Scientists briefly estimated that Comet Bernardinelli-Bernstein, as it's now known, was the largest such icy body identified to date, perhaps more than 100 miles (160 kilometers) across. Additional observations have cast that into doubt, but given the "megacomet" a new distinction: it sprouted a tail remarkably far from the sun, suggesting more revelations to come. All told, the object offers astronomers an unprecedented opportunity to watch the antics of a comet. Although what initially stood out to Bernardinelli was the comet's weird orbital characteristics, the discovery made such a splash because of a different trait, the comet's estimated size. Based...