The Earth is plunging through the debris trail of Comet Swift-Tuttle right now. It happens every August. The debris consists mostly of rocks about half an inch across, tumbling through space along the comet’s trajectory. The rocks are small, but orbital velocities impart tremendous kinetic energy. When those pebbles strike Earth’s tenuous upper atmosphere at over 12 miles per second, the resulting fiery display of shooting stars is something we call the annual Perseid meteor shower.
This year the Perseids peak on the night of Aug. 12, though you can see them streaking across the sky for many days before and after the peak.
Comets are the primordial remnants of the early solar system. They’re giant dirty snowballs, made of ice mixed with dust and rock, averaging six miles in diameter. Billions of comets orbit the distant reaches of the solar system; of these, we know of around 3,700 with highly elongated orbits that periodically bring them into our neighborhood, the inner solar system. Halley’s Comet is the one most people have heard of. It has a 75-year orbit — look for it next in 2061. Swift-Tuttle, with an orbit of 133 years, passed by in 1992. It won’t return until 2126.
Comets like Halley and Swift-Tuttle spend most of their orbits frozen solid. It’s very cold way out there. But as they make their relatively short dash through the inner solar system, the Sun’s heat grows more intense. At around three times Earth’s distance from the Sun, the exposed ice on the surface of the comet begins to sublimate — that is, it transitions directly from solid to gas. This water vapor trails behind the comet, along with little rocks and countless specks of dust released by the sublimating ice. Sunlight illuminates this debris trail. If the trail is large and reflective enough, the comet becomes a naked-eye object during the weeks before and after the perihelion (the point when it’s closest to the Sun).
After the comet rounds the Sun and starts back towards the outer solar system, the temperature plummets, sublimation ceases, and the comet goes dark again, having permanently lost some of its mass.
If a comet’s trajectory intersects Earth’s orbit, then we pass through its debris trail once every year. These annual smash-ups are called meteor showers. The comet debris strikes our atmosphere at extremely high speed. Friction heats the dust grains and little rocks to incandescence and leaves a glowing trail of ionized air molecules. We call this flash of light streaking across the sky at an altitude of about 60 miles a meteor or shooting star. The comet debris doesn’t survive atmospheric entry. It’s too small and vaporizes in the intense heat.
Millions of meteors strike our atmosphere every day. Most are tiny grains of sand, and their fiery demise is faint and hard to see. But there are larger rocks out there, and on any night of the year you can see a shooting star every few minutes or so. What makes a meteor shower different is the frequency, brightness, and orientation of the meteors.
Under favorable conditions with a moonless sky, you can expect to see 90 Perseids per hour. They will be brighter and more dramatic than your average shooting star. Many will leave lingering trails; some will explode with a dramatic flash.
If you trace the Perseids’ path back with your eye, they will all appear to originate from the same part of the sky. The constellation that happens to be in that part of the sky gives this meteor shower its name: the Perseids originate in the constellation Perseus. (You know him. He’s the one who slew Medusa with the old reflection-in-the-shield trick.) The Leonids, another well-known meteor shower and the debris of Comet Tempel-Tuttle, occur every November.
This year, unfortunately, a nearly full moon will obscure the less bright Perseids. Even so, you can expect to see about 45 per hour if the sky is clear. Perseus is low in the northeast, but you don’t need to find him to see his meteors. Perseids can appear anywhere; just look up and be patient. The best time is after midnight and right before dawn, when Earth’s rotation points those of us on the East Coast directly towards the debris stream.
But get out there whenever you can, even if it’s not the peak night. You never know when we might pass through an unusually dense filament of debris, turning a light shower into a heavy rain – or even a downpour – of meteors. Clear skies!
Readers can contact the writer with astronomy questions at [email protected].