Whenever astronomers discover a comet headed inbound toward a close encounter the Sun, there's always buzz among observers about how bright it might get. That was certainly the case last December, when Comet Elenin (C/2010 X1) made its debut. Many hoped it would become easily visible to the unaided eye as it rounded perihelion nine months later. By April, that initial enthusiasm had waned a bit, as it became clear that Comet Elenin was small and intrinsically faint. The keys to its peak visibility were the closeness it would eventually have to the Sun on September 10th (0.48 astronomical unit,...