Tunguska 1908: the explosion over Siberia

It was morning, not night. At about a quarter past seven on 30 June 1908, in one of the emptiest parts of the Russian Empire, the sky over the Podkamennaya Tunguska River lit up. According to NASA's history office[nasa-history-2023], the few Evenki reindeer herders who saw it described a fireball trailing smoke, then a flash brighter than the Sun, then a noise like thunder. Those closest were blown off their feet and knocked unconscious; their tents were damaged or destroyed.

Further south, at the trading post of Vanavara, a man was sitting on the porch. The Science@NASA centenary feature[science-nasa-2008] says he was hurled from his chair, and the heat was so strong he felt his shirt was on fire. He was about 65 kilometres from the centre.

How many people were there? A peer-reviewed re-reading of the old accounts by Jenniskens and colleagues[jenniskens-2019] counts at least 30 people in or near the zone where trees fell. Many lost consciousness. At least three died, then or later, as a direct result.

Far away, instruments noticed too. NASA[nasa-history-2023] notes that seismic stations hundreds of miles off recorded the shock.

The witnesses in this file, then, are herders, a trader and a set of needles on paper. None of them saw what came down. They saw what it did.

For nineteen years nobody with a notebook went to look. The site was remote, and the country went through war and revolution. NASA[nasa-history-2023] records that the Soviet Academy of Sciences sent the geologist Leonid Kulik in 1921, but he could not reach the area. In 1927 he did.

What he found is still the core of the record. Near the centre, trees stood upright but stripped of branches and bark, with signs of burning. Further out, they lay flat, pointing away from the centre. Later aerial and ground surveys in the 1950s and 1960s mapped a butterfly-shaped zone of about 830 square miles, roughly 2,150 km², with millions of trees down. What Kulik did not find was a crater, or a meteorite.

Today the forest has grown back. A Landsat 8 image[nasa-eo-2024] taken on 6 July 2024 shows healthy pine, marsh and river, with labels for the approximate epicentre and one possible flight path. NASA notes that published estimates of the trajectory vary considerably.

The event also became a date in the calendar. In 2016 the United Nations[un-asteroid-day] proclaimed 30 June, the Tunguska anniversary, International Asteroid Day.

Inventory: eyewitness accounts collected years later, seismic records, Kulik's expedition notes known here through NASA, survey maps of fallen trees, a modern satellite image, and a UN resolution. No piece of the object itself.

The turn in this file is the missing crater, and what it turned out to mean.

For decades, the absence of a hole in the ground invited every kind of explanation, including, in fiction and later in fringe writing, an exploding spacecraft. No source opened for this file supports that idea, and it does not need to be stamped to be set aside.

The science went another way. In 1993 Chyba, Thomas and Zahnle[chyba-1993] showed in Nature that a stony asteroid tens of metres across, entering at high speed, would be crushed by the air and release its energy several kilometres up. An explosion in the sky leaves flattened trees and no crater. The NASA Earth Observatory[nasa-eo-2024] now gives a body of 50 to 100 metres and an airburst at 6 to 10 kilometres.

Later modelling refined the size of the blast. Boslough and Crawford[boslough-2008] argued that a low-altitude airburst carries its energy downward, so a smaller explosion than once thought can do the same damage. That matters beyond Siberia: smaller means more frequent. JPL's Don Yeomans, in the centenary feature[science-nasa-2008], put the average at one Tunguska-sized entry every 300 years.

So the grade here is Recorded, and the finding Explained: a natural cosmic airburst. What remains is detail, not mystery of kind.

What is still open about Tunguska is the object, not the event.

First, what it was made of. Most scientists, says NASA[nasa-history-2023], favour an asteroid over a comet. Microparticles found by later expeditions point to an extraterrestrial origin but could not be tied to the blast with certainty.

Second, Lake Cheko. In 2007 a Bologna team led by Gasperini[gasperini-2007] proposed that this small lake, about 8 kilometres from the epicentre, fills a crater made by a surviving fragment. A year later Collins and colleagues[collins-2008] argued it is not an impact crater at all. NASA's Earth Observatory[nasa-eo-2024] still describes the question as disputed.

Third, the size. Energy estimates differ by a factor of several, depending on the model. Boslough and Crawford[boslough-2008] argue for the lower end. This file has not read the full papers and does not pick a number.

Fourth, the witnesses. The accounts that survive were mostly written down years after 1908. Jenniskens and colleagues[jenniskens-2019] note contradictions and gaps in the written record. The Archive has not opened the original collections of Evenki and Vanavara statements.

The honest position: Recorded, a large airburst over Siberia on 30 June 1908, a flattened forest and injured witnesses. Explained, as a cosmic body exploding in the air. Open, its composition and whether any piece reached the ground.

• Stony asteroid, iron body or comet fragment?
• Is Lake Cheko a crater?
• Where are Kulik's original reports?

The folder stays open and the file stays a draft for the editor.