At about 5.00 p.m. Greenwich mean time today (27th June 2011) Asteroid 2011 passed over Australia, Southern Africa and Central America at a distance of about 12 000 km or 32 times as close as the moon, which sounds pretty close, but, to give a sense of proportion, is also 36 times as far from the Earth as the International Space Station.
The asteroid was discovered five days previously (on the 22nd June) by the Lincoln Near-Earth Asteroid Research (LINEAR) project in New Mexico and given the designation 2011 MD, which means the fourth asteroid discovered in the period 16-30 June 2011. For the purpose of naming asteroids the year is split into 24 half month periods, numbered A to Y (I is not used).
An image of 2011MD, a faster moving streak against the background of stars.
2011MD is thought to be a house sized chunk of rock, between 10 and 45 meters across. This is small enough that it would break up in out atmosphere; small chunks might reach the ground but it would be unlikely to do any significant damage. Nevertheless reports of the near miss in the popular press have been peppered with stories about dinosaur killing asteroids. These step from the 1980 theory proposed by physicist Luis Walter Alvarez that the mass extinction 65 million years ago at the end of the Cretaceous Period, most noted for the loss of all non-avian dinosaurs, was caused by a massive meteorite impact, and the subsequent discovery of a global iridium layer at the same stratigraphic level by his son Walter Alvarez. This theory was popular throughout the 80s and 90s, particularly after the discovery of an impact crater off the Yucatan Peninsula in Mexico which seemed to fit the bill. Subsequent study by geologists has suggested that this crater is in fact to early to mark the end of the Cretaceous, and the impact theory in general is not as widely supported in the geological community as might seem to be the case to an outsider. Many geologists feel that volcanic activity associated with the collision of India and Asia, particularly the massive Deccan Traps flood basalts, may have caused a catastrophic climatic breakdown which caused the extinction. However the impact theory has rather more 'Hollywood' appeal than the volcanic one so it remains the theory most likely to be seen in the popular press, and it is still widely supported by astronomers and astrophysicists, partly out of tribal loyalty, and partly because they do not tend to follow geological journals closely.
Almost certainly not how the dinosaurs died out.
This is not to say that a sufficiently large asteroid impact couldn't cause devastation on such a large scale, or indeed far, far worse. The moon is thought to have been produced as the result of a collision between the Earth and a Mars-sized planet over four billion years ago, but as the solar system has aged the number of large objects floating about has steadily decreased; the odds of such a large impact happening now, let alone with an object we had not yet seen, are vanishingly small.
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