THE asteroids are the joyriders of outer space, amalgamations of cosmic dust that never quite made it to the size of Mars or Mercury, and infest the universe, some say, as a kind of cosmic vermin.
Most of them, it must be said, are well behaved enough to confine themselves to a dense swarm in a belt which lies between the orbits of Jupiter and Mars. A minority however, follow more unusual paths, and a few of them in the course of their wanderings come close enough to Earth from time to time to constitute danger to our planet.
The most observed "Near Earth Object" was asteroid 1991 BA with a diameter of about 20 feet which passed within 100,000 miles of Earth in 1991. It was a near miss - but at least it was a miss. There have been occasions in the recent past, however, when such objects, whether asteroids or cometary debris, scored a hit.
One came to Earth in June 1908 near the Tunguska River in Siberia. It appeared one morning as a gigantic ball of fire blazing a trail across the northern sky before it exploded in a huge pillar of fire that was visible at a great distance.
Trees were levelled radically in a zone extending 30 miles from the occurrence, tremors were recorded on seismographs as far away as London, and for many nights following the "Tunguska Event" the sunsets in Europe and Asia were magnificent.
A second such happening, the "Brazil Tunguska", occurred 65 years ago today, on August 13th, 1931, in the upper reaches of the Amazon, close to the border with Peru. Details of it came to light about one year ago, when Dr Mike Bailey of Armagh Observatory discovered an eye witness report written by a Father Fidele d'Alviano for the papal newspaper L'Osservatore Romano.
The report describes how at 8 a.m. that morning the sun suddenly turned blood red, darkness fell, and a fine white ash rained down on the surrounding countryside. Then three balls of fire whistled across the sky and exploded with a massive shock. The noise was heard for hundreds of miles and the forest burned for several months thereafter.
Fortunately, as far as we know, neither of these incidents resulted in the death of a single human being. Each, however had an energy on impact equivalent to several atomic bombs of the kind that were dropped on Hiroshima, and had either landed near a large centre of population, the destruction caused would have been unimaginable.