Monday, August 17, 2009
THE Perseid meteor shower, known in medieval times as the "tears of St Lawrence", occurs each year around now when the Earth passes through the tail of Comet Swift-Tuttle. The meteors are particles of dust from its wake.
Up to 60 "shooting stars" per hour can be seen on a clear night.
But there’s another source of extra-terrestrial visitors. The Asteroid Belt, a vast ring of debris orbiting the sun between Mars and Jupiter, is all that remains of proto-planets which never got a chance to develop; they collided with each other under fierce gravitational forces when passing by Jupiter.
These, left-overs from the formation of the solar system 4,560 million years ago, are mostly small lumps of rock or metal but four of them are more than 400km in diameter. One, known as Ceres, is 950km wide, a minor planet. From time to time, items from the belt are captured by Earth’s gravity.
Travelling at enormous speeds, they glow white hot in the heat generated when they enter our atmosphere. Most of the objects disintegrate but large ones survive to reach the ground. When they do so they are known as "meteorites".
The dinosaurs were the most famous victims of a meteorite impact. At any rate they, and about half of all species alive around 65 million years ago, were wiped out in some sort of global catastrophe. A meteorite can strike at any time but the risk of being clobbered by one is minuscule.
A dog in Nakhla Egypt was struck and killed in 1911 but there is no known instance of a human fatality. A Ugandan boy, hit by fragments when a meteorite landed close to him in 1992, survived.
One that crashed through the roof of Sprucefield RUC station on April 25, 1969 seemed to have IRA sympathies.
Only eight meteorites have been recorded in Ireland. The earliest fell at Pettiswood, Co Westmeath, in 1779. The most recent struck Leighlinbridge, Co Kilkenny, on November 28, 1999.
A local grandmother, who wished to remain anonymous, found the first piece of the object. Four fragments, with a total weight of 271 grams, were eventually located.
All of the Irish meteorites came to light after "shows" – the objects were seen falling and located in subsequent searches. Nobody has found a meteorite on Irish soil purely by chance.
At nine o’clock on the morning of September 10, 1813, stones rained down on the picturesque village of Adare in Co Limerick. The "Adare aerolites" together formed the largest meteorite recorded on Irish soil. Three of the objects found are quite substantial.
The largest, known as the Brasky Mass, weighs 7kg and features in a meteorite exhibition which has opened at the National Museum in Dublin’s Collins Barracks.
To celebrate the 30th anniversary of the first moon landing, pride of place at the show goes to a piece of rock, brought back by Apollo astronauts in1972 and presented to Ireland by the United States.
Astronauts, however, were not the first bearers of moon rock, meteorites which originated on our nearest celestial neighbour are also on display.
The moon has no atmosphere so objects don’t burn up as they approach it. Some strike so violently that bits of moon rock are flung into space. Drifting towards Earth some fall here as meteorites.
Both the most extraordinary piece of rock on display is from Mars. The red planet, like the moon, is being hammered constantly by meteorites. One of them made a crater 10,000km wide. Mars is much lighter than Earth and a piece of rock flung upwards can escape from it relatively easily, to drift in space for eternity.
Some of these Martian fragments wander in our direction and fall to Earth as meteorites. Only 34 of the 30,000 objects examined to date are of Martian origin.
The Mars rock specimen on show was acquired recently in Munich by the National Museum’s Geology Curator, Dr Matthew Parkes. Rumours that it cost €25,000 are, according to Dr Parkes, grossly exaggerated. It cost, he says, "much less than that".
The Dead Zoo at Large Exhibition, at the National Museum Collins Barracks, is open daily except Mondays. Admission is free.
© Examiner Publications (Cork) Limited, City Quarter, Lapps Quay, Cork. Registered in Ireland: 73385.
