Here’s what I wrote about Comet SWAN for the Gettysburg Times about a week ago:
Current models for Comet SWAN show that it will reach a peak brightness of magnitude 2.8 on May 21. That’s pretty close to the same brightness as the faintest two stars in the Big Dipper, but since a comet is a hazy, diffuse source instead of a starlike point, it will be harder to see. For another comparison, that’s about 30 times fainter than the great Comet Hale-Bopp of 1997. While it may be visible without optical aid, you will probably want binoculars to find it, at least at first.
Since then the bad news has come that the comet brightened much less than predicted. “Yet another fizzler,” says Alan MacRobert of Sky and Telescope. It’s currently an underwhelming magnitude 6, and since each magnitude is 2.5 times brighter than the last, that’s almost 19X fainter than the peak prediction of 2.8. Too bad.
But why are comets so unpredictable? Sometimes described as “dirty snowballs,” these icy objects are leftovers from the formation of the solar system. Many orbit the sun slowly far out beyond the orbit of Neptune. Some come into the inner solar system on one-time trips; others are gravitationally captured here in recurring orbits, such as Halley’s Comet. As these icy bodies approach the sun they warm up under the influence of solar wind and solar radiation and release gas and dust that may form a hazy coma and tail. That outgassing material is what we see when we spot a comet in the sky and also what makes their appearance so hard to predict. The brightness depends on volatile processes that take place as the comet warms up, and as we’ve seen with Comet ATLAS, that can even include disintegration.
The next comet to get our hopes up is Comet NEOWISE, which has the potential to reach naked-eye visibility in July. In the meantime, here’s a song named after a famous cometary dissapointment of the 1970s.