CHICAGO — When Adler Planetarium astronomer Mark Hammergren first heard the buzz about comet ISON last fall, he felt a familiar tinge of excitement.
The brightest comet in the last century. As vivid as a full moon in the night sky over Chicago. A once-in-a-lifetime, blazing spectacle.
But as ISON zips closer to Earth’s orbit, the hunk of ice and dust has done something typically cometish: upended expectations.
Compared with original estimates, ISON is already dimmer than expected, reviving bitter…