A fiery explosion tore through the skies over Eastern Siberia on the morning of June 30, 1908, decimating more than 830 square miles of frozen taiga in what remains the largest asteroid-related blast in recorded history. Known as the Tunguska event, today it serves as a stark reminder of potential dangers presented by space objects that cross paths with our planet.

In commemoration of the 1908 incident, June 30 is recognized worldwide as International Asteroid Day, as part of an effort to raise awareness about asteroid hazards and to promote international cooperation in addressing their statistically rare, but still ever-present and potentially deadly reality.

Now, revealed in satellite imagery obtained last summer by NASA’s Operational Land Imager (OLI) on Landsat 8, the blast site as it appears today can be seen to show no direct signs of an impact, or even any damage from the blast which more than a century ago that had been large enough to level a modern city.

The Day the Sky Split Open

Eyewitness descriptions preserved from the time of the Tunguska event are still haunting today, with many reporting observations of the blazing fireball streaking across the sky at an estimated 60,000 miles per hour.

In Kirensk, observers saw a ball of fire descend toward the horizon, followed by deafening crashes and thunderous bangs. One witnessed described seeing the blazing object descending, and after several minutes, hearing “separate deafening crash[es] like peals of thunder” followed by “eight loud bangs like gunshots.”

“As it approached the ground, it took on a flattened shape,” one eyewitness reported, while another described the object as resembling “a flying star with a fiery tail” that “disappeared into the air.”

“I saw the sky in the north open to the ground and fire poured out,” another witness description reads. “The fire was brighter than the sun. We were terrified, but the sky closed again and immediately afterward, bangs like gunshots were heard. We thought stones were falling… I ran with my head down and covered, because I was afraid stones my fall on it.”

Another striking eyewitness report detailed how heat from the blast wave struck him, carrying him off the porch of the local trading station.

“Suddenly in the north … the sky was split in two, and high above the forest the whole northern part of the sky appeared covered with fire,” the witness report reads. “I felt a great heat, as if my shirt had caught fire… At that moment there was a bang in the sky, and a mighty crash… I was thrown twenty feet from the porch and lost consciousness for a moment…. The crash was followed by a noise like stones falling from the sky, or guns firing. The earth trembled…. At the moment when the sky opened, a hot wind, as if from a cannon, blew past the huts from the north.”

Damaging vegetation in the community, the witness also said that “many panes in the windows had been blown out and the iron hasp in the barn door had been broken” following the incident.

Scientific Mystery and Ongoing Investigations

Although the explosion was detected by seismic instruments more than 600 miles away, scientific expeditions to the remote Siberian site would not be undertaken until almost two decades after the blast. By the time they reached ground zero, the blast’s effects were plainly evident: trees were flattened for hundreds of miles, while those which remained standing directly beneath the blast site remained standing, albeit stripped of their bark.

Researchers found no crater—only scorched, branchless trees flattened in a radial pattern. Estimated to have been close to 50 meters in diameter, the Tunguska object disintegrated in the atmosphere in what scientists believe to have been a massive air burst, releasing enough energy to destroy a modern city.

As to the object’s specific identity, the prevailing theory today involves an asteroid airburst, although some scientists have argued that a comet could have been responsible. Several maps of the Tunguska blast zone have been produced over the years, which possesses a distinctive butterfly shape, and studies since the 1990s have uncovered evidence of particles trapped in local tree resin that appear to support the asteroid hypothesis.

Additional evidence in the form of rock fragments, shocked quartz, and anomalies in tree rings are also suggestive of an asteroid or comet airburst. Although the airburst theory is commonly accepted, some scientists have argued the possibility that nearby Lake Cheko could have been formed by a bolide fragment associated with the blast. This idea remains disputed, however, and no confirmed evidence of craters or the original object have ever been found.

Tracking Near-Earth Objects

According to NASA’s Earth Observatory, comets or asteroids must have an orbit that brings them to within 1.3 astronomical units of the Sun before they qualify as near-Earth objects (NEOs).

Based on data available from as recently as June 2025, NASA’s current catalog of known near-Earth asteroids included more than 38,000 objects—a number that has grown rapidly in recent years, with asteroid surveys adding as many as several hundreds of new entries within any given month.

For instance, within a span of just a few days in June 2025, the Vera C. Rubin Observatory reported the discovery of 2,104 new asteroids in our solar system, seven of which were deemed to be NEOs. Going forward, astronomers expect that the observatory’s massive digital camera will likely reveal the presence of at least hundreds of thousands more—if not millions—as it scans wider stretches of Earth’s night sky.

Fortunately, the majority of NEOs pose no threat to life on Earth and are unlikely to ever come close enough to intersect with its orbit. Nonetheless, in order to help mitigate the potential for a rare “city-killer” asteroid impact, in 2016 NASA created the Planetary Defense Coordination Office to identify and monitor those space objects that could present a potential hazard.

Only three years beforehand, such a potentially hazardous incident had occurred in 2013, when a meteor exploded over Chelyabinsk, Russia. That air burst, recorded with modern instruments, was as much as 33 times more powerful than the Hiroshima atomic bomb and reignited global concern over asteroid threats.

A Call for Global Preparedness

In the wake of Chelyabinsk, international coordination has seen a significant increase as space agencies and other groups from countries around the world have united to help mitigate asteroid threats.

The UN’s Office for Outer Space Affairs (UNOOSA) supports initiatives like the International Asteroid Warning Network (IAWN) and the Space Mission Planning Advisory Group (SMPAG), both of which were launched to increase international cooperation in the detection and response to any NEO threats that may arise.

“International Asteroid Day aims to raise public awareness about the asteroid impact hazard,” a statement on the UN’s website reads, emphasizing the need for coordinated crisis communication in the event of a credible threat.

Although the technologies used for the detection and monitoring of NEOs is improving at a steady pace, events like Tunguska and Chelyabinsk serve as reminders of the dangerous potential for asteroid impacts, and therefore inspire vigilance against what remains one of space’s most deadly and unpredictable dangers.

Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. He can be reached by email at micah@thedebrief.org. Follow his work at micahhanks.com and on X: @MicahHanks.