Fifty to seventy-five blazing streaks per hour every one a piece of cosmic flotsam going out in a flash of incandescent plasma. That is the show in store for those venturing outside during the height of the Perseid meteor shower, a show that has been thrilling onlookers for centuries and, in Flagstaff, Arizona, is part and parcel of a rich history of astronomical discovery and preservation of the nighttime sky.
The Perseids come from Comet Swift-Tuttle, a huge near-Earth body measuring around 26 kilometers across and taking 133 years to orbit the Sun. When the comet nears the Sun, solar heat causes dust and pebble-sized particles to be released from its icy surface. These small particles, usually no bigger than sand grains, travel along the orbital path of the comet, creating a meteoroid stream. Every August, Earth makes its way through this stream and the particles crash into the atmosphere at about 37 miles per second about 133,200 mph producing strong friction that incinerates them to about 3,000 degrees Fahrenheit. At about 96 kilometers high, they burn up in dazzling bursts of light.
Lowell Observatory astronomer Nick Moskovitz explains it concisely: “A comet can leave behind a trail of bread crumbs and that trail of bread crumbs populates what we call the meteor stream. The earth is plowing through this stream of particles and those particles are hitting the atmosphere at high speed and burn up and vaporize and produce that flash of light.” The resulting streaks emanate from the constellation Perseus and give the shower its name.
From the physical point of view, the meteoroid-atmosphere interaction is a sudden conversion of kinetic energy into heat and light. The compression and ionization of atmospheric gases ahead of the shock wave created by the meteoroid results in the glowing trail. Larger pieces may produce “fireballs” meteors brighter than Venus although Perseid meteors, which are the result of a comet and not an asteroid, are usually too weak to survive to the ground as meteorites.
Flagstaff provides an exceptionally clean perspective for such occurrences. Rendered the world’s first International Dark Sky City in 2001, it has had lighting regulations on the books since 1958, prohibiting commercial searchlights and requiring low-pressure sodium streetlights whose warm amber light reduces skyglow. Adopted by civic policy as well as popular support, these efforts permit the Milky Way to remain visible within city boundaries an exception among cities. This dedication stems from the city’s astronomical heritage: Lowell Observatory, established in 1894, was the place where Pluto was discovered in 1930 and is still a center for planetary science.
The observatory’s 2,130-meter altitude and distance from large light domes make it an optimum spot for public viewing. At times of meteor showers, there are thousands of people on its grounds, not just to see the spectacle, but to be a part of the larger context of humanity’s existence in the universe. As Moskovitz points out, these events bring their lives and the earth in a broader context of the cosmos and how our understanding of the cosmos is very much tied to our existence here on planet earth.
For best viewing, astronomers suggest the 2 a.m. to 3 a.m. period, when the observer’s position on Earth is swung into the direction of its orbital motion, essentially making the relative velocity of incoming meteoroids greater and enhancing meteor rates. Although the shower is active from mid-July through early September, the rates plummet away from the peak. Observers should allow 20 minutes for their eyes to adapt to darkness and avoid bright light sources including the Moon when it is above the horizon to maximize visibility.
The Perseids’ steadiness and luminosity have made them standard fare for amateur sky-watchers and scientists alike. Radar and optical observations have mapped fluctuations in the stream’s density, detecting fine structure associated with earlier perihelion encounters of Swift-Tuttle. Such observations not only better model meteoroid stream evolution but also guide spacecraft shielding design for missions that could encounter similar particle environments.
In Flagstaff, the dark sky is still a common heritage, conserved as much for the science of it as for its culture and beauty. On a clear August evening, when the meteors burst and vanish in the sky, the intersection of cometary physics, atmospheric chemistry, and human responsibility to darkness is as much part of the spectacle as the streaks themselves.
