holmes.

According to the group, astronomers Rachel Stevenson and David Jewitt of UCLA and Jan Kleyna of the University of Hawaii discovered that “multiple fragments” shot out of Holmes during the outburst, creating mini-comets. They began studying Holmes shortly after the outburst and continued observing as the dust cloud grew.

“The astronomers examined a sequence of images taken over nine nights in November 2007 using a digital filter” to pick out small features “that would otherwise remain undetected against the bright background of the expanding comet,” says a news release issued by the European Planetary Science Congress.

Small objects were shooting away from the nucleus at up to 280 mile per hour. “These objects were too bright to simply be bare rocks, but instead were more like mini-comets creating their own dust clouds as ice sublimated from their surfaces.”

Why did Holmes change? That’s still unknown. The scientists speculate that when it moved closer to the sun, sub-surface ice evaporated, causing internal pressure to build up, the release says.

“Surprisingly, the solid nucleus of come Holmes survived the outburst and continued on its orbit, seemingly unperturbed.”

[Photos courtesy of the European Planetary Science Congress 2009. On left is a normal image of Comet Holmes taken with the Canada-France-Hawaii telescope on Mauna Kea, Hawaii. On the right is the same view after the Laplacian spatial filter was used to emphasize small objects including mini-comets jetting away. Circular white objects surrounded by black are background stars.]

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