For thousands of years, humans cowered in fear at these phenomena, and blamed them for wars, disease, earthquakes, and animal mutations. The most famous recurring comet, Halley's, was believed to be responsible for Europe's Black Death pandemic. Pope Callixtus III contended that the comet was an ominous message from Satan himself. As recently as Halley's appearance in 1910, gullible citizens consumed "comet pills" to save them from the poisonous cyanogen gas streaming from its nucleus. Indeed, the recent meteor explosion over Russia reminds us that our innate fear of certain celestial phenomena is sometimes justified.
However, humanity's gradual embrace of science has helped it determine that, unless a comet is on a trajectory to crash into the Earth, we have nothing to fear. Comets visible to the naked eye, while relatively rare, can be the most beautiful and stunning of all celestial events, and 2013 has the potential to be the greatest year in comet-watching history.
This month, beginning on March 12, Comet Panstarrs should become visible in the Northern Hemisphere's evening sky, possibly as bright as the Big Dipper stars and trailing a long, spectacular tail. But the comet that has the astronomical community buzzing with excitement is Ison, which will approach Earth in November.
Current calculations indicate that Ison has the potential to be the brightest comet ever seen by humans, and could even be visible during daylight. Unfortunately, the words could and possibly often accompany discussions of an approaching comet's potential brightness because there are numerous factors contributing to it: comet size, nucleus activity, and distance from the Earth and sun. But, based on current estimations, astronomers are cautiously optimistic that Ison will live up to its "once in a lifetime" billing.
Recent Earth-based and robotic probe analyses of comets have confirmed earlier assumptions that they are remnants of the early solar system. In 2005, NASA's Deep Impact spacecraft shot an 820-pound projectile into Comet Tempel 1 and analyzed the material ejected by the impact. And NASA's Stardust spacecraft returned dust samples of Comet Wild 2 in 2006. Surprisingly, both probes revealed lower-than-expected amounts of water ice, but also discovered that they were not loosely cemented rubble piles, which conflicted with the generally accepted "dirty snowball" theory of comet composition first espoused by astronomer Fred Whipple in 1950. More surprises are expected next year, when Europe's Rosetta spacecraft will rendezvous with a comet and dispatch a lander to study its surface and composition.
Astronomers believe that Ison's composition is probably similar to Tempel 1 and Wild 2, and will likely survive its close swing around the sun. They also surmise that it must be a relatively large object (maybe 2 miles across) because it was discovered while still extremely far from Earth. Both of these factors, in addition to its close approach to the Earth and sun in November, are the primary reasons why it has the potential to be as bright as a full moon. Incredibly, we could get a preview of what we'll see, as NASA's Mars rover Curiosity may attempt to take a picture of Ison as it moves across the Martian skies in September. The ancients could not have imagined achieving such things.
All of us are fortunate to be living in a time when two bright comets could illuminate our skies in the same year. Like our ancestors, we will lift our children onto our shoulders so that they may better see these ancient celestial visitors. But unlike previous generations, we will teach our children to marvel, and not cower, at the sight of these comets and, in so doing, reinforce humanity's continuing triumph of science over superstition.
Chris Gibbons is a Philadelphia writer and member of the Planetary Society. He can be reached at email@example.com.