The Perseid meteor shower happens every August. The Earth plows into debris left behind from Comet Swift-Tuttle, which last passed close to Earth in 1992. The Perseids generally stretch from mid-July to late August. You can find out the meteor shower's exact peak date and exactly what to look for this year in our Perseid Meteor Shower Observing Guide.

While the comet poses no threat to the planet for the foreseeable future, it is a very large object. Its nucleus of 6 miles (9.7 kilometers) in diameter is about the same size as the one that crashed into Earth about 66 million years ago and killed the dinosaurs.

Swift-Tuttle was discovered independently by two astronomers, Lewis Swift and Horace Tuttle, in 1862. When it last made a pass by Earth in 1992, it was too faint to be seen with the naked eye. The next pass, in 2126, could make it a naked-eye comet similar in brightness to the 1997 Hale-Bopp comet — providing that predictions are correct.

A prediction from a few years ago said that Swift-Tuttle could collide with Earth during its next pass, but examining the record of past comets showed that its orbit is far more stable than that. Swift-Tuttle poses no danger to Earth for the foreseeable future, although in a millennium it is expected to pass within a million miles (1.7 million kilometers) of the planet.

Characteristics of the Perseids

Meteor showers are named after the constellation from which the meteors appear to emanate. From Earth's perspective, the Perseids appear to come approximately from the direction of the Northern Hemisphere constellation Perseus. The constellation's position in the sky is:

Right ascension: 3 hours

Declination: 45 degrees

Visible between latitudes 90 and -35 degrees

A typical Perseid meteoroid (which is what they're called while in space) moves at 133,200 mph (214,365 kph) when it hits Earth's atmosphere (and then they are called meteors). Most of the Perseids are tiny, about the size of a sand grain. Peak activity is about 60 meteors per hour during an ordinary year, according to NASA. Almost none of the fragments hit the ground, but if one does, it's called a meteorite.

Peak temperatures for Perseids are more than 3,000 degrees Fahrenheit (1,650 Celsius) as each fragment travels through the atmosphere and both compresses and heats the air in front of it. Most of the fragments are visible when they are about 60 miles (97 kilometers) from the ground.

Image 1 of 5 Dozens of Perseid meteors appear to sprint toward the Milky Way over Colorado's Great Sand Dunes National Park in this composite taken by astrophotographer Cody Limber. "I backpacked into the dunes to make sure that I was far away from all other lights," Limber told Space.com. "Although I got incredibly sandy, it was worth it and I managed to capture a ton of meteors throughout the night! Mars is also visible in this photo, glowing brighter than any other star in the sky." Limber captured this photo during the peak of the Perseid meteor shower on Aug. 12, 2018. (Image credit: Cody Limber Image 2 of 5 Night sky watcher David Kingham took this photo of the Perseid meteor shower from Snowy Range in Wyoming on August 12, 2012. (Image credit: David Kingham/ DavidKinghamPhotography Image 3 of 5 A time-lapse image of the Perseid meteor shower. (Image credit: NASA/JPL) Image 4 of 5 Plenty of meteors — including some fireballs — are visible over Sprague Lake in Rocky Mountain National Park in this photo by astrophotographer Sergio Garcia Rill. He captured these meteors during the peak of the Perseid meteor shower on Aug. 12, 2018. (Image credit: Sergio Garcia Rill Image 5 of 5 Astrophotographer Jeff Berkes sent in a photo of Perseid meteors taken at East Point Lighthouse in southern New Jersey, August 2012. (Image credit: Jeff Berkes)

The meteors are bright, and showers take place during August, which is an easy time for amateur astronomers to stay outside in the Northern Hemisphere, as it is summertime. The Perseids are also one of the most well-known showers, NASA noted. "The Perseids are typically fast and bright meteors that frequently leave trails," the agency wrote.

For years before and after Swift-Tuttle's return in 1992, the Perseids had a spectacular run where amateur astronomers would see as many as several hundred meteors an hour.

"The most likely reason was that the Perseids parent comet was itself passing through the inner solar system and that the streams of Perseid meteoroids in the comet's vicinity were larger and more thickly clumped together, leading to brighter meteors as well as much-higher-than-normal meteor rates," wrote Space.com skywatching columnist Joe Rao in a past article.

In the decades since, however, activity has fallen more to normal levels — although astronomers still characterize the shower as one of the best to watch every year. As of August 2013, the comet was more than 3.2 billion miles (5.1 billion kilometers) away from Earth.

Collision with Earth?

An astronomer calculating Swift-Tuttle's orbit once suggested that it could come dangerously close to Earth in 2126 and possibly collide with the planet. Further refinements, however, show that the comet will not, according to a primer by the Astronomical Society of the Pacific.

Swift-Tuttle's pass by Earth in the year 3044 could bring it within a million miles of our planet. That's just over twice the distance from the Earth to the moon, making the comet very close in astronomical terms.

The uncertainty came because initial projections for Swift-Tuttle's path through space came from only three months of observations in the 1860s, when the comet was first discovered, added the primer's author, Sally Stephens.

In the 1970s, astronomers noted that the number of annual Perseids meteors was increasing, suggesting that the comet would make an appearance soon. "But it failed to show, and soon afterward, Perseid meteor activity dropped sharply. Astronomers wondered if the comet had somehow come and gone unnoticed," Stephens wrote.

Brian Marsden, who was an astronomer with the Harvard-Smithsonian Center for Astrophysics, suggested in 1973 that Swift-Tuttle might be the same comet as one seen in 1737 by a Jesuit missionary in China.

Marsden suggested the comet would return in 1992, which it did, but its closest approach was 17 days off from his prediction. He continued tweaking his calculations. Although he initially predicted a possible collision in 2126, he examined the historical record and found observations of a comet in a similar track as far back as at least 188 A.D. — allowing him to calculate the comet would pose no harm.

"His new calculations show Comet Swift-Tuttle will pass a comfortable 15 million miles from Earth on its next trip to the inner solar system," Stephens wrote.

This article was updated on Dec. 10, 2018 by Space.com Associate Editor Sarah Lewin. Follow us on Twitter @Spacedotcom and on Facebook.