On this day in 1908, the largest asteroid in recorded history exploded above Tunguska in Siberia, Russia.  We now observe Asteroid Day each year on June 30, on the anniversary of what’s now known as the Tunguska explosion. Here are 5 things you didn't know about this famous explosion.

The Explosion Was The Largest In Recorded History When the blast occurred, witnesses claimed to see something of a fireball or a bluish light moving across the sky. It was almost as bright as the sun and was followed by a flash and a sound that mimicked artillery fire. The incredible sight was accompanied by a powerful shockwave that shattered windows for hundreds of miles and knocked people off their feet. 

No Crater Was Ever Found A mysterious aspect of the Tunguska event was that no crater was ever found. But, even without a crater, scientists still categorized it as an impact event. They now believe the incoming object never struck Earth, but instead exploded in the atmosphere, causing what’s known as an airburst. This type of atmospheric explosion was still enough to cause massive damage to the forest in the region. Scientists determined the object was most likely a stony asteroid approximately the size of a 25-story building. The asteroid was traveling at a speed of about 33,500 miles per hour and exploded 3 to 6 miles above Earth’s surface. 

Decades Passed Before Anyone Could Explain The Event Due to the size and speed of the asteroid, the explosion released so much energy that an estimated 80 million trees destroyed, and reindeer were killed for miles around the blast site. At the time of the explosion, it was difficult for anyone to reach that remote part of Siberia. It wasn't until almost 20 years later that Leonid Kulik sled the first Soviet research expedition to investigate the Tunguska event. 

The Cause Of The Event Is Believed To Be From An Asteroid Scientists and researchers came up with all sorts of hypotheses regarding what sparked the Tunguska explosion throughout the years.  Some theories were very colorful, such as the notion that the Earth was encountered by an alien spacecraft or that a mini-black-hole or a particle of antimatter was the culprit. It is now more widely accepted that the blast was likely the result of a stony asteroid that collided with the Earth's atmosphere. 

Two Upcoming Missions Are Planned To Travel To Asteroids Tunguska was the largest cosmic impact witnessed by modern humans. It's also representative of the type of impact we'll have to guard against in the future. Following the Tunguska incident, astronomers have been paying increased attention to the likelihood of cataclysmic comet impacts  Today, "Near-Earth Objects." observation programs are in place to keep an eye out for objects that appear to be nearing the Earth's atmosphere.  Scientists are currently laying the groundwork for two separate missions to the asteroids Didymos and Didymos. The Hera mission of the European Space Agency is scheduled to launch in 2024. DART is a NASA project that will launch at the end of this year. The DART mission will collide with Didymos' small moonlet in order to explore how we can nudge an object in space and modify its course, a task we may have to face in the future if a dangerous object is heading for Earth. The Hera expedition will travel to Didymos to study DART's impact.