Eyes on the sky watch the unpredictable Draconid meteor shower
Meteor showers — nature’s superlative fireworks displays — occur when Earth’s orbit intersects a trail of debris left behind by a comet. Meteoroids from the comet stream collide with Earth’s atmosphere and ignite, punctuating the night sky with a cascade of shooting stars. Scientists can accurately forecast the timing of meteor showers using the principles of celestial mechanics; however, the intensity of the Draconid shower, which occurs every October, has proved notoriously hard to predict. A new study employing a network of sky-watching cameras reveals that in 2011, the shower peaked at 400 meteors per hour and delivered nearly one ton of meteoritic material to Earth in less than 48 hours.
The Draconid shower graces Earth courtesy of a comet known as 21P/Giacobini-Zinner, which swings through the solar system every 6.6 years. Where it comes closest to the sun, some of the comet’s icy mass sublimates, depositing debris in space that remains suspended there for hundreds to thousands of years. Earth then passes through these streams during its yearly orbit, giving rise to a meteor shower that appears to originate from the head of the constellation Draco.
However, because of small variations in the paths of the comet and Earth induced by the gravitational pull of other planetary bodies, the intensity of the storm varies as Earth passes through more or less dense parts of old comet trails. In 1946, for example, the Draconids put on one of the most astonishing meteor storms ever recorded, with rates of activity exceeding thousands of meteors per hour when the planet intersected a very dense meteoroid stream. In other years coincident with sparse streams, observers have seen nothing at all.
So when scientists predicted that the 2011 event might rival the famous storm of 1946, astronomers made a special effort to monitor the shower. Josep Trigo-Rodriguez, an astronomer at the Spanish Institute of Space Sciences and the leader of its meteorite working group, marshaled the resources of the Spanish Meteor Network to watch the skies over Europe and North Africa during the event. The network consists of 25 stations equipped with high-resolution cameras and video cameras that allowed the scientists to observe the meteors — even though a bright moon impeded viewing conditions on Earth — and then to analyze the brightness and spectrum of each fireball to determine its size and composition.
In two recent studies published in the Monthly Notices of the Royal Astronomical Society, one led by Trigo-Rodriguez and the other by José Madiedo of the University of Huelva in Spain, the researchers reported that the 2011 shower, while impressive, failed to match expectations, probably because it originated from old streams left behind by the comet between 1873 and 1900. Precise modeling of the comet’s trajectory allows scientists to estimate the age of meteoroid streams, so researchers knew Earth would intersect these older trails. But the meteoroids proved much more fragile than anticipated.
“In interplanetary space, [meteoroids] are subjected to irradiation and collisions with other particles from their own stream, and also with other particles in the interplanetary medium,” Trigo-Rodriguez says. “Due to their fragile nature, 21P particles seem to [disintegrate] over timescales of a few centuries.” Because of this, the ton of material in the 2011 shower was “several orders of magnitude lower than the mass delivered during historic Draconid storms” involving much younger streams.
However, the age of the streams is only one possible explanation for the weaker than expected 2011 Draconid shower, says Bill Cooke, an astronomer at NASA’s Meteoroid Environment Office based in Huntsville, Ala., who was not involved in the study. “Another is that the comet produced fewer meteoroids in the streams that intersected Earth,” he says, because “cometary production rates can vary.” Another possibility is that the models that forecast the shower strength were incomplete. Regardless, he says, “the observations presented [in this study] are of very high quality” and will help improve modeling efforts to better predict meteor showers like the Draconids.
Meteoric material like that delivered by the 2011 storm is important to predict and understand for a number of reasons, Trigo-Rodriguez says. Man-made satellites and the International Space Station “are also subjected to the meteoroid influx, and there is growing interest in quantifying this hazard.” In addition, the researchers learned that the Draconid meteoroids have chemical compositions similar to carbonaceous chondrites, the kind of meteorites that may have delivered large quantities of volatile compounds like water, methane and ammonium to early Earth, setting the stage for the evolution of life. “We envision a scenario in which the disruption of fragile comets like 21P could have delivered massive amounts of organics and hydrated minerals to early Earth,” Trigo-Rodriguez says.
Look out for the 2013 Draconid shower, which is predicted to occur on October 7-8, although it will likely be weaker than the 2011 event.