Photographed from 200km away, a fireball streaks through the sky near Chelyabinsk, Russia, on February 15. 2013. It wasn't quite fireball season yet, but the sky-splitting, glass-shattering fireball made a big impression around the world. Image credit: Alex Alishevskikh via Wikimedia Commons // CC BY-SA 2.0
Yesterday, March 20, was the vernal equinox. Spring is here—and so is fireball season!
Keep an eye on the night sky over the next couple of weeks to spot particularly bright meteors called fireballs. Though fireballs can be seen throughout the year, once we reach the vernal equinox, their appearance can increase by 30 percent. If you live in an area with clear skies and little light pollution, you're sure to spot some even by happenstance.
When space debris enters Earth's atmosphere, it can create a visible stream of light called a meteor (aka a "shooting star"). As we've discussed previously here at mental_floss, known meteor showers are the result of particles from comets—and in at least one instance, an asteroid—slamming into Earth's atmosphere at tens of thousands of miles per hour. At such speeds, even a particle the size of a grain of sand can produce an extraordinary streak of light. Fireball sounds like a casual term, but it actually has a specific meaning: Any meteor brighter than Venus in the night sky is called a fireball. If it disintegrates in our atmosphere, it's called a bolide.
Below is a map of 556 fireballs that were detected by NASA's Near Earth Object Program between January 1, 1994, and December 31, 2013. The fireballs spotted during the day are in yellow, and those observed at night are in blue. The size of each fireball on the map is proportional to the amount of its radiated energy as measured in gigajoules on a scale from 1 to 1 million.
Image credit: NASA Planetary Science
Generally speaking, astronomers have a pretty good grasp on the origins of meteor showers that occur regularly. The Orionids in October come from a phantom trail of dust and rock left behind by Comet Halley. Accumulated over thousands of years, when the Earth crosses through the trail, the Orionids light up the night. The Geminid meteor shower, meanwhile, is an annual event in December produced by the Earth's orbit intersecting with a debris trail left behind by Asteroid 3200 Phaethon, the so-called "rock comet."
But here's the thing: Nobody knows the origin of fireball season. NASA's best guess is that this region of space is more littered with space debris than others. In cooperation with scientists across the country, the agency established the All Sky Fireball Network to collect data for the NASA Meteoroid Environment Office. The MEO, in turn, builds models of the meteoroid environment in order to work out what's going on, exactly, and to determine how spacecraft "in and beyond Earth's orbit" might avoid the risk of damage from meteoroids.
Here's a video overview Science @ NASA put together a few years ago about the "beautiful mystery" of fireball season.