• August 7, 2013

    Comets and Meteor Showers

    Credit: Wikimedia Commons User Navicore

    A brief streak of light crosses the night, fleeting and beautiful. You scan the sky, turning your head at the glint in the corner of your vision, hoping to be staring at the right spot when a new star is born to a bright existence and swift end.


    Shooting stars, we call them. Meteors are the scientific term. The Perseid meteor shower will peak this year before dawn on Aug. 12, bringing around 60-100 meteors per hour to viewers in the Northern Hemisphere. (Southern Hemisphere viewers will see a handful per hour.) Even the most casual skywatchers have heard of this anticipated and much-loved skywatching event. But not many know we have comets to thank for the sight.


    Annual meteor showers like the Perseids occur when Earth passes through the trails of dust left behind by comets as they loop around the Sun. As a comet’s ice vaporizes or disintegrates in the heat of the Sun, creating a coma and tail, a trail of dust is left in the comet’s wake. When Earth’s orbit intersects that trail, those fine dust particles burn up in Earth’s atmosphere, creating the bright streaks of light that flash across the sky. We name the showers after the constellations they appear to emanate from, though their actual location is determined by the path of the comet that shed the dust.


    The Perseid meteor shower is the result of Comet 109P/Swift-Tuttle, which returns to the Sun every 133 years. Typically only short-term comets cause meteor showers. That’s because short-term comets, originating from the Kuiper Belt, return repeatedly to the inner solar system. During each visit, they make a deposit of dust – dust made up of materials like iron, nickel, pyroxene and olivine, according to NASA’s Stardust mission, which plunged through a comet’s coma and snatched a sample in 2004. The built-up dust trail provides enough material to create a show in the night sky, and the comets' return trips replenish the debris that dissipates or burns up in the atmosphere.


    Long-period comets from the Oort Cloud, like Comet ISON, generally won’t leave meteor showers in their wake. These one-time visitors would have to be incredibly active comets to deposit enough material to cause a shower in a single trip. And even if they did, their orbits typically don’t intersect with Earth’s orbit.


    Some astronomers speculate that Comet ISON could, however, cause the phenomenon of noctilucent clouds – wispy, glowing , high-atmosphere clouds that appear at the horizon at sunrise and sunset. The clouds are made of water-ice crystals and could result from comet dust particles seeding the atmosphere.


    Noctilucent Clouds over Helsinki, Finland
    Credit: Timo Newton-Syms


    If watching bits of debris burn up in the atmosphere makes you want to duck – especially after seeing videos of Russia’s recent window-shattering Chelyabinsk meteorite -- it shouldn’t. That meteorite -- the term for a meteor that makes it to the ground -- was an asteroid around 54 feet (17 meters) in size that exploded around 14.5 miles (23.3 km) above the surface. Comet dust is fluffy, typically smaller than grains of sand and usually burns up around 50-75 miles (80-120 km) above the Earth. Only the Geminid meteor shower in December, which results from the asteroid 3200 Phaethon, is proven to originate from an object other than a comet.


    It seems odd that such tiny grains could cause the beautiful lightshows we witness from below, but a meteor’s light comes not from mass, but from the speed at which the dust enters the atmosphere. The bits of dust come screaming in at 25,000-160,000 miles per hour (40,200-257,500 km per hour). When the meteor hits air molecules, kinetic energy excites the atmospheric atoms, causing the flash of light.


    So this week, grab a lounge chair, go someplace far away from city lights, and keep your eyes on the sky. After midnight is the best time to spot these bits of comet tail as they end their existence in tiny blazes of glory. Remember as you watch them flash through the night that you’re witnessing the result of comets journeying for hundreds upon hundreds of years through the depths of space, the lightshow a parting gift to planet Earth from those spectacular cosmic guests that appear only rarely in our skies.