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Comet Holmes
Photo: NASA/ESA and A. Dyer

A Burst of Brightness for the Comet 17P/Holmes

On October 23, 2007, a comet discovered 115 years ago underwent an exceptional outburst of activity that turned it for a few weeks into a naked-eye object, easily spotted from the city, even with the bright full moon shining nearby!

Comet 17P/Holmes travels around the sun every 7 years on an elongated trajectory, between the orbits of Mars and Jupiter. A comet at that distance is normally very faint, around 16th magnitude, which should confine it to photographic observations. However, from October 23 to 24, the magnitude of Comet Holmes increased dramatically, reaching +2.5 — a 500,000-fold brightness increase in a matter of just a few hours! Astronomers have suggested a number of hypotheses to explain the outburst, but the exact cause will likely remain a mystery.

At the onset of the outburst, Comet Holmes appeared at first glance like an extra "star" in the constellation Perseus. To the naked eye, the comet initially appeared pointlike, just like the bright neighbouring stars. However, since October 28, the cloud of dust and gas that surrounds the comet's nucleus (the coma) has expanded considerably. For the first half of November, Comet Holmes appeared like a fuzzy star when viewed without optical aid. After mid-November, the bright moon interfered with observations for a few days. Right now, the moon rises late enough to allow good early evening observations.

The halo of dust ejected from the nucleus on October 23, 2007, continues to spread out into space. The coma is now estimated to be about 3 million kilometres across, more than twice the size of the sun! Because of this expansion, the comet is becoming dimmer. A pair of binoculars is now necessary to reveal it in light-polluted city skies. However, Comet Holmes is still a naked-eye object under dark country skies.

Dirty snowballs

The nucleus of a comet is a mountain-sized "dirty snowball" made of water ice and gases, frozen by the cold of outer space, and mixed with dust and pebbles. Small and dark, a comet nucleus is very hard to detect. It's only in the vicinity of the sun that the heat turns the icy material to gas and frees the dust particles. This dust and gas creates an enormous, luminous halo around the nucleus, called the coma. When the outgassing rate is sufficiently high, the dust and gas form a long tail that stretches away from the coma.

The crumbly texture of cometary nuclei explains why fragments can break off on occasion, suddenly releasing more gas and dust. When this happens, the brightness of a comet temporarily increases. So, while comet outburts are fairly common, the amplitude of Comet Holmes' current outburst is truly exceptional.

A fragile comet?

It may very well be that Comet Holmes' specific composition or structure gives it a natural predisposition for such large outburts. Indeed, when it was first observed in London, by English astronomer Edwin Holmes, on November 6, 1892, the comet was undergoing a similar brightness increase — which certainly played a major role in the comet being discovered at the time!

How long will this outburst last? How will it evolve? What's certain is that the comet will gradually dim as the dust that was released on October 24 continues to spread out into space — unless another outburst happens in the coming months. Indeed, that's exactly what happened following the discovery of the comet, 115 years ago! Comet Holmes truly is an object we must keep an eye on.

Comet Holmes is moving slowly, tracing a wide loop in the constellation Perseus and passing close to the bright star Algol around January 23, 2008. That area of the sky will remain ideally placed for observation for the coming months. Stay tuned…

Happy observing!

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