enormous explosion, occurring about 7:40 AM on June 30, 1908, at an altitude of 5–10 km (15,000–30,000 feet), that flattened some 2,000 square km (500,000 acres) and charred more than 100 square km of pine forest near the Podkamennaya Tunguska River in central Siberia (60°55′ N, 101°57′ E), Russia. The energy of the explosion is estimated to have been equivalent to that of about 15 megatons of TNT, a thousand times more powerful than the atomic bomb dropped on Hiroshima, Japan, on Aug. 6, 1945. It is believed that the cause was a small asteroid (large meteoroid (meteor and meteoroid)) perhaps 50–100 metres (150–300 feet) in diameter and having a stony or carbonaceous composition—or, though less likely, a comet—that entered the atmosphere at a speed of 15–30 km per second (30,000–60,000 miles per hour). Objects of this size are estimated to collide with Earth once every few hundred years on average (see Earth impact hazard). Because the object exploded in the atmosphere high above Earth's surface, it created a fireball and blast wave but no impact crater, and no fragments of the object have yet been found. The radiant energy from such an explosion would be enough to ignite forests, but the subsequent blast wave would quickly overtake the fires and extinguish them. Thus, the Tunguska blast charred the forest but did not produce a sustained fire.