Each year as Earth journeys around the sun, it slams into streams of particles that comets have spewed into space. When these particles, known as meteoroids, smash into the atmosphere, they generate streaks of light called meteors. The most reliable meteor showers—the Perseids in August and the Geminids in December—sport about 60 meteors an hour and require nothing more than a dark, moonless sky in order to appreciate them.
Early last December, however, an uncharted meteoroid stream pelted the planet with one of the year's strongest showers. "It was a complete surprise," says Peter G. Brown, an astronomer at the University of Western Ontario. "No one had predicted this." On the night of December 4, 2011, six Canadian radar stations, which send out pulses of radio waves that bounce off meteor trails, detected 50 meteors an hour.
The meteors radiated from the northern constellation Cassiopeia, which borders the constellation Andromeda, and marked a revival of the Andromedids, a shower that achieved notoriety in the 19th century. The meteors result from debris left by Biela's Comet. First seen in 1772, this faint comet swung around the sun every 6.6 years. Before the comet's 1846 passage, it broke in two, and in 1852 it returned as two separate comets.
No one ever saw them again. But in 1872 and 1885, spectacular Andromedid meteor storms took their place, shooting thousands of meteors an hour across the sky: "a real rain of fire," wrote one observer. During the 20th century, though, the Andromedids dwindled to insignificance. The surprise 2011 comeback was their best performance in more than 100 years.
Astronomer Paul Wiegert, also at Western Ontario, modeled the 2011 shower and traced the meteors' origin to particles that Biela's Comet shed in 1649, more than a century before the comet was seen. Furthermore, in work recently submitted to The Astronomical Journal, Wiegert, Brown and their colleagues predict that Earth will soon plow through this newfound meteoroid stream again.
"There are certainly a number of uncertainties involved with trying to predict the meteoroid stream which is associated with a long-gone comet," Wiegert says. "But our best bet is that the shower will return again in 2018."
Fortunately, the Andromedids are civilized visitors: Unlike most meteor showers, which are best seen in the wee hours, bits of Biela's Comet are easiest to see before midnight, when the radiant in Cassiopeia is highest. The radiant is the spot on the sky from which every meteor seems to emanate, but meteors will appear everywhere, not just in Cassiopeia and Andromeda, so it is best to look wherever the sky is darkest. In fact, a dark sky is essential: the meteors are faint, because the particles in the newfound stream are small and their speeds slow; it is kinetic energy that largely determines a meteor's brightness. Thus, observers must escape both the glow of city lights and the brightness of the moon, which washes out faint meteors.