The Tunguska Event of 1908: What Exploded Over Siberia

What happened on June 30, 1908

NASA places the event near the Stony Tunguska River in Siberia, Russia. The Smithsonian's National Air and Space Museum says trees were flattened over more than 2,000 square kilometers (800 square miles) and that the shock wave knocked people to the ground at distances of tens of kilometers from the blast point. The region was thinly populated, and the first scientific survey was not carried out until about two decades after the event.

A NASA workshop report from 2018 adds more detail. Fewer than a hundred anecdotal eyewitness reports exist, mostly collected decades later. Windows broke out to 200 or 300 km, and burns to exposed skin were reported out to 50 km. The pressure wave was recorded by instruments around the world, a seismic signal was picked up at Irkutsk about 1,000 km to the south, and bright "white nights" lingered for several nights over northern Europe.

• June 30, 1908: an airburst over the Stony Tunguska River region flattens forest across a wide area.
• 1920s: Leonid Kulik begins a series of expeditions to the site that continue into the 1930s.
• 1966: experiments by Zotkin and Tsikulin show the explosion was not a simple point blast.
• 1993: Christopher Chyba and colleagues model it as the airburst of a stony asteroid at about 10 km altitude.
• February 15, 2013: the Chelyabinsk airburst provides modern data to test the models.
• January 16, 2018: a NASA Ames workshop reexamines Tunguska, and the models converge on 10 to 20 megatons.

One small source of confusion is the date. A 2025 preprint by the independent researcher Andrei Ol'khovatov points out that, by the old Russian calendar in use in 1908, the event took place on June 17, so historical Russian records may carry that date.

What the forest shows

Most of what scientists know comes from trees. The NASA report says the data consist mainly of the distribution of fallen and burned trees, the local terrain, a handful of eyewitness accounts, seismic records, the infrasound pressure wave and the bright nights. The trees fell in a pattern often described as a butterfly, with a north-south asymmetry. Simulations by NASA Ames researchers reproduced that asymmetry only when they included the real hills and swamps of the site, which raised the peak wind speed by about 21% compared with flat ground.

The damage was not uniform. The Ol'khovatov preprint notes that the commonly quoted figure of about 2,150 square km is the area where at least some trees fell, and that a calculation from an aerial survey map puts the area of complete devastation near 500 square km. Near the center, in a zone about 3 to 5 km across, many dead trees stayed standing with their crowns torn off, and a number of groves survived. Preprints have not been through peer review, so treat these details as one researcher's reading of the older Russian records.

Heat mattered too. The NASA report says thermal radiation from the fireball was enough to start forest fires, and a radiation model gives a tree-burn zone with a radius of about 10 km.

Why there is no crater

An object that breaks up in the air does not dig a hole. The 1993 Nature study by Chyba, Thomas and Zahnle, hosted by NASA, found that the explosion released 10 to 20 megatons of high-explosive energy at about 10 km altitude, and that this is a typical fate for stony asteroids tens of meters across. The same study says comets and carbonaceous asteroids of that energy would break up too high, while typical iron objects would reach the surface and leave a crater. How impact craters form and are confirmed covers that contrast, and iron is one of the families described in the main types of meteorites.

The NASA report lists other points. No fragments have been recovered, perhaps because the object fell in a boggy area. A model run at Ames excluded a long-period comet because it would need implausibly high strength to reach the altitude that matches the damage. Physicist Jay Melosh proposed that hot air can seep into a porous, fast-moving object and speed up its breakup, which might explain why an object roughly 100 m across disintegrated so thoroughly. NASA also says that volcanic and mining explanations were ruled out for lack of physical evidence.

How big and how powerful

The estimates differ because the physical data are thin. The NASA report says the main limit is the scarcity of quantitative data, not weak computer models. The table shows the main numbers and where they come from.

A megaton is the energy of a million tons of TNT. NASA compares the blast with the 1980 eruption of Mount St. Helens. The low end of the range comes from physicist Mark Boslough, who estimates 3 to 5 megatons and argues that older estimates based on nuclear tests may run high. The 2018 workshop favored the higher figures, but its report notes that the energy cannot be turned into a size without assumptions about the object's density and path.

What Tunguska taught scientists

Through the twentieth century, Tunguska was the only known case of extensive damage from a cosmic impact. Then the Chelyabinsk airburst of 2013 supplied video, a light curve and sensor data. NASA says the models validated on that event were applied to Tunguska, and that four different computer codes reached similar conclusions. The Chelyabinsk airburst of 2013 is where those data are described.

The biggest change was in how often such events happen. The workshop report says a Tunguska-class object, with 10 to 20 megatons of energy, hits somewhere on Earth about once every 2,000 to 4,000 years, and about three times less often on land. NASA's article describes the interval as on the order of millennia, not the centuries once assumed. These are long-term averages with large uncertainty, since the report says the population of such asteroids is uncertain by a factor of two or more. They are not forecasts. The same report says current surveys of near-Earth asteroids are far from complete at this size, a gap discussed in near-Earth asteroids and how they are tracked.

Common mistakes and myths

• "A giant crater was found." None was. NASA says no crater or fragments were found, and the airburst explanation accounts for that.
• "It was a nuclear blast or an alien ship." The NASA workshop report describes nuclear, antimatter and spacecraft ideas as bizarre explanations put forward by fringe enthusiasts. Natural airburst models, meanwhile, fit the tree-fall, pressure and seismic data.
• "Every tree in 2,000 square km was flattened." The area figure marks where some trees fell. One researcher's review of older records says the fully devastated zone was much smaller.
• "Hundreds of people died." The area was very thinly populated. The NASA report says there are fairly reliable accounts of several deaths in nomad camps about 25 km from the center, and no larger toll is established.
• "It was certainly a comet." The question was debated for decades. Modern models favor a stony asteroid and find a long-period comet unlikely.

Frequently asked questions

How big was the Tunguska explosion?

The best recent estimate is 10 to 20 megatons of TNT equivalent, from a 2018 NASA workshop and a 1993 study. Published values range from about 3 to 30 megatons.

Was anyone killed?

Reliable accounts describe several deaths in nomad camps about 25 km from the center, according to the NASA workshop report. The forest was almost uninhabited, and no larger count has been established.

How often do events like Tunguska happen?

NASA's workshop report estimates about once every 2,000 to 4,000 years for an object of that energy hitting anywhere on Earth, and longer for land impacts. That is a long-term average and says nothing about when one might occur.

How do we know it was an airburst?

The trees fell outward in a radial pattern and many were scorched, with no crater and no fragments. Models of an exploding stony asteroid reproduce the damage, the pressure wave and the seismic signal.