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.
Lunar conditions will be ideal in 2018, because the moon will be new in early December. Wiegert's team predicts that the 2018 performance should be weaker than last year's, with perhaps 35 meteors an hour. Observations in 2018 will help refine predictions for the future.
However, "2023 looks pretty solid," Brown says. "We have some reasons to suspect that may be the stronger of the bunch." The 2023 event might yield as many as 200 meteors an hour, surpassing the Perseids and Geminids. "It's not a storm, but it's a very strong outburst," Brown says. The moon will rise before midnight but won't dampen the display during the early evening hours when the shower should be strongest.
Alas, as the astronomers acknowledge, forecasting meteor showers shares a trait with meteorology: uncertainty. "You're in a very awkward situation here," says David Hughes, an astronomer at the University of Sheffield in England who was not affiliated with the researchers. "You'd hate to be the person who told people, 'Don't bother looking,' and then the sky fills up with meteors!" Still, Hughes worries that, because the Andromedid meteoroid stream's orbit extends toward Jupiter, that planet's gravity may have jostled the stream about, hindering accurate predictions for future showers.
"What do you have to lose?" asks Donald Yeomans of the Jet Propulsion Laboratory. "It may disappoint you, or it may rise well above your expectations. That's just the nature of these meteor showers; you never quite know."
Indeed, without sacrificing any sleep, observers in the Northern Hemisphere can see whether the Andromedid meteors were the 21st century's one-hit wonder—or will become a more enduring presence on the celestial stage.