Comet releases dust clouds, making it visible again

Comets are unpredictable, but they are not supposed to be this unpredictable.

Ten days ago, far beyond the orbit of Mars, a dead clod of frozen dirt known as Periodic Comet Holmes suddenly came alive and brightened about a millionfold, lighting up to shine within easy view by the naked eye. It remains visible from your doorstep almost at a glance, if you know exactly where to look.

For unknown reasons, this long-forgotten dirtball, just a couple miles wide, belched out an enormous puff of fine dust that is being lit up by sunlight. The dust cloud has been expanding, enthralling astronomers with a sight unlike anything they have seen.

At first the cloud was so small that you needed a telescope to see it as anything more than a star-like point. It expanded daily, glowing pale yellow and looking in binoculars and telescopes like a frosted incandescent bulb with a dimmer switch.

It shows a tiny, bright core and a bright blob slightly offset from the middle. The blob is a fountain of gas and dust spraying from the comet's solid nucleus, which is invisibly tiny.

Now Comet Holmes has grown large enough that even to the naked eye, it is fuzzier than a star.

Through binoculars, it looks like a luminous jellyfish swimming straight at us in the depths of the night.

To spot Comet Holmes, face northeast in midevening. Look very high for the W-shaped constellation Cassiopeia, a landmark of the autumn sky. The W is standing on end, as shown here, and is about as big as your fist held at arm's length.

Look about two fist-widths to the lower right of Cassiopeia. The brightest star there is Mirfak, in the constellation Perseus. Mirfak is the top star of a smallish triangle about the size of your thumb at arm's length. The triangle's lower-left corner is Comet Holmes.

If it is cloudy tonight, don't worry. Holmes is not leaving just yet. It will probably fade gradually in the next week or two.

Watch each clear night, and you will notice that the comet is creeping toward Mirfak. It will pass close by Mirfak on the night of Nov. 19.

The outburst of Comet Holmes was a surprise, but it has happened to other comets before. Comet cores far out in space sometimes bust loose with gushers of gas and dust for no apparent reason. This comet was in a somewhat lesser outburst when it was first discovered by English amateur astronomer Edwin Holmes in November 1892. That time it faded within a few weeks, and then erupted again two months later.

In an ordinary bright comet, the icy, solid nucleus is close enough to the sun to get a lot of heat and to spray a lot of vapor and dust from its evaporating surface. But Comet Holmes was far from the sun and nearly dormant. Something happened inside it to make it suddenly spew all at once.

Some thought that it was hit by a meteoroid, but astronomers have ruled this out. Meteoroids are too few and far between - space is extremely empty - and cold comets show this type of behavior too often.

Another idea is that as a comet ages, it develops a hard, frozen-dirt shell, like a city snowpile in March. The shell's dark color helps it absorb whatever solar heat it receives. This warmth slowly vaporizes the ice inside, and eventually the shell bursts. And maybe, when the pressure releases, a mixture of ice and dry ice (frozen carbon dioxide) inside keeps fizzing and foaming like a clumsily popped champagne bottle.

Don't miss it.

Alan M. MacRobert is a senior editor of Sky & Telescope magazine in Cambridge (SkyandTelescope.com). His Star Watch column appears the first Saturday of every month.