The Tunguska Explosion and the Exploding Spaceship Hypothesis

30 June 1908. About 07:14 local time. At the Vanavara trading post, some 65 km south of the epicentre, S. Semenov sat by his house, facing north. "The sky split in two and fire appeared high and wide over the forest." The split grew until the entire northern side was covered with fire. The heat felt "as if my shirt was on fire." Then the sky shut closed. A strong thump. He was thrown a few metres. His wife ran out and led him to the house. Then came noise "as if rocks were falling or cannons were firing," and the Earth shook.

He was not alone. Chuchan, of the Evenki Shanyagir group, saw trees falling and branches on fire, "as if there was a second sun, my eyes were hurting." The Sibir newspaper reported a "strangely bright (impossible to look at) bluish-white heavenly body" that "turned into a giant billow of black smoke." Nobody described a craft. Some Evenki, historian Andy Bruno argues, blamed the spirit Ogdy.

Leonid Kulik led Academy of Sciences expeditions in 1921 and from 1927 to 1939. He found about 2,150 km² of forest flattened in a radial pattern, scorched trees still standing at the centre, and no crater.

August 1945. American atomic bombs fell on Hiroshima and Nagasaki. Soviet science fiction writers turned to Tunguska with fresh eyes. In January 1946 the engineer and writer Alexander Kazantsev published "Explosion" in Vokrug sveta. His answer was a nuclear-powered alien spacecraft that exploded over Siberia in 1908. He pointed to real clues. No impact crater. A forest left standing at the epicentre, like the trees under the Hiroshima blast. No meteorite fragments.

Felix Ziegel, then at the Moscow Planetarium, promoted the theory from 1947. The planetarium staged a dramatised lecture, with light and sound effects of a fireball crossing the taiga and a staged dispute among three hypotheses, one of them an alien ship accident. Ziegel later argued the object exploded in the air after a curving manoeuvre of some 600 km.

29 May 1948. E. L. Krinov, K. P. Stanyukovich and V. V. Fedynsky accused the planetarium in Moskovsky Komsomolets of promoting "false hypotheses." A debate in Nauka i zhizn followed in 1951. In 1959 geophysicist Alexei Zolotov proposed ways to test the idea, and young Tomsk scientists, among them Gennady Plekhanov, set out as the KSE, which Bruno calls "a distinctively Soviet form of citizen science." Kirill Florensky of the Academy traced the only radioactivity in the trees to bomb-test fallout. On 8 January 1961 physicist L. A. Artsimovich attacked Soviet UFO lecturers in Pravda.

Yet Bruno notes a twist. "Kazantsev got something right: he claimed that the blast must have been above ground." Meteorite scientists denied it at the time. Later they came to see it as correct.

August 2004. In Krasnoyarsk, Yuri Lavbin's Tunguska Spatial Phenomenon Foundation announced "two strange black stones," cubes with sides of a metre and a half, and materials said to "recall an alloy used to make space rockets." Benny Peiser called it "a rather stupid hoax." James Oberg predicted ordinary space junk. By January 2005, no findings had followed.

Today the consensus is an airburst of a stony asteroid about 50 to 60 m across, 5 to 10 km up. Some favour a comet. The spaceship endures anyway, in The Fire Came By (1976), Jacques Vallée's UFO Chronicles of the Soviet Union, The X-Files and the 1998 documentary Tunguska: The Russian Roswell. In Bruno's words, the fiction "became a hypothesis and the source of significant and acrimonious public debate about the borders of legitimate science."