Old Comets for a New Year

A reminder about Comet Holmes!

While the spotlight is now on Comet Tuttle, we should not forget about our old friend, Comet Holmes which continues to be dimly visible to the unaided eye as a diffuse, circular cloud, roughly twice the apparent diameter of the moon against the stars of the constellation Perseus.

This comet was no brighter than magnitude 17 in mid-October — that's about 25,000 times fainter than the faintest star that can normally be seen without any optical aid. But late on Oct. 23, the comet's brightness suddenly rocketed all the way up to magnitude 2.5, brightening nearly one million times in less than 24 hours!

In attempting to explain why Comet Holmes exploded, comet expert, John Bortle suggested that this comet's nucleus consists of low-density material that, over time developed into a large region with a very tenuous structure, like a honeycomb. At some point, the highly fragile bonds connecting the honeycomb of material reached a failing point and a sudden crushing collapse occurred, expelling a gigantic volume of dust into space, making this dim comet suddenly appear impressively bright.

Back in 1892, Comet Holmes suffered two major outbursts separated by about 75 days. This leads to the question as to whether this comet will undergo a similar "cosmic aftershock" in the wake of its recent late October explosion.

Bortle thinks it's a possibility, based on the theory that there may be a large amount of residual instability which might lead to a second major collapse of material on the comet nucleus. If what happens now parallels what happened in 1892, another possible explosive outburst may be imminent, so it might be wise to keep a close watch on Comet Holmes in the coming days ahead.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.