The Cometary Origins of Meteor Showers
The Cometary Origins of Meteor Shower Such as The Perseids. This year’s Perseids meteor shower might provide a good show if weather cooperates, as the first-quarter moon will set before midnight on the peak dates around Aug 10-12. Perseid counts usually increase late in the night before twilight, and as many as 50-70 per hour are predicted for darker sky locations this year.
But what exactly are the Perseid meteors?
A few years ago, I returned from the NEAF conference in April with a small meteorite specimen. Many of us have marveled at the US’s largest meteor (the Willamette meteorite, found near Portland OR) weighing in at 16 tons, on display at the Hayden Planetarium in NYC. My 50 gram NEAF specimen is a fragment of the Canyon Diablo meteorite (Meteor Crater AZ), whose ~30 tons explosively dispersed over a ~10 km radius upon impact thousands of years ago. Both of these examples are composed mostly of iron with ~5-10% nickel and 0.5% cobalt. The origin of the iron-nickel class of meteorites can be traced to the cores of large asteroids, similar in composition to earth’s core.
In contrast, meteor showers like the Perseids originate from cometary orbital material which is quite unlike the iron nickel composition of the large meteorites. Spectroscopic analysis suggests carbonaceous chondrites found on earth are similar to comets in composition, and some specimens may be from meteor showers. Of course, only the rare larger meteors ever make it to the ground, and most are only a few grams in mass or even less. Despite their low mass during entry into the atmosphere, their very high kinetic energies derive from velocities on the order of 100,000 mph, producing a brilliant transient incandescence if we are lucky enough to see them
Meteor showers result from earth crossing the orbital path of periodic comets, which over time have “leaked” small particles of dust and debris from the nucleus along the entire orbit. The Perseid meteor shower of August is associated with the periodic comet 109P/Swift Tuttle. This is a large comet with nucleus ~16 miles across, and a 133-year elliptical orbit which intersects our plane of the ecliptic at a sharp angle of ~113 degrees.. The comet crossed the earth’s orbital plane most recently most recently in 1992. According to information from the American Meteor Society, most of the Perseid particles have been part of the Perseid meteor cloud for at least a thousand years.