Is Comet Swift-Tuttle On a Collision Course?

Learn About the Comet Behind Those Pretty Perseids

October 29, 2020
Perseids Meteor Shower

A meteor streaks across the sky during the annual Perseid meteor shower. The Perseids show up every year in August when Earth ventures through trails of debris left behind by an ancient comet.

NASA/Bill Ingalls

One of the year’s best meteor showers—the Perseids—is now under way. It starts with just a few extra shooting stars per hour, and when we reach the nights of August 11 and 12, we will see a meteor every 2 minutes or so. But the comet behind these lovely shooting stars is what we should really be talking about!

Meteors are tiny specks of debris that have fallen off a comet. Most are no larger than apple seeds. These particular meteors, the Perseids, have been observed for over 2000 years and were chronicled by ancient Chinese sky watchers.

Why Are They Called the Perseids?

They all seem to streak away from the constellation of Perseus, which is why they are called the Perseids. But the orbiting path of each of these tiny bits of dirty ice shows that they share an orbit with a comet named Swift-Tuttle.

Is Swift-Tuttle on a Collision Course with Earth?

That comet came close to Earth during the civil war, when it was discovered the same week in July, 1862 by two famous astronomers, Horace Tuttle and Lewis Swift. The comet has a 133 year orbital period around the sun, locked by Jupiter’s gravity so that it is synchronized with that giant world, and makes one orbit in exactly the same time period in which Jupiter circles the Sun 11 times. Very cool. This made it next approach us in the mid-1990s. But that was an unusually poor and distant apparition, and the comet couldn’t even be seen with the naked eye.

But what makes us really sit up and pay attention is that some astronomers have called comet Swift-Tuttle the most hazardous object to human life in the entire universe. Wow.

That’s because comet Swift-Tuttle is big, has the potential to approach us so closely that sooner or later it may collide with us, and worst of all goes around the sun in the opposite direction from the way we do. So if and when it does hit us it will be a head-on impact.

The speed is actually obvious each year when we watch the summer Perseid meteors. They are very nearly as fast as meteors can possibly be. They streak into our atmosphere at 37 miles per second, which is about 80 times faster than a high velocity rifle bullet. That’s the same speed the comet would hit us. It would be more devastating than the extinction event 66 million years ago that destroyed the dinosaurs and allowed for the ascent of all the beloved mammals that now populate our world, like humans and rats.

So there’s much to think about as we observe these fine summer shooting stars.

Tips for Viewing the Perseids

In 2020, the waning Moon rises around midnight during the Perseids’ peak. See your local Moonrise here.

The Moon casts a lot of light, washing out many nighttime treasures. Therefore, considering watching just before midnight (moonrise) the nights of August 11 and 12.

You should still see some shooting stars, especially as the Perseids are very bright.

Obviously, it helps to be in a location with very dark skies (and no street lamps). If you really want to see the Perseids, take a drive away from city lights. Remember your eyes will need about 20 minutes to adjust to darkness, so be patient.

If you’re stargazing once the Moon is out, place yourself in the Moon’s shadow: Place some large object – a barn, a cabin, a mountain – between you and the Moon. You’ll see more meteors that way than if you’re standing out under moonlight. 

See my complete 2020 Perseids Meteor Shower Guide for more viewing tips!

About This Blog

Welcome to “This Week’s Amazing Sky,” the Almanac’s hub for everything stargazing and astronomy. Bob Berman, longtime and famous astronomer for The Old Farmer’s Almanac, will help bring alive the wonders of our universe. From the beautiful stars and planets to magical auroras and eclipses, he covers everything under the Sun (and Moon)! Bob, the world’s mostly widely read astronomer, also has a new weekly podcast, Astounding Universe