Many years, the Leonids are one of the best meteor showers skywatchers can catch. Every November, observers can expect peaks of 10 to 15 meteors an hour. Occasionally, the display turns sublime: every 33 years or so, there have been instances of the Leonids producing meteors at a rate of about 1,000 – or more – in an hour. However, this won't happen again until 2034. Read the full details for this year's Leonids in our Leonid Meteor Shower Guide.
The particles that make up the Leonid shower appear to emanate from the constellation Leo in the Northern Hemisphere. The source of the meteor shower is Comet Tempel-Tuttle, which is falling apart as it makes periodic runs around the sun and its ices melt from the heat. As Earth plows through the vast number of particles it leaves behind, meteors streak through the atmosphere.
NASA classifies the Leonid meteors as bright, colorful, and "some of the fastest meteors out there." A typical speed for a fragment is about 44 miles (71 kilometers) per second. The meteors take place through much of November, but come to their best at about the middle of the month.
The meteor shower can also produce fireballs, a bright meteor that can leave a streak of color that lingers for several seconds. The fireballs originate from much bigger chunks of cometary stuff, which produces the extra fireworks in the sky. Skywatchers can also spot earthgrazer meteors, or meteors that streak close to the horizon.
"The Leonids are white or bluish white, many are faint, though can appear outstandingly bright, leaving glowing trains in their wake," wrote Space.com skywatching columnist Joe Rao in 2012. "A Leonid meteor that is roughly as bright as the brightest stars, results from a meteoroid that is only a few milligrams in mass."
The Leonids are best visible after midnight, and it's possible for an annual peak to come to a meteor a minute — as long as the observer is in a dark location. City lights, or even the moon, can wash out visibility and substantially lower the count of meteors. No fancy equipment is needed to observe the meteors — just your eyes and if it's cold outside, something to shelter your body from the weather. [Gallery: Most Amazing Leonid Meteor Shower Photos]
About every 33 years, the Leonids can produce a storm of meteors visible at certain locations on Earth. NASA defines one of these storms as a situation in which meteors fall at a rate of at least 1,000 an hour, or about 16 or 17 every minute. The last one of these storms occurred in 2002, but a jaw-dropping one happened in 1966.
"Thousands of meteors per minute fell through Earth's atmosphere during a 15-minute period," NASA officials wrote. "There were so many meteors seen that they appeared to fall like rain."
One observer, James Young, recalled 50 meteors falling per second from his vantage point at the JPL Table Mountain Facility in California.
"We all felt like we needed to put on hard hats," he recounted. "The sky was absolutely full of meteors ... a sight never imagined ... and never seen since! To further understand the sheer intensity of this event, we blinked our eyes open for the same time we normally blink them closed, and saw the entire sky full of streaks ... everywhere!"
For those hoping for a repeat, however, Jupiter will put a damper in the show, Rao said. The giant planet is expected to alter the comet's path and make it "all but impossible" to see a similar storm in most of our lifetimes.
The peak in Leonids follows a similar pattern as Comet Tempel-Tuttle's path around the sun, which also takes place every 33 years. Two astronomers, Ernst Tempel and Horace Tuttle, found it independently in 1865 and 1866.
According to "Comets, A Descriptive Catalog," the comet then vanished from view for about a hundred years. Astronomers weren't able to find it in searches in 1899 and in 1932. Finally in 1965, just before the large meteor storm, it was spotted again as a 16th magnitude object.
Although it leaves behind a powerful punch of debris, the comet itself is quite small, with a nucleus of only 2.24 miles (3.6 km) in diameter.
On its last pass through the solar system, Karen Meech, Olivier Hainaut and James Bauer at the University of Hawaii spotted the comet on March 4, 1997, when it was still only a 22.5 magnitude object. It peaked at about eighth or ninth magnitude in January 1998, making it visible with a pair of binoculars.
After its closest approach just inside Earth's orbit, the comet then made its way out of Earth's neighborhood, trailing a tail behind it. It's next expected to return in 2031, but in the meantime, the Leonids will serve as an annual reminder of that return date.
This article was updated on Dec. 10, 2018 by Space.com Associate Editor Sarah Lewin.