by ANDY FLEMING
An asteroid measuring 45 metres across will whizz past Earth just 28,000 kilometres above the equator on Friday February 15, 2013– a record close approach for an object of its size. The 130,000 tonne space rock designated 2012 DA14 will miss Earth so narrowly that it will come within the orbit of some communication satellites, travelling at a speed of five miles per second – eight times the speed of a bullet from a rifle.
Although smaller asteroids have made even closer approaches the close shave, which will peak at about a thirteenth of the distance to the moon, will be the nearest for such a large object since records began. Sir Isaac Newton’s physics and the great man’s laws of gravitation are so exquisitely accurate that NASA knows for sure that 2012 DA14 will indeed miss the Earth.
Graphic depicts the trajectory of asteroid 2012 DA14 on February 15, 2013. In this view, we are looking down from above Earth's north pole. Image credit: NASA/JPL-Caltech
But one day such a large rock from space will indeed strike the Earth. Infact a rock much, much larger, perhaps over up to ten kilometres in diameter will once again impact our planet. We know for sure that it has happened before, and we know for sure that it will happen again. It’s not a question of if; it’s just a question of when.
Take what happened to poor old Tyrannosaurus Rex. It reads like an Indiana Jones blockbuster, but infact it’s an account of a 65 million year old cosmic detective story that ends with a theory, now accepted by a majority of the world’s scientists, as to what caused the extinction of the dinosaurs, and with them seventy five per cent of the Earth’s plant and animal species.
Like any riveting detective story it starts at the crime scene, commencing with a ten kilometre diameter asteroid, hurtling towards the Earth at forty thousand miles per hour. In less than a second the object hurtled through the Earth’s atmosphere and buried itself forty to fifty kilometres deep in the planet’s upper mantle at Chicxulub in the Yucatan Peninsula in Mexico. The global catastrophe that followed caused by the lava, ejector, massive tsunamis, global firestorms and associated atmospheric particulates and soot, is difficult to comprehend in scale. The melting of carboniferous limestone at the one hundred kilometre wide crater also lead to a massive increase in atmospheric carbon dioxide, which had serious ramifications for the planet’s climate.
Scientists postulated that the KT or Cretaceous–Tertiary mass extinction event was caused by an asteroid impact and evidence from Italy to the USA and on to Mexico, supported this hypothesis. The ‘smoking gun’ at the crime scene turned out to be the abundance of Iridium in the world-wide sedimentary layer marking the boundary between the Cretaceous–Tertiary, below which fossils of the extinct species, including dinosaurs are found, and above which they aren’t.
Geologists soon realised the scale of the impact, and the hunt was then on to locate the gigantic crater. Confirmation of the impact site in the Yucatan came in 1978 when two geophysicists working for the Mexican state-owned oil company were undertaking an airborne magnetic survey. Supporting evidence near the crater, included rising levels of Iridium, gravity anomalies, shocked quartz and tektites. Isotopic dating confirmed the age of the crater at 65.5 million years – the same age as the worldwide Iridium layer.
As late as 1975, many scientists thought that craters were caused by mysterious explosions in the Earth’s mantle (just as they had initially thought the lunar craters were a result of volcanism). The sea change in the opinion of scientists was created in no small part by Gene and Carolyn Shoemaker, and their work in identifying Meteor Crater in Arizona as an asteroidal impact site.
Perhaps the pivotal event that really brought public and political attention to the cataclysmic devastation wreaked by asteroid and cometary impacts was the pummelling of Jupiter’s atmosphere by the comet Shoemaker-Levy 9 in July 1994 – and the largest fragments of this impactor were only two kilometres in diameter!
Most scientists now accept that it was this cosmic catastrophe that killed off the dinosaurs. They had been the most successful group of animals to ever inhabit the Earth; indeed they had reigned unchallenged for 165 million years, compared to mankind’s couple of million years history. The KT Impact was the pivotal event that paved the way for the ascent of mammals, and ultimately us.
Given enough warning and time we know that we can deflect asteroids using a variety of practical methods from gravity to laser beams, all using known technology. Unlike the dinosaurs a mass extinction by asteroid impact is now not inevitable but more funding is needed for NASA and other space agencies to successfully identify and track PHAs or Potentially Hazardous Asteroids. And unfortunately that requires political will and the scientific literacy of our leaders.