There's perhaps no better harbinger of the waning days of summer than the Perseid meteor shower. Peaking between the ninth and 14th of August, this dazzling display of colorful meteors can sometimes number anywhere between 60-200 per hour. Unbeknownst to the many who lay gazing under their spell, however, the Perseids come with a dark twist: The source of all this streaking beauty has one of the best calculated shots of striking Earth and ending nearly all life as we know it.
'The single-most dangerous object known to humanity'
The parent of the annual Perseid meteor shower is a 16-mile-wide icy comet named Swift-Tuttle. Officially discovered in 1862 by American astronomers Lewis Swift and Horace Tuttle, this periodic comet makes a trip through the inner solar system every 133 years. Every time Swift-Tuttle passes Earth and makes a close approach to the sun — an event first recorded as early as 69 B.C. — it sheds a massive cloud of dust, rock and ice in its wake. According to estimates, this cloud is enormous, stretching some 10 million miles wide and more than 75 million miles long.
Each July and/or August, the Earth's orbit takes it through this debris cloud, with the shooting stars that result appearing to emanate in the constellation of Perseus.
Unfortunately for Earth and every living thing on it, each time Swift-Tuttle take a turn around the sun, its orbit nudges ever-so-closely to Earth's. While its most recent pass in November 1992 placed it a comfortable 110 million miles from Earth, its return in August 2126 will whittle that distance down to 14.2 million miles. Less than a thousand years later, in the year 3044, things will get really dicey, with the comet passing less than 1 million miles from Earth.
No other object this large has ever been discovered to consistently track this close to Earth.
"This date is so far in future that I hope people will not get unduly worried," astrophysicist Brian G. Marsden told the New York Times in 1992. "But some day there's a chance of a hit."
After another close pass in the year 4479, Swift-Tuttle's orbit gets more tricky to predict. Every pass around the sun can unleash jets of material that subtly alter the comet's orbit, as can influences from one of our solar system's massive gas planets. Added up over thousands of years, these little shifts can turn into one big push that will either knock Swift-Tuttle into Earth, our moon, the sun, or possibly straight out of the solar system entirely.
As radio astronomer Gerrit Verschuur wrote in his book detailing the threat of comets and asteroids, Swift-Tuttle's creeping potential to strike Earth makes it "the single most dangerous object known to humanity."
A bad day for Earth
Should the paths of Swift-Tuttle and Earth ever converge, the impact would trigger one of the largest extinction events in our planet's history. Traveling at an estimated speed of 37 miles per second, the miles-wide behemoth would unleash energy estimated to be more than 27 times greater than the Chicxulub impactor that wiped out the dinosaurs.
Researchers studying that impact event, which occurred in the Yucatán Peninsula some 66 million years ago, say that immediately after the asteroid or comet struck, massive amounts of dust, sulfur and carbon dioxide from vaporized rocks were ejected into the atmosphere. A 2017 study found that this deadly cloud would have reflected sunlight and plunged global temperatures an average of 47 degrees Fahrenheit for at least three years. Some three-quarters of the plant and animal species on Earth is believed to have been wiped out.
According to astrophysicist and author Ethan Siegel, Swift-Tuttle would be far worse. Writing for Forbes, he says that its impact would "release more than one billion MegaTons of energy: the energy equivalent of 20,000,000 hydrogen bombs exploding all at once."
The show must go on
Earth hasn't missed a Perseid meteor shower in 2,000 years. (Photo: Tucker Hammerstrom/Flickr)
Despite Swift-Tuttle's doom-and-gloom possibilities, it's still nothing we have to necessarily worry about for at least the next 2,400 years. Planetary scientists are more concerned with discovering smaller celestial bodies that, short of wiping out life on Earth, could still severely cripple modern civilization. Since 1999, more than 18,000 Near-Earth Objects (NEOs) have been discovered with orbits passing within roughly 30 million miles of Earth. An average of 40 more are discovered each week. So far, none are predicted to collide with our planet in the immediate future.
So this August, look up at the Perseids, relax, and give thanks that we're still around to admire the beauty of it all. Who knows what that next roll of the celestial dice may one day place in our path?