[krapp]

Unfortunately, there don't appear to be many credible, impartial or first-party sources linked to that wikipedia article. The best appears to be a Chicago Tribune article[0], but that only contains direct quotes from employees who claim not to have seen anything, and indirect (and sometimes contradictory) descriptions by other witnesses.

The common details seem to be that it was "dark grey" and "well defined," and displayed no lights. It can be difficult to correctly estimate the size of an object in the sky, so the disagreement about that doesn't necessarily discredit the story. The conclusion reached that it was a "weather phenomenon" doesn't sufficiently explain it.

Interestingly, there appears to be evidence that airport personnel contacted the FAA about it which turned up in a FOIA request, but I don't know if that audio is anywhere, or verified.

[0]http://articles.chicagotribune.com/2007-01-01/travel/chi-070...